Physics Notes for NEET Chapter 27 PDF

60 1532 Electronics Chapter E3 27 Electronics ID (7) They have sharp melting point. (8) Bond strengths are identical throughout the solid. (9) These are considered as true solids. (10) An important property of crystals is their symmetry. Amorphous or glassy solids (1) These solids have no definite e...

60 1532 Electronics Chapter E3 27 Electronics ID (7) They have sharp melting point. (8) Bond strengths are identical throughout the solid. (9) These are considered as true solids. (10) An important property of crystals is their symmetry. Amorphous or glassy solids (1) These solids have no definite external geometrical form. U Semiconductor electronics Solids D YG (2) Ions, atoms or molecules of these solids are not arranged in a definite fashion. It is a state of matter which has a definite shape and a definite volume. The characteristic properties of the solid depends upon the nature of forces acting between their constituent particles (i.e. ions, atoms or molecules). Solids are divided into two categories. U Crystalline solids (1) These solids have definite external geometrical form. ST (2) Ions, atoms or molecules of these solid are arranged in a definite fashion in all it’s three dimensions. (3) Examples : Quartz, calacite, mica, diamond etc. (4) They have well defined facets or faces. (5) They are ordered at short range as well as at long range. (6) They are anisotropic, i.e. the physical properties like elastic modulii, thermal conductivity, electrical conductivity, refractive index have different values in different direction. (3) Example : Rubber, plastic, paraffin wax, cement etc. (4) They do not possess definite facets or faces. (5) These have short range order, and there is no long range order. (6) They are isotropic. (7) They do not have a sharp melting point. (8) Bond strengths vary. (9) These are considered as pseudo-solids or super cooled liquids. (10) Amorphous solids do not have any symmetry. Terms Related with Crystal Structure (1) Crystal lattice : It is a geometrical arrangement of points in space where if atoms or molecules of a solid are placed, we obtain an actual crystal structure of the solid. (2) Basis : The atoms or molecules attached with every lattice point in a crystal structure is called the basis of crystal structure. Space lattice Basis containing two different ions Electronics 1533  =  = 90o and   90o KclO3, etc. Number of lattices = 4 Monoclinic a bc  FeSO4 c  a Number of lattices = 2 Triclinic  b   b a  K2Cr2O7, CuSO4 etc. a =b=c == 90o Calcite, As, Sb, Bi etc.  =  = 90o Zn, Cd, Ni etc. 90o a Rhombo-hedral or Trigonal   a bc Number of lattices = 1 ID c   b c E3 (3) Unit cell : Is defined as that volume of the solid from which the entire crystal structure can be constructed by the translational repetition in three dimensions. The length of three sides of a unit cell (3D) are called primitives or lattice constant they are denoted by a, b, c 60  b (4) a U unit cell volume unit cell or a Primitive cell : A primitive cell is 3aDminimum 2D unit cell the simple unit cell with particles Fig. only27.1 at the corners is a primitive unit cell and other types of unit cells are called non-primitive unit cells. There is only one lattice point per primitive cell. D YG (5) Crystallographic axis : The lines drawn parallel to the lines of intersection of the faces of the unit cell are called crystallographic axis. All the crystals on the basis of the shape of their unit cells, have been divided into seven crystal systems as shown in the following table.   c  b Number of lattices = 1 Hexagonal Table 27.1 : Different crystal systems System Lattice constants Cubic c   a =b=c a  and  = 120o  b  Number of lattices = 1 = =  = White 90o NiSO4 etc. (2) Plane of symmetry : It is Fig. an imaginary plane which passes through 27.2 the centre of a crystal and divides it into two equal portions such that one part is exactly the mirror image of the other. = =  = HgCl2, Number of lattices = 3 ST a =bc c Different Types of Symmetry in Cubic Lattices 90o a b Examples Diamond, NaCl, Li, Ag, Cu, NH4Cl, Pb etc. = =  = U  Angle between lattice constants (1) Centre of symmetry : An imaginary point within the crystal such that any line drawn through it intersects the surface of the crystal at equal distances in both directions. Tetragonal c    b a =bc tin, a Number of lattices = 2 Orthorhombic c    a bc 90 b a o KNO3, gallium etc. (B) Diagonal plane of symmetry (A) Rectangular plane of symmetry Fig. 27.3 1534 Electronics A cubical crystal possesses six diagonal plane of symmetry and three rectangular plane of symmetry. (3) Axis of symmetry : It is an imaginary straight line about which, if the crystal is rotated, it will present the same appearance more than once during the complete revolution. In general, if the same appearance of a crystal is repeated on rotating through an angle 360 o , around an imaginary axis, the axis is called an nn fold axis. th Let N , N and N be the number of atoms at the corners, centre and face of the unit cell respectively. Therefore the number of atoms per unit N N cell is given by N  N b  f  c 2 8 c b f 60 Table 27.2 : A cubical crystal possesses in all 13 axis of symmetry (3) Atoms per unit cell : An atom located at the corner of a unit cell of a lattice is shared equally by eight other unit cells in the three dimensional lattice. Therefore, each unit cell has 1/8 share of an atom at its each corner. Similarly, a face of the unit cell is common to the two unit cells in the lattice. Therefore, each unit cell has 1/2 share of an atom at its each face. The atom located at the centre of the unit cell belongs completely to the unit cell. (i) In sc lattice : N b  0, N f  0, N c  8 so N  1 Axis of three-fold symmetry = 4 Axis of two-fold symmetry = 6 (Because of six faces) (Because of eight corners) (Because of twelve edges) (ii) In bcc lattice : N b  1, N f  0, N c  8 so N  2 (iii) In fcc lattice : N b  0, N f  6, N c  8 so N  4 E3 Axis of four-fold symmetry = 3 ID (4) Elements of symmetry : The total number of planes, axes and centre of symmetry possessed by a crystal are termed as elements of symmetry. A cubic crystal possesses a total of 23 elements of symmetry. U Planes of symmetry  (3  6)  9 , Axes of symmetry  (3  4  6)  13 , D YG Centre of symmetry = 1. Total number of symmetry elements = 23 More About Cubic Crystals (1) Different lattice in cubic crystals : There are three lattice in the cubic system. (i) The simple cubic (sc) lattice. (ii) The body-centered cubic (bcc). (iii) The face-centered cubic (fcc). bcc The atomic packing fraction indicates how close the atoms are packed together in the given crystal structure or the ratio of the volume occupied by atoms in a unit cell in a crystal and the volume of unit cell is defined as APF. (i) For sc crystal : Volume occupied by the atom in the unit cell  4 3 a 3. Volume of the unit cell  a 3 r  3 6 Thus P.F.  (ii) For bcc : P.F.  ST (C) a C B bcc unit cell r  r 3a/4 Fig. 27.5 a r a a A a r r A a fcc unit cell r  a/2 2 (6) cell  C r r r r sc unit cell r = a/2 D r   6  0.52 = 52% 3  68% 8   74% 3 2 (2) Atomic radius : The half of the distance between two atoms in contact is defined as atomic radius. a a3 (iii) For fcc : P.F.  (B) Fig. 27.4 (A) a 3 / 6 fcc U sc (4) Co-ordination number : It is defined as the number of nearest neighbours that an atom has in a unit cell. It depends upon structure. (i) Simple cubic structure : Each atom has two neighbours along Xaxis, two along Y-axis and two along Z-axis so co-ordination number = 6. (ii) Face-centred cubic structure: Every corner atom has four coneighbours in each of the three planes XY, YZ, and ZX so ordination number = 12 (iii) Body-centred cubic structure: The atom of the body of the cell has eight neighbours at eight corner of the unit cell so co-ordination number = 8. (5) Atomic packing fraction (or packing factor or relative packing density) B Density of unit cell : Density of unit Mass of the unit cell nA nA   Volume of the unit cell NV Na 3 where n = Number of atoms in unit cell (For sc lattice n = 1, for bcc lattice n = 2, for fcc lattice n = 4), A = atomic weight, N = Avogadro’s number, V = Volume of the unit cell. (7) Bond length : The distance between two nearest atoms in a unit cell of a crystal is defined as bond length. (i) In a sc lattice : Bond length = a (ii) In a bcc lattice : Bond length  a 3a (iii) In a fcc lattice : Bond length  2 2 Electronics 1535 (5) Hydrogen bonding : Hydrogen bonding is due to permanent dipole interaction. Hexagonal Close Packed (HCP) Structure The HCP structure also maximizes the packing fraction (i) This bond is stronger than vander Waal's bond but much weaker than ionic and covalent bond. (ii) They possesses low melting point. (iii) Common examples are H O, HF etc. 2 Single, Poly and Liquid Crystals c (1) Single crystal : The crystals in which the periodicity of the pattern extends throughout the piece of the crystal are known as single crystals. Single crystals have anisotropic behaviour i.e. their physical properties (like mechanical strength, refractive index, thermal and electrical conductivity) are different along different directions. The small sized single crystals are called mono-crystals. b (A) (1) a  b  c 60 a (B) Fig. 27.6 (2) Number of atoms per unit cell = 6 (2) Poly-crystals : A poly-crystal is the aggregate of the monocrystals whose well developed faces are joined together so that it has isotropic properties. Ceramics are the important illustrations of the poly-crystalline solids. E3 (3) The volume of the hexagonal cell = 3 2 a 3  2 (4) The packing fraction  6 (5) Coordination number = 12 ID (6) Magnesium is a special example of HCP lattice structure. Bonding Forces in Crystals Energy Bands This theory is based on the Pauli exclusion principle. In isolated atom the valence electrons can exist only in one of the allowed orbitals each of a sharply defined energy called energy levels. But when two atoms are brought nearer to each other, there are alterations in energy levels and they spread in the form of bands. U The properties of a solid are mainly determined by the type of bonding that exists between the atoms. According to bonding in crystals they are classified into following types. (1) Ionic crystal : This type of bonding is formed due to transfer of electrons between atoms and consequent attraction between them. (i) In NaCl crystal, the electron of Na atom is transferred to chlorine atom. In this way Na atom changes in to Na ion and Cl atom changes into Cl ion. (ii) Cause of binding is electrostatic force between positive and negative ion. (iii) These crystal are usually hard, brittle and possesses high melting and boiling point. (iv) These are bad conductor of electricity. (v) Common example are NaCl, CsCl, LiF etc. (2) Covalent crystal : Covalent bonding is formed by sharing of electrons of opposite spins between two atoms (i) The conductivity of these solids rise with rise in temperature. (ii) These crystal posses high melting point. (iii) Bonding between H , Cl molecules Ge, Si, Quartz, diamond etc. are common example of covalent bonding (3) Metallic bonds : This type of bonding is formed due to attraction of valence (free) electrons with the positive ion cores (i) Their conductivity decreases with rise of temperature. (ii) When visible light falls on a metallic crystal, the electrons of atom absorb visible light, so they are opaque to visible light. However some orbital electrons absorb energy and reach in excited state. They then return to their normal states, remitting light of same frequency. Common examples are Na, Li, K, Cs, Au, Hg etc. (4) Vander waal's crystal : These crystal consists of neutral atoms or molecules bonded together in solid phase by weak, short range attractive forces called vander Waal's forces. (i) This bonding is weakest and occurs in solid CO , methane, paraffin, ice, etc. (ii) They are normally insulator, they are soft, easily compressible and posses low melting point. U D YG + – (3) Liquid crystals : The organic crystalline solid which on heating, to a certain temperature range becomes fluid like but its molecules remain oriented in a particular directions, showing that they retain their anisotropic properties, is called liquid crystal. These crystals are used in a liquid crystal displays (L.C.D.) which are commonly used in electronic watches, clocks and micro-calculators etc. 2 ST 2 2 E Actual separation of atoms in the crystal O Energy bands are of following r0 types r (1) Valence Fig. band : The energy bandbands formed 27.7 Origin of energy in a by solida series of energy levels containing valence electrons is known as valence band. At 0 K, the electrons fills the energy levels in valence band starting from lowest one. (i) This band is always filled with electrons. (ii) This is the band of maximum energy. (iii) Electrons are not capable of gaining energy from external electric field. (iv) No flow of current due to electrons present in this band. (v) The highest energy level which can be occupied by an electron in valence band at 0 K is called fermi level. (2) Conduction band : The higher energy level band is called the conduction band. (i) It is also called empty band of minimum energy. (ii) This band is partially filled by the electrons. 1536 Electronics (iii) In this band the electrons can gain energy from external electric field. (iv) The electrons in the conduction band are called the free electrons. They are able to move any where within the volume of the solid. (v) Current flows due to such electrons. (3) Forbidden energy gap (E ) : Energy gap between conduction band g and valence band E g  (C.B.)min  (V.B.)max (2) A hole is considered as a seat of positive charge, having magnitude of charge equal to that of an electron. (3) Holes acts as virtual charge, although there is no physical charge max. C.B. (1) When an electron is removed from a covalent bond, it leaves a vacancy behind. An electron from a neighbouring atom can move into this vacancy, leaving the neighbour with a vacancy. In this way the vacancy formed is called hole (or cotter), and can travel through the material and serve as an additional current carriers. on it. min. max. V.B. 60 (4) Effective mass of hole is more than electron. Eg (5) Mobility of hole is less than electron. min. Intrinsic Semiconductors (i) No free electron is present in forbidden energy gap. (ii) Width of forbidden energy gap depends upon the nature of substance. (iii) As temperature increases (), forbidden energy gap decreases () very slightly. Electrical conductivity Insulators 10 to 10 Ʊ/m 2 8 10 10–2 to 10–8 -m (negligible) Resistivity Band structure –8 Ʊ/m 108 -m Semiconductors –5 10 Very large; for diamond it is 6 eV U V.B. Ge 0.7 eV Si 1.1 eV GaAs 1.3 eV GaF2 2.8 eV –– Current carriers Free electrons Condition of V.B. and C.B. at ordinary temperature V.B. and C.B. are completely filled or C.B. is some what empty Temperature co-efficient of resistance Effect of temperature on conductivity Positive Zero Negative Decreases — Increases Increases — Decreases U V.B. – completely filled ST C.B. – completely unfilled Effect of temperature on resistance Examples Electron density Cu, Ag, Au, Na, Pt, Hg etc. (5) For them fermi energy level lies at the centre of the C.B. and V.B. ne(o )  nh(o )  ni ; where n e(o )  Electron density in conduction band, D YG Zero or very small holes (v h ) (6) In pure semiconductor, impurity must be less than 1 in 10 8 parts of semiconductor. (7) In intrinsic semiconductor Eg (small) V.B. gap to 10 Ʊ/m C.B. C.B. V.B. Energy (Eg) (4) The drift velocity of electrons (v e ) is greater than that of 0 105 to 100 -m C.B. Eg (large) (3) Free electrons and holes both are charge carriers and n e (in ID Conductors (2) They have four electrons in the outermost orbit of atom and atoms are held together by covalent bond C.B.)  n h (in V.B.) Table 27.3 : Types of solid Properties (1) A pure semiconductor is called intrinsic semiconductor. It has thermally generated current carriers E3 Fig. 27.8 Free electrons holes and V.B. – empty somewhat C.B. – filled somewhat n h(o )  Hole density in V.B., ni  Density of intrinsic carriers. (8) The fraction of electrons of valance band present in conduction  E / kT band is given by f  e g ; where E = Fermi energy or k = Boltzmann's constant and T = Absolute temperature g (9) Because of less number of charge carriers at room temperature, intrinsic semiconductors have low conductivity so they have no practical use. (10) Number of electrons reaching from valence band to conduction band n  AT 3 / 2 e  Eg / 2kT Extrinsic Semiconductor (1) An impure semiconductor is called extrinsic semiconductor (2) When pure semiconductor material is mixed with small amounts of certain specific impurities with valency different from that of the parent material, the number of mobile electrons/holes drastically changes. The process of addition of impurity is called doping. Impurity Wood, plastic, mica, diamond, glass etc. 1029/m3 Holes in Semiconductors — Ge, Si, Ga, As etc. Ge ~ 1019 /m3 Si ~ 1016 /m3 Intrinsic S.C. Extrinsic S.C. + Fig. 27.9 (3) Pentavalent impurities : The elements whose atom has five valance electrons are called pentavalent impurities e.g. As, P, Sb etc. These impurities are also called donor impurities because they donate extra free electron. Electronics (4) Trivalent impurities : The elements whose each atom has three valance electrons are called trivalent impurities e.g. In, Ga, Al, B, etc. These impurities are also called acceptor impurities as they accept electron. (5) The compounds of trivalent and pentavalent elements also behaves like semiconductors e.g. GaAs, InSb, In P, GaP etc. 1537 P-Type Semiconductor These are obtained by adding a small amount of trivalent impurity to a pure sample of semiconductor (Ge). Intrinsic S.C. (6) The number of atoms of impurity element is about 1 in 10 8 atoms of the semiconductor. P-type + S.C. trivalent impurity (7) In extrinsic semiconductors n e  n h (8) In extrinsic semiconductors fermi level shifts towards valence or conduction energy bands. (9) Their conductivity is high and they are used for practical purposes. Ge 60 As Ge e i (11) Extrinsic semiconductors are of two types (i) N-type semiconductor (ii) P-type semiconductor Ge Ge N-type + S.C. h e h e U Pentavalent impurity Fig. 27.10 Ge Ge D YG Ge Donor electron Ge As Ge U Ge ST (1) Majority charge carriers – Fig. electrons 27.11 Minority charge carriers – holes (2) n >> n ; i >> i e h e h (3) Conductivity   n  e (4) N-type semiconductor is electrically neutral (not negatively charged) (5) Impurity is called Donar impurity because one impurity atom generate one electron. (6) Donor energy level lies just below the conduction band. e e Conduction band Eg  1 eV EF Conduction band Eg  1 eV Ed  0.01eV Fig. 27.12 Acceptor levels EF Ea  0.01eV Valence band Fig. 27.14 Density of Charge Carriers Due to thermal collisions, an electron can take up or release energy. Thus, occasionally a valence electron takes up energy and the bond is broken. The electron goes to the conduction band and a hole is created. And occasionally, an electron from the conduction band loses some energy, comes to the valence band and fills up a hole. Thus, new electron-hole pairs are formed as well as old electron-hole disappear. A steady-state situation is reached and the number of electron-hole pairs takes a nearly constant value. For silicon at room temperature (300 K), the number of these pairs is about 7  10 m. For germanium, this number is about 6  10 /m. 15 3 19 3 Table 27. 4 : Densities of charge carriers Material Type Copper Conductor 9  1028 0 Silicon Intrinsic semiconductor 7  1015 7  1015 Silicon doped with phosphorus (1 part in 106) N-type 5  1022 Donor levels Valence band h Ge Valence electrons Ge Ge (3) Conductivity   n  e (4) P-type semiconductor is also electrically neutral (not positively charged) (5) Impurity is called Acceptor impurity. (6) Acceptor energy level lies just above the valence band. h Intrinsic S.C. Valence electrons (1) Majority charge carriers – holes Fig. 27.13 Minority charge carriers – electrons (2) n >> n ; i >> i ID These are obtained by adding a small amount of pentavalent impurity to a pure sample of semiconductor (Ge). Ge E3 valence band in a pure semiconductor then ne nh  ni2 (mass action law) N-Type Semiconductor Ge Acceptor hole (10) In a doped extrinsic semiconductor, the number density of e  of the conduction band (n ) and the number density of holes in the valence band (n ) differs from that in a pure semiconductor. If n is the number density of electron in conduction band or the number density of holes in h Ge semiconductor Density of conduction electrons (m– 3) Density of holes (m–3) 1  109 1538 Electronics 1  109 P-type Silicon doped with aluminium (1 part in 106 (1) Depletion region : On account of difference in concentration of charge carrier in the two sections of P-N junction, the electrons from Nregion diffuse through the junction into P-region and the hole from P region diffuse into N-region.  1022 5 semiconductor Conductivity of Semiconductor (1) In intrinsic semiconductors n = n. Both electron and holes contributes in current conduction. e h Due to diffusion, neutrality of both N and P-type semiconductor is disturbed, a layer of negative charged ions appear near the junction in the P-crystal and a layer of positive ions appears near the junction in N-crystal. This layer is called depletion layer (2) When some potential difference is applied across a piece of intrinsic semiconductor current flows in it due to both electron and holes i.e. i = i + i  i  eA [n e v e  n h v h ] VB h e– 60 e hole Electric field i P E3 (i) The thickness of depletion layer is 1 micron = 10 m. Fig. 27.15   e[ne  e  nh  h ] ; where v = drift velocity of electron, v = drift h v velocity of holes, E = Applied electric field  e  e  mobility of electron E vh  mobility of holes E (iii) Depletion is directly proportional to temperature. (iv) The P-N junction diode is equivalent to capacitor in which the depletion layer acts as a dielectric. (2) Potential barrier : The potential difference created across the P-N junction due to the diffusion of electron and holes is called potential barrier. For Ge D YG U (4) Motion of electrons in the conduction band and of holes the valence band under the action of electric field is shown below Conduction electron – – + – – – – – Charge density Potential P P N N Valancy band + – Distance distance E Electric field U Electric field V 0.5   5  10 5 V / m d 10 6 (3) Some important graphs Band gap + –6 So the barrier electric field E  Conduction band Hole VB  0.3 V and for silicon VB  0.7 V On the average the potential barrier in P-N junction is ~ 0.5 V and the width of depletion region ~ 10 m. – – – 1 Dopping ID e Eg –6 Fig. 27.18 (ii) Width of depletion layer  J i (3) As we know   . Hence conductivity of semiconductor E AE E N Depletion layer V and  h  + – Fig. 27.16 ST (5) At absolute zero temperature (0 K) conduction band of semiconductor is completely empty i.e.  = 0. Hence the semiconductor behaves as an insulator. P-N Junction Diode When a P-type semiconductor is suitably joined to an N-type semiconductor, then resulting arrangement is called P-N junction or P-N junction diode P-N Junction N N Distance Fig. 27.19 (4) Diffusion and drift current : Because of concentration difference holes/electron try to diffuse from their side to other side. Only those holes/electrons crosses the junction, which have high kinetic energy. This diffusion results in an electric current from the P-side to the N-side known as diffusion current (i ) df P P P Anode N Cathode As electron hole pair (because of thermal collisions) are continuously created in the depletion region. There is a regular flow of electrons towards the N-side and of holes towards the P-side. This makes a current from the N-side to the P-side. This current is called the drift current (i ). dr Fig. 27.17 Biasing Electronics 1539 It means the way of connecting emf source to P-N junction diode. It is of following two types (1) Forward biasing : Positive terminal of the battery is connected to the P-crystal and negative terminal of the battery is connected to N-crystal Reverse Breakdown E If the reverse biased voltage is too high, then breakdown of P-N junction diode occurs. It is of following two types Eb N – + (i) In forward biasing width of depletion layer decreases Fig. 27.20 (ii) In forward biasing resistance offered R  10 - 25 Forward (iii) Forward bias opposes the potential barrier and for V > V a forward current is set up across the junction. B (1) Zener diode : It is a highly doped p-n junction which is not damaged by high reverse current. It can operate continuously, without being damaged in the region of reverse background voltage. In the forward bias, the zener diode acts as ordinary diode. It can be used as voltage regulator is  Saturation current, In the exponent e = 1.6  10 C, -19 ID k = Boltzmann's constant (v) Cut-in (Knee) voltage : The voltage at which the current starts to increase rapidily. For Ge it is 0.3 V and for Si it is 0.7 V. (vi) df – diffusion Unregulated voltage R IL U Forward current in mA (2) Avalanche breakdown : At high reverse voltage, due to high electric field, the minority charge carriers, while crossing the junction acquires very high velocities. These by collision breaks down the covalent bonds, generating more carriers. A chain reaction is established, giving rise to high current. This mechanism is called avalanche breakdown. Special Purpose Diodes (iv) The current is given by i  is (e eV / kT  1) ; where N P Idf Idr Inet D YG dr – drift 60 P (1) Zener breakdown : When reverse bias is increased the electric field across the junction also increases. At some stage the electric field becomes so high that it breaks the covalent bonds creating electron, hole pairs. Thus a large number of carriers are generated. This causes a large current to flow. This mechanism is known as Zener breakdown. E3 + – Knee voltage Forward voltage Fig. 27.21 (2) Reverse biasing : Positive terminal of the battery is connected to the N-crystal and negative terminal of the battery is connected to P-crystal Load RL Regulated voltage (A) Zener diode as a voltage regulator (B) Symbol of zener diode Fig. 27.24designed diodes, which give (2) Light emitting diode (LED : Specially out light radiations when forward biases. LED’S are made of GaAsp, Gap etc. These are forward biased P-N-junctions which emits spontaneous radiation. N E + Eb U – g P ST Fig. 27.25 (3) Photo diode: Photodiode is a special type of photo-detector. Suppose an optical photons of frequency  is incident on a semiconductor, such that its energy is greater than the band gap of the semiconductor ( i.e. h > E ) This photon will excite an electron from the valence band to the conduction band leaving a vacancy or hole in the valence band. Which obviously increase the conductivity of the semiconductor. Therefore, by measuring the change in the conductance (or resistance) of the semiconductor, one can measure the intensity of the optical signal. N – + (i) In reverse biasing width of depletion layer increases Fig. 27.22 (ii) In reverse biasing resistance offered R  10  5 Reverse (iii) Reverse bias supports the potential barrier and no current flows across the junction due to the diffusion of the majority carriers. (A very small reverse currents may exist in the circuit due to the drifting of minority carriers across the junction) (iv) Break down voltage : Reverse voltage at which break down of semiconductor occurs. For Ge it is 25 V and for Si it is 35 V. (v) Fig. 27.26 (4) Solar cells : It is based on the photovoltic effect. One of the semiconductor region is made so thin that the light incident on it reaches the P-N-junction and gets absorbed. It converts solar energy into electrical energy. Reverse voltage Break down voltage P-N Junction Diode asFig.a 27.27 Rectifier Reverse current Idf Idr Inet Fig. 27.23 1540 Electronics Rectifier is a circuit which converts ac to unidirectional pulsating output. In other words it converts ac to dc. It is of following two types (1) Half wave rectifier : When the P-N junction diode rectifies half of the ac wave, it is called half wave rectifier Input ac output dc RL Input + ac signal + V0 or I0 – (i) During positive half cycle – Diode : D V or I + Diode : D obtained (iii) Fluctuating dc not obtained (iii) Output voltage is obtained across the load resistance R. It is not constant but pulsating (mixture of ac and dc) in nature. L ; I0   V0 rf  RL D YG (r = forward biased resistance) f (v) r.m.s. output : Irms I V  0 , Vrms  0 2 2 (vi) The ratio of the effective alternating component of the output voltage or current to the dc component is known as ripple factor. 2     1      I I r  ac   rms Idc  Idc  1/2 = 1.21 U (vii) Peak inverse voltage (PIV) : The maximum reverse biased voltage that can be applied before commoncement of Zener region is called the PIV. When diode is not conducting PIV across it = V ST (viii) Efficiency : It is given by %  0 (v) Average output : Vav  2 V0 , Iav  2 I0 (vi) r.m.s. output : Vrms  V0 , Irms  I0  2  2 (vii) Ripple factor : r = 0.48 = 48% (viii) Ripple frequency : The ripple frequency of full wave rectifier = 2  (Frequency of input ac) (ix) Peak inverse voltage (PIV) : It's value is 2V 0 81.2 (x) Efficiency : %  for r > r then  = 40.6% L (iv) Output voltage is obtained across the load resistance R. It is not constant but pulsating in nature. 1 Pout 40.6  100  rf Pin 1 RL 2 constant dc. Filter U (iv) Average output in one cycle  obtained due to D only ID Output signal V0 forward biased 2 Output signal reverse biased and Vdc  reverse biased 1 D (ii) During negative half cycle I0 1 (ii) During negative half cycle Output signal Diode obtained due to D only Output signal V0 or I0 Fig. 27.28 forward biased Diode reverse biased 2 E3 + (i) During positive half cycle Idc  forward biased 1 D Output dc signal 60 V or I D3 Vout 3 Electronics (2) A transistor is mostly used in the active region of operation i.e. emitter base junction is forward biased and collector base junction is reverse biased. (3) From the operation of junction transistor it is found that when the current in emitter circuit changes. There is corresponding change in collector current. (4) In each state of the transistor there is an input port and an output port. In general each electrical quantity (V or I) obtained at the output is controlled by the input. Table 27.6 : Circuit diagram of PNP/NPN transistor NPN – transistor N P N P N E C C P Ib Ie Ib + – mA N B P C E C Fig. 27.32 B In PNP transistor holes are majority charge carriers and flow from emitter to base. In the symbols of both NPN and PNP transistor, arrow indicates the direction of conventional current. U ST Table 27.5 : Different modes of operation of a transistor Operating mode Emitter base bias VCB Ic Ib 5% emitter holes combine with the VCBresulting electrons VinEB the base region in small base current. Remaining 95% holes enter the collector region. Ie > Ic , and Ie = Ib + Ic Ie > Ic , and Ie = Ib + Ic Transistor Configurations A transistor can be connected in a circuit in the following three different configurations. Common base (CB), Common emitter (CE) and Common collector (CC) configuration. (1) CB configurations : Base is common to both emitter and collector. PNP Ie Ic E C + B – RL Ib VCB = output – VEB = input (iii) Cut-off mode : Denotes operation like an open switch where only leakage current flows. (iv) Inverse mode : The emitter and collector are inter changed. + 5% emitter electron combine with the holes in Vthe in EB base region Vresulting CB small base current. Remaining 95% electrons enter the collector region. (1) There are four possible ways of biasing the two P-N junctions (emitter junction and collector junction) of transistor. (ii) Saturation mode : Maximum collector current flows and transistor acts as a closed switch from collector to emitter terminals. + (i) Input current = I (ii) Input voltage = V e EB Fig. 27.33 (iii) Output voltage = V (iv) Output current = I CB Collector base bias mA Ie Ic Working of Transistor (i) Active mode : Also known as linear mode operation. – A mA VEB U P D YG E Ib Ib In NPN transistor electrons are majority charge carriers and flow from emitter to base. (ii) PNP transistor : It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductor Ib VCB ID B Fig. 27.31 P + Ie B N Ie – A mA VEB N PNP – transistor 60 (1) The name of this electronic device is derived from it's fundamental action transfer resistor. (2) Transistor does not need any heater or hot filament, transistor is small in size and light in weight. (3) Transistor in general is known as bipolar junction transistor. (4) Transistor is a current operated device. (5) It consists of three main regions (i) Emitter (E) : It provides majority charge carriers by which current flows in the transistor. Therefore the emitter semiconductor is heavily doped. (ii) Base (B) : The based region is lightly doped and thin. (iii) Collector (C) : The size of collector region is larger than the two other regions. (6) Junction transistor are of two types : (i) NPN transistor : It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors E3 Transistor E 1541 C With small increase in emitter-base voltage V , the emitter current I increases rapidly due to small input resistance. EB Active Forward Reverse Saturation forward Forward Cut off Reverse Reverse Inverse Reverse Forward e (v) Input characteristics : If V = constant, curve between I and V is known as input characteristics. It is also known as emitter characteristics CB e VCB = – 10 V Ie (mA) VCB = – 20 V VEB (in volt) VCB = 0 EB 1542 Electronics Output characteristics : Variation of collector current I with V can be noticed for V between 0 to 1 V only. The value of V up to which the I changes with V is called knee voltage. The transistor are operated in the region above knee voltage. C CE CE CE C CE 70 Input characteristics of NPN transistor are also similar to the above figure but I and V both are negative and V is positive. Dynamic input resistance of a transistor is given by EB 50 CB  V Ri   EB  Ie    VCB  constant Ic (mA) { R is of the order of 100 } IB=100 A IB=50 A IB=0 mA 30 20 i (vi) Output characteristics : Taking the emitter current i constant, the curve drawn between I and V are known as output characteristics of CB configuration. 10 e C IB=200 A IB=150 A 40 60 e IB=300 A IB=250 A 60 CB 0 5 Ic (mA) 10 15 VCE (volt) Ie = 40 mA Ie = 30 mA E3 Fig. 27.38  V Dynamic output resistance R0   CE  IC Ie = 20 mA Ie = 10 mA ID The low input impedance of the junction transistor is a handicap in certain applications. In addition, it is difficult to incorporate large numbers of them in an integrated circuit and they consume relatively large amounts of power. The field-effect transistor (FET) lacks these disadvantages and is widely used today although slower in operation than junction transistors. VCB (in volt)     ie  constant P-type gate Input signal U  27.35 V Fig. Dynamic output resistance Ro   CB   iC (2) CE configurations : Emitter is common to both base and collector. D YG The graphs between voltages and currents when emitter of a transistor is common to input and output circuits are known as CE characteristics of a transistor. PNP Ib – RL VCE = output – VEB = input + U Input characteristics : InputFig. characteristic curve is drawn between base 27.36 current I and emitter base voltage V , at constant collector emitter voltage V. b ST EB 100 80 Ib (A) Source – B Ie VCE=0 VCE=1V VCE=2V VCE=5V 60 Drain + + N-type channel E + Depletion region – Ic C     I B constant Field-Effect Transistor Ie = 0 mA CE 20 Fig. 27.39 An n-channel FET consists of a block of N-type material with contacts at each end together with a strip of P-type material on one side that is called the gate. When connected as shown, electrons move from the source terminal to the drain terminal through the N-type channel. the PN junction is given a reverse bias, and as a result both the N and P materials near the junction are depleted on charge carriers. The higher the reverse potential on the gate, the larger the depleted region in the channel and the fewer the electrons available to carry the current. Thus the gate voltage controls the channel current. Very little current passes through the gate circuit owing to the reverse bias, and the result is an extremely high input impedance. FET is uni-polar. Transistor as an Amplifier A device which increases the amplitude of the input signal is called amplifier. 40 20 0 0.2 0.4 0.6 0.8 VBE (volt)  27.37 Fig. V  Dynamic input resistance Ri   BE   IB V 1.0 Amplifier Input signal Output amplified signal CE constant Fig. 27.40 Electronics The transistor can be used as an amplifier in the following three configuration (i) CB amplifier (ii) CE amplifier (iv) Power gain  (iii) CC amplifier (2) Transistor as CE amplifier NPN ic ie C – RL B + + VCB ib VCC VCC ic  i (i) ac current gain  ac   c  ib Fig. 27.41 b e C e (iv) Power gain  (ii) V < V CC (iii) Net collector voltage V = V – i R CB CC C L c (iv) Input and output signals are in same phase RL E VCE Ie + – + VCC D YG VBB – ic Fig. 27.42 (i) ie  ib  iC ; i = 5% of i and i = 95% of i b (ii) V > V CC BB e C CC e C (2) In an oscillator the frequency, waveform, and magnitude of ac power generated is controlled by circuit itself. (3) An oscillator may be considered as amplifier which provides it's own input signal. (4) The essential of a transistor oscillator are (i) Tank circuit : Parallel combination of L and C. This network resonates at a frequency  0  1 2 1. LC L U (iv) Input and output signals are 180° out of phase. Different Gains in CE/CB Amplifiers Small change in collectorcurrent (ic ) Small change in collectorcurrent (ie ) ST (ii) Amplifier : It receives dc power from the battery and converts into ac power. The amplifier increases the strength of oscillations. (1) Transistor as CB amplifier (i) ac current gain  ac   1 Audio frequency oscillators generates signals of frequencies ranging from a few Hz to 20 kHz and radio frequency oscillators have a range from few kHz to MHz. (iii) Net collector voltage V = V – i R CE 1  or   U B Input signal ~  (1) It is defined as a circuit which generates an ac output signal without any externally applied input signal. ic Output signal ie (3) Relation between  and  :   Transistor as an Oscillator (2) NPN transistor as CE amplifier C L ID CB NPN (v) Trans conductance (g ) : The ratio of the change in collector current to the change in emitter base voltage is called trans conductance. ic A i.e. g m . Also g m  V ; R = Load resistance V EB RL m When the input signal (signal to be amplified) is fed to the emitter base circuit, it will change the emitter voltage and hence emitter current. This in turn will change the collector current (i ). This will vary the collector voltage V. This variation of V will appear as an amplified output. CB Po 2   ac  Resistance gain Pi E3 EE CE Vo   ac  Resistance gain Vi (iii) Voltage gain : Av  (i) ie  ib  iC ; i = 5% of i and i = 95% of i V = constant ic ib (ii) dc current gain  dc  – +     60 VEE – iC RL Output signal E ~ Change in output power(Po ) Change in input power(Pc ) 2  Power gain   ac  Resistance gain (1) NPN transistor as CB amplifier Input signal 1543 (iii) Feed back circuit : This circuit supplies a part of the collector energy to the tank circuit. Amplifier Out put V (constant) B Collector current(ic ) (ii) dc current gain  dc (or )  Emitter current (ie ) LC network valve of  lies between 0.95 to 0.99 dc (iii) Voltage gain Av  Change in output voltage(Vo ) Change in input voltage (Vi ) Fig. 27.43 (5) A basic common-emitter NPN oscillator is shown in the figure. L'  A =   Resistance gain v C ac B VCC E L C VBB Fig. 27.44 1544 Electronics The oscillations can be transferred to an external circuit by mutual induction in a coil connected in that circuit. (1) Decimal number system : In a decimal number system, we have ten digits i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. A decimal number system has a base of ten (10) e.g. = 1000 + 900 + 70 + 1 1971 = 1  10 + 9  10 + 7  10 + 1  10 3 2 1 0 LSD MSD LSD = Least significant digit ID (6) Need for positive feedback : The oscillations are damped due to the presence of some inherent electrical resistance in the circuit. Consequently, the amplitude of oscillations decreases rapidly and the oscillations ultimately stop. Such oscillations are of little practical importance. In order to obtain oscillations of constant amplitude, we make an arrangement for regenerative or positive feedback from the output circuit to the input circuit so that the losses in the circuit can be compensated. Decimal and Binary Number System E3 On completion of the circuit electrical oscillations are developed in the tank circuit. The circuit amplifies these oscillations. A part of the amplifies signal in the collector circuit is fed back in the base circuit by the coupling between L and L'. Due to this feed back amplitude of oscillation builds up till power dissipation in the oscillatory circuit becomes equal to power fedback. In this state the amplitude of oscillations becomes constant. 60 A tank circuit (L-C circuit) is connected in the base-emitter circuit, in which the capacitance C is kept variable. By changing C oscillations of a desired frequency can be obtained. An inductance coil L' connected in the collector-emitter circuit is coupled to coil L. MSD = Most significant digit U (2) Binary number system : A number system which has only two digits i.e. 0 (Low) and 1 (High) is known as binary system. The base of binary number system is 2. D YG A t (i) Each digit in binary system is known as a bit and a group of bits is known as a byte. (ii) The electrical circuit which operates only in these two state i.e. 1 (On or High) and 0 (i.e. Off or Low) are known as digital circuits. Table 27. 8 : Different names for the digital signals State Code Fig. 27.45 Table 27.7: Comparison between CB, CE and CC amplifier Characteristic Amplifier CE CC Input resistance (Ri)  50 to 200  low  1 to 2 k medium  150 – 800 k high Output resistance (Ro)  1 – 2 k high  50 k medium  k low ST U CB Current gain 0.8 – 0.9 low 20 – 200 high 20 – 200 high Voltage gain Medium High Low Power gain Medium High Low Phase difference between input and output voltages Zero 180o Zero Used as amplifier for current Digital Electronics Power Voltage Name for the State 1 0 On Off Up Down Close Open Excited Unexcited True False Pulse No pulse High Low Yes No (3) Decimal to binary conversion (i) Divide the given decimal number by 2 and the successive quotients by 2 till the quotient becomes zero. (ii) The sequence of remainders obtained during divisions gives the binary equivalent of decimal number. (iii) the most significant digit (or bit) of the binary number so obtained is the last remainder and the least significant digit (or bit) is the first remainder obtained during the division. For Example : Binary equivalence of 61 2 61 Remainder 2 30 1 LSD 2 15 0 Electronics 2 2 2 7 3 1 0 1 1 1 1 MSD A T + 1545  (61) = (111101) 10 2 (4) Binary to decimal conversion : The least significant digit in the binary number is the coefficient of 2 with power zero. As we move towards the left side of LSD, the power of 2 goes on increasing. A OFF Lamp ON A ON Contact at T is broken Lamp OFF For Example : (11111100101) = 1  2 + 1  2 + 1  2 + 1  2 + 1  2 + 1  2 + 0  2 + 0  2 + 1  2 + 0  2 + 1  2 = 2021 10 9 8 7 6 2 4 3 2 1 (3) Basic Boolean postulates and laws Fig. 27.50 (i) Boolean Postulates : 0 + A = A, 1 · A = A, 1 + A = 1, 0 · A = 0, 0 Voltage Signal (1) Analogue voltage signal : The signal which represents the continuous variation of voltage with time is known as analogue voltage signal 60 5 A A 1 A + A = A, A·A=A E3 (ii) Identity law : AA (iii) Negation law : (iv) Commutative law : A + B = B + A, A · B = B · A (v) Associative law : (A+B) + C = A + (B+C), (A · B) · C = A · (B · C) (vi) Distributive law : A · (B+C) = A · B + A · C (A + B) · (A + C) = A + BC (vii) Absorption laws : A + A·B = A, A · (A + B) = A + V0 V Time – V0 ID (2) Digital voltage signal : Fig. The27.46 signal which has only two values. i.e. either a constant high value of voltage or zero value is called digital voltage signal +V A ·( A  B)  A · B Time D YG Fig. 27.47 U (viii) Boolean identities : A  A B  A  B , A( A  B)  AB , O Boolean Algebra (1) In Boolean algebra only two states of variables (0 and 1) are allowed. (2) The variables (A, B, C ….) of Boolean Algebra are subjected to three operations. (i) OR Operation : Represented by (+) sign A U B ST Boolean expression Y = A Fig. + B27.48 When switch A or B is closed – Bulb glows (ii) AND Operation : Represented by (·) sign A  BC  ( A  B) ( A  C) , ( A  B) ·( A  C)  AC  AB (ix) De Morgan’s theorem : It states that the complement of the whole sum is equal to the product of individual complements and vice versa i.e. A  B  A  B and A  B  A  B Logic Gates and Truth Table (1) Logic gate : The digital circuit that can be analysed with the help of Boolean algebra is called logic gate or logic circuit. A logic gate has two or more inputs but only one output. There are primarily three logic gates namely the OR gate, the AND gate and the NOT gate. (2) Truth table : The operation of a logic gate or circuit can be represented in a table which contains all possible inputs and their corresponding outputs is called the truth table. To write the truth table we use binary digits 1 and 0. The 'OR' Gate (1) It has two inputs (A and B) and only one output (Y) Boolean expression Y = A · B When switches A and B both are closed – Bulb glows A B (2) Boolean expression is Y = A + B and is read as " Y equals A OR B" A Y B Fig. 27.51 : Logical symbol of OR gate Fig. 27.49 (3) Realization of OR gate (iii) NOT Operation : Represented by bar over the variables Boolean expression Y  A D1 A Y R B D2 1546 Electronics None of the diode conducts the out voltage at Y= Battery voltage =1 (4) Truth table for 'AND' gate A B Y=A.B 0 0 0 0 1 0 Both diodes D and D do not conduct and hence Y = 0 1 0 0 (ii) A = 0, B = 1 1 1 1 (i) A = 0, B = 0 1 2 D = Does not conducts, D = Conducts, hence Y = 1 2 The 'NOT' Gate (iii) A = 1, B = 0 D = Conducts, D = Does not conduct, hence Y = 1 (iv) A = 1, B = 1 Both D and D conducts, hence Y = 1 1 (1) It has only one input and only one output. 2 (2) Boolean expression is Y  A and is read as ‚y equals not A‛ 2 E3 1 60 1 (4) Truth table for 'OR' gate A A B Y=A+B 0 0 0 0 1 1 1 0 1 1 1 1 Y Fig 27.55 : Logical symbol of NOT gate (3) Realization of NOT gate : The transistor is so biased that the collector voltage V = V (Voltage corresponding to 1 state) CC ID The resistors R and R are so chosen that if the input is low i.e. O, the transistor is in the cut off and hence the voltage appearing at the output will be the same as applied V. Hence Y = V (or state 1) 1 The 'AND' Gate If the input is high, the transistor current is in saturation and the net voltage at the output Y is 0 (in state 0) U (1) It has two inputs (A and B) and only one output (Y) (2) Boolean expression is Y = A · B is read as " Y equals A AND B" A Y D YG B 27.53 : Logical symbol of AND gate Fig. VCC = V R (3) Realization of AND gate D1 A Y B V R V B D2 0 Fig.: 27.56 (4) Truth table for NOT gate A Y A 0 1 1 0 Fig. 27.54 ST (i) A = 0, B = 0 The voltage supply through R is forward biasing diodes D and D (offers low resistance) the voltage V would drop across R 1 The output voltage at Y = the voltage across diode = 0 (ii) A = 0, B = 1 2 Combination of Logic Gates (1) The ‘NAND’ gate : From ‘AND’ and ‘NOT’ gate D = conducts, D = Not Conducts 1 E V U 0 R1 A 1 Y C Y = A · B 2 the out voltage at Y= The voltage across the diode (D ) =0 1 A AND NOT B (iii) A = 1, B = 0 D = Conducts, D = Not conducts 1 2 the out voltage at Y= The voltage across the diode (D ) =0  2 iv) A = 1, B = 1 Fig. 27.57 A B Y Electronics Boolean expression and truth table : Y  A  B B Y = A  B Y 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 1 1 0 (4) The exclusive nor (XNOR) gate XOR + NOT XNOR A Y B A  (2) The ‘NOR’ gate : From ‘OR’ and ‘NOT’ gate A 0 1 Y 60 A 1 B Y = A + B OR 1547 NOT Fig. 27.60 Boolean expression : Y = A  B = A B  AB B Logic Gates Using 'NAND' Gate  E3 A The NAND gate is the building block of the digital electronics. All the logic gates like the OR, the AND and the NOT can be constructed from the NAND gates. Y B Fig. 27.58 (1) Construction of the ‘NOT’ gate from the ‘NAND’ gate Boolean expression and truth table : Y  A  B Y = A + B Y 0 0 0 1 0 1 1 0 1 0 1 0 1 1 1 (i) When both the inputs (A and B) of the NAND gate are joined together then it works as the NOT gate. ID B A 0 D YG It may be noted that if both the inputs of the XOR gate are high, then the output is low (i.e., 0). Y '  A. B A U B ST Fig. 27.61 (ii) Truth table and logic symbol (3) The ‘XOR’ gate : From ‘NOT’, ‘AND’ and ‘OR’ gate. Known as exclusive OR gate. or The logic gate which gives high output (i.e., 1) if either input A or input B but not both are high (i.e. 1) is called exclusive OR gate or the XOR gate. A Y B U A Y Input Output A=B Y 0 1 1 0 (2) Construction of the ‘AND’ gate from the ‘NAND’ gate (i) When the output of the NAND gate is given to the input of the NOT gate (made from the NAND gate), then the resultant logic gate works as the AND gate Y Y A B Fig. 27.62 (ii) Truth table and logic symbol A A B Y Y 0 0 1 0 0 1 1 0 1 0 1 0 1 1 0 1 B Y ' '  A. B B  A Y B Fig. 27.59 Boolean expression and truth table : Y = A  B = A B  A B A B Y 0 0 0 0 1 1 (3) Construction of the ‘OR’ gate by the ‘NAND’ gate (i) When the outputs of two NOT gates (obtained from the NAND gate) is given to the inputs of the NAND gate, the resultant logic gate works as the OR gate A A Y 1548 Electronics (5) Four processes of electron emission from a metal are (i) Thermionic emission (ii) Photoelectric emission (iii) Field emission (iv) Secondary emission Thermionic Emission (ii) Truth table and logic symbol A B Y 0 0 1 1 0 0 1 1 0 1 1 0 0 1 1 1 1 0 0 1 (1) The phenomenon of ejection of electrons from a metal surface by the application of heat is called thermionic emission and emitted electrons are called thermions and current flowing is called thermion current. 60 B (2) Thermions have different velocities. (3) This was discovered by Edison (4) Richardson – Dushman equation for current density (i.e. electric current emitted per unit area of metal surface) is given as E3 A J  AT 2 e  W0 / kT  AT 2 e Valve Electronics  qV kT  AT 2 e  11600 V T where A = emission constant = 12  10 4 amp/ m -K , k = Boltzmann’s constant, T = Absolute temp and W = work function. (5) The number of thermions emitted per second per unit area ( J) depends upon following : 2 2 ID 0 (i) J  T 2 (ii) J  e W0 Table 27.9: Types of thermionic emitters U Directly heated emitter D YG Electron Emission from Metal (1) Free electron in metal experiences a barrier on surface due to attractive Coulombian force. Indirectly heated emitter Plate Plate e– e– Cathode Filament Filament Cathode (2) When kinetic energy of electron becomes greater than barrier potential energy (or binding energy E b ) then electron can come out of the Cathode is directly heated by passing current. Cathode is indirectly heated. surface of metal. Thermionic current is less. Thermionic current is more. Energy consumption and life is small. Energy consumption and life is more. (3) Fermi energy (E ) : Is the maximum possible energy possessed by free electron in metal at 0K temperature f U (i) In this energy level, probability of finding electron is 50%. (ii) This is a reference level and it is different for different metals. ST (4) Threshold energy (or work function W ) : Is the minimum energy required to take out an electron from the surface of metal. Also W = E – E W0 Eb Ef W0 = Eb – Ef 0 Pure tungsten (W ) 0 Throated tungsten (W ) 0 Oxide coa ted tungsten = 4.5 eV F Vacuum Tubes and Thermionic Valves (1) Those tubes in which electrons flows in vacuum are called vacuum tubes. 0 0 Ionised energy level b f (2) These are also called valves because current flow in them is unidirectional. (3) Vacuum in vacuum tubes prevents the emission of secondary electrons and burning of heated filament (which will happen if we use air in place of vacuum) Fermi energy level (4) Every vacuum tube necessarily contains two electrodes out of which one is always electron emitter (cathode) and another one is electron collector (anode or plate). Ground energy level (5) Depending upon the number of electrodes used the vacuum tubes are named as diode, triode, tetrode, pentode…. respectively, if the number of electrodes used are 2, 3, 4, 5….. respectively. Fig. 27.64 Work function for different materials (W ) F W0 Diode Valve = 2.6 eV Heater = 1 eV Temp. Cathode (K) Anode (P) Fig. 27.65 P Glass e– Filament C F F Electronics 1549 4 mek 2 amp / m 2  k 2 h3 A = Emission constant = S = Area of emitter in m 2 ; T = Absolute temperature in K 0 =Work function of metal in Joule; k =Boltzmann constant The small increase in ip after saturation stage due to field emission is known as Shottkey effect. (4) Diode resistance P also increases and finally becomes maximum plate current, then the ratio Vp / ip is called the dynamic resistance, or the ‘plate resistance’ of the diode rp  iP mA VPP Rh F Fig. 27.67 ip. (iv) In TLR : R p  rp and rp  . (5) Uses of diode valve (i) As a rectifier (iii) As a transmitter (ii) As a detector (iv) As a modulator Diode Valve as a Rectifier Rectifier is a device which converts ac into dc (1) Half wave rectifier : The circuit of half wave rectifier is shown below. In the first half cycle of ac input the diode conducts and in the second half cycle it does not conducts. Thus half of the input cycle appear as output. U F – V p (iii) In SCLR : rp  R p , ID + VP. E3 ( i p ) flows in the circuit (because some emitted electrons reaches to plate). P C ip (ii) Dynamic or ac plate resistance : If at constant filament current, a small change ∆V in the plate potential produces a small change ip in the (4) Working : When plate potential ( V p ) is positive, plate current If + V p increases i p (saturation). Vp 60 (i) Static plate resistance or dc plate resistance : R p  (1) Inventor : Fleming (2) Principle : Thermionic emission (3) Number of electrodes : Two D YG (5) Space charge : If V p is zero or negative, then electrons collect around the plate as a cloud which is called space charge. space charge decreases the emission of electrons from the cathode. F F Output voltage Characteristic Curves of a Diode A graph represents the variation of i p with V p at a given filament D1 ~ VOut RL current ( i f ) is known as characteristic curve. t E C ip (mA) (A) Half wave rectifier T2 D T1 Fig. 27.69 (i) Output voltage is not constant but pulsating in nature. B Stopping potential A (ii) It is a mixture of ac and dc. TLR or SR (iii) The dc values of the half wave output are given by U LR O SCLR VP (volt) Vd.c.  Fig. 27.68 ST The curve is not linear hence diode valve is a non-ohmic device. (1) Space charge limited region (SCLR) : In this region current is space charge limited current. Also ip  Vp3 / 2  ip  kV p3 / 2 ; where k is a constant depending on metal as well as on the shape and area of the cathode. This is called child’s law. (2) Linear region (LR) : In this region ip  Vp (3) Saturated region (SR) or temperature limited region (TLR) : In this part, the current is independent of potential difference applied between the cathode and anode. ip  f (Vp ) , ip  f (Temperature) The saturation current follows Richardson Dushman equation i.e. i  AST 2e 0 / kT ; Here (B) Output signal V0  and id.c.  i0  (iv) The r.m.s. values of the half wave output are given by Vrms  V0 i and irms  0 2 2 (v) The ratio of the effective alternating component to the direct component of the output voltage or current is called ripple factor 2 r i  ia.c.     rms   1     1  1.21  121%. id.c. i 2 d c..   2 (vi) Efficiency of half wave rectifier is given by 1550 Electronics  Triode Valve Pd.c. 40.6  100%  % r Pa.c. 1 p RL Control grid (G) Cathode (K) The maximum efficiency (for R >> r ) = 40.6% L (vii) Form factor  Anode (P) p Glass irms V   rms   1.57 id.c. Vd.c. 2 P G (viii) Ripple frequency = Frequency of input ac =  (2) Full wave rectifier : It consist of two diodes D and D. They conducts alternately during positive and negative half cycle of input ac and a unidirectional (or dc) current flows in output Base pins Fig. 27.71 (1) Inventor : Dr. Lee De Forest (2) Principle : Thermionic emission + V0 ~ Output voltage RL D1 D2 D1 O F F (ii) Plate or anode (P) : It collect the electrons. When grid is given positive potential then plate current increases but in this case triode cannot be used for amplifier and therefore grid is normally not given positive potential. (ii) It is a mixture of ac and dc U (iii) The r.m.s. values of the half wave output are given by i0 and irms  2    2 2  When grid is given negative potential then plate current decreases but in this case grid controls plate current most effectively. (4) Working : Plate of triode valve is always kept at positive potential D YG (iv) Ripple factor r  (iii) Control grid : It is a third electrode, also known as control grid, which controls the electrons going from cathode to plate. As a result grid controls the plate current. It is kept near the cathode with low negative potential. ID (B) Output signal (i) The average or dc output values are Fig. 27.70 2V0 2i and id.c.  0 Vd.c.    2 (i) Filament (F) : It emits electron on heating. D2 t (A) Full wave rectifier V0 (3) Number of electrodes : It consists of three electrodes. E3 Output voltage D2 2 w.r.t. cathode. The potential of plate is more than that of grid.    1  0.48  48%.   iP (v) Efficiency of half wave rectifier is given by  (B) Symbol (A) Triode valve F F D1 Vrms  K 2 60 1 Pd.c. 81.2  100%  % r Pa.c. 1 p RL L U ST Filter circuits smooth out the fluctuations in amplitude of ac ripple of the output voltage obtained from a rectifier. (i) Filter circuit consists of capacitors or/ and choke coils. Fig. 27.72 The variation of plate potential affects the plate current as follows 3/2 V   ip  k  VG  p  ; where  = Amplification factor of triode valve, k =    Constant of triode valve. The valve of V for which the plate current becomes zero is known as Vp the cut off voltage. For a given V p , it is given by VG  .  G Characteristics of Triode (iii) A choke coil offers high resistance to high frequency ac, and almost zero resistance to dc. It is used in series. as (v) For voltage regulation choke input filter (L-filter) is best. VPP F (ii) A capacitor offers a high resistance to low frequency ac ripple (infinite resistance to dc) and a low resistance to high frequency ac ripple. Therefore, it is always used as a shunt to the load. (iv)  - Filter is best for ripple control. RL + i V   1.11 (vii) Form factor  rms  rms  id.c. Vd.c. 2 2 Filter Circuit VP F p (viii) Ripple frequency = Double of frequency of input ac = 2 K – The maximum efficiency (for R >> r ) = 81.2% mA P G The triode characteristics can be obtained under two sets of condition Static characteristics and dynamic characteristics (1) Static characteristics : Graphical representation of V or V and i without any load p g p Electronics (i) Static plate characteristic curve : Graphical representation of i and p V at constant V. P g ip (a) Points at which load line cuts the plate characteristic curves are called operating points. dip (b) The slope of load line AB  – 2V – 4V Vg = 0 1551  dVp 1 RL (c) In graph, OA  Vpp  intercept of load line on V axis and P OB  Vpp / R L  intercept of load line on i p axis. O Fig. 27.73 (ii) Static mutual characteristics curve : Graphical representation of i and V when V is kept constant P P ip Constants of Triode Valve VP=120V VP=100V VP=80V (1) Plate or dynamic resistance (r ) E3 G (d) Static plate characteristic + load line Dynamic plate characteristic Static mutual characteristic + load line Dynamic mutual characteristic 60 VP P (i) The slope of plate characteristic curve is equal to 1 plate resistance +Vg 3 2 1 Fig. 27.74 (iii) Constant current characteristic curve : Graphical representation between V and V when i is constant. g p ip 2 Vg Vp 1 D YG ip U p Fig. 27.75 (2) Dynamic characteristics : The curve plotted between i , V and V when the triode contains load in the plate circuit are called dynamics characteristics of diode. p p g (i) Load line : Voltage drop i R across load R which decreases the plate potential will be less then the supply voltage. L L Vpp 1 Vp  RL RL U p Plate voltage V = V – i R  ip   p pp p L ST This equation represents a straight line on the static plate characteristics, joining the points ( V pp , 0) on plate voltage axis and (0, Vpp / R L ) on plate current axis. This line known as load line. ip  VPP  0 ,  RL    B or It is the ratio of small change in plate voltage to the change in plate current produced by it, the grid voltage remaining constant. That is, Vp rp  , VG  constant. ip ID  Vg  Vg  Vg O – 2V  BC  rp     AB  A iP B C O VP VP Fig. 27.77 (ii) It is expressed in kilo ohms (K). Typically, it ranges from 8K to 40K. The r can be determined from plate characteristics. It represents the reciprocal of the slope of the plate characteristic curve. p (iii) If the distance between plate and cathode is increased the r increases. The value of r is infinity in the state of cut off bias or saturation state. (2) Mutual conductance (or trans conductance) (g ) p p m (i) It is defined as the ratio of small change in plate current (ip ) to the corresponding small change in grid potential (Vg ) when plate  i potential V p is kept constant i.e. gm   p  Vg  gm  Vg=0 Vg=0 – 2V – 4V AC BC A    Vp is constant ip VP =100 V ip B C A O (VPP, 0) Fig. 27.76 O iG Fig. 27.78 VP VG (ii) The value of g is equal to the slope of mutual characteristics of triode. m 1552 Electronics (iii) The value of g depends upon the separation between grid and cathode. The smaller is this separation, the larger is the value of g and vice versa. m m (4) The linear portion of the mutual characteristic with maximum slope is chosen for amplification without distortion. iP (mA) (iv) In the saturation state, the value of ip  0 , g m  0 (3) Amplification factor () : It is defined as the ratio of change in plate potential (Vp ) to produce certain change in plate current (ip ) to Linear part Output the change in grid potential (Vg ) for the same change in plate current  V  ; negative sign indicates that V and V (ip ) i.e.     p   Vg   I p  a constant are in opposite phase. (i) Amplification factor depends upon the distance between plate and cathode (d ), plate and grid (d ) and grid and cathode(d ). pg i.e.   d pg  d pk  gg O gk – Vgg– Vi Input (i) For the positive half cycle of input voltage (V ) : V becomes less Fig. 27.81 negative, so i increases P p rp  ip 1 / 3 , gm  ip1 / 3 ,  does not depend on i. (5) Voltage amplification  ipA C Fig. 27. 82 : Equivalent circuit of triode amplifier Current through the load resistance is given by ip   ipB B VP 1  VP 2 VG 2  VG1 VO VP2 D YG I  I PB  PA , VG1  VG 2 Output RL –Vi VP1 A ip Triode U VP 1  V P 2 , I PA  I PB g (iii) The phase difference between the output signal and input signal is 180° (or ) Above three constants may be determined from any one set of characteristic curves. ip i p rp p g (ii) For the negative half cycle of input voltage (V ) : V becomes more negative, so i decreases ID The rp and g m depends on i in the following manner gm – Vgg+Vi i 1 d gk (ii) The value of  is greater than one. (iii) Amplification factor is unitless and dimensionless. (4) Relation between triode constants : The triode constants are not independent of each other. They are related by the relation   rp  g m rp  Vg E3 pk –V g 60 p VG1 VG2 O VG Fig. 27.79 Triode as an Amplifiers Numerically A  Amplifier is a device by which the amplitude of variation of ac signal voltage / current/ power can be increased (1) The signal to be amplified (V ) is applied in the grid circuit and amplified output is obtained from the plate circuit  Vi RL rp  RL  V0  ip RL  R L  rp  R L  Voltage gain  Vi rp  R L V0 R L  Vi rp  R L  rp 1 RL (i) If R =   A will be maximum and A =  L max U i P ST G Vi Vg RL RL K (ii) If r = R  A = p L  2 (iii) Power at load resistance P  ip V0  ip2 RL C Condition for maximum power R = r + – Vgg Vo Output voltage (Practically A < ) L Vpp  Vi  Pmax    RL  RL – + p 2   2 Vi2   RL   4 RL  Fig.sum 27.80 (2) The voltage at grid is the of signal V and grid bias V. Vg  Vgg  Vi i gg (3) Small change in grid voltage results in a large change in plate current so results in a large change in voltage across RL (V0  ip RL  V0  ip RL )  The most efficient packing of atoms in cubic lattice structure occurs for fcc. Electronics 1553  The lattice for NaCl crystal is fcc. CE configuration.  The space lattice of diamond is fcc. (The diamond structure may be  MOSFETS : In a MOSFET, a type of three-terminal transistor, a viewed as two fcc structures displaced from each other by one quarter of a body diagonal).  Carbon, silicon, germanium, tin can crystallize in the diamond structure.  At room temperature  Ge   Si potential applied to the gate terminal G controls the internal flow of electrons from the source terminal S to the drain terminal D. Commonly, a MOSFET is operated only in its ON (conducting) or OFF (not conducting condition. Installed by the thousands and millions on silicon wafers (chips) to form integrated circuits, MOSFETs form the basis for computer hardware.  When a PN junction is forward biased, it can emit light, hence can forward bias. So that it may exert a large attractive force on the charge carriers to enter the collector region.  Ge is more sensitive to heat since it's forbidden energy gap is smaller than that of silicon. Electrons from the valence band of Ge requires less energy to move from the valence band to conduction band. serve as a light-emitting diode (LED). The wavelength of the emitted c hc light is    f Eg 60  In a transistor circuit the reverse bias is high as compared to the  The fermi energy of a given material is the energy of a quantum state that has the probability 0.5 of being occupied by an electron.  Number of conduction electrons per unit volume E3  (ni )Ge ~ 2.4  1019 / m 3 and (ni )Si ~ 1.5  1016 / m 3  Both N-type as well as P-type semiconductor are neutral. (Material' s density) (Molar mass M)/N A   Semiconductor devices are current control devices.  The semiconductor devices are temperature sensitive devices. (N = Avogadro's number = 6.02  10 /mol) 23 A  The occupancy probability P(E) : Electrical conduction of a metal of 3  10 V/m for Ge and 7  10 V/m for Si. depends on the probability that if an energy level is available at energy E, is it actually occupied by an electron. 5 5 ID  The electric field setup across the potential barrier is of the order  An ideal junction diode when forward biased offers zero resistance. Voltage drop across such a junction diode is zero. In reverse biased diode offers infinite resistance and voltage drop across it is equal to voltage applied. the expression for occupancy probability P(E) is given by capacitor with P and N regions acting as the plates of the capacitors and depletion layer as the dielectric medium.  A good emitter should have low work function, high melting point, D YG U  A P-N junction diode can be considered to be equivalent to a 1  E  EF exp   kT  The mobility of electron is two-three times the mobility of holes. Therefore NPN devices are fast and hence preferred.  If Eg ~– 0 eV , the material is good conductor or metal and if Eg ~– 1 eV , the material is a semiconductor. If Eg ~– 6 eV then the Fermi-Dirac statistics P(E)    1  ; E =Fermi energy F high working temperature, high electrical and mechanical strength.  When triode amplifier are in series, total voltage gain A = A A A.......... 1 2 3  When two triode valve are in parallel T.V.-1  A P-N junction or diode acts like a valve or voltage controlled 1 1 1 Total plate resistance   rp rp1 rp switch. When forward biased, it acts like ON switch. When reverse biased, it acts like an OFF switch. Total mutual conductance Gm  gm1  gm 2  The current due to minority carriers in the junction diode is Total amplification factor  = G R independent of the applied voltage. It only depends upon the temperature of the diode. Voltage amplification A  material is an insulator. U ST  Voltage obtained from a diode rectifier is a mixture of alternating and direct voltage.  Cross sectional area of base is very large as compared to emitter. Cross sectional area of collector is less than base but greater than emitter.  C.C (common collector) amplifier is called power amplifier or T.V.-2 2 m p R L rp  R L  NOR gate is a universal gate because it can be used to perform the basic logic function, AND, OR and NOT.  Output in Ex-OR gate is '1' only when inputs are different.  If both inputs of NAND gate are shorted then it will become 'NOT gate current booster or emitter follower.  Devices like tunnel diode, tetrode and thyrisisters have negative resistance.  Transistor provides good power amplification when they are use in – A A NOT gate