Chapter 14 Semiconductor Electronics Material Devices (PDF)

CHAPTER SEMICONDUCTOR ELECTRONICS: MATERIALS, 14 DEVICES AND SIMPLE CIRCUITS No. of Hrs. No. of 1 Marks 5 Marks Total PU Board Required 2 Mark...

CHAPTER SEMICONDUCTOR ELECTRONICS: MATERIALS, 14 DEVICES AND SIMPLE CIRCUITS No. of Hrs. No. of 1 Marks 5 Marks Total PU Board Required 2 Marks 3 Marks Marks Allotted Sessions (MCQ) FIB LA NP 9 15 1 1 1 - 1 - 9 Learning Objectives After studying this unit, you will be able to understand the following  Classification of solids on the basis of their conductivity  Energy bands in solids  Difference between metals, insulators and semiconductors using band theory  Intrinsic semiconductor  Extrinsic semiconductors  p-n junction diode  V-I characteristics of p-n junction diode  p-n junction diode as rectifier Introduction: Devices in which a controlled flow of electrons can be obtained, are the basic building blocks of all electronics circuits, Earlier, vacuum tubes (also called valves) were used to serve this purpose. The discovery of semiconductor diode and transistor revolutionized the development of electronics. The vacuum tubes are being replaced by diodes and transistors because of their several advantages. Some of the major disadvantages of vacuum tubes are i) they are very bulky ii) Consume large power iii) Operational voltage is very large (more than 100v) iv) Less reliable The study of electrical properties of solids, in particular semiconducting material, is important to understand the electronic circuits with semiconductor devices. Classification of solids: Solids can be classified into conductors (metals), insulators and semiconductors, on the basis of (i) Conductivity(σ) or resistivity (ρ) (ii) Energy bands 156| II PU Physics Classification of solids on the basis of electrical conductivity: 1. Metals: These are the solids which have high conductivity or low resistivity. The conductivity 2 2 8 1 of the metals is in the range of 10 to10 S m and resistivity is in the range 10 to 108  m. Ex: Aluminum, copper, silver, gold etc. 2. Insulators: These are the solids which have low conductivity or high resistivity. The conductivity 11 19 1 of the insulators is in the range of 10 to 10 Sm and resistivity is in the range 1011 to 1019  m. Ex: Rubber, plastic, mica, glass etc. 3. Semiconductors: These are the solids which have conductivity or resistivity intermediate to 5 6 1 metals and insulators. The conductivity of the semiconductors is in the range of 10 to10 Sm 5 and resistivity is in the range 10 to 10  m. The semiconductors are of two types. 6 (i) Elemental semiconductors: Silicon, germanium (ii) Compound semiconductors:  Inorganic: CdS, GaAs, etc.  Organic: Anthracene doped pthalocyanines, etc.  Organic polymers: Polypyrrole, polyaniline, etc. Band theory of solids: According to the Bohr atomic model, in an isolated atom the energy of any of its electron is decided by the orbit in which it revolves. But when the atoms come together to form a solid, they are close to each other. So, the outer orbits of electrons from neighboring atoms would come very close or could even overlap. This would make the nature of electron motion in a solid very different from that in an isolated atom. Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. Energy band: The group of closely spaced energy levels is known as energy band. Valence band: The energy band which includes the energy levels of the valence electrons is called the valence band. Conduction band: The energy band which includes conduction electrons is called the conduction band. Energy gap  Eg  : The separation between highest energy level in valance band and lowest energy level in conduction band is known as energy gap or forbidden energy gap. If EC denotes the lowest energy level in conduction band and EV denote the highest energy level in valence band, then the energy gap EG  EC  EV 14.1 Semiconductor Electronics | 157 Classification of solids on the basis of band theory of solids: Based on separation between valence band and conduction band, the solids are classified as conductors, semiconductors and insulators. 1. Conductors: In conductors either the conduction band is partially filled and valence band is partially empty or conduction band and valence band overlap. When the conduction band overlaps on the valence band the electrons can move freely from valence band to conduction band. Energy gap Eg  0 eV. Energy band structure for a conductor is shown in figure 14.1(a) and 14.1(b) 2. Semiconductors: In semiconductors there exist a finite but small energy gap between valence band and conduction band. Because of small energy gap, above absolute zero temperature some of the electrons in valence band may jump to conduction band. This makes the material to exhibit partial conductivity. Energy gap Eg < 3eV. Energy band structure for a typical semi-conductor is shown in fig(14.2) 3. Insulators: In insulators there exist a large energy gap between valence band and conduction band. Electrons in the valence band cannot reach the conduction band even due to thermal agitation. The absence of electrons in conduction band makes the material non conducting. Energy gap Eg > 3eV. The energy band structure for an insulator in shown in fig (14.3) 158| II PU Physics Q. Distinguish between a conductor and a semiconductor on the basis of band theory of solids. (March-15) Q. Explain ‘Conduction band’, ‘Valence band’ and ‘Energy gap’ in semiconductors. (March-19) Q. Explain the formation of energy bands in solids. On the basis of energy bands distinguish between a conductor, a semiconductor and an insulator. (March-14) Semiconductor Electronics | 159 Intrinsic semiconductor: An intrinsic semiconductor is a pure semiconductor having poor conductivity. At absolute zero temperature, an intrinsic semiconductor behaves like an insulator. Its conductivity can be increased by increasing the temperature. Ex: germanium, silicon. In a crystal of germanium or silicon, each atom forms four covalent bonds (valence bonds) by sharing its four valence electrons with four electrons of neighboring four atoms. At 0K all the covalent bonds are complete and there are no free charge carriers in the crystal. This situation is described in fig (14.4a) Hence an intrinsic Ge or Si at 0K has a completely filled valence band and a completely empty conduction band as shown in fig (14.4b) As temperature increases, thermal energy becomes available to the electrons. Consequently some of the electrons may absorb sufficient energy and break away from the covalent bond. The electron which leaves the covalent bond is known as a free electron. The vacancy created at the sight of a atom due to the release of the electron is known as a hole. Hole behaves as an apparent free particle with effective positive charge. The creation of electron hole pairs is shown in fig (14.5a) 160| II PU Physics In other words, some of the electrons move from valence band to the conduction band, leaving holes in the valence band. Both holes and electrons play an important role in electrical conduction in semiconductors. The electrons excited into the conduction band and holes created in the valence band is represented in fig(14.5b) In intrinsic semiconductors, the number of free electrons ne is equal to the number of holes nh. That is n e = n h =ni 14.2 Where n i is called intrinsic carrier concentration. It may be noted that apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole. When an electric field is applied, electrons move towards the positive potential and holes move towards negative potential. If I e is the current due movement of electrons and I h is the current due to the movement of holes, the net current I=Ie + Ih 14.3. Q. What is intrinsic semiconductor? (June-15) Extrinsic semiconductor: A semiconductor to which an impurity is added to increase its conductivity is known as an extrinsic semiconductor or impurity semiconductor. The process of adding impurities to an intrinsic semiconductor is known as doping. The impurity atoms are called dopants. There are two types of dopants used in doping the tetravalent Si or Ge: (1) Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc. (2) Trivalent (valency 3); like Indium (In), Boron (B), Aluminum (Al), etc. Depending on the kind of impurity added the extrinsic semiconductors are classified into two types, i) n-types semiconductor ii) p-types semiconductor Semiconductor Electronics | 161 n-type semiconductor: When a small amount of pentavalent impurity is added to pure semiconductor it becomes n-type semiconductor. Consider the case of silicon (Si) doped with phosphorus (P). The impurity atom gets surrounded by four Si atoms. Four of the five valence electrons of P are involved in covalent bonds. The fifth valence electron becomes almost free (see fig 14.6) and can move to the conduction band easily. Energy of 0.05eV in case of silicon is sufficient a liberate this electron which s very small compare to the energy gap (1.1eV). This leaves behind an immobile positive ion in the crystal. Since an impurity atom donates a free electron to the semiconductor, impurity atom is called donor. At room temperature, electron-hole pairs are created due to the breaking of covalent bonds. It produce equal number of electrons and holes. Over and above these electrons in the conduction band, there are additional number of electrons contributed by donor atoms. Therefore number of electrons are greater than number of holes. The excess of electron population in conduction band increases the rate of recombination which suppresses the holes in the valence band Thus, in n-type semiconductor electrons are the majority charge carriers and holes are minority charge carriers. i.e., ne  nh 14.4 In the energy band diagram of n-type semiconductor, the donor energy level E D is slightly below the bottom E C of the conduction band and electrons from this level move into the conduction band with very small supply of energy. The conduction band will have most electrons coming from the donor impurities. 162| II PU Physics P - type semiconductor: When a small amount of trivalent impurity is added to pure semiconductor it becomes p-type semiconductor. Consider the case of silicon (Si) doped with boron (B) The impurity atom gets surrounded by four Si atoms. But it provides only three valence electrons to make three covalent bonds. The vacancy that exist in the fourth bond constitutes a hole. This hole accepts electron from the neighborhood, creating a vacancy or hole at its own site. Thus, the hole is available for conduction. When impurity atom accepts an electron from adjoining Si atom, it becomes an immobile negative ion. Since an impurity atom accepts an electron from the semiconductor, an impurity atom is called acceptor. At room temperature, electron- hole pairs are created due to the breaking of covalent bonds. Due to additional number of holes contributed by accepter atoms, the numbers of holes is the valence band are greater than number of electrons. Thus, in p-type semiconductor holes are the majority charge carriers and electrons are minority charge carriers. i.e., nh  ne 14.5 In the energy band diagram of p-type semiconductor, the acceptor energy level EA is slightly above the top EV of the valence band. With very small supply of energy an electron from the valence band can jump to the level EA and ionise the acceptor negatively. At room temperature, most of the acceptor atoms get ionised leaving holes in the valence band. Semiconductor Electronics | 163 The differences between intrinsic and extrinsic semiconductors. Intrinsic Semiconductors Extrinsic Semiconductors 1. Electrical conductivity depends 1. Electrical conductivity depends on both only on temperature temperature and dopant concentration 2. The number of free electrons is 2. The number of free electrons is not equal equal to the number of holes to number of holes The differences between n-type and p-type semiconductors. n-Type Semiconductor p-Type Semiconductor 1. These are extrinsic semiconductors 1. These are extrinsic semiconductors obtained by doping pure Ge or Si obtained by doping Ge or Si crystals crystals with pentavalent dopants with trivalent dopants like aluminium. like Phosphorus. 2. Free electrons are the majority 2. Free electrons are minority charge charge carriers and holes are carriers and holes are majority charge minority charge carriers carriers. Q. Distinguish between n-type and p-type semiconductors. (March-17, 18, 19, June-14, 16) 164| II PU Physics p – n junction When a semiconductor crystal (Si or Ge) is doped in such a manner that one-half is p-type and the other half is n-type, the contact surface dividing the two halves is called p-n junction. p-n junction formation Two important processes occur during the formation of a p – n junction - diffusion and drift. During the formation of p – n junction due to the concentration gradient across p – side and n – side, holes diffuse from p – side to n – side  p n  and electrons diffuse from n – side to p – side  n p . This motion of charge carriers give rise to diffusion current across the junction. When an electron diffuses from n p , it leaves behind an ionized immobile donor (positive charge) on n-side. As the electrons continue to diffuse from n p , a layer of positive charge on n- side of the junction is developed. Similarly, when a hole diffuses from p n due to the concentration gradient, it leaves behind an ionised immobile acceptor (negative charge) on p- side. As the holes continue to diffuse, a layer of negative charge on the p – side of the junction is developed. The space charge region on either side of the p-n junction having no mobile charge carries known as depletion region or depletion layer. Because of positive ion layer on n-side and negative ion layer on p-side, an electric filed is setup across the junction. This electric field sets a barrier at the junction which opposes the further diffusion of majority charge carries into opposite regions. This is called potential barrier. It facilitates the flow of minority charge carriers across the junction. The motion of these charge carriers due to electric field is called drift. The current due to minority charge carriers facilitated by the electric filed is called drift current, which is opposite in direction to the diffusion current. Initially the diffusion current is large and drift current is small. As the process of formation of junction builds up the diffusion current decreases and drift current increases, until the two becomes equal and an equilibrium is established. This completes the formation of p-n junction. Semiconductor Electronics | 165 Note: 1. In a p-n junction under equilibrium there is no net current. 2. A p-n junction at equilibrium (under no external bias) and potential difference across the junction is as shown in fig 4.11. 3. The positive and negative ions set up a potential difference across the p-n junction called barrier potential denoted by V0. 4. The barrier potential V0 opposes diffusion of majority charge carriers but supports drift process. 5. Potential barrier is of the order of 0.1 V to 0.7 V 6. At room temperature potential barrier for Si is 0.7 V and for Ge is 0.3 V. 7. The thickness of depletion region is about 10-6to 10-7m. Semiconductor diode: Semiconductor diode consists of a p-n junction with metallic contacts provided at the ends for the application of an external voltage. It is a two-terminal device as shown in fig (14.12a).The barrier potential can be altered by applying an external voltage across the diode. The symbolic representation of semiconductor diode is as shown in fig (14.12b). Representation Of p-n junction diode under equilibrium (without bias) p-n junction diode under forward bias: When an external voltage V is applied across a semiconductor diode such that p-side is connected to positive terminal of the battery and n-side to the negative terminal of the battery, then it is said to be forward biased. 166| II PU Physics Due to relatively large resistance of the depletion region compared to p – and n – regions, approximately entire applied forward bails voltage V will appear across the depletion region. Since V is opposite to V0 , the effective barrier voltage reduces from V0 to V0  V  (Fig 14.13(b)) consequently the depletion region becomes thin. This will allow electrons from n-side cross the depletion region and reach p-side. Similarly holes from p-side cross the junction and reach the n-side where they are minority carriers. This process under forward bias is known as minority carrier injection. An electric current will flow due to the migration of these carriers across the p-n junction called forward current. The total forward current is sum of hole diffusion current and electron diffusion current. The magnitude of this current is usually in mA. Note: 1. Effective barrier height in the forward bias is  Vo  V . 2. If the applied voltage is small, the barrier potential  Vo  will be reduced only slightly below the equilibrium value and current will be very small. As applied voltage increases, the barrier height will be reduced and current also increases. 3. A diode under forward bias offers negligibly small resistance called forward resistance few ohms). 4. The magnitude of current depends upon the applied forward voltage. p-n junction diode under reverse bias: When an external voltage V is applied across a semiconductor diode such that n-side is connected to positive terminal of the battery and p-side to the negative terminal of the battery, then it is said to be reverse biased. In reverse bias, positive voltage in the battery pulls the electron on n-side away from the junction and negative voltage pull the holes on the p-side away from the junction. Thus width of depletion region increases and applied voltage adds up to the junction potential so that the effective barrier height becomes  Vo  V . This suppresses the flow of electrons from n p and holes from p n. Thus, diffusion current decreases and high resistance path is established. With the reverse applied voltage the flow of minority carriers across the junction constitute drift current in the opposite direction called reverse saturation current. It is of the order of few  A. Note: 1. The saturation current is not affected by increase in applied voltage but increases with increase in temperature. 2. If the reverse bias is increased to a high value, the covalent bond near the junction break down and a large number of electron hole pairs are liberated. Thus, the reverse current increases abruptly to a very high value. This phenomenon is called breakdown and this value of reverse voltage is called breakdown voltage  Vbr  which can damage or destroy the diode due to overheating. Semiconductor Electronics | 167 Q. Describe with suitable block diagram action of p-n junction diode under forward and reverse bias conditions. Also draw I –V characteristics. (June-18) Characteristics of p-n junction diode: The variation of current with the applied voltage across the junction diode gives the characteristics of p-n junction diode. I. Forward bias characteristics: The circuit diagram to study forward bias characteristics and graph of I v/s V is shown below. It consists of rheostat to vary the applied potential and milliammeter to measure current. For different values of voltages, the value of the current is noted. As the forward battery voltage increases, the barrier potential starts decreasing and a small current begins to flow. Beyond certain value of voltage called threshold voltage or cut in voltage the current sharply increase even for a very small increase in the diode bias voltage. The graph becomes almost linear. Note: The cut in voltage for Ge Is ~ 0.2 V and for Si is ~ 0.7 V. II. Reverse bias characteristics: The circuit diagram to study reverse bias characteristics and graph of I v/s V is shown below. It consists of rheostat to vary the applied potential and micro ammeter to measure current. For different values of voltages, the value of the current is noted. For reverse bias, the current is very small (μA) and almost remains constant with change in bias voltage. This current is called reverse saturation current. At very high reverse bias voltage called breakdown voltage, there is sudden rise in reverse current. Commonly diodes are used well below the breakdown voltage. 168| II PU Physics Typical V-I characteristics of a silicon diode (forward and reverse bias): Dynamic resistance: The forward bias resistance is low as compared to the reverse bias resistance. This property is used for rectification of ac voltages. For diodes, dynamic resistance is defined as the ratio of small change in voltage V to a small V change in current I. i.e., rd  I It is equal to the reciprocal of the slope of the V-I characteristics at a given dc voltage. Semiconductor diode as a rectifier: Rectifier is a device which is used for converting alternating current/voltage into direct current/voltage. The process of converting alternating current (AC) into direct current (DC) is known as rectification. Junction diode conducts only when forward biased and it does not conduct when reverse biased. It acts as a valve. This fact makes the junction diode to work as rectifier. p-n junction diode as a half wave rectifier: Circuit of half wave rectifier is as shown in figure (14.16a), which consists of a step down transformer, one diode and a load resistor R L. Step down transformer Steps down high ac voltage to the required value and is given to the diode. The output is taken across a load resistor RL. Semiconductor Electronics | 169 Working:  During the positive half cycle of the input ac voltage, end A of transformer secondary is positive whereas end B is negative. Therefore diode gets forward biased and it conducts. Current flows through R L and hence we get the output voltage.  During the negative half cycle of ac input, end B becomes positive and end A becomes negative. Therefore diode gets reverse biased and hence it will not conduct. Therefore there is no current through R L and hence no output voltage.  The input and and output waveforms are shown in fig(14.16b) Q. What is rectification? Describe with a circuit diagram the working of p-n junction diode as a half wave rectifier with input and output waveforms. (March-20) (June-19) p-n junction diode as a full wave rectifier: A center tap full wave rectifier is shown in fig. (14.7a). In centre tap transformer, there is tapping provided exactly at the centre. Hence both ends of secondary will be at equal voltages and centre tap (point Z) will be at zero potential. The ends A & B are connected together through diodes D1 & D2 and a load resistance is connected across the junction of D1 & D2 and the centre tap as shown in the figure. The output voltage is taken across R L. The circuit diagram of centre tapped full wave rectifier is as shown in fig (14.17a) 170| II PU Physics Working:  During the positive half cycle of ac input voltage, end A becomes positive and end B becomes negative. Therefore diode D1 gets forward biased and D 2 is reverse biased. Hence D1 conducts but D 2 does not conduct. Current flows through the load resistance R L and hence we get output voltage.  During the negative half cycle of ac input, end A becomes negative and end B becomes positive. Therefore diode D 2 gets forward biased and diode D1 gets reverse biased. Hence D1 does not conduct but D 2 conducts. Current flows through R L in the same direction as in previous cycle and an output voltage is obtained across it. The input and output waveforms are shown in figure (14.17b). The frequency of the output is twice that of input. Output voltage is obtained for both positive and negative half cycles and the current flows in the same direction. Since output voltage appears for both cycles of ac input, this circuit gets its name as full wave rectifier. Q. What is rectification? With relevant circuit diagram and waveforms, explain the working of a p-n junction diode as a full wave rectifier by drawing input and output waveforms. (March-16, 17, 18, 22) (June-14, 15) Semiconductor Electronics | 171 Multiple Choice Question 1. In an n-type silicon, which of the following statement is true: a) Electrons are majority carries and trivalent atoms are the dopants. b) Electron are majority carries and Pentavalent atoms are the dopants. c) Hotel are minority carries and pentavalent atoms are the dopants d) Holes are majority carries and trivalent atoms are the dopants 2. Which of the statement is true for p-type semiconductos a) Electrons are majority carries and trivalent atoms are the dopants. b) Electron are majority carries and Pentavalent atoms are the dopants. c) Hotel are minority carries and pentavalent atoms are the dopants d) Holes are majority carries and trivalent atoms are the dopants 3. Carbon, silicon and germanium have four valence electrons each. These are characterized by valence and conduction bands separated by energy band gap respectively equal to  Eg  ,  Eg  and  Eg . Which of the following statement is true? C Si Ge a)  Eg    Eg    Eg  b)  Eg    Eg    Eg  Si Ge C C Ge Si c)  Eg    Eg    Eg  d)  Eg    Eg    Eg  C Si Ge C Si Ge 4. In an unbiased p-n junction, holes diffuse from the p- region to n- region because a) free electron in the n – region attract them. b) they move across the junction by the potential difference c) hole concentration in p- region is more as compared to n – region d) All the above. 5. When a forward bias is applied to a p – n junction, it a) raises the potential barrier b) reduces the majority carrier current to zero c) lowers the potential barrier d) None of the above. Multiple Choice Questions Answer Keys Q 1 2 3 4 5 A b d b c c Two mark questions with answers: 1. Name the charge carriers in the following at room temperature: (i) conductor (ii) semiconductor. Ans: 2. Name the factors on which electrical conductivity of a pure semiconductor depends at a given temperature. Ans:. 172| II PU Physics 3. Mention the necessary conditions for doping. Ans: 4. Name one impurity each, which when added to pure Si produces (i) n-type and (ii) p-type semiconductor. Ans: 5. Give two differences between intrinsic and extrinsic semiconductors. Ans: 6. Give two differences between n-type and p-type semiconductors. Ans: 7. What happens to the width of the depletion layer of a p-n junction when it is (i) forward biased? (ii) reverse biased? Ans: Three mark questions with answers: 1. What is intrinsic semiconductor? Explain the formation of a hole in the covalent bond structure of a Si crystal. Ans: = Semiconductor Electronics | 173 2. How is an n-type semiconductor formed? Name the majority charge carriers in it. Draw the energy band diagram of an n-type semiconductor. Ans 3. How is a p-type semiconductor formed? Name the majority charge carriers in it. Draw the energy band diagram of a p-type semiconductor. Ans 174| II PU Physics Assignment Two mark questions with answers: 1. What is energy gap or energy band gap? How do you distinguish a semiconductor from an insulator based on energy gap? Ans: 2. What are intrinsic and extrinsic semiconductors? Ans: 3. Name one dopant which can be used with germanium to form i) n-type semiconductor ii) p-types semiconductor. Ans: 4. Define the term i) depletion region ii) barrier potential. Ans: 5. What is i) diffusion current? Ii) Drift current? Ans Semiconductor Electronics | 175 6. What is rectifier? Which property of the diode is used for rectification? Ans: 7. Write the input and output wave forms for a half wave rectifier. Ans: 8. Write the input and output wave forms for a full wave rectifier. Ans: Parent’s Signature Mentor’s Signature ********