Semiconductor Devices PDF

# Half-Wave Rectifier ## Part-11 **By Indresh sir** ## Application of Junction Diode **Rectifier**: It is a device which is used for converting alternating current into direct current. Diode can be used as a rectifier as it is a unidirectional device. ## Half-Wave Rectifier ### Working: - **For po...

# Half-Wave Rectifier ## Part-11 **By Indresh sir** ## Application of Junction Diode **Rectifier**: It is a device which is used for converting alternating current into direct current. Diode can be used as a rectifier as it is a unidirectional device. ## Half-Wave Rectifier ### Working: - **For positive half cycle**: - During the first half (positive) of the input signal, S₁ is at positive and S₂ is at negative potential. So the PN junction diode D is forward biased. The current flows through the load resistance R₁ and output voltage is obtained across R₁. - **For negative half cycle**: - During the second half (negative) of the input signal, S₁ is at negative potential and S₂ is at positive potential. The PN junction diode D is reversed biased. In this case, practically no current would flow through the load resistance. So there will be no output across R₁. - Thus, corresponding to an alternating input signal, we get a unidirectional pulsating output called rectified output. # Full-Wave Rectifier ## Part-12 **By Indresh sir** ## Centre Tap Full Wave Rectifier **We need two diodes working for both the half cycles. Also, we need a centre-tap arrangement for transformer.** ### Working: - **During the positive half of the input signal**: S₁ positive and S₂ negative. In this case, diode D₁ is forward biased and D₂ is reverse biased. So only D₁ conducts and hence the flow of current in the load resistance R₁ is from A to B. - **During the negative half of the input signal**: S₁ is negative and S₂ is positive. So D₁ is reverse-biased and D₂ is forward biased. So only D₂ conducts and hence the current flows through the load resistance R₁ again from A to B. - It is clear that whether the input signal is positive or negative, the current always flows through the load resistance in the same direction and thus output is called a full wave rectified. # Application of p-n junction ## Part-13 ## Applications of p-n junction **Light Emitting Diode (L.E.D)** A Light Emitting Diode converts electrical current into light. LEDs are heavily doped p-n junctions and operated under forward bias. Outer material must be transparent so that photons can escape. ### Working: - When LED is forward biased then electrons move from N→P and holes move from P → N. At the junction boundary these are recombined. On recombination, energy is released in the form of photons of energy equal to or slightly less than the band gap. - When the forward current of the diode is small, the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum. Further increase in the forward current results in decrease of light intensity. LEDs are biased in such a way that the light emitting efficiency should be maximum. - In case of Si or Ge diodes, the energy released in recombination lies in the infra-red region. Therefore to form LED, such semiconductors are to be used which have band gap from 1.8 eV to 3 eV. Hence GaAs₁-xPx is used in forming LED. ## Advantages of LED : 1. Low operational voltage and less power 2. Fast action with no warm up time 3. Emitted light is nearly monochromatic 4. They have long life # Photodiode ## Part-14 **By Indresh sir** ## Photodiode A photo-diode converts light into electrical current or voltage. Photo-diode are moderately doped p-n junctions and operated under reverse bias. Outer material must be transparent so that photons can enter. ### Working - When light of energy "hv" falls on the photodiode (Here hv > energy gap) more electrons move from valence band to conduction band, due to this current in circuit of photodiode in "Reverse bias", increases. As light intensity is increased, the photo current goes on increasing. So, photo diode is used "to detect light intensity". Example used in "Video camera". # Solar Cell ## Part-15 ## Solar Cell It converts light energy into electrical energy. It has same working principle as that of photo-diode but working method is different. It is operated unbiased. The surface layer of p-region is made very thin so that the incident photons may easily penetrate to reach the junction which is the active region. ### Working - When light falls on, emf generates due to the following three basic processes: generation, separation and collection- (i) generation of e-h pairs due to light (with hv > Eg) in junction region; (ii) separation of electrons and holes due to electric field of the depletion region. Electrons are swept to n-side and holes to p-side by the junction field; (iii) On reaching electrons at n-side and holes on at p-side. Thus n-side becomes negative and p-side becomes positive potential and giving rise to photovoltage. # Zener Diode ## Part-16 ## Zener Diode Special purpose diode invented by C. Zener. Heavily doped p-n junction. Works like a normal diode in forward bias. Mainly operated in reverse bias breakdown region. ### Working - It can handle large variation in current without change in Zener voltage, and hence it will be used in regulation. ## In reverse biasing, breakdown can occur in two ways: 1. Avalanche Breakdown 2. Zener Breakdown # Avalanche Breakdown - It occurs in p-n junction having low doping. Breakdown voltage is very high. Depletion layer width is also more. Charge carriers crossing the depletion region gain enough kinetic energy and make collision with other atoms, and hence it starts the avalanche effect. Damage to the diode is permanent. # Zener Breakdown - Occurs in heavily doped p-n junction. Breakdown voltage is very small. Depletion layer is very thin. Breaking of covalent bonds is mainly due to electric field. Hence damage is not permanent. # Zener diode as a voltage regulator - Regulation is possible because in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes. # Basics of Transistor ## Part-17 ## Transistor - A transistor is a three terminal electronic device made up of semiconductor material. It is a lot more complex than diode, but with complexity the number of applications increases. - Transistors have many uses, such as: - Amplification - Switch - Voltage regulation - Modulation of signals # Types of Transistors - Transistor are of two types - NPN transistor - PNP transistor ## NPN Transistor - If a thin layer of P-type semiconductor is sandwiched between two thick layers of N-type semiconductor, then it is known as NPN transistor. ## PNP transistor - If a thin layer of N-type of semiconductor is sandwiched between two thick layer of P-type semiconductor, then it is known as PNP transistor. # Each transistor has three terminals - **Emitter**: It is moderate size and heavily doped. It supplies a large number of majority carriers for the current flow through the transistor. - **Base**: It is very thin & lightly doped as we want very less recombination of charge carriers. - **Collector**: It is moderately doped and larger in size as compared to the emitter. Larger size helps in proper heat dissipation. # Transistor has two p-n junctions: 1. Base-Emitter 2. Base-Collector Based on the biasing of these junctions, its region of operation and application will be different. | Base-Emitter | Base-Collector | Region | Application | |---|---|---|---| | Forward | Reverse | Active | Amplification | | Forward | Forward | Saturation | Switch "ON" | | Reverse | Reverse | Cut-Off | Switch "OFF" | | Reverse | Forward | Inverse-Active | --NA-- | # Working of Transistor ## Part-18 ## Working of NPN Transistor - The emitter base junction is forward biased and base collector junction is reversed biased to study the behaviour of transistor. It is called active state of transistor. N-P-N transistor in circuit and symbolic representation is shown in figure. - In active state of n-p-n transistor majority electrons in emitter are sent towards base. - The barrier of emitter base junction is reduced because of forward bias therefore electrons enter into the base. About 5% of these electrons recombine with holes in base region results very small current (IB) in base. - The remaining electron (95%) enters into the collector region because these are attracted towards the positive terminal of battery results collector current (lc) - The base current is the difference between IE and Ic and proportional to the number of electron hole recombination in the base. - IE = IB +IC. ## Working of PNP Transistor - When emitter-base junction is forward biased, holes (majority carriers) in the emitter are repelled towards the base and diffuse through the emitter base junction. The barrier potential of emitter-base junction decreases and hole enters into the n-region (i.e. base). A small number of holes (5%) combine with electrons of base-region resulting small current (18). The remaining holes (95%) enter into the collector region because these are attracted towards negative terminal of the battery connected with the collector-base junction. These holes constitute the collector current (lc). - As one hole reaches the collector, it is neutralized by the battery. As soon as one electron and a hole is neutralized in collector, a covalent bond is broken in emitter region and an electron hole pair is produced. The released electron enters the positive terminal of battery and holes moves towards the collector. So IE = IB + IC. # Configuration of Transistor ## Part-19 ## Configuration of Transistor - The transistor is connected in either of the three ways in circuit. - Common base configuration - Common emitter configuration - Common collector configuration # Common Emmiter Configuration ## Part-20 ## Common Emmiter Configuration - Input signal is provided between the emitter and the base. Output signal appears across the collector and the emitter. ### Explanation: - The configuration in which the emitter is connected between the collector and base is known as a common emitter configuration. The variation of emitter current (IB) with Base-Emitter voltage (VBE), keeping Collector Emitter voltage (VCE) constant. - The current gain (ẞoc) in CE mode of a transistor is given by, $B_{DC} = \frac{I_c}{I_b}$ - The current gain (BAC) in CE mode of a transistor is given by, $B_{AC}= \frac{\varDelta I_c}{\varDelta I_b} $ - $ẞ$ is in the range of about 50 to 300. # Transistor Characteristics ## Part-21 ## Common emitter transistor characteristics ### Input characteristics: - The variation of base current (IB) (input) with base emitter voltage (VEB) at constant collector emitter voltage (VCE) is called input characteristic. - Keep the collector-emitter voltage (VCE) constant (say VCE = 10 V) - Now change emitter base voltage VBE in steps of 0.1 volt and note the corresponding values of base current (IB). - Plot the graph between VBE and IB. ### Output characteristics: - The variation of collector current Ic (output) with collector-emitter voltage (VCE) at constant base current (IB) is called output characteristic. - Keep the base current (IB) constant (say Iв = 10 μА) - Now change the collector-emitter voltage (VCE) and note the corresponding values of collector current (lc). - Plot the graph between VCE and Ic. - A set of such curves can also be plotted at different fixed values of base current (say 20 μΑ, 30 μΑ etc.) # Relation Between α, β and γ ## Part-22 ## Relation Between α, ẞ and γ | α, β | β. γ | α, γ | |---|---|---| | $I_E = I_B + I_c$ divide by $I_c$ | $I_E = I_B + I_c$ divide by $I_B$ | $I_E = I_B + I_c$ $γ = 1 + β$ | | $α = \frac{I_c}{I_E} = 1 + \frac{I_B}{I_c}$ | $β = \frac{I_c}{I_B} = 1 + \frac{I_c}{I_B} $ | $γ = 1 + \frac{α}{1-α} $ | | $α = \frac{1}{1 + \frac{I_B}{I_c}}$ | $γ = 1 + β$ | $γ = \frac{1}{1 - α}$ | | $β = \frac{α }{1 - α} = 1 + \frac{α}{1 + β} $ | | $α . γ = β$| # Comparison between Common Base, Common Collector, Common Emitter ## Part-24 ## Comparison between Common Base, Common Collector, Common Emitter | Comparision factors | Common Base (CB) | Common Emitter (CE) | Common Collector (CC) | |---|---|---|---| | Circuit Diagram | image | image | image | | Input Resistance | Low (100Ω) | High (750Ω) | Very High ≈750 ΚΩ | | Output resistance | Very High | High | Low | | Current Gain | (Αi or a) | (A or β) | (Αi οι γ) | | | α<1 | Β>1 | γ>1 | | | $ α = \frac{I_c}{I_E} < 1$ | $β = \frac{I_c}{I_B} > 1$ | $γ = \frac{I_c}{I_B} > 1$ | | Voltage Gain | Av =$\frac{V_o}{V_i} = \frac{I_c R_L}{I_E R_i} = \frac{α R_L}{R_i} $ | Av = $\frac{V_o}{V_i} = \frac{I_c R_L}{I_B R_i} = \frac{β R_L}{R_i} $ | Av = $\frac{V_o}{V_i} = \frac{I_c R_L}{I_B R_i} = \frac{γ R_L}{R_i} $ | | Power Gain | A_p = $\frac{P_o}{P_i} = \frac{I_c^2 R_L}{I_E^2 R_i} = \frac{α^2 R_L}{R_i}$ | A_p = $\frac{P_o}{P_i} = \frac{I_c^2 R_L}{I_B^2 R_i} = \frac{β^2 R_L}{R_i}$ | A_p = $\frac{P_o}{P_i} = \frac{I_c^2 R_L}{I_B^2 R_i} = \frac{γ^2 R_L}{R_i}$ | | Phase difference (between output and input) | Same phase | Opposite phase | Same phase | | Application | For High Frequency amplifier | For Audible frequency amplifier | For Impedance Matching | # Transistor as Amplifier ## Part -26 ## Transistor as Amplifier - The process of increasing the amplitude of input signal without distorting its wave shape and without changing its frequency is known as amplification. - A device which increases the amplitude of the input signal is called amplifier. ### Common Emitter Amplifier NPN Transistor - To operate the transistor as an amplifier it is necessary to fix its operating point somewhere in the middle of its active region. If we fix the value of VBB corresponding to a point in the middle of the linear part of the transfer curve then the dc base current Is would be constant and corresponding collector current Ic will also be constant. The dc voltage VCE = Vcc - Ic Rc would also remain constant. The operating values of VCE and IB determine the operating point, of the amplifier. - If a small sinusoidal voltage with amplitude ui is superposed in series with the VBB Supply, then the base current will have sinusoidal variations superimposed on the value of IB. As a consequence the collector current also will have sinusoidal variations superimposed on the value of Ic producing in turn corresponding change in the value of Vo. **Note:** However, it should be realised that transistor is not a power generating device. The energy for the higher ac power at the output is supplied by the battery Vcc. # Boolean Algebra ## Part-28 ## Analogue v/s Digital Electronics ### Analogue Signal - A continuous signal value which at any instant lies within the range of a maximum and a minimum value. ### Digital Signal - A discontinuous(discrete) signal value which appears in steps in pre-determined levels rather than having the continuous change. ## Digital Circuit - An electrical or electronic circuit which operates only in two states (binary mode) namely **ON/1** and **OFF/0** is called a Digital Circuit. - To understand the Digital circuits, we need to understand the Boolean algebra. - Boolean Algebra was invented by George Boole. - Boolean algebra have binary variables that take only 2 discrete values (0 and 1). ## Boolean Algebra | 1 | 0 | |---|---| | High | Low | | On | Off | | True | False | | Present | Absent | | Positive | Negative | | 5 Volt | 0 Volt | ## Boolean algebra have three basic logic operations: | **OR** | **AND** | **NOT** | |---|---|---| | 0+0=0 | 0.0=0 | **0'**=0=1 | | 0+1=1 | 0.1=0 | **1'**=1=0 | | 1+0=1 | 1.0=0 | | | 1+1=1 | 1.1=1 | | # Logic Gates ## Part-29 ## Logic Gates - A logic gate is a digital circuit which is based on certain logical relationship between the input and the output voltages of the circuit. ## Basic Logic Gates 1. **OR Gate** ### Representation: > image ### Boolean expression: > Y = A + B ### Truth table: | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 1 | **Note:** Output is ON if any of the inputs are ON. ### Waveforms: > image ### Electric analogous circuit: > image 2. **AND Gate** ### Representation: > image ### Boolean expression: > Y = A. B ### Truth table: | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 1 | **Note:** Output is ON if and only if all the inputs are ON. ### Waveforms: > image ### Electric analogous circuit: > image 3. **NOT Gate (Inverter)** ### Representation: > image ### Boolean expression: > Y = A ### Truth table: | A | Y | |---|---| | 0 | 1 | | 1 | 0 | ### Waveforms: > image **Note:** The output of a NOT gate attains the state ON if and only if the input does not attain the state ON. ### Electric analogous circuit: > image # Universal Gates ## Part-30 ## Universal Gates 1. **NAND Gate** ### Representation: > image ### Boolean expression: > Y = A.B ### Truth table: | A | B | Y'=A-B | Y=A-B | |---|---|---|---| | 0 | 0 | 0 | 1 | | 0 | 1 | 0 | 1 | | 1 | 0 | 0 | 1 | | 1 | 1 | 1 | 0 | **Note:** The output is low only when both the input are high. ### Waveforms: > image ### Electric analogous circuit: > image 2. **NOR Gate** ### Representation: > image ### Boolean expression: > Y = A+B ### Truth table: | A | B | Y'=A+B | Y=A+B | |---|---|---|---| | 0 | 0 | 0 | 1 | | 0 | 1 | 1 | 0 | | 1 | 0 | 1 | 0 | | 1 | 1 | 1 | 0 | ### Waveforms: > image **Note:** The gate give high output only when both the inputs are low. ### Electric analogous circuit: > image ### Digital Circuit (Realizing circuit): > image # Special Purpose Gates ## Part-32 ## Special Purpose Gates 1. **XOR Gate** ### Representation: > image ### Boolean expression: > Y = AB+ A-B or Y = A⊕B ### Truth table : | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 0 | 2. **XNOR Gate** ### Representation: > image ### Boolean expression: > Y = A-B+A-B or Y = A⊙B ### Truth table: | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 1 | # Illustration ## Illustration 3. - Given electrical circuit is equivalent to which logic gate, also draw its symbol and truth table. - **Solution:** - **OR gate** > image ## Illustration 4. - Write the truth table for the logical function **D = (A AND B) OR B** - **Solution:** | A | B | X = A AND B | D = X OR B | |---|---|---|---| | 0 | 0 | 0 | 0 | | 0 | 1 | 0 | 1 | | 1 | 0 | 0 | 0 | | 1 | 1 | 1 | 1 | ## Illustration 5. - Identify the logic gates P and Q in given circuit. Also write down relation in A, B and X. - **Solution:** - P is NOR gate & Q is AND gate, *X=(A+B)-B=(A.B) B=A-(BB)=Ā-0=0* > image ## Illustration6. - Write down the equivalent function performed by given circuit. Explain your answer. - **Solution:** - AND gate, *Z=A+B+B+C=AB-BC=ABC=ABC (X+Y=X-Y)* > image ## Illustration 7. - If inputs A and B are inverted before entering into NAND gate as shown in diagram. Write down the logical symbol and truth table by using A, B, A, B, Y. - **Solution:** - **Y=A.B=A+B** so logical symbol >image - **Truth table** | A | B | A | B | Y | |---|---|---|---|---| | 0 | 0 | 1 | 1 | 1 | | 0 | 1 | 1 | 0 | 1 | | 1 | 0 | 0 | 1 | 1 | | 1 | 1 | 0 | 0 | 0 | > image