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UNIT-2 Semiconductor Diode 1 2.1 P-N JUNCTION:• PN-junction is formed by growing a single crystal of Si or Ge, which is half P-type and half N-type. Fig.1 PN Junction 2 2.2 Formation of Depletion Region Fig.2 PN Junction with Depletion Region 3 Depletion region or space charge region. barrier potential or junction potential. The barrier potential for Si is 0.7V & for Ge is 0.3V at 25°C. 4 2.3 Bias of P-N Junction:Bias of PN junction Zero bias or no bias Forward Bias Reverse Bias 5 Zero or No Applied Bias (VD=0 V) • ..No current in ckt. 6 P-N Junction with Forward Bias Fig. 3 PN Junction with Forward Bias 7 • • • • • Applied voltage opposes the contact potential. Net barrier potential VB is reduced. Diffusion current increases. Drift current slightly decreases. The forward biasing current is I f I dif I dr 8 P-N Junction with Reverse Bias Fig. 4 PN Junction with Reverse Bias 9 • • • • VB increase. Idif due to majority carriers reduces to almost zero. Idr slightly increases. The net current Ir remains constant till breakdown, hence called saturation current Is • Reverse saturation current. • It is in Si of the order of nA & in Ge it is of the order of μA 10 2.4 PN JUNCTION DIODE:..also known as crystal diode ..two terminal of diode are cathode and anode ..unidirectional two terminal device .. Used as circuit element P N (a) Anode Short Ckt. (c) P + N - Cathode (b) Rf VT= 0.7V (d) Fig.1 (a) Circuit symbol (b) schematic symbol (c) Ideal FB diode (d) Practical diode equivalent ckt. 11 2.5 V-I Characteristics of PN junction Diode:V-I Characteristic Forward Characteristic .. It offers negligible impedance Reverse Characteristics .. It offers high impedance 12 V-I Characteristics Curve of Si & Ge Diode Forward Characteristics Breakdown voltages Knee or cut in voltage Reverse Characteristics 13 Breakdown voltage Knee voltage or cut-in-voltage Maximum forward current Peak inverse voltage (PIV) 14 2.6 Temperature Effect on V-I Characteristics 15 2.7 Summary of Biasing Conditions 16 2.8 Comparison of Si & Ge Diode:S. No. 1. 2. 3. Parameter 4. 5. Material Used Cut-in Voltage Reverse Saturation current Effect of temp. Breakdown voltage 6. Application Si Diode Ge Diode 17 2.9 Diode Current Equation:The diode current equation can be given as, I I 0 eV / VT 1 ….(1) η is the constant, η=1 for Ge & η= 2 for Si VT=kT/e = T/11,600 Volt equivalent of temperature if the diode is reverse biased then the diode current is, I I0 e V / VT 1 …….(2) 18 If V>>VT then the term e V /VT 1 so, I I 0 Two parameter I0 & VT are the temp. dependent. Breakdown voltage , when Temp. Reverse Saturation current , when temp. 19 2.10 DEPENDENCE OF I0 ON TEMPERATURE :The dependency of I0 on temperature is given as, I 0 KT e m EG /VT Where K is the constant which is independent of temp. m = 2 for Ge & 1.5 for Si • Reverse saturation current doubles its value for every 10ºC rise in temperature. 20 2.11 DIODE RESISTANCE:An ideal diode offer zero resistance in F.B. & infinite resistance in R.B. Diode resistance d.c. Forward Resistance or Static Resistance .. Static resistance RF= VF/IF a.c. Forward Resistance or Dynamic Resistance .. Dynamic resistance rF= ΔVF/ΔIF Static or DC Resistance 22 Dynamic or AC Resistance 23 Summary Table for Resistance Level 24 25 26 27 28 29 2.12 Ideal Diode: • Ideally, a diode should have RF = 0 and RR = V-I characteristics of an ideal diode 30 2.13 Linear Piece-wise or Approximate Circuit Model of a Diode: 31 2.14 Simplified Circuit Model: 32 Summary of Diode Models: 33 2.15 DIODE CAPACITANCE:Diode Capacitance Diffusion or Storage Capacitance Depletion or Transition Capacit. .. Occur in FB junction .. The amount of stored charge Represents the magnitude of diffusion capacitance. .. CD= τIF/ηVT Here, CD.. Diffusion Capacitance η is constant VT is the volt equivalent of temp. τ mean life time of carrier IF is the forward current .. The typical value for CD is 0.02μF .. Occur in RB condition .. Due to the +ve & -ve immobile Ions acts as dielectric medium so P & N acts as two plate of capacitor. .. CT= K/V1/2 Here, CT.. transition Capacitance K is constant V is the applied voltage 34 Transition and diffusion capacitance versus applied bias for a silicon diode. 35 2.16 Application of PN diode:1. Rectifier ckts. 2. Clipping and clamping ckt. 3. Voltage multiplier 4. log & antilog amplifier ckt using OP-AMP 5. Freewheel diode 36 37 38 39 2.17 VARACTOR DIODE:…voltage variable capacitance also known as varicaps and voltcaps. …. Used in Reverse bias condition … The barrier capacitance is not a constant but varies with applied voltage. ..Used for balancing bridge, Tuning of any LC circuit Fig. (a) Circuit Symbol (Under Reverse bias) Fig. (b) Circuit Model (Under Reverse bias) 40 Fig. Varicap Characteristic CT Vs VR In term of applied reverse bias, the transition capacitance is given by, 41 42 2.18 Regulated Power Supply:- Fig. D.C. Regulated Power Supply Transformer Rectifier Filter Regulator 44 2.19 RECTIFIERS:- A.c. to d.c. converter Rectifiers Half Wave Rectifier 1-Φ HWR ..Used to convert a.c voltage into pulsating d.c. voltage Full Wave Rectifier 3-Φ HWR Bridge FWR Center tap 1-Φ FWR 3Φ FWR 45 2.20 HALF WAVE RECTIFIER:- Fig.2 Half wave Rectifier 46 47 Fig.3 Input & output voltage wave form of HWR 48 Performance Parameters of HWR:(a) Average Current or d.c. current:- 49 2Vm Vdc 0.318Vm 2 so the value of o/p voltage is 31.8% of max. a.c. i/p voltage, the avg. or d.c. value of current is, I avg Vdc I m I dc 0.318I m RL 50 Total RMS Value : 51 RMS Value of AC Components : 52 (b) Ripple Factor:The a.c. component present in the o/p is called the ripple. 53 (c) Peak Inverse Voltage for HWR:…..for HWR the PIV is Vm 54 (d) Efficiency of HWR:The rectifier efficiency is expressed as, 55 The efficiency will be max. when RL rF So, ηmax . 0.406 40.6% 56 (e) Transformer Utilization Factor (TUF):TUF is the ratio of the d.c. power delivered to the load and the a.c. Rating of the transformer secondary. Vdc .I dc D.C. power delivered to the load TUF AC power rating of the transform er Vrms .I rms Assuming R L rF for HWR then we get, Vm I m π π 2 2 TUF 2 π Vm I m 2 2 TUF 0.287 28.7% Ideal value of TUF is 100% practicall y it should be . 57 If we take rF into considerat ion then w e have, Now, 0.287 RL TUF RL rF 58 (f) Voltage Regulation:The variation of d.c. o/p voltage as a function of d.c. load current is called voltage regulation. Vno load - Vfull load % Voltage Regulation 100% Vfull load An ideal power supply has full load voltage equal to the its no-load Voltage and hence zero % regulation. 59 2.21 Center Tapped Full Wave Rectifier:- Fig.2.16 (a) & (b) Center Tapped FWR 60 2.22 FULL WAVE BRIDGE RECTIFIER:- Fig.2.17 (a) Full Wave Bridge Rectifier The problem of center tapped transformer is eliminated in bridge rectifier. The diode in bridge rectifier is required to have PIV rating of only Vm. Only disadvantage is that it requires four diodes. 61 62 2.23 Performance Parameter of Full Wave Rectifier:- 63 Total RMS Value : 64 Ripple Factor : 65 Rectification Efficiency : 66 Peak Inverse Voltage for center Tap & bridge FWR:……for FWR the net PIV is 2Vm 67 2.24 Comparison of Various Rectifier:Sr. Property No. 1. No. of diode 2. Transformer Req. 3. Efficiency 4. Ripple Factor 5. PIV 6. O/P Frequency 7. RMS current 8. DC current 9. Voltage Regulation 1-Ф HWR 1 No 40.6% 1.21 Vm fi Im/2 Im/π Good Center-Tap Bridge FWR FWR 2 4 Yes No 81.2% 81.2% 0.482 0.482 2Vm Vm 2fi 2fi Im/1.414 Im/1.414 2Im/π 2Im/π Better Good 68 2.25 FILTER CIRCUIT:Filters circuits are used to remove the a.c. component those are very undesirable in rectifier o/p. There are two ways to do it : 1. Ripples are bypassed around a the load by using a shunt capacitor. 2. Ripples can be limited to a low value by a series inductor. A filter circuit is generally a combination of capacitors and inductors. Filtering action depends upon the fact that: Capacitor allows ac only to pass. Inductor allow dc only to pass. 70 2.26 (a) Shunt Capacitor Filter: 71 (a) Initial charging of the capacitor (diode is forward-biased) happens only once when power is turned on. (b)The capacitor discharges through RL after peak of positive alternation when the 72 diode is reverse-biased. (c)The capacitor charges back to peak of input when the diode becomes forwardbiased 73 OUTPUT: 74 Operation of Capacitor Filter: During the conduction period, the capacitor gets charged and stores energy. During the non-conduction period, the capacitor discharges through the load resistance delivering energy to it. Capacitor gets charged to the peak value quickly because charging time constant is almost zero. Discharging time constant is quite large, because it discharges through the load resistance. 75 As XC << RL, the ripples are bypassed through the capacitor and only dc component flows through the load resistance. The Ripple Factor (r) is an indication of the effectiveness of the filter and is defined as where Vr(pp) is the peak-to-peak ripple voltage and VDC is the dc (average) value of the filter’s output voltage 76 • The variable Vp(rect) is the unfiltered peak rectified voltage. 77 Q. Determine the ripple factor for the filtered bridge rectifier with a load as indicated in Figure below Solution:- The transformer turns ratio is n = 0.1 The peak primary voltage is 78 79 80 81 82 83 84 2.27 CLIPPER CIRCUITS: • A circuit that clips off or removes a portion of the input signal without distorting the remaining part. • A clipper can process any type of signal. • It is also known as wave shaping circuit. • Clipper Circuit Positive Clipper Series .. Diode & Load in series Negative Clipper Shunt …Diode & load in parallel Biased Clipper Combinational Clipper 85 Positive Clipper:- Input wave Series +ve Clipper Output wave 86 The Input. Series +ve Clipper The Output. 87 Input wave. • Shunt +ve Clipper Output wave The diode is put in parallel with the load. Practical Aspects of Parallel Clipper: • If we take into account the threshold voltage, VT = 0.7 V, the clipping level is not zero, but 0.7 V. 88 Negative Clipper:- Input wave. Series -ve Clipper The transfer characteristics Output wave 91 Input wave. Series -ve Clipper Output wave 92 2.28 Biased Clippers: Bias means applying a dc voltage to change the dc level of a circuit. 93 Guidelines to Solve: • • • • Determine the transition level at which the diode turns ON. With diode ON, find relation between vo and vi. Draw the transfer characteristic of the clipper. Plot the waveshape of vo for given input. 94 We find that • • • • • Diode is ON for vi > VB. Therefore, vi(tr) = VB. When diode is ON, vo = vi – VB. When diode is OFF, vo = 0. Plot the transfer characteristic of the clipper. 95 Now, draw the output. शक्र ु वार, 5 मई 2017 Clippers and Clampers-1 96 Example 1 • Determine the output waveform for the clipper circuit, if the input is a sinusoidal wave of peak value 15 V. 97 Solution: • The direction of the diode suggests that it will be ON for positive values of vi. • At transition level, vd 0 V and id 0 V; so that vo id RL 0 Writing KVL, vi (tr) 3 0 vi (tr) 3V शक्र ु वार, 5 मई 2017 Clippers and Clampers-1 98 98 After diode is ON, vo vi 15 99 Transfer characteristic Draw the output. Clippers and Clampers-1 100 Example 2 • Determine the output waveform for the clipper circuit of Example 1, if its input is as follows 101 Solution: Problem is simpler. Only two levels : vi = +15 V and vi = -5 V Clippers and Clampers-1 102102 Now, you can draw the output. Clippers and Clampers-1 103103 Note that 1. Total swing of vi is 15 – (-5) = 20 V. 2. Total swing of vo is 18 – 0 = 18 V. 3. Clipper circuit clipped off 2 V, and raised the dc level by 3 V. 104 Example 3 • Determine the output of the parallel biased clipper for the given input. 105 Solution: 106 Now, draw the transfer characteristic Now, draw the output wave, and get credit. 107 Example 4 • Repeat Example 3, taking a silicon diode with VT = 0.7 V, instead of an ideal diode. Solution : To determine transition level, we use the condition,id 0 A at vd 0.7 V VR id RS 0 V 108 Applying KVL, Vi (tr) VT VB 0 Vi (tr) VB VT 4 0.7 3.3 V For inputs less than 3.3 V (including negative values), the diode is ON, and vo = 3.3 V For inputs greater than 3.3 V, the diode is OFF, and vo = vi as shown in figure. 109 Draw the output, Note that VT reduces Vi(tr) to 3.3 V from 4 V. 110 2.29 Combination Clippers:- • D1 clips off positive parts above the positive bias level. • D2 clips off below negative level. • This circuit is called a combination clipper. 111 112 2.30 Applications of Combination Clippers • If the input voltage is very large compared to the bias level, the output signal is a SQUARE WAVE. • Thus, the circuit can be used for wave-shaping. • It can also be used in a completely different way, as a limiter used to protect a sensitive circuit (e.g., OPAMP, Galvanometer). • The diodes conduct only when something abnormal happens. Sensitive Circuit 113 2.31Clampers (Electronic Circuits): • Also called DC Restorers. • It clamps (or holds, or ties) either the positive or the negative peak of a signal to a definite level. • The circuit has a capacitor, a diode, and a resistor. • In addition, it may have a dc supply to introduce additional shift. • Time constant τ = RC is made much larger than T (time period) of the signal. • The capacitor does not discharge when diode is not conducting. 114 Positive Clamper:- (a) The input. • (b) The circuit. On first negative cycle, the diode turns ON. The capacitor starts charging. At negative peak, the circuit is as shown in (c). The capacitor charges to Vm. 115 (c) (d) Slightly beyond negative peak, the diode turns OFF. Capacitor does not discharge much because of high RC. At positive peak, the circuit is as shown in (d). The net output is shown in (e). 116 (e) The output. The charged capacitor acts like a battery of Vm. This is the dc voltage that is added to the signal, as seen in (e). The output sits on 0 V level. The output is shifted in positive direction. 117 Negative Clamper:- (a) Input Wave • • (a) The circuit. (b) The output The diode is turned around. The capacitor voltage reverses, and the circuit becomes negative clamper. It is clamped to zerovolt level, but always remaining below 0 V. Memory Aid : The diode points in the direction of shift. 118 Note : • The total swing of the output is the same as that of the input. • A clamper can also have an added dc voltage. It is then called biased clamper. • Start the analysis of the circuit for that part of input, for which diode is ON. • Assume that the capacitor charges to voltage level decided by the circuit. • Assume that when diode is OFF, the capacitor does not discharge. 119 Example 1 (a) The input. • (b) The circuit. Is it a positive or negative clamper ? • Ans. : Biased positive clamper. 120 Solution : The time constant of the circuit, RC 100 kΩ 1μF 100 ms The time period of the input signal, 1 1 T 1 ms f 1 kHz T • Thus, the capacitor holds the charge when the diode is OFF. • We begin analysis with the period from T/2 to T; the diode is ON. • The circuit is as shown in (a) 121 (a) • • • • The output is across R, but it is also across 3-V battery. Hence, vo = 3 V, during this period. Applying KVL, -15 + VC -3 = 0; VC = 18 V. For the period from T to 3T/2, the circuit is as shown in (b). 122 (b) • Applying KVL, vo = 5 + 18 = 23 V. • Thus, the output is as shown in (c). • Note that the output swing is also 20 V. (c) 123 Example 124 2.32 VOLTAGE MULTIPLIER: • A Voltage multiplier is that of circuit which produces an O/P d.c. voltage whose value is multiple of peak a.c. I/P voltage. • It is a combination of two or more peak rectifier circuit. • Voltage Multiplier (it contain diode & • Capacitor) Voltage Doubler Half-Wave Voltage Doubler Voltage Tripler Full Wave Voltage Doubler Voltage Quadr-upler 125 Half Voltage Doubler: Fig. Half-wave voltage doubler Fig. Double operation,showing each half-cycle of operation: (a) positive half-cycle; 126 Fig. Double operation,showing each half-cycle of operation: (b) negative half cycle. 127 Full Voltage Doubler: Fig. Full-wave voltage doubler 128 Fig. Alternate half cycles of operation for full-wave voltage doubler. 129 2.33 ZENER DIODE:.. Two terminal device e.g. anode & cathode. ..Heavily doped P-N junct. so depletion layer is about 100A◦ ..Operates in breakdown region. .. Breakdown voltage can be set by controlling doping. Cathode + IZ _ VZ rz Anode + _ VZ (a) Ckt. Symbol of Zener diode + _ VZ (b) Practical Equivalent (c) Approx. zener equiv. ckt. of zener diode ckt. Here rz is zener dynamic resistance of zener diode so rz = ΔVz/ΔIz & 130 value of rz varies from few Ω to several hundred Ω. 2.34 Biasing of Zener Diode:- Biasing of Zener Diode Both process due to of VRB Reverse Bias Forward Bias .. It is identical to the ordinary P-N Zener Breakdown Avalanche Breakdown ..Observed in zener diode Junction diode. ..Observed in zener diode having Vz >8V having Vz between 5 to 8V ..Breakdown Voltage as Temp ..Breakdown Voltage as Temp ..VI characteristic with ..VI characteristic with zener Avalanche breakdown is Breakdown is very sharp. gradually increases. .. Due to colliding ..due to Electric field minority carrier 131 2.35 V-I Characteristics of Zener Diode:- 132 2.36 Zener Diode Application :1. Zener diode as a voltage regulator. 2. Used as a peak clipper in wave shaping circuit. 3. It is used as fixed reference voltage in transistor biasing circuit. 4.Used for meter protection. 133 2.37 Zener Diode as a Voltage Regulator :..Used to maintain a constant o/p d.c. voltage RS IS IZ + _ IL RL VS Regulated VL Supply VZ Fig. 1 Zener Diode Shunt Regulator For operation of circuit, The necessary condition VS>VZ The I/P current is calculated by, VS = ISRS+VZ So IS = (VS-VZ)/RS …….eq.(1)134 Here Vs is the unregulated I/P voltage, VZ is the Zener voltage. For practical zener diode, zener resistance rz is taken into account then load voltage is given by, VL=VZ+IZ.rz ......eq.(2) if rz is neglected Then, VL=VZ The current through the load resistance is IL=VL/RL ……eq.(3) The I/P current is given by, IS=IL+IZ ……eq.(4) 135 Case1.Regulation when I/P voltage is Varied:- Fig. (a) I/P Voltage is Varied Case2.Regulation when Load Resistance is Varied:- Fig. (b) Load Resistance is Varied 136 DRAWBACK:Large power dissipated in series resistance. Large power wastage. Diode Parameters:1. Zener Voltage (VZ) 2. Maximum Power Dissipation (PZmax) 3. Maximum Current (IZmax) 4. Minimum Current (IZmin) IZmin = 10 % of IZmax 137 138 139 • This voltage (82.5 V) is more than VZ (= 60 V). • Hence, diode is ‘ON’. • Replace the diode by its equivalent. 140 141 142 143 This is more than VZ. Hence diode is ‘ON’. We now calculate the minimum value of zener current. 144 145 When V1 becomes 120 V : 146 Problem:3 (a) For the network of fig. below determine the range of RL and IL that will result in VRL being maintained at 10V. (b) Determine the maximum wattage reading of the diode. 147 148 149 2.38 Zener Diode as Sinusoidal Regulation: (a) The Input. (b) 40 volt peak to peak sinusoidal regulator 150 (b) Circuit operation at Vi=10V (c) The Output. 151 2.39 Zener as Square Wave Generator:- (a) The Input. (b) Square wave Generator (c) The Output. 152 2.40 Schottky Diode:• Schottky diodes are high-current diodes used primarily in high-frequency and fast-switching applications. • It is also known as hot-carrier diodes. • It is formed by joining a doped semiconductor region (usually n-type) with a metal such as gold, silver, or platinum. (a) Simplified geometry of Schottky diode. (b) Circuit symbol 153 Forward bias V-I characteristic of Schottky and pn junction diode. 2.41 Light Emitting Diode (LED):• The light-emitting diode (LED) is a solid-state light source. • A light-emitting diode (LED) is a diode that gives off visible light when forward biased. • LED are made by using elements like gallium, phosphorus and arsenic. (b) Symbol of LED Forward bias LED. 155 LED voltage and current Advantages of LED (i) Low voltage (ii) Longer life (more than 20 years) (iii) Fast on-off switching Applications of LED (i) As a power indicator. (ii) Seven-segment display. 156 2.41 Tunnel Diode:• A tunnel diode is a pn junction that exhibits negative resistance between two values of forward voltage. • doping of p and n regions much more heavily than in a conventional diode. • It is made with germanium or gallium arsenide. • It is also known as Esaki diode. (b) Circuit symbol of tunnel diode 157 • The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling. • Valence electrons seem to tunnel through the forbidden energy band. • It useful in oscillator and microwave amplifier applications. 158