JUNCTION DIODES PDF

JUNCTION DIODES Basic Electronics(EC − 201), KIIT-BBS. By: Mr. Srinivas Ramavath 1. Operation of pn-junction diode (a) The semiconductor diode is formed by bringing p and n type materials together (constructed from the same base Ge or Si) by...

JUNCTION DIODES Basic Electronics(EC − 201), KIIT-BBS. By: Mr. Srinivas Ramavath 1. Operation of pn-junction diode (a) The semiconductor diode is formed by bringing p and n type materials together (constructed from the same base Ge or Si) by processes like alloying, diffusion, ion implantation.....etc. Figure 1: pn-diode (b) Holes and electrons recombine at junction and uncover bound charges. So a layer of positive fixed charges is formed on the n side and negative fixed charges is formed on the p side of the junction. Figure 2: pn-diode with depletion region (c) A pn junction can be step graded or linearly graded. (d) A potential barrier or built in potential Vb i is formed across the junction. ( ) ( ) KB T NA ND NA ND Vbi = ln = VT ln (1) q n2i n2i Vbi = 0.7V (Si), Vbi = 0.3V (Ge). (e) An electric field is produced because of uncovering of bound charges. dV E=− (2) dx  (f) Two types of current flow in pn junction diode. Diffusion current. Drift current. (g) Diffusion current is due to concentration gradient. Diffusion of holes from p type to n type. Diffusion of electrons from n type to p type. (h) Due to the electric field E, minority carriers start flowing. It gives rise to drift current. Drift of holes from n type to p type. Drift of electrons from p type to n type. (i) Under steady state drift current is equal to diffusion current. So net current is zero under open circuit. (j) Width of the depletion region Wdep of an open circuit pn junction is given by √ ( ) 2ϵ 1 1 Wdep = + Vbi (3) q NA ND (k) The pn junction can be used as a switched, ON switch when forward biased and OFF switch when reverse biased. (l) The diode symbol is (m) A pn junction diode can be destroyed by a high level of forward current over- heating the device and a large reverse voltage causing the junction to break- down. (n) In general physically large diodes pass the largest currents and survive the largest reverse voltages. small diodes are limited to low current and low reverse voltages. low current diode (100mA, 75V ). Medium current diode (400mA, 200V ). High current diode (many Ampers, several hundred volts). 2. Diode characteristics 2 (a) When a voltage V is applied across a pn junction, the total current I flowing through the junction. qV ( ηK ) I = Irs [e B T − 1] (4) Irs -The reverse saturation current. q- electron charge= 1.602 × 10−19 C KB -Boltzmann constant= 1 × 10−23 J/k T-Absolute temperature. η-Dimension less number = 1 (Ge or GaAs) or 2 (Si). (b) Forward biased pn-junction Figure 3: pn-junction in forward biased i. The holes on the p side are rippled from the positive terminal and driven toward the junction. ii. Similarly the electrons on the n side are repelled from the negative terminal toward the junction. iii. The result is that the depletion region width and built in potential are reduced. √ ( ) 2ϵ 1 1 Wdep−f b = + Vbi − Vf (5) q NA ND iv. The current flows because of a majority charge carries is moving from one side to another and the junction is said to be forward biased. v. A very little forward current flows until Vf exceeds the junction build in potential Vbi. (c) Reverse biased pn-junction i. The holes on the p side are attracted to the negative terminal. 3  Figure 4: pn-junction in reverse biased ii. The electrons on the n side are attracted to the positive terminal. iii. The electrons and holes are attracted away from junction this causes the depletion region and build in potential to be increased. √ ( ) 2ϵ 1 1 Wdep−rb = + Vbi − Vr (6) q NA ND iv. The is no possibility of majority charge carrier current flow across the junction and the junction is said to be reverse biased junction. v. A very small reverse bias voltage is necessary to move all minority charge carriers across the junction. vi. Further increases in Vr do not increase the current. This current is known as reverse saturation current Irs. vii. Irs small quantity (nA, mA). It is depending on the junction area temper- ature and semiconductor material. (d) V-I characteristic of a pn junction. Figure 5: normal diode characteristics 4 3. Ideal diode (a) Ideal diode is in forward bias short circuit (V = 0, I = 0). (b) Ideal diode is in reverse bias open circuit (V = 0, I = 0). (c) Ideal diode characteristic is 4. Breakdown mechanisms (a) The maximum reverse bias voltage that can be applied to a p-n diode is limited by breakdown. Breakdown is characterized by the rapid increase of the current under reverse bias. The corresponding applied voltage is referred to as the breakdown voltage. (b) Two mechanisms can cause breakdown, namely avalanche breakdown and zener breakdown. Figure 6: breakdown characteristics (c) Avalanche breakdown mechanism i. Avalanche breakdown is when the minority carriers are accelerated in the electric field near the junction to sufficient energies that they can excite valence electrons through collisions. 5  ii. Avalanche breakdown occurs in lightly-doped pn-junctions where the de- pletion region is comparatively long. iii. Avalanche breakdown is caused by impact ionization of electron-hole pairs. iv. Avalanche breakdown occurs in a normal diode at high reverse voltage. (d) Zener breakdown mechanism i. Zener breakdown occurs when the electric field near the junction becomes large enough to excite valence electrons directly into the conduction band. ii. Zener breakdown occurs in heavily doped pn-junctions. The heavy doping makes the depletion layer extremely thin. iii. Zener breakdown is caused by field ionization of electron-hole pairs. iv. The tunneling current is obtained from the product of the carrier charge. v. Zener breakdown occurs in a zener diode at low reverse voltage. vi. A zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse current value. vii. The zener diode maintains a nearly constant voltage across its terminals for values of different reverse currents. 5. Zener diode and Voltage regulator (a) Zener diode is specifically designed to operate in the reverse breakdown region. (b) If there change in load currant or change in input voltage we still get output constant voltage. Figure 7: Voltage regulator circuit IR = Iz + IL (7) 6  (c) Variable input voltage:. IR = Iz + IL (d) When input voltage Vin fluctuates, load current constant and Zener current Iz fluctuates, but Zener voltage constant. (e) Variable load resistor : IR = Iz + IL (f) When load resistance RL fluctuates, Zener current and load current Iz fluctu- ates, but input current constant. Figure 8: Zener characteristics 6. Rectifier circuits (a) Rectifier circuits are converting from AC (voltage) to DC (voltage). (b) Rectifier circuit is one of the most important application of diodes. (c) half wave rectifier circuit Figure 9: half wave circuit 7  i. During the positive half cycle of the input voltage, the diode is forward biased (Ideal diode short ckt). ii. Thus during the positive half cycles of the input ac voltage, the current flows through circuit and all input voltage appears across the load. iii. During the negative half cycle of the input voltage, the diode is reverse biased (Ideal diode open ckt). iv. Thus during the negative half cycles of the input ac voltage, no current flows through load resistance and voltage across the load is zero. v. The input and output voltage Waveforms of Half-wave Rectifier are shown in Figure 10 respectively. The output is unidirectional and has a finite average value, DC component. Figure 10: wave forms (d) Filters: The basic components of filters are the inductors and capacitors. i. An inductor allows dc only and capacitor allows ac only to pass. ii. An inductor offers a high impedance to the ac components but offers almost zero resistance to dc components. iii. The capacitor offers a high impedance to the dc components but offers low resistance to ac components. 8  iv. Different types of filters. (e) half wave rectifier circuit with capacitor filter Figure 11: half wave circuit with capacitor filter i. The large value capacitor is connected in parallel with the rectifier output. ii. During the positive cycle (from 0 to π/2 ) of the input voltage, the diode is forward biased (Ideal diode short ckt), the capacitor charges up to maximum input voltage. iii. During the input voltage (from 0 to π ), the diode is reverse biased (Ideal diode open ckt), the capacitor discharges through the load. Further ap- plying positive input voltage diode is still open circuit when input voltage less then capacitor voltage. Figure 12: output waveform with capacitor filter (f) when the input voltage greater then capacitor voltage, the capacitor start charging up to maximum input voltage. 9 (g) The waveform produced by this filtered half-wave rectifier is shown (h) Here, ripple is defined as the difference between the maximum and minimum voltages on the output waveform. (i) Bride full wave rectifier circuits Figure 13: Full wave circuit i. During the positive half cycle of the input voltage, the diode D2 and D4 are forward biased (ON switch) while the diode D1 and D3 are reverse biased (OFF switch). ii. Thus during the positive half cycles of the input ac voltage, the current flows through circuit and all input voltage appears across the load. Figure 14: Full wave circuit iii. During the negative half cycle of the input voltage, the diode D1 and D3 are forward biased (ON switch) while the diode D2 and D4 are reverse biased (OFF switch). iv. Thus during the negative half cycles of the input ac voltage, the current flows through circuit and all input voltage appears across the load. v. The input and output voltage waveforms of bride wave rectifier are shown in figure 16 respectively. 10  Figure 15: Full wave circuit Figure 16: full waveforms (j) Centre tapped transformer full wave rectifier circuits Figure 17: Centre tapped transformer circuit i. During the positive half cycle of the input voltage, the diode D1 is forward biased (ON switch) while the diode D2 is reverse biased (OFF switch). 11  ii. Thus during the positive half cycles of the input ac voltage, the current flows through circuit and all input voltage appears across the load. iii. During the negative half cycle of the input voltage, the diode D2 is forward biased (ON switch) while the diode D1 is reverse biased (OFF switch). iv. Thus during the negative half cycles of the input ac voltage, the current flows through circuit and all input voltage appears across the load. v. The input and output voltage waveforms of centre tapped Rectifier are shown in Figure 20 respectively. Figure 18: centre tapped Rectifier waveforms 7. Centre tapped transformer full wave rectifier circuits with capacitor filter Figure 19: centre tapped circuit with capacitor filter (a) During the positive half cycle of the ac input the diode D1 gets forward biased and quickly charges the capacitor C to peak value of the supply voltage. 12  (b) During the input voltage less then capacitor voltage diode gets reverse biased and so the capacitor C discharges through load resistance RL. (c) The voltage across C (Vc ) decreases exponentially with time constant = RL C along the curve dc. (d) The capacitor bypasses the ac components as its reactance is much lower in comparison with the load. Figure 20: wave forms (e) The ripples get bypassed through capacitor C and only dc component flows through the load resistance RL. 8. Parameters (a) Average DC value (Idc ): The load current of a rectifier is unidirectional but fluctuating since it is a periodic function of time, by Fourier’s theorem. ∫ 1 T Idc = i(t)dt T 0 Half wave ∫ 2π 1 Idc = i(t)dt 2π 0 ∫ π 1 Idc = Im sin (ωt) dt 2π 0 Im Idc = π 13  Full wave ∫ 1 π Idc = i(t)dt π 0 ∫ 1 π Idc = Im sin (ωt) dt π 0 2Im Idc = π (b) Ripple factor (γ): ′ Irms γ= I √dc ′ Irms 2 = Irms − Idc 2 √ 2 Irms − Idc 2 γ= Idc Half wave √ ∫ 2π 1 Irms = i(t)2 dt 2π 0 √ ∫ 2π 1 Irms = (Im sin ωt)2 dt 2π 0 Im Irms = 2 Im Idc = π γ = 1.1 Full wave √ ∫ 1 π Irms = i(t)2 dt π 0 √ ∫ 1 π Irms = (Im sin ωt)2 dt π 0 Im Irms = √ 2 2Im Idc = π γ = 0.485 14 (c) Efficiency (η): The ratio of the dc output power to the ac input power. Pdc η= Pac 2 Pdc = Idc RL 2 Pac = Idc (Rf + RL ) Idc η= Irms Half wave Idc η= Irms Im π η= Im 2 η = 0.405 Full wave Idc η= Irms 2Im π η= Im √ 2 η = 0.810 (d) Peak inverse voltage (PIV): The peak inverse voltage is the maximum reverse voltage when diode is non-conducting. Half wave P IV = Vm (8) Bridge wave P IV = Vm (9) 15  Centre tapped transformer P IV = 2Vm (10) 9. Positive series clipper 10. Negative series clipper 16 11. Positive parallel clipper 12. Negative parallel clipper 17 13. Biased Clipper 14. Biased Clipper 18 15. Biased Clipper 16. Biased Clipper 19 17. Biased Clipper 18. Biased Clipper 20 19. Biased Clipper 20. Biased Clipper 21 21. Negative Clamper 22. Positive Clamper 22 23. Biased Clamper 24. Biased Clamper 23 25. Biased Clamper 26. Biased Clamper 24 27. Photo diode (a) A visible light is focused on the junction through a lens. The incident light produces free electrons and holes. when the light intensity increases, reverse bias current increase. Figure 21: Photo diode circuit (b) The Photo diode symbol is (c) The total current I = Ic + Irs Ic -Conduction current due to photo electric effect. Irs -reverse saturation current. (d) The photo detector is the Electron excitation from the valence band to con- duction band by photons. (e) The light fall on semiconductor atom than band will break up gives electron and hole pair charges carriers is called “Photo conductivity”. (f) The Photo diode characteristic is 25 28. Light emitting diode (LED) (a) When a pn diode is forward biased. The carriers injected across the junction enhance the concentration above the thermal equilibrium values. (b) These excess carriers recombination and emit light energy. (c) The Light emitting diode symbol is Figure 22: LED circuit (d) LED’s emit no light when reverse biased. (e) A metal film is applied to the bottom of the substrate for reflecting as much high as possible to the surface of the device and also to provide cathode con- nection. (f) LED’s operate a voltage levels from 1.5V to 3.3V. (g) The visible wavelength is from 0.45µ m to 0.7µm. (h) The least band gap of the semiconductor for use in the visible region 1.8 eV. 29. Seven segment Light emitting diode (a) The arrangement of a seven segment LED numerical display. (b) Any desired numerical from 0 to 9 can be indicated by passing current through the appropriate segments. (c) The LED’s in a seven segment display may be connected in common anode or in common cathode configuration. (d) (a)Common anode configuration and (b)Common cathode configuration. 26