1. Electronics Devices & Circuits/Unit I/Diodes & its Applications Department of Electronics & Telecommunication Engineering 1DMIETR

2. Outline  The p-n junction (forward bias and reverse bias)  Ideal Diode  V-I Characteristics  Application of P-N jun.diode  Avalanche Breakdown  Zener Break Down  Dynamic Characteristics  Types of Diode 2DMIETR

3. Introduction to Semiconductor Devices Semiconductor p-n junction diodes p n 3DMIETR

4. 4 Ideal Diodes Forward bias (on) Reverse bias (off) DMIETR

5. The I-V characteristic of the diode ISIS 5DMIETR

6. The experimental I-V characteristic of a Si diode 6DMIETR

7. p- n diode circuit notation p n When “plus” is applied to the p-side, the current is high. This voltage polarity is called FORWARD. When “plus” is applied to the n-side, the current is nearly zero. This voltage polarity is called REVERSE. 7DMIETR

8. p- n diode applications: Light emitters P-n junction can emit the light when forward biased p-typen-type Electrons drift into p-material and find plenty of holes there. They “RECOMBINE” by filling up the “empty” positions. Holes drift into n-material and find plenty of electrons there. They also “RECOMBINE” by filling up the “empty” positions. The energy released in the process of “annihilation” produces PHOTONS – the particles of light 8DMIETR

9. +-+- p- n diode applications:Photodetectors P-n junction can detect light when reverse biased p-typen-type When the light illuminates the p-n junction, the photons energy RELEASES free electrons and holes. They are referred to as PHOTO-ELECTRONS and PHOTO-HOLES The applied voltage separates the photo-carriers attracting electrons toward “plus” and holes toward “minus” As long as the light is ON, there is a current flowing through the p-n junction 9DMIETR

10. Avalanche Breakdown  Impact ionization mechanism  Predominant breakdown mechanism Total current during avalanche multiplication I n (w) = M * I no 10DMIETR

11. Zener Breakdown Zener effect Doping level > 10 18 /Cm 3 In case of Ge, Si E (field) > 10 6 V/m  Highly doped junction ( narrow W)  Mechanism is termed tunnelling or zener breakdown 11DMIETR

12. Dynamic Resistance Kristin Ackerson, Virginia Tech EE Spring 2002  Inverse of the slope of the transconductance curve  The equation for dynamic resistance is: r F =  V T r F =  V T I D I D  It is used in determining the voltage drop across the diode.  The ac component of the diode voltage is found using the following equation: v F = v ac r F v F = v ac r F r F + R S r F + R S  The voltage drop through the diode is a combination of the ac and dc components and is equal to: V D = V  + v F V D = V  + v F 12DMIETR

13. NEGATIVE RESISTANCE DEVICE It is a device which exhibits a negative incremental resistance over a limited range of V-I characteristic. It is of two types :- 1. Current controllable type : V-I curve is a multi valued function of voltage and single valued function of current.eg:- UJT, p-n-p-n diode 2. Voltage controllable type : V-I curve is a multi valued function of current and single valued function of voltage. eg:- SCS, Tunnel diode - 13DMIETR

14. TUNNEL DIODE (Esaki Diode) It was introduced by Leo Esaki in 1958. Heavily-doped p-n junction –Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n junction diode Width of the depletion layer is very small (about 100 A). It is generally made up of Ge and GaAs. It shows tunneling phenomenon. Circuit symbol of tunnel diode is : EVEV 14DMIETR

15. - Ve Resistance Region Vf Vp IpIp Vv Forward Voltage Revers e voltage Iv Reverse Current Forward Current Ip :- Peak Current Iv :- Valley Current Vp:- Peak Voltage Vv:- Valley Voltage Vf:- Peak Forward Voltage CHARACTERISTIC OF TUNNEL DIODE 15DMIETR

16. A Zener is a diode operated in reverse bias at the Peak Inverse Voltage (PIV) called the Zener Voltage (V Z ). Common Zener Voltages: 1.8V to 200V Zener Diode 16 DMIETR

17. Types of Diodes and Their Uses Schottky Diodes: Very fast switching time Very fast switching time Great diode for digital circuit applications. Great diode for digital circuit applications. They are very common in computers because of their ability to be switched on and off so quickly.They are very common in computers because of their ability to be switched on and off so quickly. AK Schematic Symbol for a Schottky Diode Shockley Diodes: The Shockley diode is a four-layer diode while other diodes are normally made with only two layers. These types of diodes are generally used to control the average power delivered to a load. AK Schematic Symbol for a four- layer Shockley Diode 17DMIETR

18.  It is designed with a very large band gap.  Lower band gap LEDs (Light-Emitting Diodes) emit infrared radiation.  While LEDs with higher bandgap energy emit visible light.  LEDs are extremely bright and last longer than regular bulbs for a relatively low cost. A K Schematic Symbol for a Light-Emitting Diode The arrows in the LED representation indicate emitted light. Light-Emitting Diodes : 18DMIETR

19. Photodiodes:  Photodiodes are sensitive to received light. They are constructed so their p n junction can be exposed to the outside through a clear window or lens.  In Photoconductive mode the saturation current increases in proportion to the intensity of the received light - used in CD players.  In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light, the diode generates voltage and can be used as an energy source - used in the production of solar power. AK A K Schematic Symbols for Photodiodes 19DMIETR

20. 20 Rectifier circuits

21. DMIETR21 Half-wave rectifier Positive pulse is rectified

22. DMIETR22 Full-wave rectifier Current goes through load in same direction for + VS. VO is positive for + VS. Requires center-tap transformer

23. DMIETR23  Entire input waveform is used

24. DMIETR24 Bridge rectifier A type of full-wave rectifier Center-tap not needed Most popular rectifier A type of full-wave rectifier Center-tap not needed Most popular rectifier

25. DMIETR25 Bridge rectifier VO is 2VD less than VS

26. DMIETR26 Filter Capacitor acts as a filter. Vi charges capacitor as Vi increases. As Vi decreases, capacitor supplies current to load.

27. DMIETR27 Filter When the diode is off, the capacitor discharges. Vo = Vpexp(-t/RC) Assuming t  T, and T=1/f VP - Vr = Vpexp(-1/fRC) half-wave rectifier (t  T) VP - Vr = Vpexp(-1/2fRC) full-wave rectifier (t  T/2)

28. DMIETR28 Basic Zener Characteristics Zener diodes are operated in their reverse breakdown mode to provide voltage regulation in a circuit. The point where the reverse current begins to increase is called the knee voltage. The current at this point is the knee current.

29. DMIETR29 Zener Voltage Regulator