1. Physics of Semiconductor Devices

2. Formation of PN - Junction When a P-type Semiconductor is joined together with an N-type Semiconductor a PN junction is formed. And it is also known as a Semiconductor Diode. Semiconductor diodes are widely used in Rectifiers which converts input AC signal into DC output signal.

3. P N - - - - - - - - + + + + + + + + Space charge region (OR) Depletion region Junctio n Junctio n Ionized acceptors Ionized donors Potential barrier height(V 0 ) Potential barrier width (W) (W)

4. Depletion Region & Space Charge The diffusing majority carriers from the two regions recombine near the junction and disappear. The uncompensated Acceptor and Donor ions set up an Electric field which halts majority carrier Diffusion and causes minority carrier Drift. The two kinds of majority carriers diffusing across the junction meet each other near the junction and undergo recombination's, leaving negative ions on the P-side and positive ions on the N-side of the junction. This distribution of Positive and Negative Charges is called Space charge.

5. NP CathodeAnode _ + Diode Symbol

6. Fermi level Depletion region PN - junction E Fn Valence band Conduction band N P E Fp E eV B EvEv EcEc EvEv EcEc Energy level diagram

7. V - I Characteristics of PN Junction The diode can be operated in two different ways, as Forward and Reverse bias. When positive terminal of the battery is connected to the P-type & negative terminal is to the N-type of the PN-junction diode, known the diode is kept in forward bias. When negative terminal of the battery is connected to the P-type & positive terminal is to the N-type of the PN-junction diode, known the diode is kept in reverse bias.

8. P N - - - - - - - - + + + + + + + + Space charge region Open circuit PN -junction N P - - - - + + + + VFVF Forward bias

9. P N - - - - - - - - + + + + + + + + Space charge region Open circuit PN -junction - - - - - - - - - - - - + + + + + + + + + + + + PN Reverse bias VRVR

10. Forward Bias Reverse Bias V I Current Reverse break down current Forward Current Knee Voltage

11.  The region between knee voltage & breakdown voltage is known as non-ohmic region.  Above the knee & breakdown voltage the current increases.  Breakdown voltage is due to thermally broken covalent bonds.  Diode is conducting in forward bias & non-conducting in reverse bias.

12. Rectifiers A Rectifier is a device which converts alternating current to direct or unidirectional current. Rectifiers are mainly three types… 1.Half wave Rectifiers 2.Full wave Rectifiers 3.Bridge Rectifiers

13. Half – Wave Rectifier A.C input D.C output transformer A B

14. Full Wave Rectifier D.C output A.C input C A B Center tapped transformer

15. Efficiency of a Rectifier The efficiency of a Rectifier is defined as the ratio of D.C. out put power to the A.C. input power supplied to the Rectifier.

16. Light emitting diodes: LED’ s are the most visible type of diode, that emits a fairly visible colored light, invisible infra- red or laser type light when a forward current is passed through them. Principle: Basically LED are made from a very thin layer of fairly heavily doped semiconductor material. When the diode is forward biased, electrons from conduction band combine with holes from in valence band, releasing sufficient energy to produce photons of light.

17. Semicondu ctor material Wave length ColorV f @ 20mA GaAs850-940nmInfra-Red1.2V GaAsP630-660nmRed1.8v GaAsP605-620nmAmber2.0v GaAsP:N585-595nmyellow2.2v GaP550-570nmGreen3.5v SiC430-505nmBlue3.6v GAInN450nmwhite4.0v

18. _ + CathodeAnode Cathode

19. V Current I(MA)

20. Photo diode A photodiode is a diode optimized to produce an electron current flow in response to irradiation by ultraviolet, visible, or infrared light. Silicon, Ge & gallium arsenide is most often used to fabricate photodiodes. Few electron-hole pairs in the N and P–regions, and most in the depletion region contribute to photo current.

21. Photo diode Top metal contact P-diffusion Depletion region N-type N + contact region Bottom metal contact + Sun radiation

22. _ + CathodeAnode Photo diode Symbol N+N+ PN Depletion region Heavily doped & very thin Lightly doped