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. Formation of a PN-junction
Ionized acceptors
Ionized donors
Junction P N - - + +
Space charge region
(OR)
Depletion region
Potential barrier height(V0)
Potential barrier width
(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. Diode Symbol N P
Cathode
Anode _ +

6. Energy level diagram Depletion region N P E PN - junction
Fermi level
Depletion region
PN - junction
EFn
Valence band
Conduction band N P
EFp E
eVB Ev Ec

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 - - P - N N Forward bias VF Open circuit PN -junction + + +
Space charge region P N - +
Forward bias VF

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

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

11. From the graph the following points are noted.
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 A.C input D .C output Center tapped transformer AB
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. Semiconductor material
Wave length
Color
20mA
GaAs nm
Infra-Red
1.2V
GaAsP nm
Red
1.8v nm
Amber
2.0v
GaAsP:N nm
yellow
2.2v
GaP nm
Green
3.5v
SiC nm
Blue
3.6v
GAInN
450nm
white
4.0v

18. _ +
Cathode
Anode
Cathode

19. Current
I(MA) V

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 + Sun radiation Top metal contact P-diffusion
Depletion region
N-type
N+ contact region
Bottom metal contact +
Sun radiation

22. + Photo diode Symbol N+ P N _ Cathode Anode Depletion region
Heavily doped & very thin
Lightly doped _ +
Cathode
Anode