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
Potential barrier width
(W) P N - +
Space charge region
(OR)
Depletion region
Junction
Ionized acceptors
Ionized donors
Potential barrier(V0)
Formation of a PN-junction

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 Diagram of PN diode:
(c)
& (b) Energy level diagrams of p – type and n-type semiconductor respectively
(c) Energy level diagram of PN junction
(d) Formation of potential barrier across the junction Vp Vn
VB = Vn - Vp
(d)

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 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 in reverse bias.

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

9. 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.

10. Diode as a rectifier: Rectifiers are mainly three types
A Rectifier is an electronic circuit which converts alternating current to direct current
(OR) unidirectional current.
Rectifiers are mainly three types
1.Half wave rectifiers
2.Full wave rectifiers
3.Bridge rectifiers

11. 1.Half-wave rectifier: An electronic circuit which converts
alternating voltage (OR) current for
half the period of input cycle hence
it is named as half-wave rectifier.

12. Half – Wave Rectifier A.C Input rf Pulsated D .C Output transformer RLB RL rf

13. Rectifier efficiency(η)
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
D.C. power output
A.C. power input
η =

14. η = D.C. power output = ( Im /π ) 2 x RL & ( Im /π ) 2 x RL Therefore,
(Im /2 ) 2 x (rf + RL )

15. η =
4 /π2 x RL
rf + RL =
0.406 x 100%
40.6%
since,
rf « RL
Conclusion:

16. 2.Full-wave rectifier: An electronic circuit which converts
alternating voltage (OR) current into
pulsating voltage (OR) current during
both half cycle of input is known as
full-wave rectifier.

17. Full Wave Rectifier
D .C Output A B
Center tapped
transformer rf RL

18. Rectifier efficiency(η)
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
D.C. power output
A.C. power input
η =

19. &
Therefore,
η =
(2 Im /π ) 2 x RL
(Im / √2 ) 2 x (rf + RL )

20. η =
x RL
rf + RL =
0.812 x 100%
81.2%
Conclusion:

21. Diode equation Consider Intrinsic Semiconductor Electron Concentration
Holes Concentration
Equation 1 & 2 holds good for both intrinsic and extrinsic semiconductors
under Thermal equilibrium condition.

22. Electron Concentration
For N type Material
For P type Material

23. Hole Concentration
For P type Material
For N type Material

25. Similarly diffusion current of holes is given by

26. I = ie + ih Therefore, total current is given by
When the junction is in reverse bias V = - V ,
then the reverse current is given by

27. I0 is called reverse saturation current
At room temperature (T)
Therefore,
I0 is called reverse saturation current
Eq(15) is known as the diode equation.

28. NOTE: But, practically the diode equation is written as.
β is a constant depends on the material
of the diode. For Ge = 1 & for Si = 2

29. Light emitting diode (LED)
Definition:
LED is a semiconductor PN-junction diode
which converts electrical energy to light
under forward biasing. It emits light in
both visible & IR region.
NOTE:
LEDs are typically made of compound
semiconductors (OR) direct band gap
semiconductors like gallium arsenide.

30. Injection Luminescence LED is a highly doped diode
Principle:
Injection Luminescence
When LED is forward biased, the majority charge carrier
moves from P to N & similarly from N to P region and
becomes excess minority charge carriers.
Then these excess minority charge carriers diffuse
through the junction and recombines with the majority
charge carriers in N & P region respectively to produce
light.
LED is a highly doped diode

31. Construction:
substrate VF N P
Ohmic
Contacts(Al)
Sio2
Photons

32. Therefore, a shallow PN-junction is
The PN-junction is made by doping silicon by GaAs crystal. Here silicon can act both as donor & acceptor.
Therefore, a shallow PN-junction is
formed on GaAs substrate such that
P-layer is formed by diffusion on
N-layer as shown in figure.

33. In order to increase the probability
of radiative recombinations, the
thickness of the N-layer is taken
higher than that of the thickness
of the P-layer.
The top layer of the P material is left
uncovered for the emission of light.

34. Working: If the diode is properly biased the charge
carriers move across the junction.
If the biasing voltage is further increased, these
excess minority carriers diffuse away from
the junction and directly recombine with
the majority carriers.
Therefore, electron-hole recombination
occurs more & more and thereby light is
emitted through the top layer of the
P-material which is left uncovered as
shown in figure.

35. Symbol: + -
Cathode
Anode

36. Current
I(MA)
V (volts)

37. Advantages: They are smaller in size. Its cost is very low.
It has long life time.
They are available in different colours
at low cost.
They can operate with low voltage,
faster response ≈ 10-9 seconds .
its intensity controlled easily & operate
wide range temperature………

38. Its intensity is less than laser.
Disadvantages:
Its output power is low.
Its intensity is less than laser.
Its light can not travel through
longer distance.
Its light will not have directionality,
incoherent & not in phase……