July 2, 2019 by Bhagya

Semiconductor Electronic: Material,

Devices And Simple Circuits Class 12
Notes Class 12 Notes Chapter 14
1. Metals They possess very low resistivity or high conductivity.
ρ ~ 10-2.10-8 Ωm, σ ~102. 108 Sm-1
2. Semiconductors They have resistivity or conductivity intermediate
to metals and insulators.
ρ ~ 10-5. 106 Ωm, σ ~ 10+5 .10-6 Sm-1
Types of Semiconductors Types of semiconductors are given below:
(i) Elements Semiconductors These semiconductors are available in
natural form, e.g. silicon and germanium.
(ii) Compound Semiconductors These semiconductors are made by
compounding the metals, e.g. CdS, GaAs, CdSe, InP, anthracene,
polyaniline, etc.
3. Insulators They have high resistivity or low conductivity.
ρ ~ 1011 . 1019 Ωm, σ ~ 10-11. 10-19 Sm-1
4. Energy Band In a crystal due to interatomic interaction, valence
electrons of one atom are shared by more than one atom in the
crystal. Now, splitting of energy level takes place. The collection of
these closely spaced energy levels are called an energy band.
5. Valence Band Valence band are the energy band which includes
the energy levels of the valence electrons.
6. Conduction Band Conduction band is the energy band above the
valence band.
7. Energy Band Gap The minimum energy required for shifting
electrons from valence band to conduction band is called energy band
gap (Eg ).
8. Differences between conductor, insulator and semiconductor on the
basis of energy bands are given below:
 9. Fermi Energy It is the maximum possible energy possessed by free
electrons of a material at absolute zero temperature (i.e. 0K)
10. On the basis of purity , semiconductors are of two types:
(i) Intrinsic Semiconductors It is a pure semiconductor without any
significant dopant species present
ne = nh =ni
where , ne and nh are number densities of electrons and holes
respectively and ni is called intrinsic carrier concentration.
An intrinsic semiconductor is also called an undoped semiconductor or
i-type semiconductor
(ii) Extrinsic Semiconductors Pure semiconductor when doped with
the impurity, it is known as extrinsic semiconductor.
Extrinsic semiconductors are basically of two types: (a) n-type
semiconductors
(b) p-type semiconductors
NOTE: Both the type of semiconductors are electrically neutral.

11. In n-type semiconductor, majority charge carriers are electrons and

minority charge carriers are holes, i.e. ne> nh .
Here, we dope Si or Ge with a pentavalent element, then four of its electrons
bond with the four silicon neighbours, while fifth remains very weakly bound
to its parent atom.
Formation of n-type semiconductor is shown below:

12. In p-type semiconductor, majority charge carriers are holes and

minority charge carriers are eletron i.e. nh > ne .
In a p-type semiconductor, doping is done with trivalent impurity atoms,
i.e. those atoms which have three valence electrons in their valence
shell.
Formation of p-type semiconductor is shown below:

13. At equilibrium condition, ne nh = ni2

14. Minimum energy required to create a hole-electron pair, hv > E g where,
Eg is energy band gap.
15. Electric current, I = eA(neve + nhvh) where, A is area of cross-section.
where, ve and vh are speed of electron and hole respectively.
18. p-n Junction A p-n junction is an arrangement made by a close contact
of n-type semiconductor and p-type semiconductor.
19. Formation of Depletion Region in p-n Junction During formation of
p-n junction, due to the concentration gradient across p and n sides, holes
diffuse from p-side to n-side (p —> n) and electrons diffuse from n-side to p-
side (n —> p).
This space charge region on either side of the junction together is known as
depletion region.
Depletion region is free from mobile charge carriers. Width of depletion
region is of the order of 10-6 m. The potential difference developed
across the depletion region is called the potential barrier.

20. Semiconductor Diode/p-n Junction Diode A semiconductor diode is

basically a p-n junction with metallic contacts provided at the ends for the
application of an external voltage.

The direction of arrow indicates the conventional direction of current (when

the diode is under forward bias).
21. The graphical relations between voltage applied across p-n junction and
current flowing through the junction are called I-V characteristics of
junction diode.
22. (i) Junction diode is said to be forward bias when the positive terminal of
the external
battery is connected less to the p-side and negative terminal to the n-side of
the diode. The circuit diagram and I-V characteristics of a forward biased
diode is shown below:
The circuit diagram and I-V characteristics of a reverse biased diode is shown
below.

23. The DC resistance of a junction diode,

rDC = V/I
24. The dynamic resistance of junction diode,
rAC = ∆V/∆I
25. Diode as Rectifier The process of converting alternating
voltage/current into direct voltage/current is called rectification. Diode is
used as a rectifier for converting alternating current/voltage into direct
current/voltage.
There are two ways of using a diode as a rectifier i.e.
(i) Diode as a Half-Wave Rectifier Diode conducts corresponding to
positive half cycle and does not conduct during negative half cycle. Hence,
AC is converted by diode into unidirectional pulsating DC. This action is
known as half-wave rectification.
Circuit diagram of p-n junction diode as half-wave rectifier is shown below:
The input and output wave forms have been given below:

(ii) Diode as a Full-Wave Rectifier In the full-wave rectifier, two p-n

junction diodes, D1 and D2 are used. The circuit diagram or full-wave rectifier
is shown below:
The input and output wave forms have been given below:

Its working based on the principle that junction diode offer very low
resistance in forward bias and very high resistance in reverse bias.
26. The average value or DC value obtained from a half-wave rectifier,

27. The average value or DC value obtained from a full-wave rectifier,

28. The pulse frequency of a half-wave rectifier is equal to frequency of AC.

29. The pulse frequency of a full-wave rectifier is double to that of AC.
30. Optoelectronic Devices Semiconductor diodes in which carriers are
generated by photons, i.e. photo-excitation, such devices are known as
optoelectronic devices.
These are as follows:
(i) Light Emitting Diode (LED) It is a heavily doped p-n junction diode
which converts electrical energy into light energy.

LEDs has the following advantages over conventional incandescent low

power lamps
(a) Fast action and no warm up time required
(b) It is nearly monochromatic
(c) Low operational voltage and less power consumed
(d) Fast ON-OFF switching capability.
(ii) Photodiode A photodiode is a special type of junction diode used for
detecting optical signals. It is a reverse biased p-n junction made from a
photosensitive material. Its symbol is

Its V-I characteristics of photodiode are shown below:

We observe from the figure that current in photodiode changes with the
change in light intensity (I) when reverse bias is applied.
(iii) Solar Cell Solar cell is a p-n junction diode which converts solar energy
into electrical energy. Its symbol is

V-I characteristics of solar cell are shown below:

The materials used for solar cell are Si and GaAs.

31. Zener Diode Zener diode is a reverse biased heavily doped p-n junction
diode. It is operated in breakdown region.

32. Zener Diode as a Voltage Regulator When the applied reverse

voltage (V) reaches the breakdown voltage (Vz) of the Zener diode there is a
large change in the current. So, after the breakdown voltage V z, a large
change in the current can be produced by almost insignificant change in the
reverse bias voltage i.e. Zener voltage remains constant even though the
current through the Zener diode varies over a wide range. The circuital
arrangement is shown as follows.