Semiconductor
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INTRODUCTION
The word "electronics' is derived from electron+dynamics which means the study of the behaviour of an electron
under different conditions of externally applied fields.
This field of science deals with electronic devices and their utilization. An electronic device is a device in which
conduction takes place by the movement of electron - through a vacuum, a gas or a semiconductor.
Some familiar devices are :
(i) Rectifier (ii) Amplifier (iii) Oscillator etc.
Application of Electronics
Communication Entertainment Defence Medical
Telephone TV Broadcast Radar X-rays
Telegraph Radio Broadcast Guided missiles Electro cardio graph (ECG)
Mobile phone VCR, VCD CRO display
FAX E.E.G. (Electro Engio Graph)
FM mic
* Main application of electronics is computer which is used in every field.
*All electronics equipments required D.C. supply for operation (not A.C. supply
ENERGY BAND THEORY
Based on Pauli's exclusion principle
In an isolated atom, electrons are present in sharply defined energy levels. But in solids atoms are very close to
each other. So because of their interactions, each electron doesn't have fixed energy.
It has different energy levels in a certain (small) range called energy band.
The number of energy levels in a band depends upon the number of interacting atoms.
The energy band including valence electrons is called valence band (VB) and the energy band including conducting
(free) electrons is called conduction band (CB).
Band gap or Forbidden Energy gap (FEG) (DEg)
energy gap
D Eg = (C B)min –- (V B)max
(i) It is the energy gap between CB and VB.
(ii) It is also called forbidden energy gap because free electrons can not exist
in this gap
(iii) Width of forbidden energy gap depends upon the nature of substance.
(iv) Width is more, then valence electrons are strongly attached with nucleus.
(v) Width of forbidden energy gap is represented in eV.
(vi) As temperature increases forbidden energy gap decreases (very slightly).
CLASSIFICATION OF SOLIDS ACCORDING TO ENERGY BAND THEORY
According to energy band theory, solids are conductor, semiconductor and insulator
Conductor
In some solids conduction band and valence band are overlapped so there is no band gap between them,
it means D Eg = 0.
Due to this a large number of electrons are available for electrical conduction and therefore its resistivity is low
(r = 10–2 – 10–8 W-m) and conductivity is high [s =102 – 108 (W-m)–1]
Such materials are called conductors. For example gold, silver, copper etc.
Insulator
In some solids energy gap is large (Eg > 3 eV).
So in conduction band there are no electrons and so no electrical conduction is possible. Here energy gap is so
large that electrons cannot be easily excited from the valence band to conduction band by any external energy
(electrical, thermal or optical).
Such materials are called as "insulator". Their r > 1011 W-m and s < 10–11 (W-m)–1
Semiconductor
In some solids a finite but small band gap exists (Eg < 3eV).
Due to this small band gap some electrons can be thermally excited to "conduction band".
These thermally excited electrons can move in conduction band and can conduct current. Their resistivity and
conductivity both are in medium range, r ; 10–5 – 106 W-m and s ; 10–6 – 105 W-m–1
Example of semiconducting materials
Elemental semiconductor : Si and Ge
Compound semiconductor Inorganic : CdS, GaAs, CdSe, InP etc.
Organic : Anthracene, Doped pthalocyanines etc.
Organic polymers : Poly pyrrole, Poly aniline, polythiophene
PROPERTIES OF SEMICONDUCTOR
l Negative temperature coefficient (a), with increase in temperature resistance decreases.
l Crystalline structure with covalent bonding [Face centred cubic (FCC)].
l Conduction properties may change by adding small impurities.
l Position in periodic table ® IV group (Generally)
l Forbidden energy gap (0.1 eV to 3 eV)
l Charge carriers : electron and hole.
l There are many semiconductors but few of them have practical application in electronics like
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