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Electronic Devices

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Electronic Devices PDF Notes, Important Questions and Formulas

Electronics – SemiConductor

INTRODUCTION

The word "electronics' is derived from electron+dynamics which means the study of the behaviour an electron under different conditions of externally applied fields.
That field of science which deals with electron 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 familliar devices are :

(i) Rectifier

(ii) Amplifier

(iii) Oscillator etc.

Communication

Entertainment

Defence

Medical

Telephone

TV broadcast

Radar

X-rays

Telegraph

Radio Broadcast

Guided missiles Electro cradio

 

 

 

 

graph(ECG)

Mobile phone

VCR, VCD

 

CRO display

FAX

 

 

E.E.G

FM mike

 

 

(Electro Engio Graph)


1. Main application of electronics is computer which is used in every field.

2. All electronics equipments required D.C. supply for operation (not A.C. supply)

 

ENERGY BANDS IN SOLIDS

Based on Pauli's exclusion principle Range of energy possessed by electron in a solid is known as energy band.

(i) Valence Band (VB)

Range of energies possessed by valence electron is known as valence band.

(a) Have bonded electron

(b) No flow of current due to such electron

(c) Always fulfill by electron

(ii) Conduction Band (CB)

Range of energies possessed by free electron is known as conduction band.

(a) Also called empty band of minimum energy.

(b) In general Partially filled by electron.

(c) If conduction Band is empty, then conduction is not possible.

 

(iii) Forbidden Energy gap (FEG) (DEg)

Δ E= (C B)min - (V B)max

 

Energy gap between conduction band and valence band, where no free electron can exist.

  • Width of forbidden energy gap depends upon the nature of substance.
  • Width is more, then valence electrons are strongly attached with nucleus
  • Width of forbidden energy gap is represented in eV.
  • As temperature increases forbidden energy gap decreases (very slightly).

 

ACCORDING TO ENERGY BAND THEORY

Explanation of conductor, semiconductor and insulator


EFFECT OF TEMPERATURE

At absolute zero kelvin temperature

At this temperature covalent bonds are very strong and there are no free electrons and semiconductor behaves as perfect insulator.

Above absolute temperature

With increase in temperature some covalent bonds are broken and few valence electrons jump to conduction band and hence it behave as poor conductor.

 

CONCEPT OF "HOLES" IN SEMICONDUCTORS

Due to external energy (temp. or radiation) when electron goes from valence band to conduction band (i.e. bonded electrons becomes free), vacancy of free e– create in valence band. This electron vacancy called as"hole"

Which have same charge as electron but positive, this positively charged vacancy moved randomly in semiconductor solid.


Properties of holes :

It is missing electron in valence band.

It acts as positive charge.

It is effective mass is more than electron.

It is mobility of hole is less than electron.

Hole acts as virtual charge, although there is no physical charge on it.


Hole Current
At room temperature, due to breaking of some Covalant bonds some free electrons are produced. By applying electric field current flow due to free electrons. This current called hole current.

 

EFFECT OF IMPURITY IN SEMICONDUCTOR
Doping is a method of addition of "desirable" impurity atoms to pure semiconductor to increase conductivity of semiconductor.

GOLDEN KEY POINTS
The concentration of dopant atoms be very low, doping ratio is vary from impure : pure :: 1 : 106 to 1 : 1010

Ingeneral it is 1 : 108

There are two main method of doping.

  1. Alloy method
  2. Diffusion method (The best) The size of dopant atom (impurity) should be almost the same as that of crystal atom. So that crystalline structure of solid remain unchanged.

 

CLASSIFICATION OF SEMICONDUCTOR

 

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