Semiconductor and Semiconductor devices

Classification of solids on the basis of conductivity

(i) Conductor:-  Conductors are those substances through which electricity can pass easily, 

e.g., all metals are conductors.

(ii) Insulator:- Insulators are those substances through which electricity cannot pass,

 e.g., wood. rubber, mica etc.

(iii) Semiconductor Semiconductors are those substances whose conductivity lies between conductors and insulators. 

e.g., germanium, silicon, carbon etc.

Energy Bands of Solids

1. 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 levels takes place. The collection of these closely spaced energy levels is called an energy band.

2. Valence Band:-This energy band contains valence electrons. This band may be PartIally or completely filled with electrons but never be empty. The electrons in this band are not capable of gaining energy from external electric field to take part in conduction of current.

3. Conduction Band:-This band contains conduction electrons. This band is either empty or Partially filled with electrons.Electrons present in this band take part in the conduction of current.

4. Forbidden Band:-This band is completely empty. The minimum energy required to shift an electron from valence 

band to conduction band is called band gap (Eg)

FIGURE:- Energy bands

FIGURE:- difference between conductor, Insulator and semiconductor 

Types of Semiconductor

(i) Intrinsic Semiconductor:- A semiconductor in its pure state is called intrinsic semiconductor.

FIGURE:-  intrinsic semiconductor at 0K

FIGURE:- intrinsic semiconductor at normal temperature 

(ii) Extrinsic Semiconductor:- A semiconductor doped with suitable impurity to increase its impurity, is called extrinsic semiconductor.

On the basis of doped impurity extrinsic semiconductors are of two types

(A) n-type Semiconductor:- Extrinsic semiconductor doped with pentavalent impurity like As, Sb, Bi, etc in which negatively charged electrons works as charge carrier, is called n-type semiconductor.

FIGURE:- n-type semiconductor 

Every pentavalent impurity atom donate one electron in the crystal, therefore it is called a doner atom

(B) p -type Semiconductor:- Extrinsic semiconductor doped with trivalent impurity like Al, B, etc, in which positively charged holes works as charge carriers, is called p-type semiconductor.

FIGURE:-  p-type semiconductor 

Every trivalent impurity atom have a tendency to accept one electron, therefore it is called an acceptor atom.

Note Energy gap for Ge is 0.72 eV and for Si it is 1.1 eV. 

p-n Junction

An arrangement consisting a p -type semiconductor brought into a close contact with n-type semiconductor, is called a p -n junction.

FIGURE:- p-n junction

Terms Related to p-n Junction

(i) Depletion Layer:- At p-n. junction a region is created, where there is no charge carriers. This region is called depletion layer. The width of this region is of the order of 106 m.

(ii) Potential Barrier:- The potential difference across the depletion layer is called potential barrier.

Barrier potential for Ge is 0.3 V and for Si is 0.7 V.

(iii) Forward Biasing:- In this biasing, the p -side is connected to positive terminal and n-side to negative terminal of a battery.

In this biasing, forward current flows due to majority charge carriers.

The width of depletion layer decreases.

FIGURE:- Forward Biasing

(iv) Reverse Biasing:- In this biasing, the p-side is connected to negative terminal and n-side to positive terminal of a battery.

In this biasing, reverse current flows due to minority charge carriers.

FIGURE:-  Reverse Biasing 

The width of depletion layer increases.

A p-n junction diode can be utilized as a rectifier.

Zener diode, photo-diode, light-emitting diode, etc are specially designed p-n. junction diodes.

p-n Junction Diode

The current through p-n junction flow only from p toward n and not from n toward p.

The maximum voltage that a junction diode can bear without break is called zener voltage and the junction diodes possessing this voltage is known as zener diode.

FIGURE:-  p-n junction diode 

FIGURE:- characteristics of p-n junction diode 

Resistance of diode R = V / I

Rectifier

A device which convert alternating current or voltage into direct current or voltage IS known as rectifier. The process of converting AC into DC IS caned rectification.

Half-Wave Rectifier

A half-wave rectifier converts the half cycle of applied AC signal into DC signal. Ordinary transformer may be used here.

FIGURE:- half wave rectifier 

Full-Wave Rectifier

A full-wave rectifier converts the whole cycle of applied AC signal into DC signal. Centre top,transformer is used here.

[Half-wave rectifier converts only one-half of AC Into DC while full wave rectifier rectifies both halves of AC input.]

FIGURE:- full wave rectifier 

FIGURE:- full wave rectifier during positive half cycle

FIGURE:- full wave rectifier during negative half cycle