Classification of material based on band gap is classified in three categories that are insulators, conductors and semiconductors.
CLASSIFICATION OF MATERIAL BASED ON BAND GAP
The material which does not readily conduct electricity because the forbidden gap (band gap) in an insulator is very high, approx 15 eV (electron volt). Insulators are also called poor conductors of electricity.
Normally, in insulators, the valence band is fully occupied with electrons and the conduction band is empty; there is no electron present in the conduction band.
In order to move electrons from valence band to conduction band a large amount of external energy is required which must be equal to the forbidden gap, only then current will flow.
For examples: Rubber, wood, plastic, thermocol, glass etc.
The material which easily allows electric current to flow through them are called conductors. In conductors valence band and conduction band overlap each other. Therefore, there is no forbidden gap in conductors.
Normally, in conductors valence band is partially occupied with electrons and in conduction band large numbers of electrons are present.
In order to move electrons from valence band to conduction band a small amount of external energy is enough. Therefore more electrons can easily move to the conduction band. When valence band electrons move to conduction band they become free electrons.
For example: Metal such as aluminium, iron, copper, silver etc.
The first semiconductor device was invented by Jagadis Chandra Bose in 1901 named as “Cat whiskers”, it was a point-contact semiconductor rectifier used for detecting radio waves. A transistor is a device or electronic component composed of semiconductor material. William Shockley, John Bardeen & Walter Brattain all three co-invented the transistor in 1947 at Bell Labs.
The material which behaves like both conductor as well as insulator is called semiconductor, because, In an insulator small forbidden gap is present, approx 1.3 eV (electron volt) unlike conductor which has no forbidden gap and insulator which has large forbidden gap.
At low temperature, the valence band is completely filled by electrons and the conduction band is empty, that’s why it behaves like an insulator at room temperature.
At room temperature, electrons in the valence band get enough energy in the form of heat and few electrons move from valence band to conduction band.
So, when the temperature goes up, the number of electrons moving from the valence band to the conduction band also increases. This shows that electrical conductivity of semiconductor increases with increase in temperature or any form of energy in the material.
Pure semiconductor can’t flow much current but with such a small gap, the presence of a small percentage of a doping material can increase the conductivity dramatically.
Example: Silicon, germanium, graphite etc.
ENERGY BAND GAP OF DIFFERENT MATERIALS
Energy gap (eV) at
|0 K||300 K|
LED & BATTERY CONNECTIONS
BAND THEORY OF SOLIDS