Introduction to Semiconductors

 Introduction to Semiconductors

Based on the electrical conductivity all the materials in nature are classified as insulators, semiconductors, and conductors.

a) Insulator:

An insulator is a material that provides little (or negligible) conductivity when a voltage is applied. For example: paper, mica, glass, quartz. Typical insulator resistance levels are around  1010 to 1012 Ω-cm. The band structure of the insulator is shown in Figure. The band structure of a material determines the range of energy levels that an electron can occupy. The valence band is the energy range of an electron in which the electron remains bent with the atom and does not contribute to an electric current. Conduction twist is the range of electron energy above the valence band where, under the influence of an external voltage source, electrons are freely accelerated resulting in a flow of charge. The energy band between the valence band and the conduction band is called the forbidden band gap. This is the energy required for an electron to move from the equilibrium band to the conduction band, that is, the energy required for a valence electron to become a free electron.

1 eV = 1.6 x 10-19 J
For insulators, there is a  forbidden band gap greater than 5Ev as shown in Figure 1.1. Because of this large gap, there are few electrons in the CB, so the insulator conducts poorly. Even an increase in temperature or an applied electric field is not sufficient to transfer electrons from VB to CB.

Energy band diagram insulator, semiconductor and conductor