Dear Balsam Sofi,

Explaination for your Simplified diagram of the filled valence band of insulators and semiconductors.

The band model of insulators

In insulators the valence band (VB) is fully occupied with electrons due to the covalent bonds. The electrons can not move because they're "locked up" between the atoms. To achieve a conductivity, electrons from the VB have to move into the conduction band (CB). This prevents the band gap, which lies in-between the VB and CB. Only with considerable energy expenditure (if at all possible) the band gap can be overcome; which leads to a negligible conductivity.

The band model of semiconductors

Even in semiconductors, there is a band gap, but compared to insulators it is so small that even at room temperature electrons from the VB can be lifted into the CB. The electrons can move freely and act as charge carriers. In addition, each electron also leaves a hole in the VB behind, which can be filled by other electrons in the VB. Thus one gets wandering holes in the VB, which can be viewed as positive charge carriers.

There are always pairs of electrons and holes, so that there are as many negative as positive charges, the semiconductor crystal as a whole is neutral.

A pure undoped semiconductor is known as intrinsic semiconductor. Per cubic centimeter there are about 1010 free electrons and holes (at room temperature).

Since the electrons always assume the energetically lowest state, they fall back into the VB & recombine with the holes if there is no energy supply. At a certain temperature an equilibrium is arranged between the electrons elevated to the CB and the electrons falling back. With increasing temperature the number of electrons that can leap the band gap is increased, and thus increasing the conductivity of semiconductors.

I hope I have answered your question.

With Best Wishes,

Samir G. Pandya

Samir G. Pandya

P. Contreras

Thank you very very much for your response.

Samir G. Pandya

P. Contreras

Please, is there an example for two elements clarify your idea more, one of them is semiconductor and the other is insulator?

Well expound @Pedro