At a 0°K temperature, the semiconductor crystal behaves as an insulator. However, at room temperature, some of the covalent bonds will be broken because of thermal energy supplied to the crystal, and conduction is made possible. This is shown in Fig. If an electron in the valence band gets sufficient energy, it can jump across the forbidden energy gap and enter the conduction band.
As shown in Fig. 1.2 (a) suppose covalent bond breaks due to the departure of an electron, and a vacancy is left in the valence band which is called a positive hole.
Suppose the valence electron at A has become a free-electron due to thermal energy, this creates a hole. The hole is a strong attraction for an electron. A valence electron from nearby covalent bond becomes to fill in the hole at the A.creation of the hole at B. In this way by a succession of electron movement, a hole would appear at G and the negative charge would have moved from G to A. But it is more convenient to consider a positive hole that has moved from A to G.
It should be noted that holes are filled by electrons that move from adjacent atoms without passing through the forbidden energy gap. It means that hole movement takes place in the valence band only. The movement of the hole is less compared to the movement of conduction electrons.
