Semicondutor Theory 2

1.06 Holes (virtual particles!)

The movement of electrons in the valence band is complicated and as they move to fill the vacant electron positions the position of the hole appears to move in the opposite direction to the electrons. In fact it is easier to consider the movement of the hole and we can imagine it to be a positively charged particle because of the direction it moves in (opposite to that of negatively charged electrons).

At first this may seem a strange idea but later you will see that by considering the movement of these virtual particles it is much easier to understand how semiconductor devices work. However it is important to remember that although we regard holes as positively charged particles they are not real particles and all the effects we see are actually caused by the movement of electrons in the valence band.

When an external voltage is applied the negatively charged electrons in the conduction band will move towards the positive terminal and the positively charge holes in the valence band will move towards the negative terminal.

movement of free electrons and holes

Leakage current
It is important to realise that although electron-hole pair generation in semiconductors means that there will be some current flow when a voltage is applied, this current is very small (typically millionths of an Amp) compared to the current which would flow through a conductor with the same voltage applied. This current is called leakage current.

Comparison of Silicon and Germanium
All of the above applies equally to both Silicon and Germanium except for the actual value of leakage current. Germanium has a smaller energy gap between the valence and conduction band. Therefore more electron hole pairs are produced in Germanium resulting in a higher leakage current at any given temperature.

Comparison of Conductor, semiconductor and insulator energy band diagrams
In conductors the valence band and conduction band overlap therefore electrons can move freely into the conduction band. The vast number of electrons in the conduction band drift to produce large currents when a voltage is applied. In insulators the energy gap between the valence and conduction band is very large. Therefore very few electrons manage to jump into the conduction band and leakage currents are extremely small.