What is Conduction Process?
As the temperature is increased, some of the valence electrons acquire heat energy greater than the forbidden energy gap EG and so move to the conduction band. These are now free electrons in the sense that they can move about under the influence of even a small applied force. These free or conduction electrons are indicated schematically by dots in figure. The insulator has now become slightly conducting i.e. semiconductor. In this transfer, these electrons leave holes in the valence band and this thermal breaking of valence bands and formation of electron hole pairs gives rise to intrinsic conductivityof a semiconductor. The absence of electron i.e. the hole in the valence band is represented by a small circle as shown in figure.
When an electron is freed from the valence band, It takes an additional energy. On entering a hole in a valence band, this free electron releases this excess energy as a quantum heat or light. This quantum is absorbed by another electron to break away from its valence band and creates a new electron hole-pair.
The electrons reaching the conduction band due to thermal excitation leave equal number of holes in valence band.
When an external electric field is applied to the pure semiconductor, the conduction through the semiconductor is by both free electrons and holes. The free electrons in the conduction band move towards the +ve terminal of the battery while the holes in the valence band moves towards the –ve terminal of the battery i.e. the electrons and holes move in the opposite directions. As the holes reach the –ve terminal B, the electrons reaching their combine with the holes and holes are thus destroyed. At the same time, the loosely held electrons near the +v e terminal A are attracted away from their atoms into the +ve terminal. Now new holes are created which again drift towards the –ve terminal B. The total current inside the semiconductor is thus the sum of currents owing to free electrons and holes but current in the external wires is only because of electrons. The mobility of electron is more than that of a hole because the probability of an electron having the energy required to move to an empty state in the conduction band is much greater than the probability of an electron having the energy required to move the empty state in valence band.
Thus the current due to electron flow in the conduction band is greater than the current due to the hole in the semiconductor. The mobility of hole is about half that of an electron. The net current is very small.
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