Carrier Generation and Recombination - Engineering Physics

 Carrier Generation and Recombination

Carrier generation is defined as the process whereby electron and holes are created and recombination is the process whereby electrons and holes are annihilated.

When a covalent bond in a semiconductor is broken due to thermal energy, the removal of one electron leaves behind an empty space or electron deficiency known as a hole. Thus, a free electron and a hole are generated due to thermal excitation and are commonly known as electron-hole pairs. Both free electrons and holes are available on the free carriers of electric charge. When an electric field is applied across the germanium crystal, the free electron released from the bands will travel towards the positive terminal and the holes will travel towards the negative terminal of the voltage source. Hence both free electrons and holes contribute to the current flow. In addition to the generation of electron-hole pair due to absorption of thermal energy, there is an opposite process called ‘’recombination.’’ When a free electron moves in a random manner in the crystal lattice, it may come across one of the broken covalent bonds and collide with it. It is possible that the free-electron may recombine with a hole due to collision and as a result, an electron-hole pair will be lost. The effect of these two opposite processes of generation and recombination may be expressed by an equation similar to a chemical equation.

In the equilibrium condition at a constant temperature the rate of the generation of electron-hole pair on the absorption of energy should be equal to the rate of destruction of the same by the process of recombination. When the equilibrium is reached at any constant temperature, there is always a definite number of electron pairs per unit volume of a particular semiconductor.

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