The Environmental Engineering Hub

Up until now we have only described the properties of a system in equilibrium. However, it is important to introduce what happens to a semiconductor when it is illuminated with light.The process of interest is known as photogeneration of charge carriers. Here electron-hole pairs are generated as light is absorbed in a semiconductor. During the course introduction we have already mentioned that mobile electrons and holes are formed in a semiconductor as a result of light absorption. Now, I will explain how this process happens. Here we see a photon incident on a semiconductor. At some point this photon can be absorbed by the material. Absorption is a process, in which the electromagnetic radiation interacts with atoms of a semiconductor, in particular with the valence electrons.This interaction can lead to a liberation of an electron from a covalent bond. If this happens, a valence electron is excited from an energy state in the valence band to an available energy state i

Now, that we know what doping is, let’s see how we can determine the charge carrierconcentration in doped semiconductors. Once we do that, I will also show to you how tocalculate the position of the Fermi levels of doped semiconductors as well.As we saw in a previous post, the product of electron and hole concentrations in asemiconductor is independent on the position of the Fermi-energy level. This same equationcan be applied for doped semiconductors.You can remember that ‘n’ is the concentration of electrons that occupy energy states in theconduction band. ‘P’ is the concentration of holes that occupy energy states in the valenceband. N_i is the intrinsic carrier concentration. N_C and N_V are the effective densities ofstates in the conduction and valence band, respectively. ‘E_G’ is the bandgap. ‘k_B is theBoltzmann constant and ‘T’ is the temperature. Such relation is called the “law of massaction for semiconductors”.Though the values of ‘n’ and ‘p’ will change unde