**2.1.1 Intrinsic semiconductors**

Pristine semiconductors are generally called "intrinsic semiconductors". In an intrinsic semiconductor (IS) the Fermi-level (EF), which can be the highest occupied energy level at T = 0 K, lies near the middle of the band-gap and the corresponding Fermi-Dirac distribution *f(E)* is plotted in Fig. 1. At T = 0 K the probability of an electron to occupy a state in the CB is zero and the VB is totally full (see the thick black line). At room temperature (blue sigmoid curve) some electrons can jump into the CB and fill the states closed to the bottom of the CB. In this case the tail of *f(E)* is extended into the CB, thus, there is a probability to have electrons there. In this condition an equal number of holes exists close to the top of VB. EF is now the energy level at which the probability of occupancy is half and it lies near the middle of the band-gap.
