Session 4 The Semiconductor in Equilibrium

In a semiconductor，two types of charge carrier，the electron and the hole，can contribute to a current. Since the current in a semiconductor is determined largely by the number of electrons in the conduction band and the number of holes in the valence hand，an important characteristic of the semiconductor is the density of these charge carriers. The density of electrons and holes is related to the density of statesfunction and the Fermi distributionfunction. The density of states function is the density of available quantum states as a function of energy，given in units of number per unit energy per unit volume. The Fermi-Dirac probability function describes the statistical distribution of electrons among available energy states and the probability that an allowed energy state is occupied by an electron. Fermi energy EF is the energy below which all states are filled with electrons and above which all states are empty at T=0K. For temperatures above absolute zero，there is a nonzero probability that some energy states above EF will be occupied by electrons and some energy states below EF will be empty.