Intrinsic Germanium

Author: Leonard Krugman

In those cases where the germanium is extremely pure, or where there are equal numbers of donor and acceptor atoms, the germanium is called intrinsic. Conduction can take place if electrons are forced out of their valence bonds by the addition of external energy to the crystal in the form of heat or light. Although the disruption of the covalent bonds by these processes creates equal numbers of electrons and holes, intrinsic conduction is invariably of the N type, because the mobility of the electrons is approximately twice as great as that of the holes.

In the case of thermal excitation, the higher the temperature, the greater the number of electrons liberated and the higher the germanium conductivity becomes. This explains why germanium has a negative temperature coefficient of resistance, i.e., the higher the temperature, the lower the resistance. Intrinsic conductivity can adversely affect impurity-type conductivity. As the temperature is increased to 80° C, the electrons produced by thermal excitation cause the conductivity of the germanium to become too high for satisfactory transistor operation.

The disruption of covalent bonds by the addition of light energy is discussed under P-N junction photocells in Chapter 2.