Impurity Conduction in Silicon

Impurity conduction at low temperature was investigated for various $p$- and $n$-type silicon samples. Emphasis was placed on the study of samples of low impurity concentration where the conduction is attributed to charge exchange between impurity centers which are partially ionized by some compensating impurity. A new method was used to determine the compensation. Donors were added to $p$-type samples by heat treatment. From changes in the room temperature resistivity and the Hall co-efficient in the deionization range, the added compensation and the original compensation were determined. The measurement of various samples gave the dependence of the activation energy of conduction on impurity concentration and degree of compensation. The activation energy was much larger for the Ga- and Al-doped samples than for the B-doped samples of comparable impurity concentration. However, it was found that the high activation energies may be the result of ion-pairing between gallium or aluminum atoms and the compensating impurity in the sample. The conductivities of the various samples may be correlated by an expression of conductivity which involves the impurity concentration and the radius of impurity wave function. The results are discussed in the light of current theories of impurity conduction.