Impurity Concentration

Author: Leonard Krugman

It is interesting to note the important role that donor and acceptor atoms play in determining the conductivity of germanium. If one impurity atom is added for every 100,000,000 germanium atoms, the conductivity increases 16 times. This concentration forms germanium suitable for transistor work. If one impurity atom for each 10,000,000 germanium atoms is added, the conductivity increases 160 times, and is too high for transistor applications.

Other types of impurities which are neither trivalent nor pentavalent may be present in the crystal. These impurities are not desirable. Although they do not affect the conductivity, they introduce imperfections in the structure, and cause degradations in the transistor characteristics. Conductivity is affected, however, by the presence of N-type impurities in P-type germanium and by P-type impurities in N-type germanium, since in either case the holes furnished by the P type will cancel the electrons furnished by the N type. If both N and P types were present in equal amounts, the germanium would act as if no impurities were present. To avoid these possibilities, the germanium is purified so that the impurity ratio is considerably less than 1 part in 100,000,000 before the desired impurity atoms are added.