Studies of Impurity Profiles in Silicon p-n Junction Rectifiers

Abstract

THE operating properties of semiconductor devices utilizing minority carrier phenomena are strongly influenced by very low concentrations of chemical impurities and structural defects that act as charge carrier recombination centers. The time variation of the concentration of such impurities and defects affects the reliability of highly reliable devices. The temporal change in the relative concentrations of the electronic states arising from these imperfections may be determined from the measurement of such parameters as charge carrier lifetime, reverse leakage current, and the capacity-voltage characteristics of a p-n junction device. This paper will treat a detailed study of the net ionized impurity concentration, NA - ND, obtained from the capacity-voltage measurements of silicon rectifiers as a function of stress temperature and applied reverse voltage for times in excess 10,000 hours. The variation of the capacity-voltage characteristic with time is attributed to changes in both bulk and surface effects, which may be separately identified. Surface effects are related to the adsorption during stress of a negative charge on the region where the depletion layer of the reverse-biased junction intersects the surface, and the adsorption of a positive charge on the surface of the base region beyond the depletion layer. The concentration of surface charge required to account for the observed change in junction capacity is approximately 6.5 × 1010 cm-<sub>2</sub?, resulting in an apparent change of NA - ND of about 2%. Variations in NA - ND with stress time of as much as 20% have been observed.