Experiments on the Interface between Germanium and an Electrolyte

Abstract

Measurements have been made of the electrode potential of p- and n-type germanium in contact with aqueous solutions of KOH, KCl and HCl as a function of anodic and cathodic current and of incident light intensity. For anodic currents, the measured electrode potential can be separated into three parts: the reversible electrode potential corresponding to the anodic reaction, depending only on the solution; an overvoltage of the usual form; and a term (kT/e) ln (p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> /p), where p is the equilibrium hole concentration and p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> is the concentration just inside the space-charge region of the germanium. The anodic current is determined by flow of holes to the surface, so that the current saturates for n-type germanium but not for p-type. The saturation current is determined by body and surface generation of holes and by creation of excess holes by light. There is a current gain of 1.4 to 1.8. In addition, there is a small “leakage” current not dependent on hole supply. Similar statements may be made for cathodic current, except that the electrode potential and current are determined respectively by the concentration and supply of electrons instead of holes, the current gain is of the order of unity, and the leakage current is larger. Complicating time changes were observed for cathodic but not for anodic currents. The measurements may be understood in terms of simple thermodynamic considerations, based on the idea that the anodic reaction is with holes, the cathodic reaction with electrons, in the semiconductor; the behavior for very small currents depends on a competition between the anodic and cathodic reactions, which may be treated by simple rate process considerations. A comparison is made with experiments on the germanium-gas interface by Brattain and Bardeen, to which similar considerations may apply.