In a planar transistor low current d‐c gain is the most sensitive parameter to changes in the properties of the oxide‐silicon interface. It is dominated by the nonideal base current, . In the transistors investigated here, is made up of a Sah‐Noyce‐Shockley recombination‐generation current and a channel current . The two components can be separated by analyzing the temperature dependence of IBN, making it possible to determine the failure mechanism of the transistor gain degradation.In order to control gain degradation, a correlation was sought between the components of and physical processes affecting the interface. This paper shows the effect of heat‐treatment in various ambients on . Heat‐treatment in hydrogen and water vapor improve the gain primarily by reducing the recombination‐generation current. This improved gain corresponds to the state of a transistor completed by standard processing. The high transistor gain may be degraded by baking in dry inert gases or with sodium contamination. Baking in inert gases increases the recombination‐generation component of . Hydrogen ambient accelerates, and water vapor impedes the migration of sodium through the oxide relative to dry inert gases. A model is presented to explain the effect of hydrogen on the diffusion of sodium in the oxide. Experiments with MOS capacitors are in good agreement with transistor results.