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, IBN. In the transistors investigated here, IBn is made up of a Sah-Noyce-Shockley recombination-generation current (Isns) and a channel current (IcH). 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 IBN and physical processes affecting the Si-SiO2 interface. This paper shows the effect of heat-treatment in various ambients on IBN. 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 recombinationgeneration component of IBn. 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.The understanding and control of the Si-SiO2 interface strongly affects the performance of semiconductor devices. Changes in the surface state (1) and surface charge (1) densities influence both initial device characteristics and reliability. Physical measurements (2) and radioactive tracer experiments (3) have shown that the major source of the surface charges is the diffusion of alkali ions through the oxide to the Si-SiO2 interface (1,2). The surface states on the other hand are probably associated with lattice defects (4) and unsaturated silicon bonds (5) at the interface. It follows, therefore, that the surface charge density can be reduced by the control of impurities during processing, while the surface states can be affected by treatments altering the structure and the stoichiometry of the oxide, such as heat-treatments in various ambients.The effects of sodium concentration and heat-treatment in various ambients have been evaluated on npn planar transistors by monitoring low current d-c gain (hFE). The following discussion is divided into three parts. (i) First, the correlation between the oxide-silicon interface and the low current gain is discussed. (ii) This is followed by the effect of sodium contamination and the heat-treatment in various ambients on hFE, and on the fiat band voltage on MOS capacitors. (iii) Finally, a mechanism is offered for the migration of sodium ions through the oxide to explain the effect of the ambient on the diffusion rate.
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