A Study of the Charge Control Parameters of Transistors

The present status of the concept of charge control of transistor action is considered, and what appear to be the most significant performance parameters for circuit design purposes are defined; also methods of measuring them surveyed. These parameters are the collector time constant τC, the saturation time constant τS, the "on demand current gain" ßS, the collector capacitance charge QV and the dc current gaini ß. The manner in which these parameters may be expected to vary with dc current level is analyzed, as far as possible, for homogeneous base transistors, and is considered qualitatively for graded base types. A distinction is drawn between the technique of charge analysis of transistor action, and the concept of charge control of transistor performance. It is pointed out that only devices whose performance is determined by charges which are under the control of external circuitry can be regarded as charge controlled devices. This distinction strictly speaking excludes from the class of charge controlled devices those exhibiting the "wiggle" effect or possessing carrier storage in the collector region during operation in saturation. Although charge parameters can still be used for such devices, other parameters may be desirable as additions or alternatives.