Theory of a Wide-Gap Emitter for Transistors

In order to obtain a high current amplification factor, it is important in transistors that the ratio of the injected minority carrier current over the total emitter current, γ, be close to unity, or that the quantity 1-γ, called the injection deficit, be as small as possible. It is shown that the injection deficit of an emitter can be decreased by several orders of magnitude if the emitter has a higher band gap than the base region. This effect can be utilized either in addition to the commonly used high emitter doping in order to eliminate the alpha falloff with current, or to decrease the high emitter doping in order to obtain a lower emitter capacitance. Decreasing the emitter capacitance in high-frequency transistors may be utilized either to extend their frequency range or to increase their power capabilities by increasing the area.