Emitter efficiency, transit times and current gain of bipolar transistors

Abstract

A generalized theory of emitter efficiency, transit times, and current gain of bipolar transistors have been developed by taking non-uniform doping and carrier concentrations, bandgap narrowing, mobility, diffusion coefficient, bulk and surface recombinations, and others into consideration. The final expressions exhibit forms identical to the traditional ones for non-degenerate semiconductors with position-independent device parameters. Numerical calculations carried out on a simpler model resulting from the use of approximate models for various parameters demonstrate effects of these parameters on emitter efficiency, transit times and current gain. These calculations point, in particular, to the importance of optimum doping concentrations in the emitter and base regions for obtaining maximum current gain, and of bandgap narrowing in determining the performance limitations of bipolar transistors.