Influence of high doping on the design of bipolar transistors

The classical analysis of bipolar transistors relies on the assumption that current flows by drift and diffusion. However, if heavy doping concentrations vary with the position, as occurs in the emitter and the base of bipolar transistors, a mechanism in addition to drift and diffusion gives rise to a flow of mobile carriers. This effect arises from distortion of the quantum density of states and the consequent presence of quasi-electric fields. Incorporation of this distortion in the transport equations lowers β by several orders of magnitude, whereas the intrinsic f_T becomes smaller to an extent depending on the profile. Quantitative experimental observations still unexplained by classical bipolar transistor physics, can be understood on the basis of these generalized transport equations.