Collector model describing bipolar transistor distortion at low voltages and high currents

Abstract

A model is presented for the stored collector charge in a bipolar transistor. Exponential functions are used to describe base and substrate doping. The model takes account of space charge regions as well as injection regions. Furthermore, it includes dissipation losses in the neutral collector, the Kirk effect and built-in electric fields in the neutral regions. Smooth transistions are made between the various space charge and injection modes. The model provides a direct relation between doping profiles, bias conditions and stored collector charge. The model is applied to the problem of signal distortion, to which stored collector charge is a major contributor, especially at low voltages and high currents. Distortion measurements are performed on a transistor in a test circuit. To describe the linear parts of the transistor crude estimates of relevant transistor parameters like e.g. Early voltage and saturation current are made. Therefore a simple neutral base model is used. Finally, circuit analysis has to be performed to calculate distortion. This is done using an almost periodic Fourier transform in order to calculate intermodulation products arising from two input signals to the circuit. Good agreement between model and experiment is shown. The large influence of the stored charge on distortion is demonstrated. The model is suited for predicting distortion properties of new designs as well as for assessing the effects of process spreading.