Analytical study of collector-base capacitance and cutoff frequency of n+- p- n- n+ bipolar junction transistors

Abstract

Modern high-speed bipolar junction transistors (BJTs) tend to operate with saturated carrier velocity in the base-collector space-charge region and at high current density. In the past, there have been several discussions on the analytical expressions for the collector-base capacitance and the transition frequency for n- p- n- n + BJTs. However, the expressions did not complely take into account the effect of velocity saturation. Taking into account this effect completely, analytical expressions for these device quantities of n +- p- n- n + BJTs are presented. We found that the effect of velocity saturation appears for devices with shorter epitaxial collector. In the model, the device parameters can be calculated analytically using external voltages.