Modeling the temperature-dependent early voltage of a silicon germanium heterojunction bipolar transistor

This paper compares the temperature-dependent Early voltage of a silicon germanium heterojunction bipolar transistor (HBT) to that of a silicon bipolar junction transistor (BJT) fabricated with identical geometry. Derived is an isothermal expression for the forward Early voltage specifically suited to the base composition of a SiGe HBT, The expression includes two fit factors, one for the Si1-xGex alloy and the other for the nonuniform doping density. The fit factors are functions of the device temperature and are determined through pulsed bias measurements.