Early effect of high-current-gain heterojunction bipolar transistor

The current gain ß of a high-ß heterojunction bipolar transistor is usually limited, at normal operating range, by the ratio of the injected electron current to the recombination current in the space-charge region. The Early effect in these transistors is very important because the base layer is very thin in order to obtain high ß. In this case, the classical transistor models are not very suitable, and an Early voltage smaller than for a homojunction bipolar transistor is commonly observed. In the letter an analysis has been carried out taking into account the emitter current crowding effect and heterojunction characteristics. A simple analytic equation of the Early voltage is derived, and experimental results are presented.