Early effect of high-current-gain heterojunction bipolar transistor

The Early voltage and current expressions for a heterojunction bipolar transistor have been obtained. Experimental and predicted results are in good agreement. At a low emitter current crowding level, the Early effect is the same as in the case of the homojunction bipolar transistor with the same structure, but at high emitter crowding levels the Early effect is much more important. This result is valid for all bipolar devices. In particular, it explains the difference in Early voltage between high-current-gain phototransistors and microwave transistors.