Integration of a superlattice limiter into the HBT emitter for improved operational reliability

A common problem limiting the output power of multiple finger heterojunction bipolar transistors (HBT's) is nonuniform current flow in the fingers, resulting from an underlying nonuniform temperature distribution. We have fabricated HBT devices containing an integrated superlattice region to help overcome this problem. We demonstrate that the superlattice functions as a temperature-dependent resistive current limiter in a single finger device at dc and RF. Furthermore, the RF performance of the HBT was not compromised by the inclusion of the superlattice structure.