CURRENT GAIN COLLAPSE IN MICROWAVE MULTIFINGER HETEROJUNCTION BIPOLAR-TRANSISTORS OPERATED AT VERY HIGH-POWER DENSITIES

The rapid development of heterojunction bipolar transistor (HBT) technologies has led to the demonstration of high power single-chip microwave amplifiers. Because HBT's are operated at high power densities, the ultimate limits on the performance of HBT's are imposed by thermal considerations. This work addresses a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phenomenon, referred to as the collapse (of current gain), occurs when suddenly one finger of the HBT draws most of the collector current, leading to an abrupt decrease of current gain. A quantitative model and the condition separating the normal operation region and the collapse are presented. Critical difference of the collapse in the constant I(b) and constant V(be) modes of operation is discussed for the common-emitter I-V characteristics. The collapse in the common-base I-V characteristics and its relationship with avalanche breakdown are also described. A solution to eliminate the collapse is experimentally verified.