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

The rapid development of heterojunction bipolar transistor (HBT) techn ologies has led to the demonstration of high power single-chip microwa ve amplifiers. Because HBT's are operated at high power densities, the ultimate limits on the performance of HBT's are imposed by thermal co nsiderations. This work addresses a thermal phenomenon observed when a multifinger power HBT is operating at high power densities. This phen omenon, referred to as the collapse (of current gain), occurs when sud denly one finger of the HBT draws most of the collector current, leadi ng 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) a nd constant V(be) modes of operation is discussed for the common-emitt er I-V characteristics. The collapse in the common-base I-V characteri stics and its relationship with avalanche breakdown are also described . A solution to eliminate the collapse is experimentally verified.