Reduction of base-collector capacitance by undercutting the collector and subcollector in GaInAs/InP DHBTs

The total base-collector capacitance (C/sub BC/) of GaInAs/InP double heterojunction bipolar transistors (DHBTs) has been reduced by the etching away of the semiconductor layers below the extrinsic base region, resulting in an undercut structure. The reduction was further enhanced by using a novel composite subcollector structure. A 54% reduction of total C/sub BC/ and improvement of microwave characteristics (an increase of 20% in f/sub T/ and 38% in f/sub max/) were observed as a result of the undercut process.