Improved emitter transit time using AlGaAs-GaInP composite emitter in GaInP/GaAs heterojunction bipolar transistors

A new emitter structure based on composite graded AlGaAs-GaInP approach is described, which allows significant reduction of CBE and improved high-frequency performance. A theoretical study of the composite and conventional emitter HBTs is performed to prove the superiority of composite emitter HBTs using Monte Carlo simulation of their transport properties. The self-aligned HBTs fabricated in this study are grown by CBE with TBA/TBP precursors. The current gain cutoff frequency (f/sub T/) was 62 GHz for the composite emitter design HBT, and 45 GHz for conventional emitter design HBT. The CBE achieved with the composite emitter designs was at least 3 times lower than that of conventional designs and does not show significant variation with collector current. This leads to enhanced f/sub T/ characteristics by 15% for composite emitter HBT designs and confirms the theoretical expectations.