Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolar transistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 mu m/sup 2/. The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF/sub 3/. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5*2 mu m/sup 2/ at submilliampere collector currents. The maximum f/sub T/ and f/sub max/ obtained from a 0.5*2 mu m/sup 2/ emitter HBT are 46 and 42 GHz, respectively at I/sub C/=1.5 and more than 20 GHz even at I/sub C/=0.1 mA.<
>