Self-aligned AlGaAs/GaAs HBT with low emitter resistance utilizing InGaAs cap layer

A self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT) with an InGaAs emitter cap layer that has very low emitter resistance is described. In this structure, a nonalloyed emitter contact allows the emitter and base electrodes to be formed simultaneously and in a self-aligned manner. The reduction of emitter resistance greatly improves the HBT's transconductance and cutoff frequency. In fabricated devices with emitter dimensions of 2 mu m*5 mu m, a transconductance-per-unit-area of 16 mS/ mu m/sup 2/ and a cutoff frequency of 80 GHz were achieved. To investigate high-speed performance, a 21-stage ECL ring oscillator was fabricated using these devices. Propagation delay times as low as 5.5 ps/gate were obtained, demonstrating the effectiveness of this structure. >