Reduction of extrinsic base resistance in GaAs/AlGaAs heterojunction bipolar transistors and correlation with high-frequency performance

Improved high-frequency performance in GaAs/AlGaAs heterojunction bipolar transistors (HBT's) by reduction of extrinsic base resistance is demonstrated. A new self-aligned process which is very simple, yet capable of producing 0.25-µm emitter-to-base contact gaps, is described. By the use of AuBe, we have also been able to produce contact resistances to p-type GaAs (p = 5 × 1018) as low as 1.2 × 10-7Ω.cm2. This is the lowest value reported to p-type GaAs considering the relatively low doping levels used. By employing these techniques, we have produced HBT's with 2.5-µm-wide emitters having current gain cutoff frequencies fTthat appear to be greater than 35 GHz and maximum oscillation frequenciesfₘₐₓof 22 GHz.