GaAs/AlGaAs heterojunction bipolar transistors with very low base sheet resistance

We have made mesa-type Npn heterojunction bipolar transistors with an AlGaAs emitter, an 800 Å thick GaAs drift field base, and a GaAs collector. The bases of these transistors have sheet resistances that are the lowest reported, to our knowledge. The fabrication uses a combination of selective wet and dry etching which allows us to contact the base directly using a self-aligned shallow Zn implant. DC and high-frequency (to 18 GHz) tests of these devices show that they have a base sheet resistance as low as 310 ohms/sq, while maintaining a quite adequate hfe of 30. The frequency of unity current gain for 3 micron wide emitter devices, constructed in the normal emitter-up configuration on an n+ substrate, is greater than 28 GHz.