A GaAlAs/GaAs heterojunction bipolar transistor has been developed and tested. A double base structure was adopted to facilitate critical selective etching problems. An emitter-collector breakdown voltage VCEO as high as 80 V was achieved. Maximum frequency of oscillationfmax of 850 MHz and a cutoff frequency fT of 1.6 GHz were realised. The current-gain/temperature curve shows that the transistor can be operated at temperatures up to 350°C.