A GaAlAs/GaAs heterojunction transistor has been made using liquid-phase epitaxy. The emitter stripe was 10µm wide by 100µm long. The base and emitter had equal impurity content around 5 × 1023/m3but, with the wide band gap GaALAs emitter, low-frequency current gains approaching 2000 were found. This is amongst the highest so far recorded for this type of transistor. The alpha-cutoff frequency was estimated to be around 1 GHz, but the unity gain frequency was experimentally as low as 100 MHz. This was attributed to high r.f. resistance, possibly caused by the Schottky barrier contact to the base region. When the basecollector region was driven close to primary breakdown, current densities in excess of 3 × 107A/m2could be obtained without secondary breakdown occurring, at least for the duration of the bias pulse. This appeared to be about an order of magnitude better than for a comparable Si transistor. Avalanche multiplication could then increase the unity gain frequency to 400 MHz.