[en] Solid-state devices formed from compound semiconductor materials like GaAs, GaP, SiC, and (Al, Ga)As have long been viewed as candidates for use in electronic circuits functioning at temperatures greater than 300⁰C. To address the specific needs of power semiconductor devices operating simultaneously at high currents, voltages, and high temperatures, heterojunction devices formed from combinations of GaAs and (Al,Ga)As have recently been proposed. These novel heterojunction structures display reduced resistive and voltage parasitics when compared to wide-bandgap GaP or SiC homojunction diodes without seriously compromising control of thermally generated leakage currents. In this study a prototype, low-power, (Al, Ga)As/GaAs, heterojunction bipolar transistor (HBT) is described which has demonstrated excellent electrical characteristics in the 300 to 400⁰C temperature range. At 350⁰C, the HBT has a common-emitter current gain of 14 (V/sub CE/ = 5V, I/sub C/ = 10 mA) and collector-base leakage of 6.4 x 10^-2 A/cm² (V/sub CB/ = 5 V). These studies and others imply that a heterojunction, (Al Ga)As/GaAs, power semiconductor electronics technology is feasible in the near future