In this paper a high-temperature N-p-n AlGaAs/GaAs HBT with wide-bandgap emitter is presented which can be operated up to a ambient temperature of 350-degrees-C. It is demonstrated that a high Al mole fraction of 0.45 in the emitter in combination with a GaAs base layer yields excellent temperature stability. A useful common-emitter small-signal current gain h(FE) higher than 35 was measured in the range between room temperature and 350-degrees-C.