The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses different semiconductor materials for the emitter and base regions, making a heterojunction. The HBT can handle signals of much higher frequencies, (up to several hundred GHz) than BJT. HBT is commonly used in modern ultrafast circuits, mostly radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in mobile phones. The idea of using a heterojunction is as old as the conventional BJT, dating back to a patent from 1951.