Abstract: A heterojunction bipolar transistor having an n- type epitaxial indium phosphide collector layer grown on a semi-insulating indium phosphide substrate with an n+ buried layer, a p- type indium phosphide base and an epitaxial, n- type boron phosphide wide gap emitter. The p- type base region is formed by ion implantation of magnesium ions into the collector layer. The transistor is applicable to millimeter wave applications due to the high electron mobility in the indium phosphide base. The wide gaps of both the boron phosphide (2.2 eV) and indium phosphide (1.34 eV) permit operation up to 350.degree. C. The transistor is easily processed using metal organic-chemical vapor deposition (MO-CVD) and standard microelectronic techniques.