A bipolar transistor with vertical geometry comprises a base region (1) provided with a base contact (21), emitter and collector regions (2,3) arranged to extract minority carriers from the base region, and an excluding structure for counteracting entry of minority carriers into the base region via the base contact, wherein the base region has a bandgap of greater than 0.5 eV and a doping level greater than 10¹⁷ cm^-3. As shown the base includes an excluding heterojunction (4) preventing entry of carriers from the base contact (21), but alternatively the base region could comprise a "high-low" doping homojunction. The construction shows improved resistance to thermal runaway even in multi-finger transistors. It is particularly useful for high power, high frequency transistors, e.g. base on gallium indium arsenide. The collector region preferably has a heterostructure.