A transistor device having: a collector region provided by a first region of semiconductor of a first type; a collector terminal associated with the collector region; an emitter region provided by a second region of semiconductor of the first type; an emitter terminal associated with the emitter region; a base region (4A, 4B) provided by a third region (101,103) of semiconductor lying between and interfacing with both the collector region and emitter region; a base terminal associated with the base region; wherein the base region (4A, 4B) includes: a sub-region of semiconductor of a second type; and a channel of semiconductor of the first type; wherein the base terminal contacts the sub-region; the sub-region interfaces with the channel to provide a first diode junction, and interfaces with both the emitter region and the collector region to form further diode junctions; the channel interfaces with and interconnects the collector region and the emitter region; the net doping concentration of the channel is less than the net doping concentration of the emitter and collector regions (2, 3); the channel has a depth extending away from the first diode junction; and characterised in that there is a separation between the collector and the emitter regions (2, 3) which is sufficiently small such that when implemented in a circuit where there is a voltage across the emitter and base terminals such as to cause a current through the base terminal, a current between the collector and emitter terminals is at least predominantly attributable to bipolar conduction, said separation being less than or equal to 1.5 microns; and that the sub region lies in a semiconductor substrate layer of the first type. a collector region provided by a first region of semiconductor of a first type; a collector terminal associated with the collector region; an emitter region provided by a second region of semiconductor of the first type; an emitter terminal associated with the emitter region; a base region (4A, 4B) provided by a third region (101,103) of semiconductor lying between and interfacing with both the collector region and emitter region; a base terminal associated with the base region; wherein the base region (4A, 4B) includes: a sub-region of semiconductor of a second type; and a channel of semiconductor of the first type; a sub-region of semiconductor of a second type; and a channel of semiconductor of the first type; wherein the base terminal contacts the sub-region; the sub-region interfaces with the channel to provide a first diode junction, and interfaces with both the emitter region and the collector region to form further diode junctions; the channel interfaces with and interconnects the collector region and the emitter region; the net doping concentration of the channel is less than the net doping concentration of the emitter and collector regions (2, 3); the channel has a depth extending away from the first diode junction; and characterised in that there is a separation between the collector and the emitter regions (2, 3) which is sufficiently small such that when implemented in a circuit where there is a voltage across the emitter and base terminals such as to cause a current through the base terminal, a current between the collector and emitter terminals is at least predominantly attributable to bipolar conduction, said separation being less than or equal to 1.5 microns; and that the sub region lies in a semiconductor substrate layer of the first type.