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 provided by a third region 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 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 ; 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 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 provided by a third region 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 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 ; 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 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. <EOS>