A silicon‐on‐sapphire lateral thin‐film bipolar junction transistor is unique in that recombination on the interface between the overlying silicon dioxide layer and the silicon semiconductor underneath must be taken into account. A simplified model of recombination is developed and then applied to determine the recombination currents in the bulk of the base of a bipolar junction transistor and in adjacent space‐charge regions. The expressions for these currents are used to modify the conventional Gummel–Poon model to include recombination on the top surface of a lateral bipolar junction transistor. In the modified form essential constants retain their original meanings.