Heat flow in short emitter length bipolar devices in trench-isolated technologies is investigated through three-dimensional numerical thermal simulation, and thermal conduction through the trench walls is shown to be important for these structures. A new model is presented which predicts the thermal resistance of bipolar transistors in trench-isolated technologies down to emitter lengths of 1.2 mum. The effect of the parasitic thermal path introduced by emitter metal is also included in the new model. The prediction of this model is compared to numerical simulation and measurement, and found to be in excellent agreement. (C) 2002 Elsevier Science Ltd. All rights reserved.