The thermal behavior of trench-isolated bipolar transistors is thoroughly investigated. Fully 3D numerical simulations are performed to analyze the impact of all technological parameters of interest. Based on numerical results, a novel strategy to analytically evaluate the temperature field is proposed, which accounts for the heat propagation through the trench and the nonuniform heat flux distribution over the interface between the silicon box surrounded by trench and the underlying substrate. The resulting model is proved to compare with numerical simulations more favorably than the other approaches available from the literature. As a consequence, it can be employed for an accurate, yet fast evaluation of the thermal resistance of a trench-isolated device as well as of the temperature gradients within the silicon box. (C) 2009 Elsevier Ltd. All rights reserved.