To satisfy the increasing demand for small power amplifiers in advanced cellular phones, we have investigated the thermal performance of multi-finger InGaP/GaAs collector-up HBTs with a heat-dissipation packaging configuration. The thermal interaction between collector fingers and the size effect on the maximum operation temperature within the transistor have been scrutinized. In addition, the thermal handling for a stable operation in the device has been optimized through the variation of finger pitches. The superior results show that the thickness of the heat-dissipation structure can be reduced by more than 35%, and the achieved thermal resistance can be effectively improved over 40%. Based on appropriate approaches from the 3-D numerical simulation for thickness-adjusting evaluation and the analytical analysis for finger-pitch optimization, a highly-compact packaging design is proposed for the miniaturization of collector-up HBTs in future mobile communication systems. (C) 2010 Elsevier Ltd. All rights reserved.