The thermal coupling between emitter fingers in a state-of-the-art InP DHBT process technology is characterized using a novel test-structure. It is discussed how thermal coupling impacts multi-finger transistors. A simple model describing the coupling effect is incorporated into a scalable HICUM/L2 model. It is shown how the model can be utilized to design thermally optimized InP HBT structures so as to accurately reproduce characteristics of a multi-finger transistor.