For many years, HBTs have been vertically and laterally scaled down to improve high-frequency performance. For the very small devices of recent process, some parameters cannot be scaled down properly and an alternative scaling-law is required. In this paper, we describe the optimization issues for high-speed InP/InGaAs SHBTs and offer a design guideline to accommodate the scaling limit. From a 0.25 mu m SHBT designed by the scaling law, the maximum extrapolated f(max) of about 687 GHz with f(T) of 215 GHz can be achieved. We also investigate the effect of key geometrical parameters such as emitter geometry and base/collector layer thicknesses on the device RF performance. (c) 2006 Elsevier Ltd. All rights reserved.