It is necessary to take quantum effects into account in the design of nanometerscale high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs). To achieve this, an effective potential method is applied to drift-diffusion (DD)-based simulations of the threshold-voltage characteristics of nanometer-scale InP-based HEMTs and to ensemble Monte Carlo (EMC) simulations of transit times in InP-based HBTs. The simulated results are compared with the experimental results in order to validate the method. We conclude that device simulations incorporating the effective potential method are very useful for designing millimeter-wave HEMTs and HBTs.