GaAs pHEMT thermal reliability test structures are introduced which incorporate on-wafer heating using Thin Film Resistors (TFR) and a DC gate metal temperature measurement method. Results from 3D Finite Element Method (FEM) thermal simulations are compared with measurements and used to investigate the frequency response of device self-heating. Comparisons are made with existing thermal models. The influence of individual device structures on the thermal characteristics of an entire device is investigated and the epitaxial layers are seen to have a large impact on overall performance. Bias dependent self-heating, independent of thermal dissipation is observed and attributed to confinement of the thermal source as the drain voltage is increased.