- AlGaN/GaN high electron mobility transistor, AlGaAs/InGAs/GaAs pseudomorphic HEMT, and InAlAs/InGaAs metamorphic HEMT (MHEMT) epitaxial structures have been characterized using surface photovoltage spectroscopy. The effects of the transistor top and bottom delta-doping levels δtop, δbot, and surface charge Qsur on the spectrum features have been studied using numerical simulations. Based on the latter, an empirical model has been developed, which allows extraction and comparison of δtop, δbot, and Qsur and is applicable for both double-sided and single-sided delta-doped structures. Prediction of the final device performance by the model is shown for two MHEMT structures. Devices produced on these structures show maximum drain currents, which correlate well with δtop values calculated using the model. © 2003 American Institute of Physics.