Surface states affect the operation of AlGaN/GaN high electron mobility transistors (HEMT). Undesired effects as current collapse, threshold voltage shifts, and other dispersion effects have been attributed to trapping of charges that originate from surface states.1 Therefore, monitoring of surface state distribution could be of great value to the device engineer. To this end, the commonly employed methods are the photo-assisted capacitance-voltage (CV) and deep level optical spectroscopy (DLOS).2,3 However, the spectral distribution of surface states in the AlGaN/GaN HEMT is still not fully clear. Here, we present a simple electro-optical alternative, which provides a high-resolution result. Our method is based on optical excitation of electrons trapped in surface states using subbandgap monochromatic light. Electrons excited over the AlGaN CBM are swept by the strong built-in electric field into the 2DEG. This results in an increase in the channel conductivity, which is observed as channel photocurrent (PC). The higher the energy of the impinging photon, the deeper are the depopulated surface states with respect to the conduction band minimum (CBM). As the photon energy is scanned over the measurement range, the difference in the photocurrent between two sequential photon energies is proportional to the charge density trapped in surface states at an energy corresponding to the photon energy used (Fig. 1). Using a constant photon flux, and assuming a uniform capture cross section, the photocurrent spectrum should be proportional to the cumulative distribution function of the surface state charge. The first derivative of the photocurrent should, therefore, be proportional to the energy distribution profile of the surface state charge. To maintain the transistor in its original state, the measurement is carried out under small perturbation conditions, i.e. low illumination power. However as a result, the surface states cannot be fully depopulated, and this renders the profile qualitative and only proportional to the real distribution. On the other hand, when a set of spectra are measured under varying gate voltages, our method can profile the exact energy range of surface charge being depopulated at each gate voltage step.