A comparative study of semi-insulating GaAs substrate, p-Al<sub>x</sub>Ga<sub>1-x</sub> As/ semi-insulating GaAs and p-GaAs/p-Al<sub>x</sub>Ga<sub>1-x</sub>As/ semi-insulating GaAs structure has been done using the surface photovoltage (SPV) spectroscopy in metal–insulator–semiconductor (MIS) configuration. Which space charge region (SCR) dominated contribution to SPV in a certain wavelength range was determined. The SPV signals were calculated in a similar way as the open circuit voltage of an illuminated photodiode. One-dimensional continuity equations was adopted for determine the distribution of excess
minority carrier. The ideality factor of MIS configuration was investigated in air ambient. The contributions for SPV
signal of different layers were discussed in detail. At last the minority carrier diffusion length of different layers and
surface or interface recombination velocity were simulated.