© 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.In this study, p-NiO/n-GaAs heterojunctions were fabricated using the thermal evaporation method that least affects the physical properties of the target sample. For the n-GaAs  crystal, all the cleaning process was carried out according to the conventional procedures. After making an ohmic contact with Au (99.9%) metal on the back mat face of the n-GaA single crystals, NiO was thermal evaporated on the front polished face of the crystal. Optical transmittance values of the NiO thin films were determined by a UV–Visible (UV–Vis) spectrophotometer. The optical band gaps of non-annealed and annealed NiO thin films were determined as 3.54 and 3.48 eV, respectively. XRD, SEM, EDX and AFM measurements were also performed to examine the effect of annealing in NiO thin films. Current–voltage (I-V) measurements of NiO/GaAs heterojunctions were made to determine photoelectric and electrical properties under illumination and in the dark. The energy dependence of interface states (Nss) were extracted from the I-V measurements by assuming the voltage dependence of the barrier height Фb and the ideality factor n. The NiO/GaAs contacts were found to have a good rectifier and photodiode properties from the I-V graphics.