Studies carried out on gallium nitride showed that they have a high defect density in their band gap. These defects have direct influence on the electrical characteristics of the devices, by contributing leakage currents and the trap assisted tunnel currents. In this article, we carried out numerical calculations based on the experimental data, in order to study the contribution of each component of the current. A comparison has been made between the devices based on GaN and on AlGaN. We study also the capacitance–frequency characteristics, which enable us to locate defects in the band gap. We discuss the effect of the series resistance on the current–voltage characteristics, as well as the influence of defects on the photodetector response time and on the rejection ratio.

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Based on the spectral response and I–V measurements, the physical mechanism responsible for electrical conduction and optical response have been suggested. The Schottky diode response remains quite flat for energies above the gap, an advantage in comparison with the p–i–n photodiode. The obtained result showed a high leakage resistance for Schottky photodiode explained by the presence of the defect at the metal/semiconductor interface.

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