The distribution of bound state energies of self-assembled quantum dots in quantum dot infrared photodetector structure is addressed using its temperature-dependent dark currents. The temperature-dependent dark currents have been fitted by the modified Arrhenius equation, which includes the distribution of activation energies. The fitting allows us to find the standard deviation and the average activation energy. It is found that the peak of photocurrent spectra, which correspond to intersubband transition energy, is coincided with the average of the activation energies. This was never achieved with the conventional Arrhenius equation.