This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 times10⁶ rad of ⁶⁰Co gamma rays.