The spontaneous emission of far-infrared radiation from a single GaN/AlGaN heterojunction under heating of two-dimensional electrons in the lateral electric field is investigated experimentally. Dependence of the hot electron temperature on applied electric field is determined with help of current-voltage characteristic analysis. It is shown that the observed radiation emission is well described in the framework of thermal emission of the hot two-dimensional electrons. Effect of nonequilibrium optical phonon accumulation on the integral intensity of radiation is discussed.