The technique of spatially resolved photoluminescence (PL) spectroscopy was used to determine the local channel temperatures on GaAs/GaInAs/GaAlAs pseudomorphic high electron mobility transistors. By focusing a laser beam onto the different regions of the DC-biased transistor, it is shown that the channel temperature can be determined from the energy shift of one of the peaks in the PL spectra, with a spatial resolution of about 1 µm and a temperature resolution in the order of 1 °C. In particular, an asymmetry in the temperature distribution between the drain and source sides is observed. Using this approach, detailed temperature maps of the devices were obtained, as a function of the gate-source voltage VGS. These experimental temperature values are also compared with predictions derived from an analytical model of the thermal resistance in these devices.