This study is focused on the experimental investigation of noise at microwave frequencies for scaled gallium arsenide high-electron-mobility transistor's (HEMT's). The light activation of noise has been achieved by laser exposure in the visible range. The devices have 0.25 μm gate length and 100–200–300 μm gate widths. Their DC characteristics, linear scattering and noise parameters were measured both in dark condition and under continuous wave light exposure in the 2–18 GHz frequency range. Previous results had shown a remarkable influence on all the above-measured parameters under illumination, with a special concern for the noise performance. Therefore, the authors investigated the origin of this light-activated noise in terms of the intrinsic noise sources, by extracting a noise temperature circuit model for each HEMT.In addition, the noise model formulation based on the P, R and C as well as the K _g, K _r and K _c coefficients is used to enlighten the key aspects of the optically activated noise on the device performance. It is observed that the degradation of the minimum noise figure can be attributed to the noise coefficient R, related to the gate noise source that is strongly affected by the charge generation related to light exposure.