Variations of the voltage response between source and drain electrodes of an AlGaN/GaN high electron mobility transistor by the green laser light or ultraviolet (UV) illumination are studied. A subsequent green laser illumination causes the voltage between the electrodes to increase with background UV illumination. Interactions between surface states and excess carriers generated by the UV light are proposed to be responsible for the voltage increase. Excess electrons are captured by the positively charged surface states after injection into the surface region with the assistance of the green laser light and result in a reduced two-dimensional electron gas density.