We studied the dynamical behaviors of the depletion-mode AlGaAs/GaAs high-electr on-mobility transistor under optical illumination. The photovoltage effect and t he photogenerated carriers contribution to the space charge concentration were t aken into account. The pinch-off voltage, the sheet concentration of two-dimensi onal electron gas (2-DEG) located at the interface of the heterojunction, the I- V characteristic curve, and the transconductance were investigated by using the charge-controlling model. We found that the pinch-off voltage was lowered and th e sheet concentration of 2-DEG was increased because of the optical illumination , which, in turn, resulted in an increase in the current gain and the transcondu ctance of the device.