Abstract
We investigated optical responses of high-electron mobility transistors (HEMTs) by the use of a tightly focused optical beam. Positive and negative optical responses in the HEMTs were observed according to the irradiation position of the optical beam. The positive optical response was obtained by the optical beam irradiation close to the gate electrodes of the HEMTs with appropriate DC-bias voltages. While the negative optical response was obtained by the optical beam irradiation away from the gate electrodes. The light-induced increase and decrease of the drain currents were observed over 1 mA by the irradiation of a 10 µW CW light beam. The increase and decrease by as much as 5 dB of the small signal gain of the HEMTs in the microwave frequency range were also observed. We also applied these irradiation-position-dependent positive and negative optical responses to the optical control of the microwave oscillator composed of the HEMT. Microwave oscillations at 11.7 GHz from the oscillator were switched between "ON" and "OFF" by the selective irradiation of the successive focused light pulses.