Electrical and optical properties of area-variable varactor diode and GaAs MESFET with Bragg-reflector buffer

Abstract

A novel area-variable varactor diode was proposed to obtain various C-V characteristics on a single wafer. The capacitance ratio could be controlled by the area ratio of the layout pattern. The quasistatic capacitance ratios were well matched to those of the layout patterns. And its microwave characteristics and photoresponses were measured and the capacitance ratio at 1 GHz was 18.5. The varactor diode showed large capacitance variations under photo-illuminations. The capacitance variation increased with the input light power and its maximum change of 450% was demonstrated. As a result, the area-variable varactor diode can be utilized in optically-controlled microwave circuits. New photosensitive MESFET with the Bragg reflector buffer (P-MESFET) was proposed to increase optical coupling efficiency between the incident light power and the active channel layer. The P-MESFET designed for 780 nm light absorption was fabricated and characterized. The P-MESFET gave larger increase of the drain current at the gate bias voltages not too close to the pinchoff voltage and smaller threshold voltage shift than the conventional MESFET. With the oblique incidence of the optical signal instead of the conventional normal incidence, the P-MESFET could be fully utilized and the change of the drain current increased by more than 350%, compared with the change of the drain current of the conventional MESFET less than 200%. The dependence of the drain current on the incident optical power was measured and the P-MESFET showed greater sensitivity to the optical signal. Under steady illuminations, RF performances of the conventional MESFET and the P-MESFET were compared and the resistance elements of the P-MESFET showed a little more decrease than the conventional MESFET. As the optically-controlled semiconductor device with improved optical coupling efficiency and larger sensitivity, the P-MESFET is expected to be widely used in the applications such as high gain optical detection ...