The microwave, low-noise GaAs-metal-semiconductor, field-effect transistor (MESFET) has a cutoff frequency higher than 80 GHz and possesses a good optical responsiveness over a very wide spectral range, from visible to near-infrared wavelengths. The drain-current variation of the MESFET is proportional to the input optic intensity. If the light is modulated, MESFET detects the high-speed subcarrier. Besides the low-noise and amplification-gain advantages, the MESFET photodetector operates at a low bias voltage, normally, below 6 V. The common-gate configuration at zero drain voltage also can function in a photocapacitive mode in which the capacitance changes with the change in polarized light intensity. The S-parameter of the device has been characterized up to Ku-band. The high-frequency optical receiver is ready to be designed and optimized using this charaterization. For high-speed fiber-optic communications, the monolithic receiver, the ILD driver, and the low-cost repeater all can be fabricated by using the MESFET's.