Field-effect transistors for efficient integrated optoelectronic sensors

Integrated receivers associating an amplifier with a photodiode are studied. The field effect transistor used for the amplifier has been designed and characterized. The FET tran-sistor has been fabricated on a GaAs semi-insulating substrate using Liquid Phase Epitaxy. The structure consists of two epitaxial layers, i.e. one N-GaAs layer for the channel and a P-GaAlAs layer for the gate. The design, technological process and current-voltage characteristics are described. Transconductance values over 10 mA/V have been achieved and the transistor have shown a threshold voltage of -4.6 volts. Integrated receivers whose field effect transistor is associated with a photodiode, enhance the detector sensitivity. The results measured on the device show current gain of 15 with a photodiode load resistance set to 2 kSt… Optical sensitivity and current amplification better than 15 are found on transistors with a high doping level ND. The results shown by the FET will be applied to a type of sensor presenting static optical responsivity.