Use of active loads with MSM photodetectors in digital GaAs MESFET photoreceivers

Illuminated metal-semiconductor-metal (MSM) photodetectors display a current-voltage characteristic that saturates with increasing bias voltage and resembles the output characteristics of a field-effect transistor (FET). It is shown that operating an MSM photodetector with a GaAs FET active load can produce output voltage signal swings of over 80% of the power supply voltage from less than a 1 decade change in the MSM photocurrent, which may in turn be produced by only a 0.1 mW change in the input optical power. This swing allows the circuit to be used as an extremely compact optical input to high-speed digital gate circuits without the need for any intervening amplifiers. For fully monolithic prototype optical input circuits, less than -6 dBm of peak optical input power provided noise-free switching of a standard buffered FET logic (BFL) inverter from DC up to 25 MHz