AN EXTREMELY LOW-NOISE PHOTODETECTOR BASED ON THE SINGLE-ELECTRON TRANSISTOR

We have demonstrated the use of the single electron transistor (SET) a s an amplifier for a photodetector operated at 20 mK. The unparalleled low input noise of the SET permits the detection of wry small numbers of charge carriers generated in a bulk p-type Si substrate. We presen t data showing the response of the detector when it is illuminated by extremely low levels of red light (lambda = 650 nm). Although we are n ot able to prove that individual charge carriers are being detected in our system, the magnitude of the detector response and the level of n oise in the system are in good agreement with single charge, and there fore single photon, detection. From the ''dark current'' noise of 0.06 electron/s, we estimate a noise-equivalent power NEP = 2x10(-21) W/sq uare-root Hz for infrared light with lambda = 30 mum, and from this ca lculate a detectivity D = 8x10(17) cm-square-root Hz/W.