"For many years, the semiconductor industry has relied on photodetectors for optoelectrical conversion, followed by low noise electronic amplifiers to convert optical signals into electronic signals with amplification to enable information detection and processing," explained Yu-Hwa Lo, Professor of electrical and computer engineering at the University.
The highest sensitivity has been achieved by combining an electronic amplifier with a photodetector that uses an internal amplification mechanism to balance out the thermal noise of the electronic amplifier and shot noise in the photodetector.
"Avalanche photodetectors that use impact ionisation became the devices of choice and have remained so for many decades," Prof Lo continued. However, impact ionisation has its drawbacks, so the team searched for a more efficient intrinsic amplification mechanism that would work at lower voltage and noise than the current method.
"We've discovered a far more efficient mechanism – the cycling excitation process (CEP) – to amplify the signal," Prof Lo noted.
The device primarily has a p/n junction similar to that found in a semiconductor device. "The only unique feature is that both sides of the p/n junction contain a substantial amount of counter doping – a large number of donors exist in the p-region, with acceptors in the n-region," Prof Lo explained.
Counter impurities in the compensated p/n junction enable the highly efficient amplification process. Electrons or holes crossing the depletion region gain kinetic energy and, in turn, excite new electron-hole pairs using the compensating impurities as intermediate states.
With further improvements, says the team, the mechanism can be used in a variety of devices and semiconductors.
"With an efficient gain mechanism at an operation voltage compatible with CMOS integrated circuits, it's possible to produce communication and imaging devices with superior sensitivity at a low cost," Prof Lo concluded.
