Despite massive improvements in solid-state light sensors in recent years, the detection of extremely low light levels has remained stubbornly resistant to the incursion of solid-state devices. The problems have been how to deal with the excessive dark count once you integrate the photodiodes into a circuit and how to reduce the cost of the specialized processes that the diodes require. In a paper from last month’s IEEE Nuclear Science Symposium and Medical Imaging Conference, researchers from Philips Electronics claimed to have solved both problems.

Whenever a photon strike or a tunneling or thermal-energy event generates an electron-hole pair, the resulting tiny current pulse goes directly to a high-sensitivity inverter and becomes a digital signal for a counter. The Philips team puts an inverter on each diode and reduces the dark count by disabling any diode/inverter pair that shows excess pulses. An additional circuit on each pair quenches the avalanche photodiode after it has detected a photon and then precharges it to increase its sensitivity for the next arrival, substantially reducing operating power. Philips also integrates the diode, which requires a reverse-bias voltage of approximately 30V, with a low-voltage CMOS process.