Single-Photon Avalanche Diode (SPAD)

 SPAD photo detectors are the fundamental building blocks from which Proteus lung imaging sensors are built. The SPAD acts as an ultra-sensitive optical detector capable of resolving photon arrival times to picosecond timing precision.

The Single Photon Avalanche Diode (SPAD) is a photodiode with a unique physical structure allowing it to operate beyond its breakdown voltage, in what is called the Geiger-mode of operation. In this state, the SPAD is functioning in a semi-stable manner where it produces no output until it is triggered by an event, or the detection of a photon. Detection involves absorption of the photon resulting in creation of a mobile electron-hole pair.

Once a photon has been detected, the SPAD loses its stability and a charge buildup begins through an effect known as impact ionization: the mobile electron and/or hole may be accelerated to high enough speeds to knock other bound electrons free, creating more free-electron-hole pairs (i.e. more charge carriers), increasing the current and leading to an electron-hole avalanche. This rapid increase in current results in an almost instantaneous electrical signal that can be sensed or processed by electronic circuitry. After the detection of a photon, the SPAD recovers back to its initial point of operation with the assist of a simple electronic circuit known as the quench element, where the device is ready to detect the next arriving photon.

This behaviour highlights the two most important properties of SPAD devices. First, their single photon sensitivity as the detection of a single photon generates a discernible electrical signal representing its capture. Second, the abrupt nature of the generated electrical signal which encodes the exact moment of arrival of the photon to within tens of picoseconds of uncertainty. The large voltage pulse generated by the SPAD provides the trigger for time-to-digital conversion and histogramming functionality in the pixel. Accumulating signals from many pixels in parallel yields microscopic images of lung cells to aid diagnosis of lung conditions.