Low-Light Detection With Silicon Geiger-Mode Photodiodes

By Dr. Andrew G. Stewart, Design Engineer, SensL

Silicon photodiodes operated in Geiger mode are sensitive to single photons, have higher detection efficiencies than Photomultiplier Tube (PMT) detectors, and have a low dark count rate particularly when cooled to the moderate temperatures afforded by today's Thermoelectric Coolers (TECs). Commonly referred to as a Photon Counting Detector (PCD), a Geiger-mode device is a reverse biased photodiode that operates above the breakdown voltage of the photodiode junction. Under these conditions a single carrier can initiate a self-sustaining current avalanche or multiplication process that produces an easily detectable current. The self-sustaining nature of this avalanche current means that it must then be quenched in order to reset the photodiode such that it is ready to detect the next photon. Avalanche current quenching can be achieved by the use of passive elements or active quench circuits. PCDs typically have dimensions ranging from 20 x 20µm² to 200 x 200µm².

The need to quench and reset the diode after every detected photon is one of the main limitations of photon counting detectors and restricts the maximum count rate to tens of MHz. In addition, a PCD is a binary device that is either on or off. The output is therefore independent of the number of photons that initiated an output response. A PCD therefore lacks photon number resolution and cannot distinguish between multiple simultaneous photons.

Silicon Photon Counting Detectors offer high performance photon counting with active areas from 20µm x 20µm to 200µm x 200µm for applications such as Fluorescence-lifetime Imaging, Micro-array scanning, and Time Correlated Single Photon Counting (TCSPC) applications. Silicon Photomultipliers offer photon counting in a large area format, 1mm² and 9mm², and the ability to detect multiple simultaneous photons (photon number resolution). As an analog device, Silicon Photomultiplier (SPM) detectors now rival the performance of PMT detectors and have applications in numerous fields including homeland security, nuclear medical imaging, high-energy physics, and microscopy.

SOURCE: SensL