Large Area Avalanche Photodiodes Challenge PMTs

The mature performance of third generation devices makes these solid-state photodetectors attractive solutions in many low light applications. Many applications require a photodetector with the ability to quantitatively respond to low incident light levels. Typical examples include spectrophotometers, remote sensing LIDAR and medical imaging. For many years, only the photomultiplier tube (PMT) and PIN photodiode could provide quantitative detection over the extended visible spectrum. However, both these technologies have significant performance drawbacks. In response, the avalanche photodiode (APD) was developed to deliver some of the advantages of both devices without many of the limitations. In this article, we examine the advances made in third generation, large-area APDs, and see how these benefit a typical application. To understand the value of large area avalanche photodiodes, we must first examine the PMT and the PIN photodiode. The PMT is the older of the two, and is based on vacuum tube technology. Its method of operation is schematically illustrated in Figure 1. An incoming photon enters the vacuum enclosure through a window before impacting the photocathode. This may cause release of an electron which is then accelerated towards another electrode (or dynode) having a positive electric potential relative to the photocathode. The electron gains energy from this acceleration, so that its impact with the dynode releases multiple secondary electrons. This process is repeated through several dynodes. At the final electrode, or anode, this results in an overall gain of around one million, producing a measurable current from a very small incident photon flux.