Avalanche Photo-Diodes

Silicon APDs are a detector that can combine the advantages of solid-state photodetection with those of traditional photomultiplier tubes (PMTs). APDs are diodes that are operated with a relatively high reverse bias. When the electric field across the APD is sufficiently strong, primary generated electrons, created by incident photons or charged particles, will accelerate with enough energy and collide with bound valence electrons with sufficient energy to liberate them. This process is known as impact ionization. These electrons will then gain sufficient kinetic energy from the electric field to cause further impact ionizations, creating an avalanche of electrons. The net effect creates an amplification or “gain” to the APD electrical signal.

Silicon 9 x 9 mm2 APD

RMD’s APDs can achieve and sustain a high gain (500 to 1000) because they are fabricated using a deep diffused planar manufacturing process and, like other solid state devices, RMD’s APDs have a high quantum detection efficiency (peak efficiency ~ 70%), a wide spectral response, and are compact and rugged. RMD has the capability to produce discrete APDs ranging in area from 3 mm2 to 10 cm2 as well as monolithic, multi-element APD arrays having up to 28 x 28 elements. Discrete APDs can be used for spectroscopic and timing applications. When tiled to form arrays, they can be used for imaging applications as well.

Silicon 8 x 8 APD Array

Basic PSAPD Fabrication and Position Decoding

RMD also can produce monolithic position sensitive APDs (PSAPDs) that possess all of the attractive performance qualities of discrete APDs while also providing imaging capabilities. PSAPDs are fabricated using the same process as APDs, however, they incorporate a resistive layer on the back-side of the detector on which a four-corner anode design allows the extraction of position information based on charge sharing amongst the anodes. The back-side of the PSAPD consists simply of four anodes placed in each corner of the resistive sheet. Accurate X-Y position information can be obtained by comparing the signal measured at each corner anode. In this way, these devices produce four position-related signals that vary in a continuous manner for events across the sensing surface of the PSAPD. As a result, a large imaging area can be decoded from just five outputs (one for top cathode contact to gather energy and timing information, and four bottom corner anodes to generate position information). RMD can produce 4 x 4 mm2 up to 28 x 28 mm2 PSAPDs. The reduction in the number of electronic channels, compared to an APD array, makes PSAPD-based imaging systems much more attractive in terms of cost.

Silicon 14 x 14 mm2 position sensitive APD