VLPC's are impurity band conduction silicon diodes, designed to convert single photons in to many thousands of electrons with high quantum efficiency. In contrast to standard avalanche photo diodes, only one of the two carrier types participates in carrier multiplication, which takes place across a band gap of only 50 meV, thus reducing gain dispersion considerably. The small gap is due to the formation of an impurity band 50 meV below the conduction band, created by a high donor concentration (ca. 10**17/cm**3) and low counterdoping (ca. 10**15/cm**3) ) in the p-layer of the diode.
A photon enters the VLPC through an antireflection coating and a transparent anode metalization, and creates an electron-hole pair across the standard valence-to-conduction band gap in the intrinsic (non-doped) zone. The hole drifts through the high field depletion region (`gain layer') and into the impurity band p zone (`drift layer'). Upon collision with a neutral donor, it frees an electron which starts the electron avalanche by impact ionization on neutral donor impurities. Due to space charge effects (created by the slowly drifting cloud of D+'s) the avalanche process is self-limiting, and the gain saturates at a few 10**4. The precise value depends on the layer geometry and dopant concentrations.
A VLPC differs from a Solid State Photomultiplier (SSPM), its infrared-sensitive predecessor, only in so far as there is no need for an extended impurity band layer beyond the gain region. In the SSPM, infrared photons are converted in this layer. In the VLPC only a small `spacer region' of highly doped material is necessary to prevent break-through of the positive space charge to the buried cathode contact, which consists of the substrate silicon doped beyond the semiconductor-metal transition.