  In the 1990s, a novel semiconductor photon-sensor operated in Geiger-mode was
invented in Russia (Silicon PhotoMultiplier), which consists of many tiny
pixels and has a single photon level sensitivity. Since then, various types of
the sensor with this scheme, Pixelized Photon Detectors (PPD), have been
developed in many places in the world. For instance, Hamamatsu Photonics K.K.
in Japan produces the PPD as a Multi-Pixel Photon Counter. While the internal
mechanisms of the PPD have been intensively studied in recent years, the
existing models do not succeeded to fully reproduce the output characteristic,
such as waveforms at low temperature. We have developed a new model with the
transient multiplication and quenching of the realistic avalanche process and
have succeeded in reproducing the output waveform of the PPD at various
temperature. In this paper, we discuss our improved model.
