J. Mc Intyre [2] was to implement a semiconductor photodiode with characteristics suitable for the triggered avalanche operation mode and therefore able to detect single photons [3-6] (whence the name Single Photon Avalanche Diode – SPAD). When a p-n junction is inversely biased 10 �� 20 % above the breakdown voltage value, a single charge carrier entering inside the high field region of the depleted volume can trigger the avalanche multiplication process (operating in Geiger mode). The fast leading edge (rise time less then 1 ns) of the corresponding current pulse can be used for detecting and timing the photo-generated carriers. In that condition, due to the ��detectable�� value of the flowing current, single optical photons can be detected.Diode current is negligible until the first carrier enters or is generated in the junction depletion layer; a suitable circuit, usually called quenching circuit (passive or active), senses the rise of the diode current and quenches the multiplication process by lowering the bias voltage down below the breakdown [17].To be used as SPAD, a diode must have a structure that fulfills some basic requirements: (i) the breakdown must be uniform over the whole active area in order to produce a standard macroscopic current pulse; (ii) the dark counting rate must be sufficiently low; (iii) the probability to generate afterpulses should be low. Operating in a dark environment, the carrier sources are essentially two: the diffusion current by quasi neutral regions, which is normally negligible [7], and the generation of electrons or holes from trap levels located in the depletion layer. In order to satisfy (ii) and (iii) prescriptions, both the effects of thermal carrier generation and trapping should be minimized.A complete characterization of the SPAD device require an estimation of some important figures of merit: dark counting rate (thermal and http://www.selleckchem.com/products/U0126.html afterpulsing components), photon detection efficiency, time resolution, maximum excess bias voltage, optimal working temperature, etc.Today modern technology gives also the possibility to produce SPAD detectors with an integrate quenching mechanism based on a Metal-Resistor-Semiconductor structure. Precise resistive elements are embedded for each individual micro-cell of the array and provide effective feedback for stabilization and quenching of the avalanche process [8]. Such technology allows the production of large numbers of micro-cells on a common substrate (with or without a read-out circuitry) in order to achieve new imaging devices [27] or high-resolution and high-sensitivity Silicon Photomultipliers (SiPM) [9,26,28]. In this paper we present the results of a characterization work on prototypes of the next generation devices.2.?The fabrication processFigure 1 (left) shows the cross-section of the SPAD structure, as manufactured by ST-Microelectronics in Catania.