This work reports on the electrical (static and dynamic) as well as on the optical characteristics of a prototype matrix of Silicon Photomultipliers (SiPM). The prototype matrix consists of 4×4 SiPM’s on the same substrate fabricated at FBK-irst (Trento, Italy). Each SiPM of the matrix has an area of 1×1mm2 and it is composed of 625 microcells connected in parallel. Each microcell of the SiPM is a GM-APD (n+/p junction on P+ substrate) with an area of 40×40mm2 connected in series with its integrated polysilicon quenching resistance. The static characteristics as breakdown voltage, quenching resistance, post-breakdown dark current as well as the dynamic characteristics as gain and dark count rate have been analysed. The photon detection efficiency as a function of wavelength and operation voltage has been also estimated.