Silicon photomultipliers are nowadays considered a promising alternative to conventional vacuum tube photomultipliers. The physical mechanisms operating in the device need to be fully explored and modeled to understand the device operational limits and possibilities. In this work we study the dark current behavior of the pixels forming the Si photomultiplier as a function of the applied overvoltage and operation temperature. The data are well modeled by assuming that dark current is caused by current pulses triggered by events of diffusion of single minority carriers (mostly electrons) injected from the boundaries of the active area depletion layer (dominating at temperatures above 0°C) and by thermal emission of carriers from Shockley-Read-Hall defects in the depletion layer (dominating at temperatures below 0°C).