The phenomenon of and the physical mechanisms for the generation of minority carriers in the substrate of NMOS and CMOS are studied. Secondary impact ionization is not responsible. The responsible mechanisms are hot-electron-induced photocarrier generation and, under extreme conditions, forward biasing of the source-substrate junction. The photon generation is believed to be due to the bremsstrahlurg of the channel hot electrons. A theoretical model based on the lucky electron concept and the bremsstrahlung mechanism is proposed. The calculated characteristics of photon generation agree well with experimental results. About 2 X 10-5photogenerated minority carriers are generated for every (primary) impactionization event in NMOSFET. Photocarrier induced leakage current can be fitted with either an inverse square dependence on distance or an exponential dependence with an effective decay length of about 780 μm.