The characteristics and reliability of nitrided-oxide gate n- and p-MOSFET’s with less than 1 atom % nitrogen concentration in the gate films were investigated in detail. These very light nitridations were accomplished using NH3 gas at low temperatures—from 800° C to 900° C. Nitrogen concentrations as low as 0.13 atom% were successfully measured by SIMS and AES. The region of optimum nitrogen concentration for deep-sub-micron devices is discussed. We explain how good drivability and good hot-carrier reliability were attained simultaneously with a nitrogen concentration of around 0.5 atom%, which is equivalent to that of oxynitride gate MOSFETs using N2O gas. The suppression of boron penetration is also discussed. Light nitridation by ammonia gas is particularly desirable for deep-sub-micron processes because it can be accomplished at a relatively low temperature of about 900° C.