This paper analyzes in detail the substrate enhanced gate current injection mechanism and the resulting hot-carrier degradation in n-channel MOS transistors and compares the results with conventional channel hot carrier injection mechanism. The degradation mechanism is studied for different values of substrate voltage over a wide range of channel length and oxide thickness. Stress and charge pumping measurements are carried out to study the degradation under identical bias (gate, drain, substrate) and gate current condition. The influence of device dimensions on the gate injection efficiency and hot carrier degradation is also studied. Results show that the degradation under negative substrate voltage operation is strongly dependent on the transverse electric field and spread of the interface trap profile. The possible mechanism responsible for such trends is discussed. It is also found that, under identical gate current (programming time in flash memory cells), the degradation is less for higher negative substrate bias, which is helpful in realizing fast and reliable flash memories.