Hot-carrier degradation of n-channel MOSFETs with 4 and 9 nm gate oxides is studied under intermediate gate voltage stress (V-g = V-d/2). An alternating injection method is proposed for determining the electron trapping effect on saturated drain current degradation. On this basis, an improved method for determining the critical energy for interface trap generation is developed. It can extract the critical energy more accurately than that through the traditional method. This method is applicable for n-MOSFETs with either thinner or thicker oxides under any stress mode between intermediate (V-g = V-d/2) and higher gate voltage (V-g = V-d).