Process Dependent Build-Up of Interface States in Irradiated N-Channel MOSFETs
The build-up of fast-states (¿Dit) continues to occur in the Si/SiO2 interface region even after the irradiation has stopped. For the devices described in this paper, the process of conversion of trapped positive charges (¿Qot) into ¿Dit appears to be responsible for the build-up. The presence of ambient hydrogen is necessary for the conversion to occur in well annealed MOS devices without SiN-Caps. The decrease in the highest temperature after the source/drain implant, dictated by the demand for shallow junctions, leaves residual neutral traps in the interface region which cause increase in the rate of conversion of ¿Qot into ¿Dit. The low-field effective mobility (¿eff) of the inversion layer electrons has been characterized as a function of the gamma-radiation dose. The reduction of ¿eff due to gamma rays is predominantly attributed to the decrease in the number of carriers available for conduction because of the fast-states. A universal model of ¿eff is presented; it is a very powerful tool in studying the Si/SiO2 interface properties.