Threshold-voltage temperature drift in ion-implanted MOS transistors

Ion implantation has been widely employed to adjust the threshold voltage of MOS transistors made on low-conductivity substrates. However, the temperature variation of device characteristics associated with the ion implantation has not been fully understood. In this paper, an analysis of the effect of the ion implantation on the threshold-voltage drift is presented, and simple but accurate design equations for predicting the temperature coefficients of the threshold voltages of implanted devices relative to those of unimplanted devices are given. Experimental results from four basic types of devices exhibit close agreement with theory. The analysis presented here is directly applicable to the design of an on-chip NMOS voltage reference based on the threshold voltage difference due to the ion implantation.