Threshold adjustment of N-channel enhancement mode FETs by ion implantation

Ion implantation of¹¹B⁺directly into the channel region was used to adjust the threshold voltage of n-channel enhancement mode FETs. Implantation was performed through a 500Å gate oxide grown during a standard four-mask FET process. Threshold voltage shift was determined as a function of implant dose and energy by measuring the difference in threshold voltage between implanted and unimplanted devices on the same wafer. Threshold voltage shift varied sublinearly with implant dose. These are believed to be the first results reported accurate enough to show this effect. Threshold voltage shift for a given dose is also reduced as implant energy (or channel depth) is increased. These results are easily understood in terms of a simple model in which the collapse of the depletion region at inversion, due to the additional doping in the channel, partially compensates for this additional doping. Data are compared with a numerical solution that for the first time fully accounts for the nonuniform doping profile, with excellent agreement.