We collected data of ion-implantation profiles using Monte Carlo simulation, and showed that these profiles can readily be expressed by a joined half Gaussian function. We extracted parameters of the joined half Gaussian function from the Monte Carlo data. We also derived a model of the shift in threshold voltage caused by the penetration of ion-implanted impurities. Based on the set of profile data. we evaluated the ion-implantation conditions for As, P, B, and BF2 to suppress the threshold voltage shift.