Effects of deep ion implantation on the characteristics of the short channel n-MOSFET have been investigated by two-dimensional numerical analysis and verified experimentally. By the analysis, it has been found that the anomalous drain current which flows in the relatively deep region between the source and the drain has been effectively suppressed by the deep ion implantation of acceptor impurities into the channel region. The structure of a short channel n-MOSFET with a deep ion-implanted layer has been optimized by computer simulation to suppress the anomalous drain current. Experimentally, the low and steep subthreshold current characteristics have been obtained by deep ion implantation for short channel n-MOSFETs with L//E//F//F equals 1. 2 mu m. L//E//F//F is the metallurgical channel length. Furthermore, the back gate bias dependence of the threshold voltage of the implanted short channel device can be made almost like that of the unimplanted long channel device. The text of this paper is in digest form.