In order to control the electrical characteristics of semiconductor fine structures, several tens of single-dopant ions were implanted one by one into sub-micron semiconductor regions by means of single ion implantation (SII). The flat-band voltage of the implanted test samples (a back-gate MOSFET) were measured by the extrapolation of the linear part of substrate bias (V-BG)-drain current (I-D) characteristics to V-BG axis. The flat-band voltage decreased linearly with the number of implanted ions. The linear relationship between the flat-band voltage and the number of implanted ions verifies the controllability of device characteristics with the SII. The increase in the flat-band voltage per one dopant atom has been found to be -4.5 mV/ion in this study.