A Ga+ focused ion-beam (FIB) technique utilizing both lithographic and nanoparticle formation processes has been introduced to create nanocrystals acting as Coulomb islands at room temperature. High-resolution transmission electron microscopy results show that the nanocrystals acting as Coulomb islands are created in the source-drain active layer by using a Ga+ FIB. The results for the drain current and the conductance as functions of the drain voltage with an open gate voltage at room temperature show a Coulomb staircase and conductance oscillations, respectively. Nanoscale particles of Al with an amorphous phase are created in the source-drain channel by the defects due to the radiation effect of the Ga+ FIB, and collisions between Ga+ ions and Al atoms produce secondary electrons, that interact with the nanoparticles, which are acting as Coulomb islands, to form the crystal phase. © 2001 American Institute of Physics.