It is demonstrated that Be ion implantation using a focused ion beam (FIB) can be used for the fabrication of fine feature size two-dimensional electron gas devices. A careful investigation to reduce the width of the FIB-induced insulating region was performed by varying the dose of the implantation. The narrowest constriction obtained using 200 keV Be2+ was 0.5 mum, smaller than those fabricated using 100 keV Ga+. Numerical simulations of the band diagram and charge distribution of the implanted structure were done to describe this behavior. A comparison with experimental results indicates the presence of channeling for Ga implantation and the existence of a mechanism that is more than compensation of donors for the creation of barriers induced by Be implantation.