We have demonstrated the possibility ofinsitutwo‐dimensional electron gas (2DEG) fabrication in GaAs/AlGaAs heterostructures, using focused ion beam implanter and molecular beam epitaxy (FIBI‐MBE) crystal growth system. 2DEGs are formed in a GaAs channel layer which is overgrown on an AlGaAs electron supply layer implanted selectively with Si‐FIBs. Implanted Si atoms are activated by rapid thermal annealing (RTA). We confirmed 2DEG formation by Shubnikov–de Haas oscillation. We clarified the influence of ion implantation damage on the quality of overgrown epilayers and obtained an extremely high 2DEG mobility of 32 000 cm2/V s at 77 K and 48 000 cm2/V s at 20 K. We also discussed possible quantum‐size doping by direct FIB implantation, considering the lateral scattering of incident Si ions in the substrate.