In situ focused ion beam doping during molecular beam epitaxial growth allows the fabrication of novel semiconductor structures and devices otherwise unobtainable using standard lithographic, planar processing. In this technique, the thermal dopant effusion cell used in conventional GaAs molecular beam epitaxy (MBE) is replaced by a scanning Si source focused ion beam column. It is therefore possible to write dopant patterns directly into the semiconductor wafer during crystal growth. Precise control over the elemental composition in the growth (z) direction afforded by MBE is thus combined with the high spatial resolution of the focused dopant ion beam in the lateral try) plane. High quality bulk doped GaAs was grown with a Si ion landing energy of 100 eV using this technique, attaining a peak mobility of 3400 cm(2) V-1 s(-1) (185 K) at a carrier concentration of 1.1 x 10(17) cm(-3). The dependence of free carrier concentration in GaAs as a function of incident ion energy, and the fabrication of GaAs/AlGaAs modulation doped heterostructures will be discussed.