Focused ion beam doping during molecular beam epitaxial (MBE) growth is a novel technique that allows the in situ fabrication of unique three-dimensional semiconductor structures with doping profiles unobtainable using standard planar lithography. Conventional MBE growth uses a thermal Si effusion cell as the dopant source during two-dimensional layer growth of III-V semiconductor material. In the technique described here, a scanning Si focused ion beam (FIB) has been added onto the growth chamber to introduce the dopant atoms selectively in a maskless lithographic process. As the FIB is rastered in the xy plane under computer control during crystal growth in the z direction, it is possible to generate specific three-dimensional dopant patterns embedded within the semiconductor. Furthermore, the patterned semiconductor crystal requires no post growth anneal as the dopant ions are decelerated by a retarding electric field on the sample. The dopant is thus deposited on the epilayer surface rather than impinging at high energy, minimizing crystal damage. In this article, we report the successful fabrication and electrical measurements of highly doped GaAs/AlGaAs structures, directly written by focused ion MBE (FIMBE). The scanning Si FIB has been used to form both lateral and vertical, low resistance Ohmic contacts to two-dimensional electron gases at low temperatures. Lateral patterning was initially exploited to form extended contacts to an electrostatically induced electron gas. This was achieved by FIMBE using both selective modulation doping (forming two-dimensional electron gas sheet contacts to the induced gas) and selective n(+) doping directly in the quantum well. Field-effect transistor action and Shubnikov de Haas oscillations were observed, thus demonstrating that the conventionally difficult constraint of self-alignment in undoped GaAs/AlGaAs field-effect transistor structures can be eliminated. Vertical patterning was used to form degenerately doped n(+) columns embedded within a conventional high electron mobility transistor structure. The resulting I-V characteristics, recorded at room temperature and at 1.5 K in the presence of a 5 T magnetic field, will be discussed. The ability to introduce specific doping within a structure opens the possibility of forming three-dimensional integrated conducting pathways which would not be possible by any other means.