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Channeled ions offer a means of patterning a wafer with a two‐dimensional electron gas (2DEG) buried deep below the surface. The implantation of 60–140 keV Si++ ions into a 580‐nm‐deep 2DEG formed at a GaAs/AlGaAs heterojunction has been characterized, with respect to ion energy, dose, and angle of incidence. Transverse electron focusing measurements have been used to investigate the roughness of the implanted boundary, leading to measured values for the specularity coefficient of about 0.5 at 1.7 and 4.2 K. Similar measurements at lower temperatures (120 mK) have shown fine structure in the magnetoresistance, in addition to the classical transverse electron focusing effect. The fine structure is attributed to electron interference effects at, or close to, the boundary. Channeled ion implantation has been used to define an in‐plane‐gated transistor which, at 300 mK, shows clear evidence of ballistic electron transport.