An in situ ultra-high vacuum processing technique using a direct-write focused ion beam (FIB) implantation in combination with an epitaxial regrowth by molecular beam epitaxy (MBE) is reported. The process is suitable for the realization of buried confinement structures and current blocking layers in novel devices. By using a Ga Γ· FIB process highly conducting n +-GaAs (38 f~/D) layers are converted into highly resistive regions (108 O/D), which are thermally stable up to temperatures above 500 Β°C. The ion depth distribution is investigated by photoluminescence (PL) measurements on InGaAs-GaAs multiple-quantum-well samples. Owing to channeling effects the penetration depth is drastically enhanced. Excellent regrown GaAs-InGaAs-A1GaAs MODFET layers on FIB implanted wafers are fabricated and characterized by variable-field Hall and PL measurements. 300 K two-dimensional electron gas mobilities of 6500 cm 2 V-t s 1 and carrier densities of 1.7 Γ 1012 cm -2 are achieved both on FIB implanted and non-implanted regions. These values are comparable with conventionally grown reference samples. Lateral ion straggling and depletion zone effects are investigated on planar resonant tunnelling diodes (RTDs) defined by FIB implantation. Sub-micrometer current path RTDs with a 300 K peak-to-valley current ratio above 6 are fabricated. Lateral FIB dose-dependent depletion zone effects significantly reduce the effective electrical width of the current channel.