Focused ion beam implantation is a powerful technology for the fabrication of opto- and microelectronic devices. Optoelectronic devices like gain coupled distributed feedback lasers and nonabsorbing waveguides can be defined in semiconductor heterostructures by the band gap shift due to highly spatially resolved implantation induced thermal intermixing, Single mode emitting devices were fabricated with emission wavelengths of 1 and 1.55 mu m in the material systems GaInAs/(Al)GaAs and GaInAsP/InP, respectively. Band gap shifts of more than 65 meV could be reached in GaInAsP quantum film structures which simplifies the integration of nonabsorbing waveguide sections with, e.g., lasers, modulators, and detectors. In highly doped semiconductor layers semi-insulating areas could be defined by focused ion implantation. Depletion lengths down to 50 nm can be controlled and were demonstrated on current injection restricted resonant tunneling devices. By using this technique collector-up heterobipolar transistors were fabricated which exhibit current amplification factors up to 45.