inspection by microtomography of active devices, tuning and repair of iii-v microstructures and microcircuits have been performed using the capabilities of combined scanning-electron microscope (sem) and low-voltage (0.5-20 kv) focused-ion-beam (fib), or focused-droplet beam (fdb) system. in this instrument, the ion or droplet beams are focused simultaneously on the same point of the sample as the electron beam. in situ (sem) images of the region to be machined or coated are displayed before, during, and after erosion or metal deposition. gallium and indium fib probes of 10 to 20 kv were used to cut functional parts of devices such as heterojunction bipolar transistors or field-effect transistors strong chemical contrast in the sem mode displays very distinctly the different levels of the structure. this constitutes a promising method for fast nondestructive on-line control testing of a given device. cuts of electrical connections have been realized. surface resistance effects induced by the ion impact have been observed accurate resistor adjustment has been realized by fib milling and controlled by in situ monitoring of the resistance. indium and gold droplet beams have been focused on gaas substrates. best emission and focusing conditions have been determined giving deposited domains smaller than the structures previously obtained. conducting connections have been fabricated by scanning the fdb and monitored by in situ recording the resistance during the establishment of the conducting bridges