A Fe–Ge alloy was used to fabricate a liquid metal ion source (LMIS) for our focused ion beam system. The iron ion and germanium ions can be utilized to create semi‐insulating regions for device isolation andn‐type doping in InP. The properties of the Fe–Ge LMIS were characterized by measuring the ion current‐extraction voltage characteristics, the mass spectrum of the ion species in the ion beam, and the stability of the source current. The changes of resistivity in InP before and after Fe++implant and after thermal annealing were measured in mesa‐etched four‐terminal test structures. The resistivity of as‐implanted samples increases about four orders of magnitude compared with original resistivity. After rapid thermal annealing at 400 °C for 10 s, the resistivity is about two times larger than that of as‐implanted sample.