The evolution of the sheet resistance (Rs) in n‐type GaAs layers during ion irradiation was studied using light mass projectiles like proton, deuterium, and helium ions at various energies. For all the cases, at the beginning of the irradiation, Rs increases with the accumulation of the dose. After reaching ≊109 Ω/☒, Rs saturates, forming a plateau. This plateau is succeeded by a decreasing of Rs with the increase of the dose, denoting that conduction via damage‐related mechanisms is taking place. The threshold dose to convert the conductive layer to a highly resistive one at room temperature or at 100 °C is found to scale with the inverse of the estimated number of displaced lattice atoms along the depth of the doped layer. Antisite defects formed by the replacement collisions are invoked to play the major role in isolation formation in GaAs by virtue of their lower sensitivity to dynamic annealing compared to other point defects. © 1996 American Institute of Physics.