Numerous studies have shown that the threshold voltage (Vth) of field‐effect transistors fabricated on Czochralski‐grown semi‐insulating GaAs substrates using silicon ion implantation is sometimes dependent and sometimes independent of proximity to dislocations, according to the processing method used. We present a comprehensive model that reconciles these diverse observations and semiquantitatively describes the dependence ofVthon EL2 concentration, boron concentration, silicon fluence, proximity to a dislocation, and post‐implant annealing method. Under conditions holding in most of the cited studies, silicon fluence between 2×1012cm−2and 5×1012cm−2at 60 keV followed by an anneal with a SiN cap, we find thatVthdepends on the substrate concentration of EL2 and on the concentration of the boron‐silicon acceptor complex BGaSiAs. We suggest that a nonuniform stress field across a wafer reduces the binding energy of BGaSiAscomplexes within 50 &mgr;m of a dislocation resulting in a local increase in silicon activation and a more negativeVth. When the post‐implant anneal is done under a SiO2cap we suggest that the infusion ofVGadue to the out‐diffusion of Ga eliminates the stress field nonuniformities and results inVthshowing no dislocation‐proximity effect. Under these latter conditions we find that any variation in local substrate EL2 concentration across the wafer is reflected in a variation ofVth.