Silicon is an amphoteric impurity of gallium arsenide that is present in different defect configurations. This work reports on the effects of heat treatment on defect reactions of heavily silicon‐doped gallium arsenide crystals. The distribution of segregation, lattice parameter, and electrical properties was studied for several heat treatments. When the crystals were bulk‐annealed at between 700 °C and 1000 °C for 20 h, zone‐distributed segregation was observed at the area with a silicon concentration of about 1×1019cm−3by etching and x‐ray topography. Regions observed to have silicon‐related segregation exhibited a decrease in lattice parameter when the crystals were annealed below 850 °C for 20 h then quenched. In addition, such regions exhibited little change in lattice parameter when the crystals were annealed above 850 °C. For electrical properties, the largest decrease in carrier concentration and mobility of such regions was observed when the crystals were annealed at 700 °C. This change in the characteristics of such regions seems to be affected not only by heavily doped silicon but also by excess arsenic.