Phosphorus (P) coimplantation raises the optimum annealing temperature, providing maximum sheet carrier concentration in Si‐implanted GaAs active layers. 3×1013cm−2coimplanted P raises the optimum annealing temperature for channel layers from 920 to 990 °C, the same temperature for contact layers. Photoluminescence measurement reveals that this is due to suppression of GaAsand SiAsacceptor generations up to about 1000 °C by P coimplantation. These features indicate that P coimplantation helps to achieve GaAs integrated circuits with high performance.