A comprehensive study of Si implants (5×1013cm−2, 150 keV) in GaAs and Al0.3Ga0.7As is made with the use of Hall‐effect measurements. A significant annealing‐temperature dependence of the electrical activation is observed. For GaAs, a clear Arrhenius plot is implied by the data, and it gives an activation energy of ∼0.79 eV. For Al0.3Ga0.7As, the electrical activation occurs dramatically from 800 °C and saturates at approximately 875 °C. Temperature‐scanned Hall‐effect measurements indicate that for GaAs the electron concentration is nearly temperature independent, which suggests highly degenerate electron statistics. For Al0.3Ga0.7As, the freezeout of electrons is an exponential function of temperature between 170 and 300 K, which provides a donor ionization energy of ∼110 meV. Low‐temperature (4.2 K) photoluminescence measurements reveal that Si‐ion implantation produces a new emission band at 1.46 eV in GaAs and strongly enhances the intensity of the 1.84‐eV band in Al0.3Ga0.7As. These bands may be due to defect‐impurity (Si) related transitions in GaAs and Si‐donor–Si‐acceptor pair recombination in Al0.3Ga0.7As.