Ionization rates in GaAs are determined from the measurement of photocarrier multiplication. Pure electron and hole initiations are achieved by using the novel crater mesa structure and appropriate optical‐injection wavelengths. The ionization rates for holes are greater than that for electrons except at highest fields. This agrees with the studies of Stillmanetal., except for the individual values. The ionization rates for electrons and holes are expressed as &agr;=5.6×106 exp(−2.41×106/E) and &bgr;=1.5×106 exp(−1.57×106/E), respectively.