Simultaneous microprofiling of semiconductor free carrier, donor, and acceptor concentrations was achieved for the first time from the absolute value of the free carrier absorption coefficient and its wavelength dependence determined by ir absorption in a scanning mode. Employing Ge‐ and Si‐doped melt‐grown GaAs, striking differences were found between the variations of electron concentration and those of ionized impurity concentrations. These results showed clearly that the electronic characteristics of this material are controlled by amphoteric doping and deviations from stoichiometry rather than by impurity segregation.