The effects of the predominant residual impurity boron on the electrical properties of In‐doped, dislocation‐free, semi‐insulating GaAs crystals are investigated. Crystals are grown from various arsenic‐rich melts using the vertical magnetic field applied, fully encapsulated Czochralski technique. It is found that carrier and neutral EL2 concentrations in as‐grown crystal and sheet carrier concentration in the Si‐implanted active layer decrease as the boron concentration increases. The effect of boron decreases as the melt composition becomes more arsenic rich. The results suggest that boron decreases the Ga vacancy concentration.