  We provide experimental evidence that the upper limit of ~110 K commonly
observed for the Curie temperature T_C of Ga(1-x)Mn(x)As is caused by the
Fermi-level-induced hole saturation. Ion channeling, electrical and
magnetization measurements on a series of Ga(1-x-y)Mn(x)Be(y)As layers show a
dramatic increase of the concentration of Mn interstitials accompanied by a
reduction of T_C with increasing Be concentration, while the free hole
concentration remains relatively constant at ~5x10^20 cm^-3. These results
indicate that the concentrations of free holes and ferromagnetically active Mn
spins are governed by the position of the Fermi level, which controls the
formation energy of compensating interstitial Mn donors.
