  We present a theory of interstitial Mn in Mn-doped ferromagnetic
semiconductors. Using density-functional theory, we show that under the
non-equilibrium conditions of growth, interstitial Mn is easily formed near the
surface by a simple low-energy adsorption pathway. In GaAs, isolated
interstitial Mn is an electron donor, each compensating two substitutional Mn
acceptors. Within an impurity-band model, partial compensation promotes
ferromagnetic order below the metal-insulator transition, with the highest
Curie temperature occurring for 0.5 holes per substitutional Mn.
