Properties of the GaN(0001) surface, fully covered by a mixture of NH3 molecules and NH2 radicals, were investigated by DFT simulations. It is shown that for extremely low ammonia content, the Fermi level at the surface is at valence band maximum (VBM), then at the 25% ammonia in coverage it is depinned, moving upward in the bandgap to reach conduction band minimum (CBM) at higher ammonia percentages. It is shown that the 25% ammonia in the coverage is corresponding to a flat band configuration in accordance with ECR arguments. The stability of the mixed NH2/NH3 coverage with respect to detachment of ammonia molecules or molecular hydrogen was also investigated, It was shown that high ammonia fraction leads to instability of the coverage due to detachment of ammonia or of molecular hydrogen. The instability point is reached for 36% and 41% fraction of ammonia in the coverage for desorption of molecular hydrogen and ammonia, respectively. The energy barrier for detachment of molecular hydrogen is consistently higher since the H-2 molecule has to be created first in the course of desorption process. The ammonia is simply detached in the molecular form so that the high energy intermediate configuration is absent. (C) 2014 Elsevier B.V. All rights reserved,