We report on the low-temperature growth of heavily Si-doped (>10(20) cm(-3)) n(+)-type GaN by N-rich ammonia molecular beam epitaxy (MBE) with very low bulk resistivity (<4 x 10(-4) Omega.cm). This is applied to the realization of regrown ohmic contacts on InAlN/GaN high electron mobility transistors. A low n(+)-GaN/2 dimensional electron gas contact resistivity of 0.11 Omega.mm is measured, provided an optimized surface preparation procedure, which is shown to be critical. This proves the great potentials of ammonia MBE for the realization of high performance electronic devices. (C) 2014 AIP Publishing LLC.