The growth of ‘thick’ (200 nm) AlN layers on sapphire at 1100–1150°C using STE3N MBE system is shown to be the key step to obtain high quality GaN-based heterostructures. An appropriate sequence of AlGaN transition layers grown on such an AlN both allows to reduce dislocation density in GaN down to (9–10)·108 cm–2 in comparison with GaN layers grown on ‘thin’ (10 nm) low temperature AlN nucleation layer. Maximum electron mobility in 1.5 µm thick GaN silicon doped layer reaches 600–650 сm2/(V·s) at electron concentrations (3–5)·1016 cm–3.