It is shown that the use of high-temperature AlN/AlGaN buffer layers grown by NH3-MBE at extremely high temperatures (up to 1200 degrees C) allows one to improve drastically the structural quality of topmost GaN layer. The influence of an ammonia flow in the temperature range 1000-1200 degrees C was investigated. AlN surface with atomically smooth terraces and RMS below 1 nm has been achieved. This approach along with using Ga as a surfactant during the buffer growth allows one to increase the 2DEG mobility in GaN/AlGaN up to 2000 cm(2)/V.s.