An optimisation of some growth parameters for the epitaxy of AlGaN-GaN based heterostructure field effect transistors (HFET) at low pressure in a new 3 * 2 " MOVPE reactor is presented. Some possible processes for the growth of semi-insulating buffers have been identified and are described. TEM analysis shows that the insulating character is not due to a high density of dislocations, whereas SIMS analysis shows that classical impurity (Si, O and C) concentrations are in the same range as in conductive undoped layers. Further studies are needed to identify the traps responsible for the compensation of the GaN layers. The properties of the two-dimensional electron gas (2DEG) located at the AlGaN-GaN interface can be tuned by modifying the characteristics of the AlGaN layer and of the insulating buffer. The best mobility (1500 cm(2) V-1 s(-1) for n similar to 6 x 10(12) cm(-2)) is obtained when using a thick buffer layer, whereas the sheet carrier density is found to increase with the Al content in. the undoped supply layer and reaches 1.1 x 10(13) cm(-2) for a composition of 24%.