This paper reports on the LP-MOCVD growth optimisation of GaAlN/GaN heterostructures grown on silicon carbide substrates for HEMT applications, and on the first device performances obtained with these structures. The critical impact of some growth parameters on the physical properties of the GaAlN/GaN epilayers has been identified and studied using high-resolution X-ray diffraction, AFM, C- V and sonogauge measurements. SiC substrates from different suppliers were evaluated and their influence on the physical properties of the GaAlN/GaN HEMT structures were investigated. We show that the static characteristics of the devices such as maximum drain current ldss or pinch-off voltage are correlated with the nucleation layer composition ( GaN or GaAlN) of the HEMT structure and with the defect density of the SiC substrate. A maximum drain current Idss around I A/mm and a pinch-off voltage of -5 V have been measured for devices with a gate length of 0.3 mum and a GaN nucleation layer, to be compared to an Idss up to 1.3 A/mm obtained for devices with the same geometry but with a GaAlN nucleation layer. I- V characteristics measured under pulsed measurements have also evidenced a clear impact of the nucleation layer composition on the trap density in the GaAlN/GaN HEMT structure. The first devices related to HEMT wafers with GaN nucleation layers were measured at 10 GHz using a load pull system. They show a CW output power in excess of 2.8 W/mm for a gate length of 0.5 mum, while devices related to HEMT wafers with GaAlN nucleation layer exhibit output power up to 4W/mm at 10 GHz. (C) 2004 Elsevier B.V. All rights reserved.