Wide Band Gap semiconductors, such as SiC and GaN, exhibit many attractive properties far beyond the capabilities of Si and GaAs. GaAlN/GaN high electron mobility transistors (HEMTs) with very impressive power densities up to 11.2 W/mm at 10 GHz have been reported by Cornell, and recently up to 30 W/mm at 4 GHz by Cree Research. This rapid progress in the performance of microwave power transistors, as compared to results published in early 1998 (2 W/mm), has been obtained thanks to the use of SiC substrates instead of sapphire. In this paper, we report on a comparative study of the physical properties of GaAlN/GaN HEMT structures grown respectively on sapphire and silicon carbide substrates . The critical steps of the MOCVD growth process of GaN on SiC (substrate surface preparation, nucleation layer composition and growth parameters) are described and their effects on the physical properties of the HEMT structures and the associated device performances will presented.
On the other hand, the GaAlN/InGaN/GaN double heterostructure, in which the channel layer is made of InGaN instead of GaN, is expected to significantly enhance device performance due to a stronger carrier confinement and a higher two-dimensional electron gas (2DEG) density at the interface in comparison with an GaAlN/GaN single heterostructure. However a strong alloy disorder scattering in GaInN and a significant interface-roughness scattering of the GaAlN/GaInN heterointerface lead to low 2DEG mobility GaInN channel structure as compared to conventional GaN channel structure (Typically 1600 cm2V-1s-1). An improvement of the 2DEG mobility ( 1100cm2V-1s-1 at RT) has been recently demonstrated by using specific GaInN channel design to suppress alloy disorder scattering [1].
GaInN based channel HEMT heterostructure appears to be promising for microwave applications. A review of the main published results will be presented together with the last GaInN based HEMT developments in our laboratory.

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1. MOCVD growth of Group III Nitrides for high power, high frequency applications.