This paper reports on the LP-MOCVD growth optimisation of bulk GaN, GaAlN materials and GaAlN/GaN heterostructures grown on Sapphire and Silicon Carbide substrates for MESFET and HEMT applications, and on the device performances obtained with these structures.
High purity GaN grown on sapphire and Silicon Carbide has been obtained, with carrier concentration lower than 1013 cm-3.
GaN/Al2O3 MESFETs based on such high purity GaN buffer layers, have exhibited very promising static and microwave performances : high beakdown voltage ~ 200V, ft = 12GHz, fmax = 25GHz and CW output power in excess of 2.2W/mm at 2GHz.. They have shown the best low frequency noise performances, with the lowest Hooge's parameter as compared to the values reported in the literature for different GaN- based FETs ( HFETs and HEMTs).
The critical impact of some growth parameters on the physical properties of the GaAlN/GaN epilayers grown on SiC has been identified and studied using High Resolution X-Ray diffraction (HR-XRD), AFM, C-V and Eddy current probe measurements.
The devices performances related to our first GaAlN/GaN HEMT structures grown on Sapphire and Silicon Carbide have confirmed the high potentiality of GaN and related alloys for high power microwave transistors.
Load-Pull measurements performed at 2 GHz on devices related to GaAlN/GaN/Al2O3 HEMT structures, have shown a remarkably high output power density (4.4W/mm) and absolute power level (3.5W for 1mm devices). These power level results for devices on sapphire substrates measured on wafer, are well in agreement with the international state of the art.
Nevertheless, the thermal effects, which lead to a significant reduction of the power density with the increase of the transistor size, have been found to be more pronounced for devices on sapphire.
Devices related to GaAlN/GaN/SiC HEMT structures have been measured at 10 GHz using a load pull system. They exhibited a CW output power in excess of 4 W/mm (Lg= 0.25 mm).
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