Optimisation of a carbon doped buffer layer for AlGaN/GaN HEMT devices

• We report on the incorporation of carbon impurities in GaN layers.
• We grew AlGaN/GaN HEMTs on SiC with carbon doped GaN buffer layer.
• The AlGaN/GaN HEMTs were successfully processed into transistors for high power RF.
• The devices were characterised in DC and RF electrical performances.
• The impact of carbon doping on the electrical performances is discussed.

This work reports on the optimisation of carbon doping GaN buffer layer (BL) for AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal–organic vapour phase epitaxy (LP-MOVPE) on 3 in. SiC semi-insulating substrates. The incorporation of carbon impurities in GaN is studied as a function of the growth conditions, without using an external carbon source. We observed that the C incorporation can be effectively controlled over more than one order of magnitude by tuning the reactor pressure and the growth temperature, without degradation of the crystalline properties of the GaN layers.HEMT structures with a specific barrier design were grown with different carbon dopings in the GaN BL and processed into transistors to evaluate the impact of the BL doping on the device performances. A significant improvement of the HEMT drain leakage current and of the breakdown voltage was obtained by increasing the carbon incorporation in the GaN BL. The RF performances of the devices show a trade-off between leakage currents and trapping phenomena which are enhanced by the use of carbon doping, limiting the delivered output power. An output power as high as 6.5 W/mm with a Power Added Efficiency of 70% has been achieved at 2 GHz by the HEMT structures with the lowest carbon doping in the BL.