Influence of barrier thickness on the high-power performance of AlGaN/GaN HEMTs

The dependence of current slump in AlGaN/GaN HEMTs on the thickness of the AlGaN barrier was observed. Power measurements on a 2 x 125 x 0.3 mum AlGaN /GaN HEMT made on Silicon Carbide (SiC) substrates with an AlGaN thickness of 10 nm gave a saturated output power of 1.23 W/mm at 8 GHz whereas a devi ce with the same dimensions fabricated on samples with an AlGaN barrier of 20 nm gave a saturated output power of 2.65 W/mm at the same frequency. RF load line measurements clearly show the reduction of RF full channel curren t as compared to de full channel current and the increase in the RF knee vo ltage compared to the dc knee voltage, with the effect being more pronounce d in thin barrier samples. Passivation improved the large signal performanc e of these devices. A 1 x 150 x 0.3 mum transistor made on AlGaN(20 mn)/GaN structure gave a saturated output power of 10.7 W/mm (40% power added effi ciency) at 10 GHz after passivation. This represents the state of the art m icrowave power density for AlGaN/GaN HEMTs. Heating of the transistors duri ng high-power operation of these devices becomes the important factor in li miting their performance after passivation.