Influence of Pinhole-Type Defects in AlGaN on rf Performance of AlGaN/GaN HFETs Grown by MOCVD

Growth pressure-dependent generation of trap states in AlGaN/GaN heterostructures was analysed. AlGaN/GaN heterostructures were grown on sapphire substrates at two different pressures of AlGaN layer growth, 150 and 200 Torr, with other growth parameters unchanged. The measured Al mole fraction in AlGaN layer by XRD was 42 and 25%, respectively. The fabricated devices using AlGaN/GaN heterostructures exhibited quite different rf performance. The device with 25% Al mole fraction showed a lower maximum drain dc current, but more than two times higher rf output power density. The results of a pulsed I-V measurement revealed that severe trap-related rf current collapse existed in the devices with 42% Al mole fraction. These trap states are believed to be generated from pinhole-type defects as shown in the AFM surface image of AlGaN/GaN heterostructure with Al mole fraction of 42%.