Performance improvements of AlGaN/GaN HEMTs by strain modification and unintentional carbon incorporation

An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% (μ s = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs’ reliability.