Technology focus: GaN HEMTs

has been developing III-nitride double heterostructures (DHs) with indium gallium nitride (InGaN) channels with a view to high-electron-mobility transistors (HEMTs) [Yi Zhao et al, Appl. Phys. Lett., vol105, p223511, 2014]. The resulting structures boast the highest reported mobility for InGaN channels and superior transport at high temperature, according to the research team. Nitride semiconductor HEMTs are being developed for microwave power and power switching applications. High-temperature operation could reduce expensive thermal management measures such as cooling or the need for high-thermalconductivity substrates. Although single heterostructures (SH) with just a barrier on a buffer/channel layer can achieve higher mobility, high-frequency transistors with short gates fabricated from such materials can suffer short-channel effects that reduce the actual performance far from the ideal. Double heterostructures add a back barrier to block off leakage currents and improve carrier confinement. The DH formation can also reduce short-channel effects such as draininduced barrier lowering (DIBL) and low punch-through voltages. The samples (Figure 1) were grown on c-plane sapphire through metal-organic chemical vapor deposition (MOCVD). The transition AlGaN layer was compositionally graded with increasing aluminium content to avoid the formation of a parasitic two-dimensional electron gas (2DEG) in the GaN buffer beneath. The composition grading was also designed to improve the crystal quality of the back-barrier layer. The carrier gas of the precursors into the MOCVD chamber was changed from hydrogen to nitrogen after the back-barrier was deposited. The AlN interlayer between the channel and top barrier was designed to suppress alloy scattering from the InAlN. Reduced electron scattering should improve carrier mobility. The top InAlN barrier was grown using pulsed MOCVD, alternating the injection of metal-organic precursors and ammonia nitrogen source. X-ray and photoluminescence analysis suggested that the AlN mole fraction of the AlGaN bottom barrier was Technology focus: GaN HEMTs