The mechanisms of ohmic contact formation and carrier transport of low temperature (600 GaN based high mobility electron transistors (HEMTs) are of interest for applications in high-temperature, high-speed and high-power devices owing to the wide bandgap, high saturation velocity and high critical breakdown field of GaN.1,2 To realize high performance in AlGaN/GaN and InAlN/GaN HEMTs, good ohmic contacts with low resistivity are critical. Although very low contact resistance can be achieved by means of regrowth technique, 3 alloyed ohmic contacts are still most widely used due to their fabrication simplicity and low cost. The typical ohmic metal stacks are Ti/Al based (e.g., Ti/Al/X/Au), where Ti is the bottom layer, followed by Al, a diffusion barrier layer X (where X = Ni, Mo and Ti, etc.) and a Au capping layer. [4][5][6][7][8][9][10][11] In order to realize a low contact resistivity for the aforementioned ohmic contact scheme, the metal stack is usually annealed at high temperature (∼800• C), which leads to the presence of randomly distributed TiN inclusions, so-called contact inclusions or contact spikes, formed at the metal-semiconductor interface. 8,12 The presence of contact inclusions has shown to be vital to achieving good ohmic properties by Kim et al. 13 for InAlN/GaN HEMTs, as contact inclusions provide direct electron transport paths through the InAlN barrier layer to the 2DEG channel. On the other hand, these contact inclusions can also lead to lower breakdown voltage for GaN based HEMTs. 14,15 In our recent study, 16 we have proposed a low thermal budget (≤600• C) Hf/Al/Ta ohmic contact scheme on unintentionally doped In 0.18 Al 0.82 N/GaN grown on Si substrate, which has demonstrated low contact resistivity, smooth surface morphology and metalsemiconductor interface. In addition, the In 0.18 Al 0.82 N/GaN HEMTs fabricated with Hf/Al/Ta ohmic contacts have shown significantly improved breakdown voltage, compared to devices with traditional Ti/Al/Ni/Au ohmic contacts, owing to the source-carrier-injection mechanism suppressed by the smooth metal-semiconductor interface in the former. Our low thermal budget Hf/Al/Ta ohmic contacts show that even with the absence of contact inclusions, good ohmic contact properties can be achieved. This is in contrast to the result of Kim et al., which has shown the necessity of contact inclusions to ohmic contact formation. We noted that our InAlN layer is much thinner than that of Kim et al., hence may not need the presence of contact inclusions for good ohmic contact formation. In addition, it is noted that although analytically studies on the transport mecha- * Electrochemical Society Active Member.z E-mail: a0068100@nus.edu.sg; elecef@nus.edu.sg nism of ohmic contacts with spike structures on AlGaN/GaN 17,18 and InAlN/GaN 13,19 heterostructures have been reported, similar studies for ohmic contacts with a smooth metal-semiconductor interface on InAlN/GaN have not been reported. Hence, the Hf/Al/Ta contacts can be used to understand the carrier transport mechani...