We present a low resistance, straightforward planar ohmic contact for Al 0.45 Ga 0.55 N/Al 0.3 Ga 0.7 N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950 • C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900 • C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900 • C anneal) and a Schottky (850 • C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρ c = 2.5 × 10 −5 cm 2 , sheet resistances of R SH = 3.9 k / , and maximum current densities of 75 mA/mm (at V GATE of 2 V). Electrical measurements from −50 to 200 • C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. III-N amplifiers and power transistors are becoming increasingly important in commercial as well as in military applications, finding uses in everything from cellular base stations, to power converters, to satellites.1-2 The advent of new materials and fabrication capabilities enables further advances in this essential technology. Wide bandgap (WBG) SiC-and GaN-based devices have already improved power electronics, reducing their size and weight, compared to Si-based technologies.2 Further, increasing the Al fraction for both the channel and barrier in AlGaN high electron mobility transistors (HEMTs) has the potential to enable the production of HEMTs with higher breakdown voltages and a more attractive tradeoff between breakdown voltage and specific on-resistance, 3 thereby offering improved figures of merit. The ultra-wide-bandgap (UWBG) of AlGaN, in contrast to the WBG of GaN, also empowers devices to have enhanced temperature stability 2 and higher power density. 4 However, the advantages of higher aluminum content come with a challenge. ohmic contacts to high Al-fraction AlGaN devices are difficult to make with low contact resistance. This can be particularly challenging for contacts to HEMT structures where the 2D electron gas is contacted under a barrier layer of very high Al content AlGaN.Previous approaches to producing ohmic contacts on AlGaN HEMTs include Si ion implant under the contacts, 5,6 recessed etching 7 and selective area re-growth under ohmic contacts, 3 or planar contacts using various metallization schemes. 4,[8][9][10][11][12][13][14] Of all of these approaches, the planar, non-implanted contacts are the simplest to fabricate. A literature summary [4][5][6][14][15][16] of AlGaN HEMT contact metallurgy and the Al-fraction of the barrier and channel layers is listed in Table I. The specific contact resistance is plotted against the Al-fraction of both the barrier (Fig. 1A) and channel layers ( Figure 1B). Regardless of contact scheme, the specific contact resistance values for HEMT devices generally adhere to exponentially increasing trend l...