An AlN barrier high electron mobility transistor (HEMT) based on the AlN/Al0.85Ga0.15N heterostructure was grown, fabricated, and electrically characterized, thereby extending the range of Al composition and bandgap for AlGaN channel HEMTs. An etch and regrowth procedure was implemented for source and drain contact formation. A breakdown voltage of 810 V was achieved without a gate insulator or field plate. Excellent gate leakage characteristics enabled a high Ion/Ioff current ratio greater than 107 and an excellent subthreshold slope of 75 mV/decade. A large Schottky barrier height of 1.74 eV contributed to these results. The room temperature voltage-dependent 3-terminal off-state drain current was adequately modeled with Frenkel-Poole emission.
AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that are promising candidates for RF and power applications. Long-channel Al x Ga 1-x N HEMTs with x = 0.7 in the channel have been built and evaluated across the −50 • C to +200 • C temperature range. These devices achieved room temperature drain current as high as 46 mA/mm and were absent of gate leakage until the gate diode forward bias turn-on at ∼2.8 V, with a modest −2.2 V threshold voltage. A very large I on /I off current ratio, of 8 × 10 9 was demonstrated. A near ideal subthreshold slope that is just 35% higher than the theoretical limit across the temperature range was characterized. The ohmic contact characteristics were rectifying from −50 • C to +50 • C and became nearly linear at temperatures above 100 • C. An activation energy of 0.55 eV dictates the temperature dependence of off-state leakage. AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors (UWBG) that are promising candidates for power and RF applications.1-9 Their promise derives from the large critical electric field, which scales as a power law with the bandgap of the material, 10 e.g. E C ∼ E G 2.5 (the exact dependence is a topic of active research) and provides favorable power and RF figures of merit. Enhancement-mode HEMTs are required for power applications, while depletion-mode HEMTs are suitable for RF applications. Power electronics applications utilize GaN/AlGaN HEMTs with dielectric insulators as a means of achieving a large gate voltage swing for high current density and low on-resistance. The dielectric insulators also suppress gate leakage, but at the expense of substantial interface charge density and the potential for hot electrons inducing unwanted trapped charges in the dielectric. These latter effects make gates employing dielectric insulators unsuitable for RF applications with GaN/AlGaN HEMTs.Al y Ga 1-y N/Al x Ga 1-x N HEMTs with high Al in the channel, x = 0.7 and larger, have promising figures of merit and are candidates for next generation power and RF applications. 1,7,11,12 With a bandgap of >5.8 eV, the Al 0.85 Ga 0.15 N barrier is practically an insulator, but since it is combined with a modest conduction band offset to the Al x Ga 1-x N channel it has both insulator and Schottky-like properties. As a crystallographic insulator-like material, it has potential for a very good interface with nearly lattice matched Al x Ga 1-x N channel adjacent to the 2-dimensional electron gas (2DEG). Even as a Schottky barrier it can have a large turn-on voltage to enhance the voltage swing and current drive capability. Although promising, Al x Ga 1-x N channel HEMTs lack maturity and do not yet match the high current density of AlGaN/GaN HEMTs due both to the difficulty of achieving good ohmic contacts and the lack of aggressive dimensional scaling to compensate for limitations of the low-field electron mobility, which is limited by alloy scattering.In this work ...
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.