Al_0.85Ga_0.15N/Al_0.70Ga_0.30N High Electron Mobility Transistors with Schottky Gates and Large On/Off Current Ratio over Temperature

Abstract

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 ...

Operation Up to 500 °C of Al0.85Ga0.15N/Al0.7Ga0.3N High Electron Mobility Transistors

“…The devices were characterized by analyzing their mobility, subthreshold slope, peak saturation current, and other relevant device parameters. These results further extend previous results on device operation to 200 • C and demonstrate the excellent performance of Al 0.85 Ga 0.15 N/Al 0.7 Ga 0.3 N HEMTs in the extreme temperature regime [9].…”

“…While the calculated thermal conductivity of AlGaN is low, by using AlN as the growth substrate, device self-heating can be reduced. From a simple consideration of the physical properties, AlN-based devices may appear to be superior; however, they have particular difficulty with the formation of low resistance Ohmic contacts, thus the use of AlGaN alloys is beneficial [6], [28], in particular, AlGaN-channel HEMTs grown on AlN benefit from low leakage current, high ON/OFF ratio, and low subthreshold swing due to the commensurate growth structure [6]- [9].…”

Ultra-wide band gap AlGaN polarization-doped field effect transistor

“…Despite achieving ohmic contacts, the I ds,max value for the PolFET is lower than the best previous reports for UWBG AlGaN HEMTs 2,3,9,10) and MESFETs. [14][15][16] Lower I ds,max for the PolFET is due in part to a larger source-to-drain spacing but primarily to a lower Al contrast in the heterostructure that results in a higher R sh and smaller knee voltage.…”

“…7) Similarly, the Johnson figure-of-merit (JFOM) increases linearly with E crit , i.e., increases as E 2:5 g . Al x Ga 1−x N with x > 0.7 is an attractive UWBG semiconductor alloy for power electronics and rf applications because it combines many desirable electronic properties including large E crit = 10 MV=cm, 8) electron mobility (μ) > 284 cm 2 9,10) high free electron sheet charge (n s ) > 10 13 cm −2 , 9) high saturation velocity (v sat ) = 2 × 10 7 cm=s, 11) and large Schottky barriers (ϕ b ) > 3 eV. 6,12,13) However, achieving ohmic contacts and simultaneously high μ and sheet density (n s ) remain significant challenges for AlGaN transistors.…”

“…18,19) Such low μ is a concern for rf devices because electrons might not achieve v sat when transiting a very short gate length (L g ). UWBG AlGaN high electron mobility transistors (HEMTs) 2,3,9,10) have also been reported with μ ≥ 284 cm 2 V −1 s −1 , 9,10) but formation of ohmic contacts on the fully depleted AlGaN barrier remains elusive. Resistive contacts cause a voltage offset in the linear region of HEMT operation that greatly increases the on-resistance (R on ) for a high voltage switch.…”