High Temperature Operation of Al_0.45Ga_0.55N/Al_0.30Ga_0.70N High Electron Mobility Transistors

Abstract

AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that have a bandgap greater than ∼3.4 eV, beyond that of GaN and SiC, and are promising candidates for RF and power applications. Long-channel Al x Ga 1-x N HEMTs with x = 0.3 in the channel have been built and evaluated across the −50 • C to +200 • C temperature range. Room temperature drain current of 70 mA/mm, absent of gate leakage, and with a modest −1.3 V threshold voltage was measured. A very large I on /I off current ratio, greater than 10 8 was demonstrated over the entire temperature range, indicating that off-state leakage is below the measurement limit even at 200 • C. Combined with near ideal subthreshold slope factor that is just 1.3× higher than the theoretical limit across the temperature range, the excellent leakage properties are an attractive characteristic for high temperature operation. Silicon carbide (SiC) and gallium nitride (GaN) have begun to enable dramatic improvements in the size, weight, and power (SWaP) of power conversion systems 1-5 compared to Si devices. The next generation of semiconductor materials, the so-called "ultra" wide-bandgap (UWBG) materials that have bandgaps larger than that of GaN (E G > 3.4 eV) will enable the next leap forward in power electronics performance. 6 Although the large bandgaps of GaN and SiC have already suggested their use in emerging radio frequency (RF) and power applications, UWBG devices with even larger bandgaps make them more suited to these applications, particularly in harsh environments, such as at high junction temperatures. In this work we'll describe and characterize emerging UWBG AlGaN-channel high electron mobility transistors (HEMTs) spanning the −50• C to +200 • C temperature range.Transport properties are critically important to transistor operation. Transport properties of AlGaN HEMTs are influenced primarily by the electron mobility, saturation velocity, and the sheet charge present in a quantum well. The latter is quantum-well design dependent and presumed to be comparable among different alloy compositions as long as the conduction band offset is comparable. During high temperature operation, alloy scattering in ternary AlGaN alloys limits the low field mobility relative to binary alloys, e.g. GaN, at temperatures where alloy scattering is dominant (200-400 K). At elevated temperatures, beyond room temperature, optical phonon scattering increases considerably and becomes the dominant factor in determining mobility for both binary and ternary alloys. Contrary to the mobility trends with alloy composition, saturation velocity is thought to be comparable in III-N binary and ternary alloys and not extremely temperature sensitive, based on Monte Carlo simulations.7 This simplified summary of a few key transport properties suggests that AlGaN HEMTs offer possible advantageous operation at high temperature, which coupled with a high critical electric field is advantageous for these devices. Previously published characteristics ...

Al0.85Ga0.15N/Al0.70Ga0.30N High Electron Mobility Transistors with Schottky Gates and Large On/Off Current Ratio over Temperature

“…The 200 • C I on /I off ratio of 5 × 10 7 derived from Fig. 3 is excellent but still less than that in Al 0.45 Ga 0.55 N/Al 0.30 Ga 0.70 N HEMTs (hereafter termed the "45/30" HEMT), which have noise limited I D leakage of ∼ 4 × 10 −12 A even up to 200 • C. 8 The 0.55 eV activation energy of the 85/70 HEMT is interpreted as the energy separation between the Fermi level and the conduction band edge in the region of the HEMT where channel current is minimal, i.e., under the gate. Lacking gate current, off-state current is dominated by source-drain leakage.…”

“…Breakdown measurements are often reported for AlGaN/GaN HEMTs and among those with large gate-drain spacing, most range between 70-110 V/μm. • C. 8 The 0.55 eV activation energy of the 85/70 HEMT is interpreted as the energy separation between the Fermi level and the conduction band edge in the region of the HEMT where channel current is minimal, i.e., under the gate. Lacking gate current, off-state current is dominated by source-drain leakage.…”

“…[1][2][3][4][5][6][7][8][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.…”

“…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][2][3][4][5][6][7][8][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.…”

Al0.75Ga0.25N/Al0.6Ga0.4N heterojunction field effect transistor with fT of 40 GHz

“…ecently ultra-wide bandgap semiconductors such as Al(Ga)N, [1][2][3][4][5][6][7][8][9][10][11] Ga 2 O 3 , [12][13][14][15][16][17][18][19][20][21] and diamond 22,23) have attracted significant research interests for electronic device applications. Particularly, aluminum rich AlGaN which is a promising candidate as a channel material for high power mm-wave transistors because of its high breakdown field and saturation velocity.…”