Analysis of 2D Transport and Performance Characteristics for Lateral Power Devices Based on AlGaN Alloys

Abstract

Predicted lateral power device performance as a function of alloy composition is characterized by a standard lateral device figure-ofmerit (LFOM) that depends on mobility, critical electric field, and sheet carrier density. The paper presents calculations of AlGaN electron mobility in lateral devices such as HEMTs across the entire alloy composition range. Alloy scattering and optical polar phonon scattering are the dominant mechanisms limiting carrier mobility. Due to the significant degradation of mobility from alloy scattering, at room temperature Al fractions greater than about 85% are required for improved LFOM relative to GaN using a conservative sheet charge density of 1 × 10 13 cm −2 . However, at higher temperatures at which AlGaN power devices are anticipated to operate, this "breakeven" composition decreases to about 65% at 500 K, for example. For high-frequency applications, the Johnson figure-of-merit (JFOM) is the relevant metric to compare potential device performance across materials platforms. At room temperature, the JFOM for AlGaN alloys is predicted to surpass that of GaN for Al fractions greater than about 40%.

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

“…4. At moderate temperatures, less than 250 K, alloy scattering will become dominant and this confirms the previous work by Coltrin et al [5], [6]. The experimental mobility fits fairly well to the modelled mobility but begins to deviate at very high temperatures.…”

“…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].…”

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

“…2-4, they are nevertheless of interest because of their high potential based on widely used figures of merit, both the conduction loss lateral figure of merit (LFOM), and the Johnson figure of merit, for RF performance potential. 31 The LFOM for Al x Ga 1-x N improves with temperature relative to other wide bandgap materials because of the lesser degradation in μ(T) compared with AlGaN/GaN. 31 It is especially favored at high x because of its high critical electric field, estimated at E C ≥ 1.1 × 10 7 V/cm for Al 0.70 Ga 0.30 N. Table I provides a compilation of Al x Ga 1-x N-channel HEMTs with x ≥ 0.5 using various approaches to ohmic contacts for comparison to the present work.…”

“…31 The LFOM for Al x Ga 1-x N improves with temperature relative to other wide bandgap materials because of the lesser degradation in μ(T) compared with AlGaN/GaN. 31 It is especially favored at high x because of its high critical electric field, estimated at E C ≥ 1.1 × 10 7 V/cm for Al 0.70 Ga 0.30 N. Table I provides a compilation of Al x Ga 1-x N-channel HEMTs with x ≥ 0.5 using various approaches to ohmic contacts for comparison to the present work. Notably, Muhtadi et al used a Zr/Al/Mo/Au planar metal stack with x = 0.65 which resulted in a current density of 250 mA/mm at a threshold voltage of approximately −10 V and an offset voltage approaching 5 V. 9 No contact resistivity numbers were provided, although they are estimated at approximately 60 -mm for V DS >5 V from the data in the paper.…”

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

“…Al x Ga 1−x N metal-semiconductor field effect transistors (MESFETs) with ohmic source and drain contacts have been demonstrated for x = 0.65, 14) and 0.70, 15,16) but μ has yet to exceed 100 cm 2 V −1 s −1 for devices with current density >100 mA=mm. Channel μ of highly conductive UWBG Al x Ga 1−x N MESFETs is likely to remain <100 cm 2 V −1 s −1 because significant improvement requires increasing x > 0.8 to reduce alloy scattering, 5,17) but n-type impurity doping becomes inefficacious for x > 0.8. 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 ).…”