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Abstract Al-rich AlGaN is a strong candidate for high-voltage power electronics, yet high-performance [>500 MW cm⁻²Baliga figure of merit (BFOM)] passivated AlGaN channel transistors on cost-effective sapphire remain limited. We demonstrate passivated, field-plated Al_0.6Ga_0.4N HEMTs achieving 2.33 kV breakdown with >2.2 MV cm⁻¹average breakdown field. The device demonstrates 8.32 mΩ·cm²on-resistance and a record 654 MW cm⁻²BFOM (>1 kV). With ∼5.5 μm gate–drain spacing, devices reach >2.5 MV cm⁻¹and 556 MW cm⁻²BFOM, highlighting excellent field management and strong potential for scalable, cost-effective sapphire-based platforms. 

Ultra-wide bandgap (UWBG) Al0.65Ga0.35N channel high electron mobility transistors (HEMTs) were deposited using a close-coupled showerhead metal-organic chemical vapor deposition reactor on AlN-on-sapphire templates to investigate the effect of transport properties of the two-dimensional electron gas (2DEG) on the epitaxial structure design. The impact of various scattering phenomena on AlGaN channel HEMTs was analyzed with respect to the channel, buffer, and AlN interlayer design, revealing that the alloy disorder and ionized impurity scattering mechanisms were predominant, limiting the mobility of 2DEG up to 180 cm2/Vs for a sheet charge density of 1.1 × 1013 cm−2. A surface roughness of <1 nm (2 μm × 2 μm atomic force microscopy scan) was achieved for the epitaxial structures demonstrating superior crystalline quality. The fabricated HEMT device showed state-of-the-art contact resistivity (ρc = 8.35 × 10^−6 Ω · cm2), low leakage current (<10^−6 A/mm), high ION/IOFF ratio (>10^5), a breakdown voltage of 2.55 kV, and a Baliga's figure of merit of 260 MW/cm2. This study demonstrates the optimization of the structural design of UWBG AlGaN channel HEMTs and its effect on transport properties to obtain state-of-the-art device performance. 

Abstract High voltage (∼2 kV) Al_0.64Ga_0.36N-channel high electron mobility transistors were fabricated with an on-resistance of ∼75 Ω. mm (∼21 mΩ. cm²). Two field plates of variable dimensions were utilized to optimize the breakdown voltage. The breakdown voltage reached >3 kV (tool limit) before passivation however it reduced to ∼2 kV after Si₃N₄surface passivation and field plate deposition. The breakdown voltage and on-resistance demonstrated a strong linear correlation in a scattered plot of ∼50 measured transistors. The fabricated transistors were electrically characterized and benchmarked against the state-of-the-art high-voltage (> 1 kV) Al-rich (>40%) AlGaN-channel transistors in breakdown voltage and on-resistance, indicating significant progress. 

In this work, we demonstrate an Al‐rich AlGaN channel metal insulator semiconductor HEMT device operating at >2 kV on a sapphire substrate. Devices with 2and 2.6 kV breakdown voltage demonstrated a Baliga figure of merit (BFOM) of 288 and 325 MW/cm²with an on‐resistance of 14.15 and 21.02 mΩ.cm², respectively. Reported BFOM of 325 MW/cm²value is 1.7 times the state‐of‐the‐art passivated AlGaN channel transistors on sapphire substrate. The obtained results represent the potential of the ultra‐wide bandgap (UWBG) AlGaN channel transistors for advanced power electronics applications. 

ScAlMgO4 (SAM) is a promising substrate material for group III-nitride semiconductors. SAM has a lower lattice mismatch with III-nitride materials compared to conventionally used sapphire (Al2O3) and silicon substrates. Bulk SAM substrate has the issues of high cost and lack of large area substrates. Utilizing solid-phase epitaxy to transform an amorphous SAM on a sapphire substrate into a crystalline form is a cost-efficient and scalable approach. Amorphous SAM layers were deposited on 0001-oriented Al2O3 by sputtering and crystallized by annealing at a temperature greater than 850 °C. Annealing under suboptimal annealing conditions results in a larger volume fraction of a competing spinel phase (MgAl2O4) exhibiting themselves as crystal facets on the subsequently grown InGaN layers during MOCVD growth. InGaN on SAM layers demonstrated both a higher intensity and emission redshift compared to the co-loaded InGaN on GaN on sapphire samples, providing a promising prospect for achieving efficient longer-wavelength emitters.