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1. High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

   https://doi.org/10.1109/LED.2019.2923085  

   Hickman, Austin; Chaudhuri, Reet; Bader, Samuel James; Nomoto, Kazuki; Lee, Kevin; Xing, Huili Grace; Jena, Debdeep (August 2019, IEEE Electron Device Letters)
2. High-current, high-voltage AlN Schottky barrier diodes

   https://doi.org/10.35848/1882-0786/ad81c9  

   Quiñones, C_E; Khachariya, D.; Reddy, P.; Mita, S.; Almeter, J.; Bagheri, P.; Rathkanthiwar, S.; Kirste, R.; Pavlidis, S.; Kohn, E.; et al (October 2024, Applied Physics Express)

   AlN Schottky barrier diodes with low ideality factor (<1.2), low differential ON-resistance (<0.6 mΩ cm²), high current density (>5 kA cm⁻²), and high breakdown voltage (680 V) are reported. The device structure consisted of a two-layer, quasi-vertical design with a lightly doped AlN drift layer and a highly doped Al_0.75Ga_0.25N ohmic contact layer grown on AlN substrates. A combination of simulation, current–voltage measurements, and impedance spectroscopy analysis revealed that the AlN/AlGaN interface introduces a parasitic electron barrier due to the conduction band offset between the two materials. This barrier was found to limit the forward current in fabricated diodes. Further, we show that introducing a compositionally-graded layer between the AlN and the AlGaN reduces the interfacial barrier and increases the forward current density of fabricated diodes by a factor of 10⁴. 

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3. In Situ Crystalline AlN Passivation for Reduced RF Dispersion in Strained‐Channel AlN/GaN/AlN High‐Electron‐Mobility Transistors

   https://doi.org/10.1002/pssa.202100452  

   Chaudhuri, Reet; Hickman, Austin; Singhal, Jashan; Casamento, Joseph; Xing, Huili_Grace; Jena, Debdeep (October 2021, physica status solidi (a))

   The recent demonstration of  W mm⁻¹output power at 94 GHz in AlN/GaN/AlN high‐electron‐mobility transistors (HEMTs) has established AlN as a promising platform for millimeter‐wave electronics. The current state‐of‐art AlN HEMTs using ex situ‐deposited silicon nitride (SiN) passivation layers suffer from soft gain compression due to trapping of carriers by surface states. Reducing surface state dispersion in these devices is thus desired to access higher output powers. Herein, a potential solution using a novel in situ crystalline AlN passivation layer is provided. A thick, 30+ nm‐top AlN passivation layer moves the as‐grown surface away from the 2D electron gas (2DEG) channel and reduces its effect on the device. Through a series of metal‐polar AlN/GaN/AlN heterostructure growths, it is found that pseudomorphically strained 15 nm thin GaN channels are crucial to be able to grow thick AlN barriers without cracking. The fabricated recessed‐gate HEMTs on an optimized heterostructure with 50 nm AlN barrier layer and 15 nm GaN channel layer show reduction in dispersion down to compared with in current state‐of‐art ex situ SiN‐passivated HEMTs. These results demonstrate the efficacy of this unique in situ crystalline AlN passivation technique and should unlock higher mm‐wave powers in next‐generation AlN HEMTs. 

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4. High-voltage kV-class AlN metal-semiconductor field-effect transistors on single-crystal AlN substrates

   https://doi.org/10.35848/1882-0786/ad85c0  

   Da, Bingcheng; Herath_Mudiyanselage, Dinusha; Wang, Dawei; He, Ziyi; Fu, Houqiang (October 2024, Applied Physics Express)

   Abstract This letter reports the demonstration and electrical characterization of high-voltage AlN metal-semiconductor field-effect transistors (MESFETs) on single-crystal AlN substrates. Compared with AlN MESFETs on foreign substrates, the AlN-on-AlN MESFETs showed high breakdown voltages of over 2 kV for drain-to-gate spacing of 15 μm and one of the highest average breakdown fields among reported AlN MESFETs. Additionally, the devices also exhibited decent drain saturation current and on/off ratio without complicated regrown or graded contact layers, which are several times higher than those of reported AlN-on-sapphire MESEFTs. This work is beneficial for the future development of ultrawide bandgap AlN power electronics. 

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5. 2.2 W/mm at 94 GHz in AlN/GaN/AlN High‐Electron‐Mobility Transistors on SiC

   https://doi.org/10.1002/pssa.202200774  

   Hickman, Austin; Chaudhuri, Reet; Li, Lei; Nomoto, Kazuki; Moser, Neil; Elliott, Michael; Guidry, Matthew; Shinohara, Keisuke; Hwang, James_C_M; Xing, Huili_Grace; et al (January 2023, physica status solidi (a))

   Aluminum nitride (AlN) offers novel potential for electronic integration and performance benefits for high‐power, millimeter‐wave amplification. Herein, load‐pull power performance at 30 and 94 GHz for AlN/GaN/AlN high‐electron‐mobility transistors (HEMTs) on silicon carbide (SiC) is reported. When tuned for peak power‐added efficiency (PAE), the reported AlN/GaN/AlN HEMT shows PAE of 25% and 15%, with associated output power () of 2.5 and 1.7 W mm⁻¹, at 30 and 94 GHz, respectively. At 94 GHz, the maximum generated is 2.2 W mm⁻¹, with associated PAE of 13%. 

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