出版者・発行元：Journal of Applied Physics  

A pseudomorphic AlN/GaN structure was grown on an AlN substrate using metalorganic vapor phase epitaxy (MOVPE). The Hall effect characteristics of two-dimensional electron gas induced at the interface between AlN and GaN and the properties of high-electron-mobility transistors (HEMTs) fabricated using the proposed AlN/GaN structures were investigated. The Hall effect measurements indicated that the electron mobility at room temperature was 294 cm² V⁻¹ s⁻¹, the sheet carrier density was 1.1 × 10¹³ cm⁻², and the sheet resistance was 1930 Ω/sq. Cross-sectional transmission electron microscopy images revealed an extremely abrupt interface between the AlN and GaN layers. By contrast, surface atomic force microscopy measurements indicated that GaN exhibited a clear step-terrace surface and the AlN barrier layer exhibited the signs of transitioning to island-like growth. Furthermore, the HEMT devices exhibited clear transistor characteristics with a source-drain breakdown voltage of 2.3 kV and an ON/OFF ratio of >10⁷. These findings demonstrate the potential of MOVPE for fabricating AlN/GaN on AlN substrates, showing the inherent potential of AlN as a substrate.

DOI： 10.1063/5.0255068

Open Access

Web of Science

Scopus

出版者・発行元：IEEE Transactions on Electron Devices  

Nearly ideal vertical AlxGa-{{1}-{x}}text{N} ({0.7} leq {x} < {1.0} ) p-n diodes are fabricated on an aluminum nitride (AlN) substrate. Distributed polarization doping (DPD) was employed for both p-type and n-type layers of the p-n junction, instead of conventional impurity doping, to overcome the major bottleneck of AlN-based material: the control of conductivity. Capacitance-voltage measurements revealed that the net charge concentration agreed well with the DPD charge concentration expected from the device layer structure. The fabricated devices exhibited a low turn-on voltage of 6.5 V, a low differential specific on-resistance of 3 text{M}Omega cm2, electroluminescence (maximum at 5.1 eV), and an ideality factor of 2 for a wide range of temperatures (room temperature - 573 K). Moreover, the breakdown electric field was 7.3 MV cm-1, which was almost twice as high as the reported critical electric field of GaN at the same doping concentration. These results clearly demonstrate the usefulness of DPD in the fabrication of high-performance AlN-based power devices.

DOI： 10.1109/TED.2024.3367314

Web of Science

Scopus

出版者・発行元：Applied Physics Express  

We developed a low-temperature ohmic contact formation process for N-polar GaN surfaces. Specific-contact-resistances of 9.4 × 10⁻⁵ and 2.0 × 10⁻⁵Ω cm² were obtained using Ti/Al metal stacks on heavily-germanium-doped GaN films, which were deposited at 500 °C and 600 °C using a radical-assisted reactive sputtering method, respectively. The electrode sintering temperature was as low as 475 °C. Carrier concentrations for the 500 °C and 600 °C samples were 2.6 × 10²⁰ and 1.8 × 10²⁰cm⁻³, respectively. These results suggest that this method is highly effective in reducing the contact resistance of GaN devices with low thermal budgets.

DOI： 10.35848/1882-0786/ad2783

Open Access

Web of Science

Scopus

出版者・発行元：IEEE Transactions on Electron Devices  

The p+-n-n - n+ structure, known as Hi-Lo structure, was investigated in gallium nitride (GaN) single-drift-region (SDR) impact ionization avalanche transit-time (IMPATT) diodes to improve the output power and efficiency. The 15 GHz GaN Hi-Lo IMPATT diode was designed according to the Scharfetter and Gummel model under realistic conditions, suppressing the tunneling current (<10-4 cm2) and breakdown voltage (< 400 V). Even in such conditions, the calculated efficiency was higher than that of the p+-n abrupt junction structure and the improvement of RF characteristics was expected. The fabricated GaN Hi-Lo IMPATT diodes showed a clear avalanche breakdown and a pulsed microwave oscillation in the frequency range from 15 to 17 GHz. The maximum peak output power of 25.5 W and the efficiency of 2% were achieved, showing the highest values on microwave band GaN IMPATT diodes, and we confirmed that the Hi-Lo structure is effective for the high-power and high-efficiency operation of GaN IMPATT diodes.

DOI： 10.1109/TED.2023.3345822

Web of Science

Scopus

出版者・発行元：IEEE Electron Device Letters  

An experimental study on the effects of junction capacitance and current density on the oscillation characteristics of GaN single-drift-region (SDR) impact ionization avalanche transit-time (IMPATT) diodes were carried out using GaN p+-n abrupt junction diodes of various diameters, 200, 150, and 100 μ m , with a depletion layer width of 2 μ m. The fabricated diodes showed a clear avalanche breakdown at 315 V and a pulsed microwave oscillation with a peak output power exceeding 30 dBm. The oscillation frequency depended on junction diameter and current density. It was widely modulated from 8.56 to 21.1 GHz with decreasing junction diameter and increasing current density. The highest oscillation frequency was obtained with a current density of 13.8 kA/cm2 and a junction diameter of 100 μ m. A numerical calculation based on Read-type small-signal theory was carried out and found to well explain the experimental results.

DOI： 10.1109/LED.2023.3285938

Web of Science

Scopus

開催年月日： 2020年9月

記述言語：英語   会議種別：口頭発表（一般）  

DOI： 10.1109/ISPSD46842.2020.9170099

Scopus