Joel T. Asubar

Effects of the ultrathin 1 nm thick AlGaN regrown layer before gate insulator deposition on the performance of ZrO2/AlGaN/GaN metal‐insulator‐semiconductor (MIS) structures are investigated. In comparison with the reference ZrO2/AlGaN/GaN MIS high‐electron‐mobility transistors (MIS‐HEMTs), MIS‐HEMTs with AlGaN regrown layer exhibit increased maximum drain current (>1050 mA mm−1) and broader full‐width at half maximum (FWHM) of transconductance profile (11 V). Moreover, the gate leakage current in the forward direction is reduced by about four orders of magnitude while the hysteresis of the transfer curves is cut‐down to more than 40%. Compared with the control MIS‐capacitors (MIS‐caps), MIS‐caps with regrowth show highly evident sudden increase in capacitance known as “spill‐over” in the capacitance–voltage (C–V) profiles, suggesting improved ZrO2/AlGaN interfaces. Extracted …

We have compared the electrical performance of a proposed normally-off GaN-based MIS-HEMTs employing ultrathin AlGaN barrier layer in the channel with those of conventional recessed-gate structure MIS-HEMTs. The proposed device exhibited much less density of interface states extracted from the measured capacitance-voltage characteristics, suggesting improved Al2O3/AlGaN interface. For corresponding three-terminal transistors, while the conventional reference device exhibited poor control of gate-to-source voltage on drain current with about 3 V hysteresis in the transfer curves, the proposed device showed well-behaved subthreshold characteristics with only 0.8 V hysteresis. Furthermore, the proposed device showed a much higher VTH of+ 5 V compared to+ 1 V of the conventional reference device.

We have fabricated and characterized AlGaN/GaN Schottky-Gate HEMTs using V/Al/Mo/Au metal stack for drain and source ohmic electrodes. The metal stack was annealed at relatively low temperature of 700 °C for 30 seconds. A very low contact resistance value of 0.34 Ω·mm was realized as measured from transfer length method test structures. For HEMT devices with gate-to-drain spacing of 10 μm, a highly competitive value of on-resistance of about 9 Ω·mm was achieved, demonstrating the promise of low-thermal budget V/Al/Mo/Au ohmic contacts for improving the performance of GaN-based devices.

In this paper, we provide a comprehensive review of our studies on the metal–organic-vapor-phase-epitaxial (MOVPE) growth of AlGaN directly on reactive-ion-etched (RIE) GaN surfaces and its application to the fabrication of vertical and lateral enhancement (E)-mode AlGaN/GaN metal–insulator–semiconductor (MIS) high-electron-mobility transistors (HEMTs). First, the peculiarities and challenges of MOVPE growth of AlGaN on RIE-GaN surfaces and their countermeasures are described and discussed. Next, the fabrication of vertical trench E-mode MIS-HEMTs using AlGaN regrowth and their output characteristics are demonstrated. Finally, the recent results on lateral recessed-gate E-mode MIS-HEMTs are described.

We report on the high performance normally-off recessed-gate GaN-based MIS-HEMTs exhibiting excellent combination of maximum drain current (>600 mA/mm) and high positive threshold voltage (+1.3 V) achieved by oxygen plasma treatment. Using the positive threshold voltage shift phenomena previously demonstrated in oxygen plasma treated planar MIS-HEMTs, we were able to achieve robust enhancement-mode operation employing a relatively thick 9-nm-AlGaN barrier layer in the channel region. The rather thick AlGaN barrier may have suppressed remote scattering from insulator/AlGaN interface states leading, to high maximum drain current.

We report on the highly improved performance of Al 2 O 3/AlGaN/GaN MIS-HEMTs using a V/Al/Mo/Au metal stack as ohmic electrodes. Transfer length method test structures using a V/Al/Mo/Au metal stack annealed at a temperature of 660 C revealed highly linear current–voltage curves and smooth surface morphology. Compared with reference MIS-HEMTs using Ti/Al/Mo/Au annealed at the standard rapid thermal annealing temperature of 880 C, V-based devices exhibited less hysteresis of transfer curves and showed higher gate controllability of the drain current, suggesting a highly improved Al 2 O 3/AlGaN interface. Measurements and analyses of capacitance–voltage characteristics of corresponding MIS-capacitors corroborated these findings. The V-based ohmic contact could open new avenues towards enhanced GaN-based MIS-HEMTs performance.

We have studied the effects of oxygen plasma treatment prior to insulator deposition on the electrical performance of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Devices treated with oxygen plasma exhibit more positive threshold voltages, less hysteresis in transfer curves and higher peak transconductance values compared with reference devices without oxygen plasma treatment. Our results suggest that oxygen plasma treatment of the AlGaN surface before insulator deposition can improve the overall electrical performance of GaN-based MIS-HEMTs.

We have fabricated and characterized ZrO2/AlGaN/GaN MIS-HEMTs with "dry-etching free" gate recess structures realized using Selective Area Growth (SAG) of AlGaN barrier on the access regions. The transfer characteristics revealed threshold voltage -0.5 V, suggesting quasi-normally-off operation. In comparison with reference planar HEMTs, the threshold voltage of SAG-HEMTs was shifted by about +5 V towards the positive voltage direction. Moreover, SAG-HEMTs showed almost parallel shift of transfer curves with respect to that of the reference planar devices, suggesting no considerable mobility degradation.

We report on the impact of the 3 nm thick ex situ AlGaN regrown layer prior to insulator deposition on the interfacial properties of Al 2 O 3/AlGaN/GaN metal–insulator–semiconductor (MIS) structures. MIS-capacitors (MIScaps) with regrown AlGaN layer exhibited anomalously excessive threshold voltage shift compared to reference sample without regrown AlGaN, suggesting highly reduced interface states density (D it). Moreover, MIScaps with regrown AlGaN layer exhibited" spill-over" in the capacitance–voltage profiles, further evidencing the improved Al 2 O 3/AlGaN interfaces. Fabricated three-terminal MIS-HEMTs with regrown AlGaN showed less hysteresis in transfer curves, enhanced maximum drain current, and increased linearity over the reference device.

In this paper, a technique to develop small signal circuit (SSC) models of AlGaN/GaN high electron mobility transistors (HEMTs) using dependent current sources is presented. In this technique, experimentally measured broadband Y-parameters of AlGaN/GaN HEMTs are mathematically modeled as a sum of pole-residue terms. By representing each pole-residue term as a dependent current source, it is possible to develop an accurate SSC models for HEMTs which otherwise may not be possible using passive resistive-inductive-capacitive elements. The accuracy of the proposed SSC model is validated against the conventional SSC model using a 2nd, 3rd and 4th order rational function representation of the admittance branches of AlGaN/GaN HEMTs. Therefore, the proposed SSC model turns out to be highly robust in nature and can take care of any form of the transfer functions of the admittance branches …

We investigated the effects of high-temperature annealed (HTA) at 880 °C of Al2O3 gate insulator on the performance of AlGaN/GaN MIS-HEMTs. HTA-Al2O3 MIS-capacitors exhibited evidences of spillover of 2DEG from AlGaN/GaN to Al2O3/AlGaN interface. Moreover, HTA-Al2O3 MIS-HEMTs also showed smaller hysteresis in transfer curves and higher maximum drain current compared to those of the reference Al2O3/AlGaN/GaN MIS-HEMTs. These suggest highly improved insulator/semiconductor interfaces by high-temperature annealing of Al2O3 insulator layer.

Mist chemical vapor deposition (mist-CVD) is a non-vacuum technique for depositing various metal oxide/sulfite films from relative non-toxic and nonpyrophoric aqueous solution, resulting in relatively simple, low-cost and fast deposition rate under atmospheric pressure. Recent reports have demonstrated the capability of mist-CVD as an able replacement for conventional vacuum deposition methods in some applications. In this work, firstly state-of-the-art literature reports on mist-CVD techniques are reviewed. Finally, we highlight the important progress in mist-CVD deposited-gate insulators for GaN-based metal-insulator-semiconductor (MIS) devices applications.

Ensemble Monte Carlo simulation was performed to simulate the current gain cutoff frequency (f T ) of an AlN/GaN HEMT. It was found that the donor density in the source ohmic neutral region should be larger than 5×10 20 cm -3 to precisely calculate the drain current. The calculation results indicated that as high as 1.6 THz of f T was achieved with a gate length of 10 nm.

We have fabricated and characterized ZrO2/AlGaN/GaN MIS-HEMTs with a 1-nm-thick AlGaN layer regrown on top of the original AlGaN barrier layer. Capacitance-voltage characteristics revealed spill-over phenomenon, suggesting good quality of the resulting ZrO2/regrown-AlGaN interface. From the transfer characteristics, maximum drain current of 1100 mA/mm and maximum transconductance of 102 mS/mm were obtained.

Traditional lumped small signal equivalent circuit models of AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOS-HEMTs) are made up of constant valued circuit elements. Such models are unable to capture the high frequency behavior (above 20 GHz) of the device. In this work, a modified small signal equivalent circuit model of AlGaN/GaN MOS-HEMTs is presented. The key feature of the proposed model is that the values of the different circuit elements in the model are considered to be frequency dependent in nature and not constants. The frequency dependent value of each circuit element is mathematically represented using polynomial functions where the coefficients of the functions are determined via a least-square curve fitting approach. This frequency dependent attribute of the circuit element values ensures that the proposed model is very accurate at high frequencies without …

We report on the fabrication of recessed gate AlGaN/GaN metal-insulator-semiconductor (MIS) high-electron-mobility transistors (HEMTs) with Al 2 O 3 gate dielectric deposited by mist-CVD method. The fabricated device exhibited a high positive threshold voltage of 1.5 V in combination of high maximum drain current of 450 mA/mm, demonstrating robust enhancement-mode operation.

a) Authors to whom correspondence should be addressed; electronic mail: joel@ u-fukui. ac. jp; yatabe@ cs. kumamoto-u. ac. jp; dagmar. Gregusova@ savba. sk; and hashi@ rciqe. hokudai. ac. jp

Amorphous Al2O3 gate insulator on AlGaN/GaN structure have been deposited by cost-effective mist chemical vapor deposition (mist-CVD) technique. The properties of deposited mist-Al 2 O 3 gate insulators are comparable to those reported for high-quality amorphous Al 2 O 3 films deposited by atomic layer deposition (ALD) method. In addition, we have fabricated AlGaN/GaN metal–insulator–semiconductor (MIS) diodes using Al 2 O 3 gate insulator deposited by mist-CVD. We obtained practically hysteresis-free with spill-over behavior in capacitance–voltage (C–V) characteristics from the fabricated mist-Al2O3 MIS diodes suggesting high interfacial quality of the mist-Al 2 O 3 /AlGaN interface.

We report on the fabrication and characterization of AlGaN/GaN metal-insulator-semiconductor (MIS) capacitors and high-electron-mobility transistors (MIS-HEMTs) using a 5 nm thick Al 2 O 3 dielectric deposited by cost-effective and environmental-friendly mist chemical vapor deposition (mist-CVD) technique. Practically hysteresis-free capacitance–voltage profiles were obtained from the fabricated two-terminal MIS-capacitors indicating high quality of the mist-Al 2 O 3/AlGaN interface. Compared with reference Schottky-gate HEMTs, mist MIS-HEMTs exhibited much improved performance including higher drain current on-to-off ratio, much lower gate leakage current in both forward and reverse directions and lower subthreshold swing. These results demonstrate the potential and viability of non-vacuum mist-CVD Al 2 O 3 in the development of high-performance GaN-based MIS-HEMTs.

Body weight fluctuation in an unrestricted free-living adult was investigated by taking daily measurements for a decade. It was found from the time series analyses that the Ornstein–Uhlenbeck process defined as the solution of the Langevin equation, is the most dominant factor affecting the body weight fluctuation. This process was shown to reasonably describe the expected weight change in nominally healthy adults in general. Our calculation reasonably expresses the relationships among specific energy, average total daily energy expenditure and the standard deviation of the energy intake and its expenditure, demonstrating the validity of the proposed stochastic model.