Hirokuni Tokuda

High Al Composition AlGaN-Channel High-Electron-Mobility Transistor on AlN Substrate

AlGaN-channel high-electron-mobility transistor (HEMT) with high Al composition of 0.51 has been developed. The epitaxial layers were grown on a free-standing AlN substrate to improve crystalline quality. The fabricated device exhibited a maximum drain current (Idsmax) of 25.2 mA/mm with a maximum transconductance (gmmax) of 4.7 mS/mm. The characteristic features of the device were a high source-to-drain breakdown voltage of 1800 V and a high applicable gate-to-source voltage of 4 V in the forward direction. Temperature dependence of DC characteristics demonstrated that the drain current degradation at elevated temperatures for the AlGaN-channel HEMT was appreciably small as compared with the conventional AlGaN/GaN HEMT. This is the first report showing successful DC operation of AlGaN-channel HEMT with high Al composition of over 0.5.

Low-Loss and High-Voltage III-Nitride Transistors for Power Switching Applications

This paper describes recent technological advances on III-nitride-based transistors for power switching applications. Focuses are placed on the progress toward enhancing the breakdown voltage, lowering the ON-resistance, suppressing current collapse, and reducing the leakage current in AlGaN/GaN high-electron mobility transistors (HEMTs). Recent publications revealed that the tradeoff relation between ON-resistance and breakdown voltage in AlGaN/GaN HEMTs exceeded the SiC limit and was getting close to the GaN limit; however, the breakdown voltage achieved was still lower than the theoretical impact ionization limit. A novel process featuring strain-controlled annealing with a metal stack, including Al gave rise to significant reduction in the sheet resistance in AlGaN/GaN heterostructures, suggesting the possibility of dramatic reduction in ON-resistance of GaN-based power devices. Some of the interesting approaches to suppress current collapse indicated that surface trapping effects must be controlled by the optimization of surface processing as well as by the reduction of bulk traps in the epitaxial layers. Close correlation between the local gate leakage current and point defects exposed on the free-standing GaN substrate demonstrated that further reduction of defects on bulk GaN substrates is truly required as future challenges.

Current Collapse Suppression by Gate Field-Plate in AlGaN/GaN HEMTs

Current collapse measurements have been performed for AlGaN/GaN high-electron-mobility transistors having identical breakdown voltages but with different field plate (FP) lengths. The results indicated that applying more positive ON-state gate biases resulted in pronounced recovery in the dynamic ON-resistance for the FP device, whereas no gate-bias effects were observed for the device without FP. The mechanism responsible for the reduced current collapse by FP is proposed, in which the key role is played during ON-state by the quick field-effect recovery of partial channel depletion caused by electron trapping at AlGaN surface states between gate and drain.

High temperature electron transport properties in AlGaN/GaN heterostructures

Hall electron mobility (μH) and sheet concentration (ns) in AlGaN/GaN heterostructures have been measured from 77 to 1020 K. The effect of the deposited Al2O3 layer is also investigated with varying its thickness. It is found that μH decreases monotonously with the temperature (T) and its dependence is well approximated with the function of μH=4.5×103 exp(−0.004T) in the temperatures over 350 K. The function is different from the commonly used one of μH=AT−α (α∼1.5), which indicates that the mobility is not only governed by the polar optical phonon scattering but also the deformation potential scattering plays a role. The sheet electron concentration (ns) has a weak dependence on the temperature, that is, slightly decreases with temperature in 300–570 K and increases gradually up to 1020 K. The decrease is explained by considering the reduction in the polarization (probably both spontaneous and piezoelectric) charge and the increase seems to be due to the parallel conduction through the interface between ...

AlGaN/GaN high-electron-mobility transistor technology for high-voltage and low-on-resistance operation

In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown field of approximately 1 MV/cm is likely limited by the premature device breakdown originating from the insufficient structural and electrical quality of GaN buffer layers and/or the GaN substrate itself. The effective lateral breakdown field is increased to 2 MV/cm by using a highly resistive GaN substrate achieved by heavy Fe doping. Various issues relevant to current collapse are also discussed in the latter half of this paper, where a more pronounced reduction in current collapse is achieved by combining two different schemes (i.e., a prepassivation oxygen plasma treatment and a field plate structure) for intensifying the mitigating effect against current collapse. Finally, a novel approach to suppress current collapse is presented by introducing a three-dimensional field plate (3DFP) in AlGaN/GaN HEMTs, and its possibility of realizing true collapse-free operation is described.

Role of Al and Ti for ohmic contact formation in AlGaN/GaN heterostructures

A mechanism for ohmic contact formation using Ti/Al based metals on AlGaN/GaN heterostructures has been investigated by measuring temperature dependence of sheet electron density (ns) and mobility (μ). It was found that both ns and μ at room temperature for Ti/Al deposited sample were increased by annealing in vacuum, while not for Al/Ti deposited one. The results, especially increase in μ, cannot be understood by the conventional ohmic formation model, including Ti-N (nitrogen) complex formation or N vacancy formation. As the most probable mechanism for the increase in ns and μ, we have proposed a model, in which tensile strain is induced by the reaction of Ti/Al and AlGaN after annealing.

Reduced gate leakage and high thermal stability of AlGaN/GaN MIS-HEMTs using ZrO2/Al2O3 gate dielectric stack

In this paper, we report on AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) fabricated using ZrO2/Al2O3 as a gate dielectric stack. Gate leakage characteristics as well as dynamic on-resistance due to current collapse have been studied for a ZrO2 (2 nm)/Al2O3 (2 nm)/AlGaN/GaN MIS-HEMT and compared with those for MIS-HEMTs with a single gate insulator of Al2O3 (4 nm) and ZrO2 (4 nm). It was found that an Al2O3 gate insulator was effective in reducing the forward gate leakage and in suppressing the current collapse, whereas the use of the ZrO2 dielectric resulted in a suppressed reverse gate leakage current. The composite ZrO2/Al2O3 MIS-HEMT exhibited superior thermal stability in both gate leakage and dynamic on-resistance up to 200 °C.

A method to increase sheet electron density and mobility by vacuum annealing for Ti/Al deposited AlGaN/GaN heterostructures

Temperature dependence of sheet electron density (ns) and mobility (μ) for Ti/Al deposited AlGaN/GaN heterostructures annealed in vacuum has been investigated using Hall effect measurements. The vacuum annealing at 1020 K caused the increase in both ns and μ at room temperature, with the amount of one order of magnitude and 65%, respectively, as compared to without annealed sample. The amount of increase was much less for only Ti or Al deposited or totally thin Ti/Al deposited sample. The origin of the increase is attributed to tensile strain induced by vacuum annealing. The method is useful for reducing the ohmic contact resistivity and/or the access resistance between source and gate in AlGaN/GaN HEMTs.

Current Collapse Reduction in AlGaN/GaN HEMTs by High-Pressure Water Vapor Annealing

We have demonstrated for the first time a remarkable reduction of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) by high-pressure water vapor annealing (HPWVA). The device subjected to HPWVA exhibited considerably low dynamic ON-resistance (RON), suggesting highly improved performance of these devices. Analyses of the results on normalized dynamic RON experiments have shown the elimination of deeper traps by HPWVA, leading to the substantially reduced current collapse. X-ray photoelectron spectroscopy (XPS) studies revealed a significant increase in the oxygen core-level O 1s peak. Moreover, angle-resolved XPS suggested the formation of surface oxide layer. These results indicate that the effective reduction of current collapse in the HPWVA-processed samples is likely due to the incorporation of active oxygen species generated by the HPWV into the AlGaN surface. These oxygen atoms eventually fill up near-surface nitrogen vacancies and promote the formation of Ga2O3 native oxide and possibly Ga2O suboxide, which is known to be an excellent III-V surface passivant. HPWVA is a relatively simple, low-damage, and low-temperature process, and hence, it is found to be a highly feasible and promising alternative for realizing AlGaN/GaN HEMTs with improved performance.

Temperature dependence of electron concentration and mobility in n-GaN measured up to 1020 K

Temperature dependence of Hall electron concentration and mobility in n-GaN has been measured up to 1020 K. The electron concentration increased monotonically with temperature and did not saturate. The measured values were fitted with the calculated ones for the whole temperature range. It is found that following two assumptions have to be made in order to obtain the best fit for both electron concentration and mobility: (i) two donor levels and one acceptor level (including dislocation) have to be taken into account; and (ii) one donor level lies in the conduction band. The obtained results in this study will contribute to the design of GaN devices operating at high temperatures.