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This work studies the dynamic breakdown voltage (BV) and overvoltage margin of a 650-V-rated commercial GaN power HEMT in hard switching. The dynamic BV measured in the hard switching circuits is over 1.4 kV, being 450 V higher than the static BV measured in the quasi-static I-V sweep. The device can survive at least 1 million hard-switching overvoltage pulses with 1.33 kV peak overvoltage (~95% dynamic BV). Recoverable device parametric shifts are observed after the 1-million pulses, featuring small reductions in threshold voltage and on-resistance. These shifts are different from the ones after the hard-switching pulses without overvoltage and are attributable to the trapping of the holes produced in impact ionization. These results suggest that the BV and overvoltage margin of GaN HEMTs in practical power switching can be significantly underestimated using the static BV. [ABSTRACT FROM AUTHOR]

The compact model of HIroshima-University Starc Igfet Model (HiSIM)_GaN for GaN-HEMT devices is reported, which solves the Poisson equation iteratively, in a similar way as the industry-standard compact HiSIM models for other semiconductor devices. The model considers all possible charges induced within the device, including the dynamically varying trap density. It is verified that the model can reproduce the 2-D-device simulation results accurately. In particular, the operation frequency dependence of the current collapse can also be captured correctly based on the trap time constant. [ABSTRACT FROM AUTHOR]

A new technique to reduce contact resistance on AlGaN/GaN HEMTs was proposed. This method introduced uneven AlGaN layer structures. 2D device simulations revealed a clear increase in two-dimensional electron gas (2DEG) concentration near the edge regions where the AlGaN layer thickness fluctuated. This fringing effect is useful because metal and high-density 2DEG can approach each other, overcoming an inherent tradeoff involving the AlGaN layer thickness. Experiments demonstrated the effectiveness of this technique, in which contact resistances obtained on some structures with uneven AlGaN layers were lower than the lowest limit for conventional flat AlGaN structures. [ABSTRACT FROM AUTHOR]

High-voltage (> 400 V) GaN high-electron mobility transistors were fabricated using two types of heterostructures with different buffer layer structures. The buffer layer structure affected the crystal defect density in grown AlGaN/GaN heterostructure. The static on-resistance under low applied voltage was independent of the buffer layer structure because it has influence on the 2-D electron-gas density. On the other hand, drain leakage current through the grown layers and the dynamic on-resistance increase caused by the current collapse phenomena depended on the buffer layer structure. The leakage current was reduced by the AlN/n-GaN/AlN layers because of the potential barrier at the AlN/n-GaN interface and no-depletion n-GaN layer. In addition, the experimental results showed the dynamic on-resistance was increased with the edge dislocation density and was not influenced by the screw dislocation density. From these results, it can be expected that edge dislocation related to the electron trapping center, which must be reduced suppress the current collapse phenomena. [ABSTRACT FROM AUTHOR]

The dynamic on-resistance increase associated with the current collapse phenomena in high-voltage GaN high-electron-mobility transistors (HEMTs) has been suppressed by employing an optimized field-plate (FP) structure. The fabricated GaN-HEMTs of 600 V/4.7 A and 940 V/4.4 A for power-electronics applications employ a dual-FP structure consisting of a short-gate FP underneath a long-source FP. The measured on-resistance shows minimal increase during high-voltage switching due to increased electric-field uniformity between the gate and drain as a result of using the dual FP. The gate-drain charge Qgd for the fabricated devices has also been measured to provide a basis for discussion of the ability of high-speed switching operation. Although Qgd/A (A: active device area) was almost the same as that of the conventional Si-power MOSFETs, RONA was dramatically reduced to about a seventh of the reported 600-V Si-MOSFET value. Therefore, RON Qgd for 600-V device was reduced to 0.32 ΩnC, which was approximately a sixth of that for the Si-power MOSFETs. The high-voltage GaN-HEMTs have significant advantages over silicon-power MOSFETs in terms of both the reduced on-resistance and the high-speed switching capability. [ABSTRACT FROM AUTHOR]

A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 mΩ ∙ cm² and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 mΩ ∙ cm² for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to + 1 V. [ABSTRACT FROM AUTHOR]

The relation between Schottky gate leakage current and the breakdown voltage of AIGaN-GaN high-electron mobility transistors (HEMTs) is discussed based on the newly introduced simple, yet useful, surface defect charge model. This model represents the leakage current caused by the positive charge in the surface portion of AIGaN layer induced by process damage such as nitrogen vacancies. The new model has been implemented into a two-dimensional device simulator, and the relationship between the gate leakage current and the breakdown voltage was simulated. The simulation results reproduced the relationship obtained experimentally between the leakage current and the breakdown voltage. Further simulation and experiment results show that the break- down voltage is maintained even if the defect charge exists up to the defect charge density of 2.5 × 1012 cm-2, provided the field plate structure is adopted, while the breakdown voltage shows a sudden drop for the defect density over 5 × 1011 cm-2 without the field plate. This result shows that the field plate structure is effective for suppressing the surface charge influence on breakdown voltage due to the relaxation of the electric field concentration in the surface portion of the AIGaN layer. [ABSTRACT FROM AUTHOR]

High breakdown voltage AlGaN-GaN power high-electron mobility transfers (HEMTs) on an insulating substrate were designed for the power electronics application. The field plate structure was employed for high breakdown voltage. The field plate length, the insulator thickness and AlGaN layer doping concentration were design parameters for the breakdown voltage. The optimization of the contact length and contact resistivity reduction were effective to reduce the specific on-resistance. The tradeoff characteristics between the on-resistance and the breakdown voltage can be improved by the optimization of the above design parameters, and the on-resistance can be estimated to be about 0.6 mω · cm² for the breakdown voltage of 600 V. This on-resistance is almost the same as that for the device on a conductive substrate. [ABSTRACT FROM AUTHOR]

Undoped AlGaN-GaN power high electron mobility transistors (HEMTs) on sapphire substrate with 470-V breakdown voltage were fabricated and demonstrated as a main switching device for a high-voltage dc-dc converter. The fabricated power HEMT realized a high breakdown voltage with a field plate structure and a low on-state resistance of 3.9 m&Ohm; · cm², which is 10 × lower than that of conventional Si MOSFETs. The dc-dc converter operation of a down chopper circuit was demonstrated using the fabricated device at the input voltage of 300 V. These results show the promising possibilities of the AlGaN-GaN power HEMTs on sapphire substrate for future switching power devices. [ABSTRACT FROM AUTHOR]

The theoretical limit of a lateral wide band-gap semiconductor (WBS) power device was estimated for SiC, GaN and diamond. The lateral WBS device realizes ultra-low on-resistance due to a very short gate-drain offset, and a power integration circuit with very high power can be easily realized even with a small chip. The lateral WBS devices with breakdown voltage of over 1 kV realized on-resistance below 1 mΩ cm2. For the breakdown voltage of over 10 kV, diamond devices are attractive due to the large critical field (5.6 MV/cm). Although the WBS device realizes drastic reduction of both the chip area and the power loss, the heat dissipation technique and high current density switching capability are also important for bringing out the potential of the WBS device. [Copyright &y& Elsevier]

AlGaN-GaN power high-electron mobility transistors (HEMTs) with 600-V breakdown voltage are fabricated and demonstrated as switching power devices for motor drive and power supply applications. The fabricated power HEMT realized the high breakdown voltage by optimized field plate technique and the low on-state resistance of 3.3 m&Ohm;cm[sup2], which is 20 times lower than that of silicon MOSFETs, thanks to the high critical field of GaN material and the high mobility in 2DEG channel. The fabricated devices also demonstrated the high current density switching of 850 A/cm[sup2] turn-off. These results show that AlGaN-GaN power-HEMTs are one of the most promising candidates for future switching power device for power electronics applications. [ABSTRACT FROM AUTHOR]

Four types of the field-plate (FP) structure were fabricated to discuss the relation between the current collapse phenomena and the electric-field peak in high-voltage GaN-HEMTs. The ON-resistance increase caused by current collapse phenomena is dramatically reduced by the single-gate-FP and dual-FP structures compared with the source-FP structure, because the gate-edge electric field was reduced by the gate-FP electrode. The dual-FP structure was slightly more effective to suppress the collapse phenomena than the single-gate-FP structure, because the two-step FP structure relaxes the electric-field concentration at the FP edge. These results show that the gate-edge peak strongly affects the ON-resistance modulation. Although the FP edge peak also causes the collapse phenomena, its influence is weak. [ABSTRACT FROM AUTHOR]

A boost converter with a 940-V/4.4 A GaN-HEMT as the main switching device was demonstrated to show the possibility of using high-voltage GaN-HEMTs in power electronic applications. The demonstrated circuit achieved an output power of 122 W and a power efficiency of 94.2% under a drain peak voltage as high as 350 V and a switching frequency of 1 MHz. The dual field-plate structure realized high-voltage switching operation with high power efficiency as dynamic on-resistance was suppressed by an increase of the current collapse phenomena. [ABSTRACT FROM AUTHOR]