Yasunobu Saito

Suppression of Dynamic On-Resistance Increase and Gate Charge Measurements in High-Voltage GaN-HEMTs With Optimized Field-Plate Structure

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 Q gd for the fabricated devices has also been measured to provide a basis for discussion of the ability of high-speed switching operation. Although Q gd /A (A: active device area) was almost the same as that of the conventional Si-power MOSFETs, R on A was dramatically reduced to about a seventh of the reported 600-V Si-MOSFET value. Therefore, R on Q gd for 600-V device was reduced to 0.32 OmeganC, 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.

A 120-W Boost Converter Operation Using a High-Voltage GaN-HEMT

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.

Field-Plate Structure Dependence of Current Collapse Phenomena in High-Voltage GaN-HEMTs

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.

O n -Resistance Modulation of High Voltage GaN HEMT on Sapphire Substrate Under High Applied Voltage

The 620-V/1.4-A GaN high-electron mobility transistors on sapphire substrate were fabricated and the ON-resistance modulations caused by current collapse phenomena were measured under high applied voltage. Since the fabricated devices had insulating substrates, no field-plate (FP) effect was expected and the ON-resistance increases of these devices were larger than those on an n-SiC substrate even with the same source-FP structure. The dual-FP structure, which was a combination of gate FP and source FP, was effective in suppressing the ON -resistance increase due to minimization of the gate-edge electric field concentration. The ON-resistance after the applied voltage of 250 V decreased by twice that at low drain voltage by the dual-FP structure. Gallium nitride (GaN), high-electron mobility transistor (HEMT), high voltage, power semiconductor device.

Effect of Buffer Layer Structure on Drain Leakage Current and Current Collapse Phenomena in High-Voltage GaN-HEMTs

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 no influence on the 2-D electron-gas density. On the other hand, the 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 of the n-GaN layer. In addition, the experimental results showed that 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 is related to the electron trapping center, which must be reduced to suppress the current collapse phenomena.

Current Collapseless High-Voltage GaN-HEMT and its 50-W Boost Converter Operation

Suppression of the on-resistance modulation caused by the current collapse phenomena in the high-voltage GaN-HEMT was successful by using dual-field plate (FP) structure and back-side FP. A 480-V/2A GaN-HEMT was designed and fabricated for power electronic applications. In this device, the on-resistance modulation was negligible as low as 5% even under an applied voltage of 300 V. Boost converter circuit was demonstrated using the fabricated device with an output power of 54 W, high power efficiency of 92.7% and high switching frequency of 1 MHz.

Switching controllability of high voltage GaN-HEMTs and the cascode connection

This paper reports that the switching controllability of high-voltage GaN-HEMTs and the cascode connection depends on the feedback capacitance design. The switching behavior of the GaN-HEMT can be controlled by the external gate resistance as the same manner as the conventional Si-MOSFETs. The switching controllability was improved by the substrate connection due to the parasitic capacitance change. The controllability of the cascode connection was slightly worse compared with the Si-MOSFET, because the effective feedback capacitance became small by the step by step switching operation.

Defect analysis in GaN films of HEMT structure by cross-sectional cathodoluminescence

Defect analysis of GaN films in high electron mobility transistor (HEMT) structures by cross-sectional cathodoluminescence (X-CL) is demonstrated as a useful technique for improving the current collapse of GaN-HEMT devices, and the relationship between crystal quality and device characteristics is also investigated. The crystal quality of intrinsic-GaN (i-GaN) and carbon-doped GaN produced clearly different peak intensities of blue luminescence (BL), yellow luminescence (YL), and band-edge emission (BE), which is independently detected by X-CL. Current collapse in GaN-HEMT devices is found to be determined by the BL/BE and YL/BE ratios at the top of the i-GaN layer, which is close to the channel. Moreover, the i-GaN thickness required in order to minimize the BL/BE and YL/BE ratios and the thickness dependency of GaN for minimizing the BL/BE and YL/BE ratios depending on the growth conditions can be evaluated by X-CL. However, there is no correlation between current collapse in GaN-HEMT devices and the YL...