In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

GaN Power Transistors on Si Substrates for Switching Applications

In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were exam- ined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance ðRonÞ and a high breakdown voltage ðVbÞ.

GaN Power Transistors on Si Substrates for Switching Applications Hybrid MOS-FET transistor devices with low on-resistance, high hold-voltages and high breakdown voltage promise to provide high-power, low-loss operation for switching applications.

This letter reports high-voltage GaN field-effect transistors fabricated on Si substrates. A halide-based plasma treatment was performed to enable normally off operation. Atomic layer deposition of Al2O3 gate insulator was adopted to reduce the gate leakage current. Incorporation of multiple field plates, with one field plate connected to the gate electrode and two field plates connected to the source electrode successfully enabled a high breakdown voltage of 1200 V and low dynamic on-resistance at high-voltage operation.

1200-V Normally Off GaN-on-Si Field-Effect Transistors With Low Dynamic on -Resistance

Gallium nitride high electron mobility transistor (GaN HEMT) has matured dramatically over the last few years. A progressively larger number of GaN devices have been manufactured for in field applications ranging from low power voltage regulators to high power infrastructure base-stations. Compared to the state-of-the-art silicon MOSFET, GaN HEMT has a much better figure of merit and shows potential for high-frequency applications. The first generation of 600 V GaN HEMT is intrinsically normally on device. To easily apply normally on GaN HEMT in circuit design, a low-voltage silicon MOSFET is in series to drive the GaN HEMT, which is well known as cascode structure. This paper studies the characteristics and operation principles of a 600 V cascode GaN HEMT. Evaluations of the cascode GaN HEMT performance based on buck converter at hard-switching and soft-switching conditions are presented in detail. Experimental results prove that the cascode GaN HEMT is superior to the silicon MOSFET, but it still needs soft-switching in high-frequency operation due to considerable package and layout parasitic inductors and capacitors. The cascode GaN HEMT is then applied to a 1 MHz 300 W 400 V/12 V LLC converter. A comparison of experimental results with a state-of-the-art silicon MOSFET is provided to validate the advantages of the GaN HEMT.

Evaluation and Application of 600 V GaN HEMT in Cascode Structure

A 175-to-350 V hard-switched boost converter was constructed using a high-voltage GaN high-electron-mobility transistor grown on SiC substrate. The high speed and low on-resistance of the wide-band-gap device enabled extremely fast switching transients and low losses, resulting in a high conversion efficiency of 97.8% with 300-W output power at 1 MHz. The maximum efficiency was 98.0% at 214-W output power, well exceeding the state of the art of Si-based converters at similar frequencies.

A 97.8% Efficient GaN HEMT Boost Converter With 300-W Output Power at 1 MHz

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.

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

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.

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

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.

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

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.

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

This paper reports the demonstration of the resonant inverter circuit for electrodeless fluorescent lamps using a high-voltage GaN-HEMT as a main switching device. A 620-V/1.4-A GaN-HEMT was designed and fabricated for power electronic applications. The dynamic on-resistance increased with current collapse phenomena was suppressed by the dual-FP structure and the switching operation could be realized under high applied voltage of over 350 V. As a high-voltage and high-frequency power supply application, a 13.56-MHz resonant inverter circuit for electrodeless fluorescent lamps was demonstrated using the fabricated device. The demonstrated circuit achieved high-voltage operation of 380 V, high-speed gate-switching of 4.5-7 ns, and lighting of the electrodeless lamp with an input power of 7-10 W. High-voltage operation realized a simple circuit composition for high-power efficiency and the discharge ignition of the lamp without the starting circuit. The power efficiency of the inverter circuit was over 90% with an input power of 9 W. These results show that high-voltage GaN devices are suitable for high-frequency switching applications under high-input voltages of several hundred volts.

Yorito Kakiuchi

Papers

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

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

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

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

Demonstration of resonant inverter circuit for electrodeless fluorescent lamps using high voltage GaN-HEMT