N. Tipirneni

The 1.6-kV AlGaN/GaN HFETs

The breakdown voltages in unpassivated nonfield-plated AlGaN/GaN HFETs on sapphire substrates were studied. These studies reveal that the breakdown is limited by the surface flashover rather than by the AlGaN/GaN channel. After elimination of the surface flashover in air, the breakdown voltage scaled linearly with the gate-drain spacing reaching 1.6 kV at 20 mum. The corresponding static ON-resistance was as low as 3.4 mOmegamiddotcm². This translates to a power device figure-of-merit V_BR ²/R_ON=7.5times10⁸ V²middotOmega^-1 cm^-2, which, to date, is among the best reported values for an AlGaN/GaN HFET

Silicon Dioxide-Encapsulated High-Voltage AlGaN/GaN HFETs for Power-Switching Applications

In this letter, new approach in achieving high breakdown voltages in AlGaN/GaN heterostructure field-effect transistors (HFETs) by suppressing surface flashover using solid encapsulation material is presented. Surface flashover in III-Nitride-based HFETs limits the operating voltages at levels well below breakdown voltages of GaN. This premature gate-drain breakdown can be suppressed by immersing devices in high-dielectric-strength liquids (e.g., Fluorinert); however, such a technique is not practical. In this letter, AlGaN/GaN HFETs encapsulated with PECVD-deposited SiO2 films demonstrated breakdown voltage of 900 V, very similar to that of devices immersed in Fluorinert liquid. Simultaneously, low dynamic ON-resistance of 2.43 mOmega ldr cm2 has been achieved, making the developed AlGaN/GaN HFETs practical high-voltage high-power switches for power-electronics applications.

An Efficient High-Frequency Drive Circuit for GaN Power HFETs

The requirements for driving gallium nitride (GaN) heterostructure field-effect transistors (HFETs) and the design of a resonant drive circuit for GaN power HFET switches are discussed in this paper. The use of wideband III-nitride (such as GaN) devices today is limited to telecom and low-power applications. The current lack of high-frequency high-power drivers prevents their application in power converters. The proposed circuit is based upon resonant switching transition techniques, by means of an LC tag, to recover part of the power back into the voltage source in order to reduce the power loss. This circuit also uses level shifters to generate the zero and negative gate-source voltages required to turn the GaN HFET on and off, and it is highly tolerant to input-signal timing variances. The circuit reduces the overall power consumed in the driver and thus reduces the power loss. This is particularly important for high-frequency driver operation to take full advantage, in terms of efficiency, of the superior switching speed of GaN devices. In this paper, the topology of the low-power-loss high-speed drive circuit is introduced. Some simulation results and preliminary experimental measurements are discussed.

Low Threshold-14 W/mm ZrO2/AlGaN/GaN Metal–Oxide–Semiconductor Heterostructure Field Effect Transistors

We report for the first time on the RF performance of a low-threshold AlGaN/GaN metal–oxide–semiconductor heterostructure field transistor (MOSHFET) with zirconium dioxide as the gate dielectric. Low gate leakage current of 5×10-7 A/mm2 and a threshold voltage which was only 1 V higher than that of an HFET were achieved. The RF power of these devices at 2 GHz was 14.32 W/mm at 50 V drain bias.

Digital oxide deposition of SiO2 layers for III-nitride metal-oxide-semiconductor heterostructure field-effect transistors

We present a digital-oxide-deposition (DOD) technique to deposit high quality SiO2 dielectric layers by plasma-enhanced chemical vapor deposition using alternate pulses of silicon and oxygen precursors. The DOD procedure allows for a precise thickness control and results in extremely smooth insulating SiO2 layers. An insulating gate AlGaN∕GaN heterostructure field-effect transistor (HFET) with 8nm thick DOD SiO2 dielectric layer had a threshold voltage of −6V (only 1V higher than that of regular HFET), very low threshold voltage dispersion, and output continuous wave rf power of 15W∕mm at 55V drain bias.

A Resonant Drive Circuit for GaN Power MOSHFET

The rapid development of the research on gallium nitride semiconductor material and the unique properties of GaN (such as high electron mobility and saturation velocity, high sheet carrier concentration at hetero-junction interfaces, high breakdown voltages, and low thermal- impedance) make the material promising in high-power, high-temperature applications. Accordingly, a design for a drive circuit for GaN switches is increasingly in demand. Until now, however, specific gate drivers for GaN switches are not available yet. In this paper, a new resonant drive circuit for GaN power MOSHFET switches is discussed. This circuit employs a resonant LC tag to recover part of the power back into the voltage source in order to reduce the power loss. It also applies a topology which can increase the voltage level relative to the energy supply, generate the zero and negative gate-source voltages required to turn the GaN MOSHFET on and off, and make the circuit highly tolerant to input signal timing variance. This function reduces the overall power consumed in the driver and thus reduces the power loss. This is particularly important for high-frequency driver operation, to take full advantage of the superior switching speed of GaN devices. In this paper, the topology of the low-power-loss, high-speed drive circuit was introduced and the simulation results were discussed

AlGaN/GaN Bidirectional Power Switch

This paper presents a novel approach to achieving BPS functionality using AIGaN/GaN HFETs and for the first time presents experimental data showing the power bidirectional capability of the devices. We present the first detailed study of the bidirectional power switching capability of single and dual gate AIGaN/GaN BPS. One approach to achieve a symmetrical voltage blocking capability in AIGaN/GaN HFET is to place the gate electrode in the middle of the source-drain spacing.

1.5 kV Power AlGaN/GaN HFETs

AlGaN/GaN power heterostructure field-effect transistors (HFETs) on sapphire substrate with up to 1600-V breakdown voltage (VBR) for power electronic applications have been fabricated. The devices have a low on-resistance (RON) of 3.4 mΩ.cm 2 , making the best VBR-RON relationship yet reported. The power device figure of merit VBR 2 / RON = 7.5x10 8 V 2 /(Ω cm -2 ) is among the best reported values for any AlGaN/GaN HEMT in their class. As silicon power semiconductor devices fail to meet the stringent frequency and temperature requirements imposed by new concepts and applications in the field of power electronics to produce the systems with increased efficiency, the SiC and GaN technologies emerge as the solutions for the future energy-conversion systems. SiC devices, although having the advantages of mature technology, possibility of vertical field effect transistor (FET) design, high breakdown voltages of several kilovolts, are limited by relatively high on-resistance and low switching frequencies. These are the key performance parameters for power conversion devices. On the other hand, AlGaN/GaN-based technology, thanks to a high-density two-dimensional electron gas (2DEG) (above 1x 10 13 cm -2 ) and high electron mobility (above 1500 cm 2 /V.s) allows for low on-resistance high-speed HFETs. For GaN- based high voltage lateral HFET the minimal value of ON-resistance, RON=VBR 2 /(qµnEC,GaN 2 ) while for vertical SiC devices it is given as RON=4VBR 2 /(erµEC,SiC 3 ). Substituting the material parameters for SiC and GaN one can see that the ratio RON,GaN/ RON,SiC ≈ 4.5 x 10 -2 . In this paper, we present a detailed study of AlGaN/GaN HFET breakdown voltage on device geometry and compare the achieved results with the theoretical limits and the best reported values. The HFETs devices were fabricated over sapphire substrate. The wafer sheet resistance was around 350 Ω, the threshold voltage, VT =-4.5V. The ohmic contacts were formed by Ti(200A)/Al(1000A)/Ti(500A)/Au(1500A) metal stacks. These were annealed at 850 °C for 1 min. in a forming gas ambient. The source-gate spacing was 2 µm, gate - drain spacing was varying from 2 to 20 µm. The Au/Ni gates with the gate length varying from 2µm to 12µm were formed using optical lithography. No field-plates or passivation layers have been deposited on the HFETs for this study. The breakdown voltage of AlGaN/GaN HFETs was found to depend linearly on the gate-drain spacing. This behavior is possible if (i) the breakdown is limited by the surface breakdown or (ii) the width of the 2DEG depletion region expands linearly with the gate- drain spacing. Detailed experiments using surface potential profiling and breakdown voltage - residual channel current dependencies have been carried out to confirm the breakdown mechanism. From these experiments, the breakdown of the HFETs was found to be surface limited. For devices with 20µm gate-drain spacing, the breakdown voltage VBD = 1600 V was measured with the corresponding value of RON = 3.4 mΩ.cm 2 . To the best of our knowledge, these are the highest reported results for the GaN based HFETs. Since the surface effects are dominating, an importance of the passivation or the encapsulation material used to cover the device will be discussed.

Low Dynamic On-Resistance Kilovolt-Range AlGaN/GaN HFETs

In the recent years, AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) have been recognized as promising novel building blocks for power conversion and switching applications. The key challenge in high-voltage switching devices is to achieving high breakdown voltage VBR and low on-resistance RON simultaneously. For lateral devices, the RON minimization translates into achieving a given VBR with a minimal gate drain spacing LGD. In most of the reported AlGaN/GaN HFET switches, the field-plated design was implemented to achieve high breakdown voltages [1, 2]. In this paper we demonstrate that high VBR values, above 1 kV, can be achieved without field-plating; we for the first time show that the essential role of the field-plate in high-voltage AlGaN/GaN switches is to suppress the excessive gate leakage currents arising from the SiN passivation. This is the first report of an AlGaN/GaN HFET with the dynamic RON as low as 4 mQ.cm2 at the breakdown voltage VBR= 1000 V. Commonly, the VBR-LGD dependencies for high-voltage AlGaN/GaN HFETs saturate at large LGD values limiting the achievable maximum breakdown voltages below 400 800 V. Recently, we have shown [3] that the saturation observed in the VBR-LGD curves for the unpassivated HFET is due to a surface flashover, not to a bulk breakdown. The suppression of the surface flashover leads to a linear VBRLGD dependence up to LGD 20 ptm resulting in the breakdown voltage of 1600V [3]. It was also shown that even that high VBR values were still limited by surface breakdown and not by the channel avalanche. These results imply that the field plate commonly used in the high-voltage fEIFET design is not required to achieve high breakdown voltage. Thus the role of field-plating in improving the performance of high-voltage high-power AlGaN-GaN HFET switches needs to be revised. In this paper, we present the first detailed study of the role of field plate on the performance of HFETs for high power switching. The fEIETs devices were fabricated over sapphire substrate. The wafer sheet resistance was around 350 Q, the threshold voltage, VT =-6V. The ohmic contacts were formed by Ti(200A)/Al(ioooA)/Ti(500A)/Au(1500A) as metal combination. These were annealed at 850 °C for 1 min. in a forming gas ambient. The source-gate spacing was 2 pim, gate drain spacing was varying from 4 to 16 ptm. After Au/Ni gate formation, the devices were tested for breakdown voltage in air ambience and in Flourinert® ambience. As expected the VBR-LGD dependence was linear yielding the VBR

0.18 μm double-recessed III-nitride metal-oxide double heterostructure field-effect transistors

In this work, we adopted a double-recess etching process and a new Digital-Oxide- Deposition (DOD) technique to fabricate 0.18 mum low-threshold AlInGaN/InGaN/GaN Metal- Oxide Semiconductor Double Heterostructure Field-Effect Transistors (MOS-DHFET). We for the first time report a high frequency operation of AlInGaN/InGaN/GaN MOS-DHFET. The gate- to-barrier aspect ratio is increased by the double recessed gate structure and the confinement of electrons is improved by a thin InGaN channel inserted between AlGaN barrier layer and GaN buffer. The ultra-thin gate oxide layer under the foot of the gate tremendously reduces the gate leakage and the drain leakage. We also compared the device performance of the InGaN channel MOS-DHFETs in Fig. 1 with that of the InGaN back-barrier MOS-DHFETs in Fig. 2.