GaN-based FETs Research Articles

Performance Comparison of Silicon- and Gallium-Nitride-Based MOSFETs for a Power-Efficient, DC-to-DC Flyback Converter

Gallium Nitride (GaN)-based devices offer many advantages over conventional electronic devices, such as lower input/output capacitances, a higher switching speed, and a compact size, resulting in higher-density power outputs and reduced switching losses. This research investigates the power and switching efficiency of GaN-based FET in an active-clamp, DC-to-DC flyback converter for step-down application (24 V to 7 V) and compares it with silicon (Si)- based devices in the same circuit topology. The operation is analyzed under various input conditions and output loads such as R, RC, RL, and RLC. The proposed topology can achieve a maximum power-conversion efficiency of 99.6% and can operate at higher frequency values above 1 MHz. The presented GaN-based flyback model can replace conventional Si-based switches in power applications which require high power-efficiency and switching speed in a compact device.

Favorable Combination of Schottky Barrier and Junctionless Properties in Field-Effect Transistors for High Temperature Applications

High-temperature capable transistors are of great interest for industrial applications, for instance, oil & gas and geothermal exploration, avionic, space, automotive and energy conversion. Especially, energy conversion electronics using wide-bandgap power transistors, i.e. SiC- or GaN-based FET, in combination with high-temperature silicon gate-driver circuits can significantly increase system efficiency and reduce costs. Our improved planar virtually dopant-free silicon-based double gate SOI FET with reactively sputtered WTiNx front-gate electrodes demonstrates ultra-low drain leakage currents at high-temperatures opening a path to record operating temperatures for silicon-based FET devices.

AlGaN/GaN Metal–Oxide–Semiconductor Heterojunction Field-Effect Transistor Integrated With Clamp Circuit to Enable Normally-Off Operation

We have developed an AlGaN/GaN-on-Si metal- oxide-semiconductor heterojunction field-effect transistor (MOSHFET) integrated with a clamp circuit to enable normally-off operation. A clamp circuit consisting of a multilayer MIM capacitor and a Schottky barrier diode was monolithically integrated with a normally ON AlGaN/GaN MOSHFET. The integrated clamp circuit shifted the input driving signal from (0, 20 V) to (-19.2, 0.8 V), allowing the normally ON AlGaN/GaN MOSHFET with a pinchoff voltage of -14 V to be operated as a normally-off device. The multichannel device with a 5-mm channel width exhibited a drain current density of ~470 mA/mm and a breakdown voltage of 900 V. In comparison with other conventional normally-off GaN-based FETs, a higher threshold voltage with high current density (i.e., low ON-resistance) can be achieved with easy device processing, no need for gate recess or complicated epitaxial growth.

Low gate interface traps AlGaN/GaN HEMTs using a lattice matched ZrZnO transparent gate design

AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) using a radio-frequency magnetron sputtered ZrZnO transparent oxide layer as a gate insulator are investigated and compared with traditional GaN HEMTs. A negligible hysteresis voltage shift in the C–V curves is seen, from 0.09V to 0.36V, as the thickness of ZrZnO films increases. The composition of ZrZnO at different annealing temperatures is observed using X-ray photoelectron spectroscopy (XPS). The ZrZnO thin film achieves good thermal stability after 600°C, 700°C and 800°C post-deposition annealing (PDA) because of its high binding energy. Based on the interface trap density analysis, Dit has a value of 2.663×1012cm−2/eV for 10-nm-thick ZrZnO-gate HEMTs and demonstrates better interlayer characteristics, which results in a better slopes for the Ids degradation (5.75×10−1mA/mmK−1) for operation from 77K to 300K. The 10-nm-thick ZrZnO-gate device also exhibits a flat and a stable 1/f noise, as VGS–Vth, and at various operating temperatures. Therefore, ZrZnO has good potential for use as the transparent film for a gate insulator that improves the GaN-based FET threshold voltage and improves the number of surface defects at various operating temperatures.

Effect of Temperature and Al Concentration on the Electrical Performance of GaN and Al<sub>0.2</sub>Ga<sub>0.8</sub>N Accumulation-Mode FET Devices

Field-effect transistors were fabricated on GaN and Al0.2Ga0.8N epitaxial layers grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrates. The threshold voltage VTH was higher when AlGaN was used as an active layer. VTH also increased with temperature due to the increased positive polarization charge at the GaN/AlN buffer/sapphire interfaces. Drain current increased at high temperatures even with more positive threshold voltage, which makes GaN-based FET devices attractive for high temperature operation.

Buffer-trapping effects on current slump in AlGaN/GaN HEMTs

Two-dimensional transient analyses of GaN MESFETs and AlGaN/GaN HEMTs are performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer. Calculated transient characteristics are compared between the two FETs, and it is shown that the deep levels affect the results essentially in a similar way. Quasi-pulsed I–V curves are derived from the transient characteristics, and are compared with steady-state I–V curves. It is shown that so-called current slump is more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because trapping effects become more significant. It is suggested that to minimize current slump in GaN-based FETs, an acceptor density in a semi-insulating GaN layer should be made low.

GaN-based FETs using Cat-CVD SiN passivation for millimeter-wave applications

We have found that SiN passivation by catalytic chemical vapor deposition (Cat-CVD) can significantly increase an electron density of an AlGaN/GaN heterostructure field-effect transistor (HFET). This effect enables thin-barrier HFET structures to have a high-density two-dimensional electron gas and leads to suppression of short-channel effects. We fabricated 30-nm-gate Al 0.4Ga 0.6N(8 nm)/GaN HFETs using Cat-CVD SiN. The maximum drain current density and extrinsic transconductance were 1.49 A/mm and 402 mS/mm, respectively. Current-gain cutoff frequency and maximum oscillation frequency of the HFETs were 181 and 186 GHz, respectively. These high-frequency device characteristics are sufficiently high enough for millimeter-wave applications.

230 W C-band GaN-FET power amplifier

A C-band high-power amplifier with two GaN-based FET chips exhibits record output powers under continuous-wave (CW) and pulsed operation conditions. At 5.0 GHz, the developed GaN-FET amplifier delivers a CW 208 W output power with 11.9 dB linear gain and 34% power-added efficiency. It also shows a pulsed 232 W output power with 8.3 dB linear gain.

Physics-based simulation of buffer-trapping effects on slow current transients and current collapse in GaN field effect transistors

Two-dimensional transient analyses of GaN metal-semiconductor field effect transistors (MESFETs) are performed in which a three level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor, and a deep acceptor are included. Quasipulsed current-voltage (I-V) curves are derived from the transient characteristics and are compared with steady-state I-V curves. It is shown that when the drain voltage VD is raised abruptly, the drain current ID overshoots the steady-state value, and when VD is lowered abruptly, ID remains at a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes quantitatively. The drain lag could be a major cause of current collapse, although some gate lag is also seen due to the buffer layer. The current collapse is shown to be more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because the change of ionized deep-donor density becomes larger and hence the trapping effects become more significant. It is suggested that to minimize the current collapse in GaN-based FETs, an acceptor density in a semi-insulating layer should be made low, although the current cutoff behavior may be degraded.

DC-to-RF dispersion effects in GaAs- and GaN-based heterostructure FETs: performance and reliability issues

The performance and reliability implications of DC-to-RF dispersion effects are addressed. The proposed physical explanations and technological counteractions are reviewed. GaAs- and GaN-based FET technologies are considered, trying to point out both similar and peculiar aspects.

MOCVD growth of group III nitrides for high power, high frequency applications

This paper reports on the LP-MOCVD growth optimisation of bulk GaN, GaAlN materials and GaAlN/GaN heterostructures grown on Sapphire and Silicon Carbide substrates for MESFET and HEMT applications, and on the device performances obtained with these structures. High purity and high resistivity GaN grown on sapphire and Silicon Carbide has been obtained. GaN/Al2O3 MESFETs based on such high purity GaN buffer layers, have exhibited very promising static and microwave performances: high beakdown voltage ∼200 V, ft = 12 GHz, fmax = 25 GHz and CW output power in excess of 2.2 W/mm at 2 GHz.. They have shown the best low frequency noise performances, with the lowest Hooge's parameter as compared to previous values reported in the literature for different GaN-based FETs (HFETs and HEMTs). The main mechanisms involved in the growth of GaAlN/GaN alloys have been studied and their impact on the physical properties of these materials determined. A parasitic reaction was clearly identified to occur in the gas phase between TMA and NH3, with a strong influence on the growth rate and the aluminium incorporation. The SiC substrate surface preparation (both ex-situ and in-situ) and the nucleation layer growth conditions (growth temperature, thickness, composition and strain) have been found to be key steps of the GaAlN/GaN/SiC growth process. Static characteristics of the devices such as maximum drain current Idss or pinch-off voltage have been correlated with the nucleation layer composition of the HEMT structure and the defect density of the SiC substrate. The devices performances related to our first GaAlN/GaN HEMT structures grown on Sapphire and Silicon Carbide have confirmed the high potentiality of GaN and related alloys for high power microwave transistors. Load-Pull measurements performed at 2 GHz on devices related to GaAlN/GaN/Al2O3 HEMT structures, have shown a remarkably high output power density (4.4 W/mm) and absolute power level (3.2 W for 1 mm devices). These power level results for devices on sapphire substrates measured on wafer, are well in agreement with the international state of the art. Nevertheless, the thermal effects, which lead to a significant reduction of the power density with the increase of the transistor size, have been found to be more pronounced for devices on sapphire. In order to improve the thermal properties of HEMT's material, the growth process of GaAlN/GaN HEMTs was transferred onto Silicon Carbide substrates. Devices related to GaAlN/GaN/SiC HEMT structures have been measured at 10 GHz using a load pull system. They exhibited a CW output power in excess of 6.2 W/mm for a gate length of 0.25 µm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On the low frequency noise mechanisms in GaN/AlGaN HFETs

The low frequency noise in GaN/AlGaN heterostructure field effect transistors (HFETs) was studied in the temperature range from 8 to 300 K. At gate biases close to the threshold, the noise came from the device region under the gate, and the Hooge parameter αch was inversely proportional to ns (α ∼ 1/ns) in the entire temperature range. This dependence might be explained by electron tunnelling from the 2D gas into the traps in the adjoining GaN or AlGaN layers. At voltages close to zero, the ungated source-gate and gate-drain regions were responsible for the noise, and the Hooge constant was two orders of magnitude larger. This result is consistent with recent studies of the mechanism of the current collapse in GaN-based FETs. A notable contribution from the generation-recombination noise with activation energy 0.24 eV was observed in the temperature interval from 50 K to 150 K.

Transient characteristics of gan-based heterostructure field-effect transistors

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed.

DC, RF, and microwave noise performances of AlGaN/GaN HEMTs on sapphire substrates

High-performance AlGaN/GaN high electron-mobility transistors with 0.18-/spl mu/m gate length have been fabricated on a sapphire substrate. The devices exhibited an extrinsic transconductance of 212 mS/mm, a unity current gain cutoff frequency (f/sub T/) of 101 GHz, and a maximum oscillation frequency (f/sub MAX/) of 140 GHz. At V/sub ds/=4 V and I/sub ds/=39.4 mA/mm, the devices exhibited a minimum noise figure (NF/sub min/) of 0.48 dB and an associated gain (Ga) of 11.16 dB at 12 GHz. Also, at a fixed drain bias of 4 V with the drain current swept, the lowest NFmin of 0.48 dB at 12 GHz was obtained at I/sub ds/=40 mA/mm, and a peak G/sub a/ of 11.71 dB at 12 GHz was obtained at I/sub ds/=60 mA/mm. With the drain current held at 40 mA/mm and drain bias swept, the NF/sub min/,, increased almost linearly with the increase of drain bias. Meanwhile, the Ga values decreased linearly with the increase of drain bias. At a fixed bias condition (V/sub ds/=4 V and I/sub ds/=40 mA/mm), the NF/sub min/ values at 12 GHz increased from 0.32 dB at -55/spl deg/C to 2.78 dB at 200/spl deg/C. To our knowledge, these data represent the highest f/sub T/ and f/sub MAX/, and the best microwave noise performance of any GaN-based FETs on sapphire substrates ever reported.

AlGaN/GaN HFETs for Automotive Applications

ABSTRACTAlGaN/GaN heterojunction field effect transistors (HFET) on sapphire substrates have demonstrated ability as power devices operating with high current densities and high breakdown voltages. Additionally, AlGaN/GaN HFET devices have a very low on-state resistance. This makes these devices ideal for automotive applications such as switching relays, DC-DC converters, and power inverters. By 2006, switching devices using GaN-based FETs are anticipated to be employed in luxury automobiles and transitioned to the mass market by 2009.In this presentation, data from AlGaN/GaN HFET's grown in an Emcore D180 MOCVD system will be presented. Typical production-scale material results (on 2” sapphire substrates) for these wafers were: μ ∼ 1000 cm2/Vs, Ns = 1.0×1013 cm−2, and Rs ∼ 450 Ω/square with <3% variation across the wafer. These wafers were then processed into devices using Pt/Au gate contacts with 2 μm gate length, 200 μm gate width, and a source to drain spacing of 13 μm. A total of 1000 FETs were combined in parallel for an effective gate width of 20 cm for high current operation (10A). These devices have a lower on-state resistance (<0.01 Ω-cm2) and higher Schottky breakdown voltages (400 V) than the theoretical limit of Si MOSFET devices. These devices demonstrate suitability for insertion in automotive electrical harnesses.

A High-Power GaN-Based Field-Effect Transistor for Large-Current Operation

A high power AlGaN/GaN hetero field-effect transistor (HFET) for large-current operation was fabricated. An Al0.2Ga0.8N/GaN heterostructure was grown using gas-source molecular-beam epitaxy (GSMBE). The mobility of Al0.2Ga0.8N/GaN was about 1200 cm2/V s at room temperature. The structure of a HFET for the large-current operation was composed of 400 units of FET. The gate width and length of a unit FET were 500 μm and 2 μm, respectively. The total gate width was 20 cm. We confirmed that an AlGaN/GaN HFET was also operated under current conditions above 15 A. The on-resistance of the HFET was 3 mΩ cm2. This is a lower value of GaN based FET. The Schottky breakdown voltage of the gate and source was 600 V. An undoped GaN layer had a high breakdown voltage above 1100 V. It was thus demonstrated that a GaN-based FET can be used for power devices.

A High-Power GaN-Based Field-Effect Transistor for Large-Current Operation

A high power AlGaN/GaN hetero field-effect transistor (HFET) for large-current operation was fabricated. An Al0.2Ga0.8N/GaN heterostructure was grown using gas-source molecular-beam epitaxy (GSMBE). The mobility of Al0.2Ga0.8N/GaN was about 1200 cm2/V s at room temperature. The structure of a HFET for the large-current operation was composed of 400 units of FET. The gate width and length of a unit FET were 500 μm and 2 μm, respectively. The total gate width was 20 cm. We confirmed that an AlGaN/GaN HFET was also operated under current conditions above 15 A. The on-resistance of the HFET was 3 mΩ cm2. This is a lower value of GaN based FET. The Schottky breakdown voltage of the gate and source was 600 V. An undoped GaN layer had a high breakdown voltage above 1100 V. It was thus demonstrated that a GaN-based FET can be used for power devices.

Application of GaN-based heterojunction FETs for advanced wireless communication

We review the features of GaN-based FETs and describe their expected development direction, GaN has a high breakdown field, but this does not necessarily mean it is suitable for high-voltage and high-power applications. The main advantage is that it enables scaling down beyond the silicon MOSFET miniaturization limitation from the Maxwell-Boltzmann distribution. Thus, fine gate patterns together with a high carrier velocity make GaN-based FETs be suited for millimeter and near millimeter wavelength high-power applications. In addition, by using large-area sapphire substrates, high-performance and low-cost MMICs can be produced on GaN. We expect that such devices will be the key to future advanced wireless communication systems.

AlGaN/GaN HEMTs on SiC with over 100 GHz f/sub T/ and low microwave noise

High performance AlGaN/GaN high electron mobility transistors (HEMTs) with 0.12 /spl mu/m gate-length have been fabricated on an insulating SiC substrate. The devices exhibited an extrinsic transconductance of 217 mS/mm and current drive capability as high as 1.19 A/mm. The threshold voltage of the devices was -5.5 V. For AlGaN/GaN HEMTs with the same gate-length, a record high unity current gain cut-off frequency (f/sub T/) of 101 GHz and a maximum oscillation frequency (f/sub max/) of 155 GHz were measured at V/sub ds/=16.5 V and V/sub gs/=5.0 V. The microwave noise performances of the devices were characterized from 2 to 18 GHz at different drain biases and drain currents. At a drain bias of 10 V and a gate bias of -4.8 V, the devices exhibited a minimum noise figure (NF/sub min/) of 0.53 dB and an associated gain (G/sub a/) of 12.1 dB at 8 GHz. Also, at a fixed drain bias of 10 V with the drain current swept, the lowest NF/sub min/ of 0.72 dB at 12 GHz was obtained at I/sub ds/=100 mA/mm and a peak G, of 10.59 dB at 12 GHz was obtained at I/sub ds/=150 mA/mm. With the drain current held at 114 mA/mm and drain bias swept, the lowest NF/sub min/ of 0.42 dB and 0.77 dB were obtained at V/sub ds/=8 V at 8 GHz and 12 GHz, respectively. To our knowledge, these are the best microwave noise performances of any GaN-based FETs ever reported.

High performance microwave power GaN/AlGaN MODFETs grown by RF-assisted MBE

High performance GaN/AlGaN MODFETs grown by MBE directly on SiC substrate have been successfully fabricated and characterised. The epitaxial layers of the device were grown by RF-assisted MBE. Excellent device uniformity, reproducibility and scalability were obtained. Maximum output density of 6.5 W/mm was obtained for a 0.1 mm device. On scaling to 1.0 mm gate-width, a total output power of 6.3 W with 38% PAE at 10 GHz was achieved. These device results demonstrate the excellent potential of GaN-based FETs as power cells for practical microwave applications.