Gallium Nitride High Electron Mobility Research Articles

Metal‐Organic Chemical Vapor Deposition Grown Low‐Temperature Aluminum Nitride Gate Dielectric for Gallium Nitride on Si High Electron Mobility Transistor

Gate dielectrics for gallium nitride (GaN) high electron mobility transistor (HEMT) technology have always been challenging because of the nonideal semiconductor–dielectric interface, which leads to electronic traps. Plasma‐enhanced chemical vapor deposition and atomic layer deposition are standard techniques, but they require surface treatment and post‐annealing to control these traps. This article explores metal organic chemical vapor deposition‐grown in situ aluminum nitride (AlN) as a gate dielectric. The interface is expected to be pristine as there is no change in the chamber environment. A thin (10 nm) AlN layer is deposited at a low temperature to minimize strain and prevent the formation of an unwanted conductive channel within the device. The electrical and structural properties of this AlN‐capped HEMT are compared to a standard GaN‐capped HEMT, including temperature‐dependent studies. The results show that the AlN‐capped HEMT retains a higher charge in the channel while having an order of magnitude lower gate leakage than the GaN‐capped sample. Furthermore, the AlN‐capped HEMT performs similarly to the GaN‐capped HEMT in terms of temperature‐dependent leakage, dynamic on‐resistance, and temperature coefficient of resistance.

An RC snubber design method to achieve optimized switching noise‐loss trade‐off of cascode GaN HEMTs

AbstractThe cascode gallium nitride high electron mobility transistors (GaN HEMTs) are very vulnerable to self‐sustained turn‐off oscillation due to their cascode configuration. This paper presents a design approach for the RC snubber of cascode GaN HEMTs to achieve the optimized noise‐loss trade‐off. At first, an analytical model is proposed to describe the instability of cascode GaN HEMTs‐based test circuits utilizing RC snubber. Based on the model, an analytical approach is proposed to achieve two optimum RC snubber designs S1 and S2. The design S1 can satisfactorily dampen the oscillation with minimum switching losses. The design S2 achieves maximum effective damping on the oscillation at a minimized cost of additional power losses. In the end, the accuracy of the proposed model is validated by the double‐pulse test and good agreement is obtained.

Adaptive impedance matching in microwave and terahertz metamaterial absorbers using PIN diodes and GaN HEMTs

Metamaterial absorbers allow electromagnetic waves to be converted into heat energy based on impedance matching. However, passive metamaterial absorbers exhibit fixed absorption characteristics, limiting their flexibility. This work demonstrates tunable microwave and terahertz absorbers by integrating adjustable resistors into the metamaterial units. First, a microwave absorber from 1 to 5 GHz was designed by embedding PIN diodes with voltage-controlled resistance. Calculations, simulations, and measurements verified two separate absorption peaks over 90% when optimized to a resistance of 250 Ω. The absorption frequencies shifted based on the resistor tuning. Building on this, a terahertz absorber was modeled by substituting gallium nitride high electron mobility transistors (GaN HEMTs) as the adjustable resistor component. The GaN HEMTs were controlled by an integrated gate electrode to modify the two-dimensional electron gas density, allowing resistance changes without external voltage terminals. Simulations revealed two absorption peaks exceeding 90% absorption at 0.34 THz and 1.06 THz by adjusting the equivalent resistance from 180 Ω to 380 Ω, and the tunable resistance is verified by DC measurement of single GaN HEMT in the unit. This work demonstrates how integrating adjustable resistors enables dynamic control over the absorption frequencies and bandwidths of metamaterial absorbers. The proposed geometries provide blueprints for tunable microwave and terahertz absorbers.

Study of Acoustic Emission from the Gate of Gallium Nitride High Electron Mobility Transistors

Nitrides are the leading semiconductor material used for the fabrication of high electron mobility transistors (HEMTs). They exhibit piezoelectric properties, which, coupled with their high mechanical stiffness, expand their versatile applications into the fabrication of piezoelectric devices. Today, due to advances in device technology that result in a reduction in the size of individual transistor elements and due to increased structural complexity (e.g., multi-gate transistors), the integration of piezoelectric materials into HEMTs leads to an interesting occurrence, namely acoustic emission from the transistor gate due to piezoelectric effects. This could affect the device’s performance, reliability, and durability. However, this phenomenon has not yet been comprehensively described. This paper aims to examine this overlooked aspect of AlGaN/GaN HEMT operation, that is, the acoustic emission from the gate region of the device induced by piezoelectric effects. For this purpose, dedicated test structures were designed, consisting of two narrow 1.7 μm-wide metallization strips placed at distances ranging from 5 μm to 200 μm fabricated in AlGaN/GaN heterostructures to simulate and examine the gate behavior of the HEMT transistor. For comparison, the test device structures were also fabricated on sapphire, which is not a piezoelectric material. Measurements of acoustic and electrical interactions in the microwave range were carried out using the “on wafer” method with Picoprobe’s signal–ground–signal (SGS)-type microwave probes. The dependence of reflectance |S11| and transmittance |S21| vs. frequency was investigated, and the coupling capacitance was determined. An equivalent circuit model of the test structure was developed, and finite element method simulation was performed to study the distribution of the acoustic wave in the nitride layers and substrate for different frequencies using Comsol Multiphysics software. At frequencies up to 2–3 GHz, the formation of volume waves and a surface wave, capable of propagating over long distances (in the order of tens of micrometers) was observed. At higher frequencies, the resulting distribution of displacements as a result of numerous reflections and interferences was more complicated. However, there was always the possibility of a surface wave occurrence, even at large distances from the excitation source. At small gate distances, electrical interactions dominate. Above 100 µm, electrical interactions are comparable to acoustic ones. With further increases in distance, weakly attenuated surface waves will dominate.

GaN HEMT and Air Core Magnetics based Power Converters Evaluations at Cryogenic Temperature

Cryogenic power electronics is both advantageous and indispensable in many applications, like deep space probe, military electric vehicle, magnetic resonance imaging etc. Among different semiconductors, the gallium nitride (GaN) high electron mobility transistor (HEMT) is the most promising candidate for cryogenic applications with significant conduction loss and switching loss reductions. Moreover, there is no carrier freeze out effect for the GaN HEMTs, which is applicable in extreme low temperature operating conditions. In this work, the efficiency of GaN HEMTs based power converters with different power levels (from several Watts to several kiloWatts) are evaluated at cryogenic temperature. Three different commercial GaN HEMTs are used in these power converters, including the Texas Instruments LMG5200 80 V GaN half-bridge power stage with integrated gate driver, the GaN Systems 650 V bottomcooled GaN HEMT GS66516B, and the 650 V top-cooled GaN HEMT GS66516T from GaN Systems. Moreover, two different cryogenic power converter evaluation methods (cryogenic chamber and liquid nitrogen channeled through cold plate) are investigated. Three of these power converters are evaluated by using a cryogenic chamber and such that the converter operating environment temperature can be regulated. One of the power converters is evaluated by using liquid nitrogen (LN2) channeled through a cold plate, where the gate driver can be designed to operate at non-cryogenic temperatures to ensure the safe operation of the system. Due to the degraded performance of conventional magnetic components, air core magnetics are used in these power converters to improve the converter efficiency. Efficiency improvements at cryogenic temperature are observed for all the GaN HEMTs and air core magnetics based power converters, which are promising for cryogenic power electronics applications.

Influence of electrical field on the susceptibility of gallium nitride transistors to proton irradiation

Radiation susceptibility of electronic devices is commonly studied as a function of radiation energetics and device physics. Often overlooked is the presence or magnitude of the electrical field, which we hypothesize to play an influential role in low energy radiation. Accordingly, we present a comprehensive study of low-energy proton irradiation on gallium nitride high electron mobility transistors (HEMTs), turning the transistor ON or OFF during irradiation. Commercially available GaN HEMTs were exposed to 300 keV proton irradiation at fluences varying from 3.76 × 1012 to 3.76 × 1014 cm2, and the electrical performance was evaluated in terms of forward saturation current, transconductance, and threshold voltage. The results demonstrate that the presence of an electrical field makes it more susceptible to proton irradiation. The decrease of 12.4% in forward saturation and 19% in transconductance at the lowest fluence in ON mode suggests that both carrier density and mobility are reduced after irradiation. Additionally, a positive shift in threshold voltage (0.32 V and 0.09 V in ON and OFF mode, respectively) indicates the generation of acceptor-like traps due to proton bombardment. high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy analysis reveal significant defects introduction and atom intermixing near AlGaN/GaN interfaces and within the GaN layer after the highest irradiation dose employed in this study. According to in-situ Raman spectroscopy, defects caused by irradiation can lead to a rise in self-heating and a considerable increase in (∼750 times) thermoelastic stress in the GaN layer during device operation. The findings indicate device engineering or electrical biasing protocol must be employed to compensate for radiation-induced defects formed during proton irradiation to improve device durability and reliability.

A new design approach of Rat-Race coupler based compact GaN HEMT power amplifier towards flat high efficiency over broadband

This paper presents a high efficiency Rat-Race Coupler based compact GaN HEMT power amplifier (PA) design over broadband for high power transmitter in wireless communication. The rat-race coupler integrated PA Compact design is proposed for the first time as per author best knowledge. The design methodology used a higher order two open stubs and a rat-race coupler (RRC) at input/output sections to control harmonics impedances. The RRC is used to enhance the i/o power, and efficiency over broadband, which provides a good insertion loss, and consumes the least power and non-crucial impedance bandwidth for the normalized frequency band of interest. As a proof of concept, a PA is fabricated using a monolithic microwave integrated circuit (MMIC) 0.15 µm gallium nitride high electron mobility transistor (GaN HEMT) process. The measured result shows that the designed PA achieves a flat power added efficiency (PAE) of 65 % − 74 %, output power (Pout) of 44.8 dBm − 46 dBm, and drain efficiency (DE) of 72 % − 85 %, over a record wide frequency of 1.8 GHz − 3.6 GHz, which is the highest one among all reported harmonic tuned PAs.

Al‐Rich AlGaN Transistors with Regrown p‐AlGaN Gate Layers and Ohmic Contacts

AbstractEpitaxial regrowth processes are presented for achieving Al‐rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p‐type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts. Utilizing a deep gate recess etch into the channel and an epitaxial regrown p‐AlGaN gate structure, an Al0.85Ga0.15N barrier/Al0.50Ga0.50N channel HEMT with a large positive threshold voltage (VTH = +3.5 V) and negligible gate leakage is demonstrated. Epitaxial regrowth of AlGaN avoids the use of gate insulators which can suffer from charge trapping effects observed in typical dielectric layers deposited on AlGaN. Low resistance Ohmic contacts (minimum specific contact resistance = 4 × 10−6 Ω cm2, average = 1.8 × 10−4 Ω cm2) are demonstrated in an Al0.85Ga0.15N barrier/Al0.68Ga0.32N channel HEMT by employing epitaxial regrowth of a heavily doped, n‐type, reverse compositionally graded epitaxial structure. The combination of low‐leakage, large positive threshold p‐gates and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high‐current, enhancement‐mode, Al‐rich AlGaN‐based ultra‐wide bandgap transistors.

Advancements in Tunable and Multifunctional Metamaterial Absorbers: A Comprehensive Review of Microwave to Terahertz Frequency Range

Abstract Over the past two decades, metamaterial absorbers have undergone significant advancements, evolving from microwave single-frequency designs to multi-frequency and broadband absorption, extending into the terahertz band. These absorbers have transitioned from unadjustable to adjustable and multifunctional configurations, enabled by the integration of adjustable materials, mechanical structures, and semiconductor devices. This article provides a comprehensive review of the progress achieved in the microwave to terahertz frequency range over the last five years. Key aspects covered include the absorbing mechanism of metamaterials in the microwave frequency band, with absorption efficiencies exceeding 90% for specific frequency ranges. The development of adjustable absorbers allows for frequency tunability within ±10% of the central frequency, while multifunctional absorbers enable concurrent control over absorption and reflection properties. In the terahertz regime, advanced electromagnetic simulations have led to absorber designs with bandwidths exceeding 50% of the central frequency, resulting in absorption efficiencies above 80% over the entire bandwidth. Integration of gallium nitride-based Gallium Nitride High Electron Mobility Transistors (GaN HMET) provides fast switching speeds below 100 nanoseconds, facilitating rapid reconfiguration of absorber functionalities. These advancements in metamaterial absorbers offer promising prospects for intelligent and integrated designs in future applications.

Fractional order capacitance behavior due to hysteresis effect of ferroelectric material on GaN HEMT devices

AbstractIn recent years, gallium nitride (GaN) high electron mobility transistors (HEMTs) have come to the forefront of the semiconductor industry because of their exceptional performance in both high‐power and high‐frequency utility. Accurate capacitance modeling is crucial to optimize performance and facilitate energy‐efficient electronic circuit design. In order to reflect the complex nature of the aluminum scandium nitride (AlScN) gate capacitance in GaN HEMTs this study investigates the use of the unique Grünwald‐Letnikov model based on fractional order calculus. The proposed model presents a powerful approach to accurately characterize capacitance since fractional order derivatives allow modeling of non‐integer order systems. Quantitative assessment of the Grünwald‐Letnikov model's accuracy is performed through various error metrics, including mean absolute error (MAE), root mean square error (RMSE), maximum percentage error (MPE), mean absolute percentage error (MAPE), and mean squared error (MSE), by comparing the model's predictions to experimental data. Notably, this model demonstrates remarkable consistency in error metrics, with maximum values of MPE = 0.21%, MAE = 0.05%, MAPE = 0.33%, MSE = 0.01%, and RMSE = 0.09% for the forward scan, and MPE = 0.32%, MAE = 0.04%, MAPE = 0.39%, MSE = 0.01%, and RMSE = 0.08% for the backward scan. These metrics affirm the model's precision in capturing the nuanced capacitance characteristics of GaN HEMT devices. Hence, herein for the first time, the novel Grünwald‐Letnikov model, augmented by fractional order calculus, proves to be a robust tool for accurately characterizing GaN HEMT capacitance. Its ability to seamlessly account for the complexities introduced by using ferroelectric material highlights its potential for advancing semiconductor design and optimizing device performance.

A Comparative Study on the Degradation Behaviors of Ferroelectric Gate GaN HEMT with PZT and PZT/Al2O3 Gate Stacks.

In this paper, the degradation behaviors of the ferroelectric gate Gallium nitride (GaN) high electron mobility transistor (HEMT) under positive gate bias stress are discussed. Devices with a gate dielectric that consists of pure Pb(Zr,Ti)O3 (PZT) and a composite PZT/Al2O3 bilayer are studied. Two different mechanisms, charge trapping and generation of traps, both contribute to the degradation. We have observed positive threshold voltage shift in both kinds of devices under positive gate bias stress. In the devices with a PZT gate oxide, we have found the degradation is owing to electron trapping in pre-existing oxide traps. However, the degradation is caused by electron trapping in pre-existing oxide traps and the generation of traps for the devices with a composite PZT/Al2O3 gate oxide. Owing to the large difference in dielectric constants between PZT and Al2O3, the strong electric field in the Al2O3 interlayer makes PZT/Al2O3 GaN HEMT easier to degrade. In addition, the ferroelectricity in PZT enhances the electric field in Al2O3 interlayer and leads to more severe degradation. According to this study, it is worth noting that the reliability problem of the ferroelectric gate GaN HEMT may be more severe than the conventional metal-insulator-semiconductor HEMT (MIS-HEMT).

Bionic optimization for cooling structure of GaN HEMTs inspired by leaf vein structure

Inspired by the leaf vein structure, this article proposes cooling structures with tapered contracting manifold and leaf vein feature, to optimize the cold plate structure for high heat flux gallium nitride high electron mobility transistors (GaN HEMTs). The correlation between the fluid flow distribution in channels and the temperature uniformity of chips is investigated. The performance of the designed Bionic Structures is compared with that of the traditional Parallel Structure in terms of fluid flow characteristics, heat transfer characteristics and comprehensive performance. Results show that the Bionic Structures with tapered contracting manifold and leaf vein feature improve the fluid flow uniformity by up to 44.1 % and the temperature uniformity of chips by up to 65.0 %, while reducing the pressure drop by up to 24.7 % and the convective heat transfer resistance by up to 32.8 %. The comprehensive performance is also significantly improved. This article demonstrates the great potential of bionic cooling structure for breaking through the thermal management limit of high heat flux power chips.

Behavioral-level modelling of GaN HEMT large signal based on Pelican-Gaussian process regression algorithm

In this paper, a novel technology named Pelican-Gaussian process regression machine learning algorithm is proposed for modelling the large-signal characteristics of Gallium Nitride High Electron Mobility Transistors (GaN HEMT). Hyperparameter optimization in traditional Gaussian process regression algorithms tends to fall into local optimums and is overly dependent on the initial values. In order to solve this problem, the Pelican optimization algorithm is introduced to optimize the hyperparameters in Gaussian process regression algorithms in the article. The Pelican optimization algorithm is able to make the global exploration and local search ability of the algorithm be effectively balanced by helping particles to escape from the local optimal position. The I–V characteristics, output power, power gain, power gain efficiency and small-signal S-parameters of GaN HEMT devices are used to verify the effectiveness of the proposed algorithm. The experimental results show that higher fitting accuracy and generalization ability is found in the improved GPR whose hyperparameters are optimized by the Pelican optimization algorithm.

Modelling and mitigating oscillation in E-mode GaN HEMT: A simulation-based approach to parasitic inductance optimization

The escalating demands in energy conversion necessitate the evolution of efficient power electronic devices beyond the limitations of traditional silicon (Si) counterparts. This research delves into the intricacies of the Enhancement mode Gallium Nitride High Electron Mobility Transistor (E-mode GaN HEMT). A focus is placed on its static characterization, parameter analysis, and the influence of external parasitic inductances, particularly regarding oscillation challenges during its hard switching process. Forward and revise bias are conducted through comparison with Si-MOSFET and Cascade-GaN HEMT, along with its static on-resistance and transfer characteristics. Through rigorous half-bridge double pulse circuit LTSPICE simulations, this study deciphers the turn-on and turn-off dynamics of the E-mode GaN HEMT. By scrutinizing various parasitic inductances, the research elucidates mechanisms instigating high-frequency voltage and current oscillations. Significantly, the research presents a hierarchy of parasitic inductance influences, employing an energy loss comparison. This analysis underscores the prominent impact of the common source parasitic inductance, which exhibits energy loss increases of 23.6 % and 22.7 % at junction temperatures of 25 °C and 75 °C, respectively. Building on these insights, the study recommends strategies to mitigate the effects of external parasitic inductance, underscoring the pivotal role of oscillation suppression in advanced GaN Printed Circuit Board (PCB) and Integrated Circuit (IC) designs. This comprehensive inquiry provides a blueprint for enhancing GaN circuit designs, paving the way for their optimal application in the evolving realm of power electronics.

An Ameliorated Small-Signal Model Parameter Extraction Method for GaN HEMTs up to 110 GHz with Short-Test Structure

An improved method of extracting small-signal equivalent circuit model parameters for gallium nitride high electron mobility transistors (GaN HEMTs) is presented. This paper intends to present a method to extract the parasitic inductance and resistance of transistors based on the short-test structure without the open-circuit test structure. The parasitic capacitance of transistors is extracted by the method based on the size scalable model. Compared with the traditional COLD-FET method, the extraction procedure is simpler and more convenient. After removing the influence of parasitic elements, the intrinsic parameters of the model can be extracted by the S-parameters measured at different bias points. The experimental results show that the simulation results have good agreement with the measured results in the range of 0.5∼110 GHz.

High-Efficiency GaN-Based Power Amplifiers for Envelope Nonlinearities’ Mitigation in VHF Wideband Polar-Mode Transmitters

Space-based communications at the very high frequency (VHF) band for air traffic management is a new technology application under development that requires energy-efficient architectures to mitigate the power limitations of satellite platforms. The usage of high-efficiency radiofrequency (RF) transmitters can help reduce the power consumption, but nonlinearities concerning the amplified signal in wide fractional bandwidth systems are a problem to solve. This paper proposes a high-efficiency (RF) power amplifier (PA) for satellite communications at the VHF band that aims to reduce the envelope distortion inherent to wide fractional bandwidth multicarrier polar-mode transmitters. Its design is based on a solution called hybrid-coupled switching voltage PA in combination with gallium nitride (GaN) high electron mobility transistor (HEMT) technology. The developed VHF PA prototype delivers up to 95 W from a 28 V power supply, with a drain efficiency about 80% within the 118 MHz to 138 MHz operating band. To test its linearity performance, operating in a polar-mode configuration, a GaN-based wideband envelope amplifier (EA) has been developed to modulate the RF PA supply port. This EA improves its power efficiency by combining it with a slow envelope power supply (SEPS). Some measurements have been taken for a 100 W peak envelope power (PEP) and 10 MHz (maximum carrier spacing) four-tone digitally-modulated test signal, where any distortion product is attenuated 46 dB below the average power of the amplified signal without applying any digital predistortion (DPD) technique.

A novel stepped AlGaN hybrid buffer GaN HEMT for power electronics applications

In this paper, a novel p-type gallium nitride (p-GaN) high electron mobility transistor (HEMT) is proposed and investigated for high power electronic applications. In the proposed device, a stepped hybrid Aluminium GaN (AlGaN) buffer layer with low Al content has been introduced to improve the performance of the device. The low aluminium mole concentration decreases the surface defects which reduces the leakage current, thereby increasing the breakdown voltage of the device. To validate the simulation results, calibration of the device models has been performed with experimental results. Results present that the proposed device exhibits high breakdown voltage, high transconductance and low small signal capacitances. The proposed device achieves improvement in breakdown voltage, transconductance and small signal capacitances as compared to the conventional hybrid buffer high electron mobility transistor (HB-HEMT). Furthermore, the processing steps have been proposed for the possible fabrication of the proposed device. Also, the optimization is performed to achieve the optimal device performance with respect to breakdown voltage. Finally, the performance of the proposed device has been compared with the state-of-the-art devices.

(Invited) Sub-Bandgap Thermoreflectance Imaging of Ultra-Wide Bandgap Semiconductors

Over the past few decades, wide bandgap semiconductors (WBG) have been used to fabricate high power and high frequency devices. Device structures, such as Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs), have begun to be implemented in commercial products such as electrical vehicles and laptop power chargers. With the continuous increase in energy demand, however, extremely high voltage (>10 kV) applications are needed to increase the grid resiliency, storage, and transmission efficiency. Ultra-wide bandgap (UWBG) Semiconductors, with bandgaps >4 eV, could potentially be used to overcome this barrier. Pushing the maximum voltage limit of these devices, however, can lead to significant power dissipation and excessive joule heating.Unfortunately, the thermal conductivity of some UWBG semiconductors, such as Gallium Oxide, is inherently low and thus requires both device and package level thermal management. Innovative solutions, such as high thermal conductivity substrates, are required to reduce the active channel temperature and extend the device lifetime and reliability. In order to successfully implement these technologies, accurate measurement of the peak temperature and thermal distribution (both vertical and lateral) is necessary. Depth-averaged vertical temperature gradients have been demonstrated via Raman thermometry but this approach can become significantly time consuming if attempting to perform transient or lateral maps. On the other hand, Transient Thermoreflectance Imaging (TTI) has shown the potential to provide high throughput thermal images with high spatial resolution (100 nm) and temporal resolution (50 ns).Traditionally, TTI has been used to probe the temperature of metals due to their inherent high reflectivity. Nevertheless, recent studies have demonstrated the ability to probe the surface temperature of the semiconductor when using near-bandgap illumination sources. Due to the high bandgap of UWBG semiconductors, the maturity of high transmission UV sources, optics and detectors has not yet been fully developed. This study investigates the applicability of sub-bandgap wavelengths to probe the active channel temperature. The thermal properties of both Gallium Oxide transistors and Transfer Length Method (TLM) structures are investigated using TTI. The accuracy of the channel temperature is assessed by direct comparison to the adjacent metal contact temperature. A finite element model (FEM) is developed to understand the thermal transport in low thermally conductive semiconductors. Finally, a 3D analytical model is used to extract thermal properties such as the thermal conductivity of the thin film layers.Figure a) Optical CCD Image of Gallium Oxide Transfer Length Method (TLM) Structure b) Transient Thermoreflectance Image (TTI) of Gallium Oxide TLM using sub-bandgap illumination wavelength. Figure 1

GaN High Electron Mobility Transistor with Floating Gate for Accurate Threshold Voltage Control

Gallium nitride (GaN) high electron mobility transistors (HEMTs) have high electron mobility, high breakdown voltage, and low on-resistance, and are expected to operate as power devices. When GaN HEMTs are normally-on devices, drain current flow even for 0-gate-voltage. Thus, the normally-off operation is essentially required to the power devices from a fail-safe standpoint. Figure A shows a schematic of the structure, which is constructed with GaN HEMT including floating gate and the injection gate. The block oxide and tunnel oxide were formed between control gate and floating gate, and between injection gate and floating gate, respectively. It is impossible to inject electrons from the channel to the floating gate by the applying voltage to the substrate because AlGaN layer is located on the GaN layer. We introduced the injection gate for injecting electrons into the floating gate, and then the normally-off operation is obtained by the injecting electrons into the floating gate. The injecting electrons decrease the floating gate potential and form the depletion region under the gate oxide. Because the channel is the same as that of a conventional GaN HEMTs during on-operation, the high electron mobility of GaN HEMTs can be maintained, and then the on-operation behaviors are the same as conventional GaN HEMTs except threshold voltage. In this study, we demonstrate the normally-off operation of the fabricated GaN HEMTs with floating gate.Figure B shows the microscopy image of the fabricated device where the gate length, gate width and thickness of gate oxide are 2 μm, 110 μm and 20 nm, respectively. In floating gate device, the coupling ratio between the induced voltage of floating gate and the applied voltage of control gate is important. To increase the coupling ratio, we extended the overlap areas of floating gate and control gate to the drain side. The area of the control gate and the thickness of block oxide are set to be 3600 μm2 and 50 nm, respectively. Then, the coupling ratio becomes 91%, and then the induced voltage of floating gate is almost the same as the applied voltage of control gate. High-quality oxides are required in our devices for block oxide, tunnel oxide and gate oxide, however high-quality oxide films cannot be deposited by the thermal oxidation processes for GaN and polycrystalline Si. To form high-quality dielectric films, we used the microwave exited Plasma Enhanced Chemical Vapor Deposition (PECVD) [1][2][3].The Id-Vcg characteristics before and after the electron injection into the floating gate are shown in figure C. To change the threshold voltage in positive value, the voltages of control gate and injection gate are determined for the electron injection. The threshold voltage before the electron injection is -10.7 V and the electron injection increases the threshold voltage, resulting in a positive threshold voltage of 3.2 V. This voltage is sufficiently high voltage for normally-off operation. The Id-Vd characteristics after electron injection are shown in the figure D. It is found that the electron mobility is 2100 cm2/Vs, which is the similar value of 1500~2250 cm2/Vs in the reported conventional GaN HEMT [4][5][6] and this indicates that the device can be maintained high electron mobility as same as the conventional GaN HEMT.We fabricated the GaN HEMTs with floating gate, which realize the normally-off and high electron mobility. The threshold voltage can be controlled to be 3.2 V and the electron mobility of 2100 cm2/Vs is achieved. It is indicated that normally-off GaN HEMT device with high electron mobility can be demonstrated.[1] T. Ohmi, et al., J. Phys. D: Appl. Phys., 39, p.R,2006. [2] H. Tanaka, et al., Jpn. J. Appl. Phys. 42, p. 1911, 2003. [3] H. Kambayashi, et al., Solid-State Electronics, 54, p. 660 [4] J.-T. Chen, et al., Appl. Phys. Lett. 106, 251601, 2015. [5] K. Shinohara et al., IEEE Trans. Electron Devices, 60, p. 2982, 2013. [6] X. Ding et al., CES Transaction on Electrical Machines and Systems, 3, p54. Figure 1

Modeling of Enhancement Mode HEMT with Π-Gate Optimization for High Power Applications

This paper presents technology computer-aided design (TCAD) modeling of an enhancement-mode aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistor (HEMT) with extensive π-gate optimization for high-power and radio frequency (RF) applications. Effects of the gate voltages on threshold (Vth), transconductance (gm), breakdown voltage (VBR), cutoff frequency (fT), maximum frequency of oscillation (fmax) and minimum noise figure (NFmin) are systematically investigated with different gate structures (π–Shaped p-GaN MISHEMT, π–Shaped p-GaN HEMT, π–Gate HEMT). A comparative study demonstrates that π–Gate with additional p-GaN and insulating layer makes the device effectively operate in the enhancement mode having a threshold voltage (Vth) = 1.72 V with a breakdown voltage (VBR) = 341 V, exhibiting better gate control with maximum transconductance (gm,max) of 0.321 S/mm. In addition, the proposed device architecture with an optimized gate structure maintains a balance between a positive device threshold and a high breakdown voltage and achieves a better noise immunity with the minimum noise figure of 0.64 dB while operating at 10 GHz with a cutoff frequency (fT) of 33.4 GHz, and a maximum stable operating frequency (fmax) of 82.3 GHz. Moreover, the device achieved an outstanding Vth, gm,max, VBR, fT, fmax and NFmin making it suitable for high-power, high-speed electronics, and low-noise amplifiers.