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In this paper, AlGaN/GaN high-electron-mobility transistors (HEMTs) with ohmic etching patterns (OEPs) "fabricated to improve device radio frequency (RF) performance for Ka-band applications" are reported. The fabricated AlGaN/GaN HEMTs with OEP structures were used to reduce the source and drain resistances (Rs and Rd) for RF performance improvements. Within the proposed study using 1 μm hole, 3 μm hole, 1 μm line, and 3 μm line OEP HEMTs with 2 × 25 μm gate widths, the small signal performance, large signal performance, and minimum noise figure (NFmin) with optimized values were measured for 1 μm line OEP HEMTs. The cut-off frequency (fT) and maximum oscillation frequency (fmax) value of the 1 μm line OEP device exhibited optimized values of 36.4 GHz and 158.29 GHz, respectively. The load–pull results show that the 1 μm line OEP HEMTs exhibited an optimized maximum output power density (Pout, max) of 1.94 W/mm at 28 GHz. The 1 μm line OEP HEMTs also exhibited an optimized NFmin of 1.75 dB at 28 GHz. The increase in the contact area between the ohmic metal and the AlGaN barrier layer was used to reduce the contact resistance of the OEP HEMTs, and the results show that the 1 μm line OEP HEMT could be fabricated, producing the best improvement in RF performance for Ka-band applications. [ABSTRACT FROM AUTHOR]

A new power receiving unit (PRU) is proposed in this paper for resonant wireless power transfer (WPT), which is characterized by the capability of attracting high power from the power transmitting unit (PTU). The resonant WPT is designed for delivering the electrical power to the PRU attached on an electrical vehicle (EV) chassis 50 cm away from a PTU installed on the ground. The proposed PRU uses only the passive elements such as inductors, diodes, and capacitors, which need no initial power from the EV. It is then applicable for charging a battery to several hundred volts for even a first-time charging battery. For a resonant WPT at a switching frequency of 4 MHz, the proposed PRU behaves as a negative impedance converter (NIC) itself in the subharmonics of 4 MHz. The NIC effect plus the subharmonic oscillation causes an instability current charging the battery connected to the PRU. In this paper, we simulated the PRU and performed the experiment. The experiment demonstrated a battery charging of 150 W from 50 cm away using three D-mode GaN HEMT transistors via the instability current ramp. The power transfer efficiency (PTE) improved as the power delivered to the load (PDL) increased. The peak PTE was 65% in the present findings. The simulation analysis showed that the circuit allowed itself be used to much higher power transfer when it is implemented with more GaN HEMT transistors connected in parallel. The theoretical derivation of the PRU circuit is also used to support both the experimental and simulation results. [ABSTRACT FROM AUTHOR]

The effect of different SiNx thicknesses on the performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated in this paper. The current, transconductance (Gm), cut-off frequency (fT), maximum oscillation frequency (fmax), power performance, and output third-order intercept point (OIP3) of devices with three different SiNx thicknesses (150 nm, 200 nm, and 250 nm) were measured and analyzed. The DC measurements revealed an increase in both the drain-source current (IDS) and Gm values of the device with increasing SiNx thickness. The S-parameter measurement results show that devices with a higher SiNx thickness exhibit improved fT and fmax. Regarding power performance, thicker SiNx devices also improve the output power density (Pout) and power-added efficiency (PAE) in the Ka-band. In addition, the two-tone measurement results at 28 GHz show that the OIP3 increased from 35.60 dBm to 40.87 dBm as the SiNx thickness increased from 150 nm to 250 nm. The device's characteristics improved by appropriately increasing the SiNx thickness. [ABSTRACT FROM AUTHOR]

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure devices have proven to be highly effective for high‐frequency power amplifiers and power switching applications with improved performance compared to those made with traditional silicon technology and other advanced semiconductor technologies. The development of enhancement‐mode (E‐mode) AlGaN/GaN high electron mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS‐HEMTs) has been a focus in recent years due to their potential applications. Arising from the concept of a flash‐memory‐like hybrid ferroelectric charge storage structure, the high‐performance hybrid ferroelectric charge storage gate (FEG) GaN HEMT has gradually gained a great deal of attention due to the concept being a useful and versatile tool to realize E‐mode operations. This article attempts to review the latest progresses in this technology, including alternative improvements and device characteristics. Future challenges for the E‐mode FEG‐HEMT are also discussed. [ABSTRACT FROM AUTHOR]

This paper presents a 250 kHz bidirectional battery charger circuit using a GaN HEMT. The charger is subjected to a high-/low-side constant voltage at 200 V/20 V. The charger circuit is a hybrid of the LLC and flyback circuit topologies. Both the power output analysis and efficiency control of this circuit are simplified when the magnetization current is minimized using the low-resistance GaN HEMT. The switching frequency is controlled to match the series resonance in a way that is analogous to conventional LLC circuit controls, while the duty ratio that determines the power output and the dead time, which determines the zero voltage switching, is controlled in an analogous manner to the flyback circuit control. The charging and discharging modes were altered by applying a double-throw relay that changes the transformer turn ratio, which is different from conventional LLC designs using the switching frequency adjustment. A nominal turn ratio with Np = 35 and Ns = 3.5 for a 200 V/20 V converter can only produce an internal series resonance with no current flowing in any charging direction. The proposed circuit using a transformer with multiple windings (Np = 35, Ns,F = 4, and Ns,R = 3) was fabricated to deliver 125 W output power from the power grid battery to the vehicle battery in the forward (charging) mode and 90 W in the reverse (discharging) mode. The conversion efficiency was calculated to be as high as 97% in the forward mode and 95% in the reverse mode. The high conversion efficiency is due to the characteristics of the GaN HEMT, including low resistive and switching losses. The equations derived in this paper associate these losses with the series resonant frequency and power conversion rate, which highlight the advantages of using a GaN HEMT in this CLLC design. [ABSTRACT FROM AUTHOR]

A high-pressure (HP) GaN nucleation layer (NL) was inserted between AlGaN buffer and an unintentionally doped (UID) GaN layer of an AlGaN/GaN HEMT on Si. The XRD and TEM showed that when the V/III ratio was optimized during the HP-GaN NL growth, the edge dislocation density in the HP-GaN NL layer could be reduced significantly. Experimental results exhibited a lower off-state leakage current, higher maximum ID and Gm (corresponding to 22.5% and 21.7% improvement, respectively), and lower on-state resistance. These results demonstrate that the electrical properties of the AlGaN/GaN HEMT can be improved through the insertion of a HP-GaN NL. [ABSTRACT FROM AUTHOR]

GaN high-electron-mobility transistors (HEMTs) have attracted widespread attention for high-power microwave applications, owing to their superior properties. However, the charge trapping effect has limitations to its performance. To study the trapping effect on the device large-signal behavior, AlGaN/GaN HEMTs and metal-insulator-semiconductor HEMTs (MIS-HEMTs) were characterized through X-parameter measurements under ultraviolet (UV) illumination. For HEMTs without passivation, the magnitude of the large-signal output wave ( X 21 FB ) and small-signal forward gain ( X 2111 S ) at fundamental frequency increased, whereas the large-signal second harmonic output wave ( X 22 FB ) decreased when the device was exposed to UV light, resulting from the photoconductive effect and suppression of buffer-related trapping. For MIS-HEMTs with SiN passivation, much higher X 21 FB and X 2111 S have been obtained compared with HEMTs. It suggests that better RF power performance can be achieved by removing the surface state. Moreover, the X-parameters of the MIS-HEMT are less dependent on UV light, since the light-induced performance enhancement is offset by excess traps in the SiN layer excited by UV light. The radio frequency (RF) power parameters and signal waveforms were further obtained based on the X-parameter model. The variation of RF current gain and distortion with light was consistent with the measurement results of X-parameters. Therefore, the trap number in the AlGaN surface, GaN buffer, and SiN layer must be minimized for a good large-signal performance of AlGaN/GaN transistors. [ABSTRACT FROM AUTHOR]

In this paper, AlGaN/GaN high electron mobility transistors (HEMTs) with etched-fin gate structures fabricated to improve device linearity for Ka-band application are reported. Within the proposed study of planar, one-etched-fin, four-etched-fin, and nine-etched-fin devices, which have 50-μm, 25-μm, 10-μm, and 5-μm partial gate widths, respectively, the four-etched-fin gate AlGaN/GaN HEMT devices have demonstrated optimized device linearity with respect to the extrinsic transconductance ( G m ) value, the output third order intercept point (OIP3), and the third-order intermodulation output power (IMD3) level. The IMD3 is improved by 7 dB at 30 GHz for the 4 × 50 μm HEMT device. The OIP3 is found to reach a maximum value of 36.43 dBm with the four-etched-fin device, which exhibits high potential for the advancement of wireless power amplifier components for Ka band applications. [ABSTRACT FROM AUTHOR]

This paper presents a study on long-distance wireless power transfer (WPT), which formulates the voltage gain in terms of the coupling coefficient between the power transmitting unit (PTU) and the power receiving unit (PRU) coils. It is proposed that maximum power transfer efficiency (PTE) can be reached when maximum voltage gain is achieved under a matching condition between the coil quality factor and the coupling coefficient. In order to achieve maximum power delivered to load (PDL), we need to elevate the input voltage as high as the high breakdown-voltage of gallium nitride (GaN) high-electron mobility transistors (HEMT) along with class E amplifier circuit topology. In order to promote voltage gain, knowledge of the coupling coefficient between two coils including the factors of the coil diameter, wire diameter, coil turns, and the coil resistance are derived. It was observed that a lower coil resistance leads to a reduced parallel quality, which facilitates long-distance wireless power transfer. Experimental results support the findings that the maximum PTE occurred at the maximum voltage gain existing at a specific distance matches the coupling coefficient between coils. A maximum power point tracking (MPPT) method is also developed to achieve maximum PDL. At a distance of 35 cm, experiments with more than 100 W successfully receive a PTE of 57% at the PRU when the received voltage reached 1.4 kV. This is used to verify the concepts and analysis that are proposed in this paper. [ABSTRACT FROM AUTHOR]

Herein, the AlGaN/GaN high‐electron‐mobility transistors (HEMTs) on silicon substrates using thick copper‐metallized interconnects with Pt diffusion barrier layer for Ka‐band application are reported. High output power density of 6.6 W mm−1 with power‐added efficiency (PAE) of 45.6% at 28 GHz is achieved for the 4 × 50 μm device in continuous‐wave (CW) mode. No obvious change in the drain–source current (IDS) is observed for the device under 40 V high‐voltage stress for 100 h and the device shows good thermal stability when annealed at 300 °C for 30 min. It demonstrates that the AlGaN/GaN HEMTs on silicon substrate with thick copper‐metallized interconnects can enhance the device performance with good reliability for future 5 G applications. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN), widely known as a wide bandgap semiconductor material, has been mostly employed in high power devices, light emitting diodes (LED), and optoelectronic applications. However, it could be exploited differently due to its piezoelectric properties, such as its higher SAW velocity and strong electromechanical coupling. In this study, we investigated the affect of the presence of a guiding layer made from titanium/gold on the surface acoustic wave propagation of the GaN/sapphire substrate. By fixing the minimum thickness of the guiding layer at 200 nm, we could observe a slight frequency shift compared to the sample without a guiding layer, with the presence of different types of surface mode waves (Rayleigh and Sezawa). This thin guiding layer could be efficient in transforming the propagation modes, acting as a sensing layer for the binding of biomolecules to the gold layer, and influencing the output signal in terms of frequency or velocity. The proposed GaN/sapphire device integrated with a guiding layer could possibly be used as a biosensor and in wireless telecommunication applications. [ABSTRACT FROM AUTHOR]

In this work, we demonstrated the thermal analysis of different flip-chip bonding designs for high power GaN HEMT developed for power electronics applications, such as power converters or photonic driver applications, with large gate periphery and chip size, as well as an Au metal heat-spreading layer deposited on top of a planarized dielectric/passivation layer above the active region. The Au bump patterns can be designed with high flexibility to provide more efficient heat dissipation from the large GaN HEMT chips to an AlN package substrate heat sink with no constraint in the alignment between the HEMT cells and the thermal conduction bumps. Steady-state thermal simulations were conducted to study the channel temperatures of GaN HEMTs with various Au bump patterns at different levels of current and voltage loadings, and the results were compared with the conventional face-up GaN die bonding on an AlN package substrate. The simulations were started from a single finger isolated HEMT cell and then extended to multiple fingers HEMT cells (total gate width > 40 mm) to investigate the "thermal cross-talk" effect from neighboring devices. Thermal analysis of the GaN HEMT under pulse operation was also performed to better reflect the actual conditions in power conversion or pulsed laser driver applications. Our analysis provides a combinational assessment of power GaN HEMT dies under a working condition (e.g., 1MHz, 25% duty cycle) with different flip chip packaging schemes. The analysis indicated that the channel temperature rise (∆T) of a HEMT cell in operation can be reduced by 44~46% by changing from face-up die bonding to a flip-chip bonding scheme with an optimized bump pattern design. [ABSTRACT FROM AUTHOR]

Normally-off ferroelectric charge trap gate stack GaN high electron mobility transistor (FEG-HEMT) was fabricated with atomic layer etching (ALE) to precisely control the device parameters including $\text {V}_{\text {th}}$ of the device. The ALE process consists of cyclic Cl2 adsorption modification steps and the Ar ion removal steps. The ALE process achieved etch-per-cycle (EPC) of 0.347 nm/cycle and superior etching morphology with RMS $=0.281$ nm. The fabricated GaN HEMT using the ALE process exhibited a high threshold voltage ($\text {V}_{\text {th}}$) of 5.06 V, high maximum drain current ($\text {I}_{\text {D,MAX}}$) of 772 mA/mm with low on-resistance ($\text {R}_{\text {on}}$) of $8.57~\Omega \cdot \text {mm}$ and high breakdown voltage (BV) of 888 V, the device also showed good $\text {V}_{\text {th}}$ uniformity. Finally, the contact resistance ($\text {R}_{\text {c}}$) was reduced from $0.46~\Omega \cdot \text {mm}$ to $0.15~\Omega \cdot \text {mm}$ by the ALE process, and the dynamic on-resistance (dyn- $\text {R}_{\text {on}}$) was improved at the same time. [ABSTRACT FROM AUTHOR]

In this study, a paradigm for modulating the light emission performance of photonic-crystal surface-emitting laser (PCSEL) via GaN high electron mobility transistor (HEMT) driving circuit is proposed for the first time. For light detection and ranging (LiDAR) system, a faster pulse repetition frequency with shorter pulse width can provide not only high resolution but also sufficiently precise range resolution. Hereupon, comprehensive analyses for such an integrated system are conducted with not only electro-optical responses but also the corresponding optical behaviors. The relevant electrical characteristics of the employed GaN HEMT are examined at first. Next, the integrated system on a matrix board with its corresponding circuit topology is discussed, illustrating the relevant operating principles. Thereby, sufficient systematical scrutinization for relevant light emissions is performed for both photodiode responses and the optical behaviors under different conditions, paving a holistic panorama for the LiDAR system. Thus, prospects for the next generation LiDAR system in high-power and high-speed operation can be expected. [ABSTRACT FROM AUTHOR]

In this work, the effect of Total Ionizing Dose (TID) on the performance of GaN on Si MIS-HEMT power devices with in-situ SiN cap layer is investigated. 13 samples were exposed to different accumulated dose (100 krad and 400 krad) of Co $^{60}~\gamma $ -ray irradiation with two bias conditions (grounded and stressed). The characteristics of the devices after irradiation experiment were evaluated by measuring VTH shift, on-resistance (RON), and dynamic RON. A comprehensive study of TID effects are demonstrated in this article to verify different mechanisms and their interactions. Simulations using PHITs were also carried out to further confirm the damages in the epitaxial structure by the accumulated dose. [ABSTRACT FROM AUTHOR]

The flyback converter with its active cell balancing topology for charging lithium-based batteries in Electrical Vehicles (EV) have been adopted recently into the industry. Electrical Vehicle battery charging requires high current operation in continuous current mode and hence, the power loss on the Schottky diode rectifier on the secondary side determines the power conversion efficiency. The depletion mode (D-mode) GaN HEMT synchronous rectifier proposed in this paper has been used to replace the Schottky diode on the secondary side of the flyback converter in order to improve the power conversion efficiency. This synchronous rectifier regulates the forward voltage drop of an external switch to about 100 mV per ampere of current flow with no concern to threshold voltage. The first challenge of converting the D-mode GaN HEMT as a synchronous rectifier is that the normally-on device must be off when the primary side inductor of the flyback converter is initially charging the magnetic energy. That is, the rectifier must behave as the normally-off device during its initialization stage. The second challenge is that the D-mode GaN HEMT must switch off as soon as the secondary current becomes zero. The third challenge is posing a fast recovery feature to reduce the drain-source voltage rise on the primary side switch, which suffices to be the main reason as to why the D-mode GaN HEMT is used instead of MOS devices. The proposed depletion mode GaN HEMT synchronous rectifier is verified to be able to overcome all challenges and in result becomes a candidate for the synchronous rectifier. [ABSTRACT FROM AUTHOR]

The presence of surface traps is an important phenomenon in AlGaN/GaN HEMT. The electrical and physical properties of these surface traps have been analyzed through the study of 2DEG electron concentration along with the variation of aluminum percentage in the barrier layer of HEMT. This analysis shows that from deep to shallow donors, the percentage change in electron density in 2DEG gets saturated (near 8%) with change in aluminum concentration. The depth of the quantum potential well below the Fermi level is also analyzed and is found to get saturated (near 2%) with aluminum percentage when surface donor states energy changes to deep from shallow. The physics behind this collective effect is also analyzed through band diagram too. The effect of surface donor traps on the surface potential also has been discussed in detail. These surface states are modeled as donor states. Deep donor (EC − ED = 1.4 eV) to shallow donor (EC − ED = 0.2 eV) surface traps are thoroughly studied for the donor concentration of 1011 to 1016 cm−2. This study involves an aluminum concentration variation from 5 to 50%. This paper for the first time presents the comprehensive TCAD study of surface donor and analysis of electron concentration in the channel and 2DEG formation at AlGaN–GaN interface. [ABSTRACT FROM AUTHOR]

A high-performance E-mode hybrid ferroelectric charge storage gate (FEG) GaN HEMT with an innovative source-connected field plate (SCFP) structure is reported for the first time. The optimized E-mode FEG-HEMT implements a novel SCFP structure, which creates a cascode-like configuration with a D-mode GaN MISHEMT. This E-mode SCFP GaN FEG-HEMT has a positive Vth of 2.81 V, a high ID,max of 757mA/mm, and a BV of 866 V. Dynamic RON reduced 25% when operated at VDSQ = 400 V. ON-state stress tests also show improved current collapse phenomena. Additionally, to address the charge storage abilities of FEG-HEMTs, a multi-cycle OFF-state (VDS = 300 V, VGS = 0 V) retention test was conducted. The SCFP FEG-HEMT showed 58.46% less Vth shift percentage than the FEG-HEMT without field plates. [ABSTRACT FROM AUTHOR]

The hybrid gate-recessed structure and ferroelectric charge trapping/storage stacked structure were used in AlGaN/GaN enhancement-mode metal–oxide–semiconductor high-electron mobility transistors (E-MOSHEMTs). The stacked structure consisted of a bottom 10-nm-thick Al2O3 tunnel oxide layer, a middle 4-nm-thick HfO2 charge trapping/storage layer, and a top 40-nm-thick LiNbO3 ferroelectric blocking layer. Using the postannealing process, a high polarized C+ domain of the pulsed laser-deposited LiNbO3 ferroelectric blocking layer was formed and the interface quality of the stacked structure was improved simultaneously. Consequently, positive threshold voltage for enhancement-mode operation was obtained. By studying the performances of the resulting devices, it was found that enhancement-mode behaviors were observed for initializing gate more than 5 V, and the threshold voltage of 1.6 V was obtained for initializing gate at 12 V. The high-frequency performances of the extrinsic unit gain cutoff frequency of 5.9 GHz and maximum oscillation frequency of 10.1 GHz were obtained. Furthermore, a low normalized noise power density of about $1.9\times 10^{-14}$ Hz−1 was achieved for the devices operating at a frequency of 10 Hz, a gate–source voltage of 5 V, and a drain–source voltage of 1 V. [ABSTRACT FROM AUTHOR]

The charge trapping effects on AlGaN/GaN HEMTs under UV illumination are investigated using the pulsed current-voltage (I-V) measurement method. The test samples are unpassivated Schottky-gate HEMTs and metal-insulator-semiconductor HEMTs (MIS-HEMTs) with SiN gate dielectric. For HEMTs, the dominant charge trapping sources are the surface trap states, whereas, for MIS-HEMTs, they are trap states in the SiN gate dielectric and GaN buffer. When these devices are shined with the UV light, the drain current increases apparently in both samples owing to the generated photocurrent. By combining the UV illumination and pulsed I-V measurement, we find out the UV light has less effect on the surface charge trapping in the unpassivated HEMTs. Moreover, in MIS-HEMTs, we observe the charge trapping in the SiN gate dielectric becomes more serious under UV illumination, whereas the charge trapping in the GaN buffer is suppressed significantly. These findings are important for designing a GaN-based HEMT for photonic applications. In addition, the different responses of the surface-, buffer-, and gate-dielectric-related charge trapping to the UV light suggest that it would be easier to distinguish the trap types by introducing the UV illumination during the pulse measurement. [ABSTRACT FROM AUTHOR]

A GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) using tri-gate architecture and hybrid ferroelectric charge trap gate stack is demonstrated for normally-off operation. Compared with the conventional planar device, the tri-gate device has the 2-D electron gas (2-DEG) channel exposed on the nanowire sidewalls, so that the trapped charges in the HfON charge-trapping layer can easily deplete the channel from the sidewalls, leading to a high positive threshold voltage (${V}_{\text {th}}$) to realize the normally-off operation. Moreover, through this electrostatic control on the sidewall, a high density of negative charge caused by hybrid ferroelectric charge trap gate stack with the optimized tri-gate structure, the tri-gate device can achieve normally-off GaN device with both low on-resistance (${R}_{ \mathrm{ON}}$) and high positive ${V}_{\text {th}}$. The designed tri-gate device exhibits a high ${V}_{\text {th}}$ of +2.61 V at current density (${I}_{\text {DS}}) = 1\,\,\mu \text{A}$ /mm, a high maximum current density (${I}_{\text {DS, MAX}}$) of 896 mA/mm, a low ${R}_{ \mathrm{ON}}$ of $5.0~\Omega \!\!\cdot \!\!\text {mm}$ and a high breakdown voltage (BV) of 788 V. To the best of our knowledge, the proposed tri-gate device shows the lowest specific on-resistance (${R}_{ \mathrm{ON}, \text {SP}}$) among reported normally-off GaN device results with BV >650 V. [ABSTRACT FROM AUTHOR]

A GaN metal-insulator-semiconductor-high electron mobility transistor (HEMT) using hybrid ferroelectric charge trap gate stack (FEG-HEMT) is demonstrated for normally-OFF operation. The ferroelectric (FE) polarization increases the number of trapped charges in the HfON charge trapping layer, leading to high positive threshold voltage ( ${V}_{\textsf {th}}$ ) shift for the normally-OFF device. Besides, under the positive bias temperature instability (PBTI) test, the internal electric field induced by FE polarization causes smoother slope of the conduction band in FE gate stack, resulting in better ${V}_{\textsf {th}}$ stability. With the proposed hybrid FE charge trap gate stack, the device exhibits a high ${V}_{\textsf {th}}$ of +2.71 V at ${I}_{\textsf {DS}} = \textsf {1}\,\,\mu \text{A}$ /mm, a high maximum current density of 820 mA/mm and low on-resistance ( ${R}_{ \mathrm{\scriptscriptstyle ON}}$ ) of 11.1 $\Omega \cdot \textsf {mm}$. The FE device also shows good ${V}_{\textsf {th}}$ –temperature stability compared to the non-FE device results. Besides, a high current device with 40 A is also fabricated in this letter to demonstrate the feasibility of the proposed FEG-HEMT device for high power device application. [ABSTRACT FROM AUTHOR]

The gate leakage mechanism for InAlGaN/GaN high electron mobility transistors (HEMTs) is systematically studied using temperature‐dependent gate current–voltage characteristics. The electric field across the barrier layer is calculated through the extracted polarization charge and dielectric constant of the InAlGaN/GaN HEMT. The gate current of the InAlGaN/GaN HEMT is analyzed by fitting the experimental data using Themionic Emission (TE), Poole–Frenkel (PF), and Fowler–Nordheim (FN) tunneling. The results show that 1) reverse leakage current in the thin InAlGaN barrier layer is dominated by FN tunneling due to triangular barrier formation; 2) for thicker barrier layer, the reverse leakage current at low electric field (<2.23 MV cm−1) is dominated by PF emission; 3) at high electric field near the threshold voltage the FN tunneling dominates. Extraction of effective barrier height by fitting the experimental data with models leads to the suggestion for the improvement of the device leakage current. [ABSTRACT FROM AUTHOR]

This letter reports an E-mode GaN MIS-HEMT using a composite La2O3/HfO2 gate insulator for power device applications. The composite dielectric formed an amorphous LaHfOx layer after post-deposition annealing. The GaN MIS-HEMT with amorphous LaHfOx gate dielectric showed good oxide film quality and excellent HfLaOx/GaN interface properties, as demonstrated by the measured C–V characteristics. Consequently, the E-mode MIS-HEMT was determined to show good V \mathrm {th} stability with only a slight increase in the dynamic R \mathrm {on} after high drain bias stress. [ABSTRACT FROM AUTHOR]

One of the epitaxial issues pertaining to the growth of AlGaN/GaN HEMTs on Si is the decrease of parasitic losses that can adversely impact the RF device performances. We characterized the microwave losses in coplanar waveguides (CPWs) on GaN-based high-electron-mobility-transistors (HEMTs) and their buffer layers on Silicon substrate, up to 40 GHz. The RF losses depend not only on the crystalline quality but also on the residual tensile stress in AlN buffer, as well as its thickness. The mechanism of interfacial lossy channel induced by the piezoelectric field is discussed. Adopting a thin high-low-high temperature (HLH) AlN buffer can help to reduce the tensile stress leading to a reduction of RF losses. We suggest that a thinner p-type AlN and/or p-AlGaN-on-thin AlN near the interface can suppress the electron interfacial lossy channel, which helps the GaN-HEMT-on-HR Si to remain in a high frequency range and at high-temperature operation. [ABSTRACT FROM AUTHOR]

High-performance GaN metal–oxide–semi-conductor high-electron-mobility transistors (MOSHEMTs) using Al2O3 gate dielectric deposited by atomic layer deposition (ALD) and damage-free neutral beam etch (NBE) gate recess process for millimeter-wave power applications are demonstrated. The high-quality ALD Al2O3 reduces the gate leakage current of the device and the NBE method eliminates the plasma-induced defects in the nitride materials. The MOSHEMT device fabricated exhibits a high maximum drain current density ( I\textsf {DS,max} ) of 1.65 A/mm and a high peak extrinsic transconductance ( g\textsf {m.ext} ) of 653 mS/mm. The MOSHEMT device also demonstrates excellent RF performances, including fT/f\textsf {MAX}= 183 /191 GHz, NF \textsf {min}= 2.56 dB with G\textsf {AS}= 5.61 dB at 54 GHz, and an output power density of 2.7 W/mm associated with a power-added efficiency of 20.9% and a linear power gain of 9.4 dB at 38 GHz. To the best of our knowledge, the noise performance at 54 GHz is the best reported so far for the AlGaN/GaN HEMTs. [ABSTRACT FROM PUBLISHER]

The electrical performances of gate-recessed AlGaN/GaN high-electron mobility transistors (HEMTs) fabricated using the damage-free neutral beam etching (NBE) method are demonstrated. The NBE method could eliminate the plasma-induced defects generated by irradiating ultraviolet/VUV photons in the conventional inductively coupled plasma reactive ion etching method. The AlGaN/GaN HEMT device fabricated using the new gate recess process exhibited superior electrical performances, including a maximum drain current density ( I\mathrm {DS,max} ) of 1.54 A/mm, low 1/f noise, a current-gain cutoff frequency ( {f}_{\mathrm {T}} ) of 153 GHz, a maximum frequency of oscillation ( {f}_{\mathrm {{MAX}}} ) of 167 GHz, and a minimum noise figure (NF \mathrm {{min}} ) of 3.28 dB with an associated gain ( {G}\mathrm {{AS}} ) of 5.06 dB at 54 GHz. Such superior characteristics confirm the inherent advantages of adopting the damage-free NBE process in fabricating GaN devices for millimeter-wave applications. [ABSTRACT FROM AUTHOR]

The piezoelectric polarization of the AlGaN/GaN high-electron-mobility transistors (HEMTs) is strongly related to the strain state in the active area. Therefore, understanding the strain behavior inside the channel is crucial to the device electrical performance improvement of devices. This paper, for the first time, reveals the potential of optimizing flip-chip structures with active-region bumps to modulate the strain state of the AlGaN/GaN HEMT for enhancing the piezoelectric effect. The thermo-mechanical strain is observed to be affected by the physical dimensions and the material properties of the package. Thus, incorporating device strain engineering into packaging design will be very important for GaN devices due to their strong piezoelectric effects. [ABSTRACT FROM PUBLISHER]

The influence of low-temperature AlN (LT-AlN) nucleation layer thickness on the material properties of the GaN layer grown on the double-step AlN layer is investigated. When GaN was grown without the LT-AlN nucleation layer, the GaN layer has low sheet resistance of 464 ohm/sq and the surface was decorated with pitted region. On the other hand, when a LT-AlN layer with a thickness of 5 nm was inserted, a GaN layer with sheet resistance higher than 10 ohm/sq was achieved. This thin nucleation layer also improved the GaN morphology, suppressed inversion domain formation, and reduced oxygen impurity incorporation. However, the surface morphology and quality of the GaN crystal were severely degraded when the LT-AlN thickness was increased to 10 nm due to the formation of disorientated grains in the LT-AlN layer. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the effect of the field plate structure on the RF performance of AlGaN/GaN High Electron Mobility Transistor (HEMT) devices. While the field plate structure helps to increase the breakdown voltage of the device through modulating the electric field locally, it induces additional feedback capacitance from drain to gate. Such feedback capacitors may impact the overall RF performance of the device especially at high frequencies. Systematic investigations on the small signal as well as power performance as functions of the drain biases are presented. [Copyright &y& Elsevier]

Abstract: A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2×108 cm−2. The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N2 radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate. [Copyright &y& Elsevier]

GaN film grown on Si substrate using multilayer AlN/AlxGa1-xN buffer is studied by the low-pressure metal-organic chemical-vapor deposition method. The AlxGa1-xN films with Al composition varying from 1 to 0.66 were used to accommodate the stress induced between GaN and the Si substrate during GaN growth. The correlation of the Al composition in the AlxGa1-xN films with respect to the stress induced in the GaN film grown was studied using high-resolution x-ray diffraction, including symmetrical and asymmetrical ω/2θ scans and reciprocal space maps. It is found that with proper design of the Al composition in the AlxGa1-xN buffer layer, crack-free GaN film can be successfully grown on 6 in. Si (111) substrates using multilayer AlN and AlxGa1-xN buffer layers. [ABSTRACT FROM AUTHOR]

Abstract: The defect structure of GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) depends on the growth temperature and thickness of the aluminum nitride (AlN) buffer layer. High-resolution X-ray diffraction was used to measure symmetric (0002) and asymmetric (101¯2) rocking curve (ω-scans) broadening, which allowed the estimation of screw threading dislocation (TD) and edge TD densities, respectively. For GaN grown on lower-temperature buffer, the density of screw TD was increased while the density of edge TD was decreased. Further examinations revealed that the edge TD was closely related to stress in GaN film and the screw TD was controlled by AlN surface roughness. Since the GaN defect was dominated by edge TD, the total TD was also effectively suppressed with the use of lower-temperature buffer with appropriate thickness. [Copyright &y& Elsevier]

In this study, an electron-beam lithography system was employed to pattern 80-nm-wide and 980-nm-spaced multi-mesa-channel for fabricating AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs). Since the structure of multi-mesa-channel could enhance gate control capabilities and reduce the self-heating effect in the channel, the performance of the MOSHEMTs could be obviously improved. The direct current performance metrics of the multi-mesa-channel-structured MOSHEMTs, such as a saturation drain-source current of 929 mA/mm, maximum extrinsic transconductance of 223 mS/mm, and on-resistance of 2.1 Ω-mm, were much better than those of the planar-structured MOSHEMTs. Moreover, the threshold voltage of the multi-mesa-channel-structured MOSHEMTs shifted toward positive voltage from −2.6 to −0.6 V, which was attributed to the better gate control capability. Moreover, the multi-mesa-channel-structured MOSHEMTs also had superior high-frequency and low-frequency noise performance. A low Hooge's coefficient of 1.17 × 10−6 was obtained. [ABSTRACT FROM AUTHOR]

In this work, a low-power plasma oxidation surface treatment followed by Al2O3 gate dielectric deposition technique is adopted to improve device performance of the enhancement-mode (E-mode) AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs) intended for applications at millimeter-wave frequencies. The fabricated device exhibited a threshold voltage (Vth) of 0.13 V and a maximum transconductance (gm) of 484 (mS/mm). At 38 GHz, an output power density of 3.22 W/mm with a power-added efficiency (PAE) of 34.83% were achieved. Such superior performance was mainly attributed to the high-quality Al2O3 layer with a smooth surface which also suppressed the current collapse phenomenon. [ABSTRACT FROM AUTHOR]

GaN HEMT has attracted a lot of attention in recent years owing to its wide applications from the high-frequency power amplifier to the high voltage devices used in power electronic systems. Development of GaN HEMT on Si-based substrate is currently the main focus of the industry to reduce the cost as well as to integrate GaN with Si-based components. However, the direct growth of GaN on Si has the challenge of high defect density that compromises the performance, reliability, and yield. Defects are typically nucleated at the GaN/Si heterointerface due to both lattice and thermal mismatches between GaN and Si. In this article, we will review the current status of GaN on Si in terms of epitaxy and device performances in high frequency and high-power applications. Recently, different substrate structures including silicon-on-insulator (SOI) and engineered poly-AlN (QST®) are introduced to enhance the epitaxy quality by reducing the mismatches. We will discuss the development and potential benefit of these novel substrates. Moreover, SOI may provide a path to enable the integration of GaN with Si CMOS. Finally, the recent development of 3D hetero-integration technology to combine GaN technology and CMOS is also illustrated. [ABSTRACT FROM AUTHOR]

Multiple-mesa-fin-channel array patterned by a laser interference photolithography system and gallium oxide (Ga2O3) gate oxide layer deposited by a vapor cooling condensation system were employed in double-channel Al0.83In0.17N/GaN/Al0.18Ga0.82N/GaN heterostructured-metal-oxide-semiconductors (MOSHEMTs). The double-channel was constructed by the polarized Al0.18Ga0.82N/GaN channel 1 and band discontinued lattice-matched Al0.83In0.17N/GaN channel 2. Because of the superior gate control capability, the generally induced double-hump transconductance characteristics of double-channel MOSHEMTs were not obtained in the devices. The superior gate control capability was contributed by the side-wall electrical field modulation in the fin-channel. Owing to the high-insulating Ga2O3 gate oxide layer and the high-quality interface between the Ga2O3 and GaN layers, low noise power density of 8.7 × 10−14 Hz−1 and low Hooge's coefficient of 6.25 × 10−6 of flicker noise were obtained. Furthermore, the devices had a unit gain cutoff frequency of 6.5 GHz and a maximal oscillation frequency of 12.6 GHz. [ABSTRACT FROM AUTHOR]

A GaN MIS-HEMT with nitrogen (N)-passivation for power device applications is demonstrated. In this letter, nitrogen radicals were adopted to recover nitrogen-vacancy-related defects which were formed due to the thermal decomposition and evaporation of nitrogen atoms from GaN surface during high-temperature process. Besides, nitrogen radicals can also remove impurities and reduce surface dangling bonds by forming Ga-N bonds on the SiN/GaN interface. With N-passivation, the device shows high ON/OFF current ratio, steep subthreshold slope, low OFF-state leakage current, high breakdown voltage, and improved dynamic ON-resistance. The device reliability under high-electric field stress was also improved as a result. [ABSTRACT FROM AUTHOR]

In this letter, a gate recessed normally OFF AlGaN/GaN MIS-HEMT with low threshold voltage hysteresis using Al2O3/AlN stack gate insulator is presented. The trapping effect of Al2O3/GaN interface was effectively reduced with the insertion of 2-nm AlN thin interfacial passivation layer grown by plasma enhanced atomic layer deposition. The device exhibits a threshold voltage of +1.5 V, with current density of 420 mA/mm, an off-state breakdown voltage of 600 V, and high on/off drain current ratio of \(\sim 10^{\mathrm {\mathbf {9}}}\) . [ABSTRACT FROM AUTHOR]

The flyback converters are widely used in low power applications. The switch typically requires 600 V breakdown voltage in order to perform large step-down voltage. Thus, slight variation on the switch control can either permanently damage the switch or decrease the efficiency of the power conversion. In order to achieve higher power efficiency, the previous literature suggested operating the flyback converter in the discontinuous current mode (DCM). It is then required to understand the critical conditions of the DCM through analyzing the dynamic behavior and discontinuous current mechanism. This paper started from the current waveform analyses, proceeded to the derivation of zero current switching (ZCS) formulation, and finally reached the necessary conditions for the DCM. The entire DCM operation was divided into three phases that subsequently affect the result of the zero voltage switching (ZVS) and then to the ZCS. The experiment shows a power efficiency of over 96% when the output power is around 65 W. The switch used in this paper is a Gallium Nitride High-Electron-Mobility Transistor (GaN HEMT) that is advantageous at the high breakdown voltage up to 800 V. The important findings from the experiments include that the output power increases with the increasing input DC voltage and the duty cycle is rather linearly decreasing with the increasing switching frequency when both the zero voltage switching (ZVS) and ZCS conditions are satisfied simultaneously. [ABSTRACT FROM AUTHOR]

The conventional cascode structure for driving depletion-mode (D-mode) gallium nitride (GaN) high electron mobility transistors (HEMTs) raises reliability concerns. This is because of the possibility of the gate to source voltage of the GaN HEMT surging to a negative voltage during the turn off transition. The existing solutions for this problem in the literature produce additional drawbacks such as reducing the switching frequency or introducing many additional components. These drawbacks may outweigh the advantages of using a GaN HEMT over its silicon (Si) alternative. This paper proposes two innovative gate drive circuits for D-mode GaN HEMTs—namely the GaN-switching based cascode GaN HEMT and the modified GaN-switching based cascode GaN HEMT. In these schemes, the Si MOSFET in series with the D-mode GaN HEMT is always turned on during regular operation. The GaN HEMT is then switched on and off by using a charge pump based circuit and a conventional gate driver. Since the GaN HEMT is driven independently, the highly negative gate-to-source voltage surge during turn off is avoided, and in addition, high switching frequency operation is made possible. Only two diodes and one capacitor are used in each of the schemes. The application of the proposed circuits is experimentally demonstrated in a high voltage flyback converter, where more than 96% efficiency is obtained for 60 W output load. [ABSTRACT FROM AUTHOR]

GaN has been widely used to develop devices for high-power and high-frequency applications owing to its higher breakdown voltage and high electron saturation velocity. The GaN HEMT radio frequency (RF) power amplifier is the first commercialized product which is fabricated using the conventional Au-based III–V device manufacturing process. In recent years, owing to the increased applications in power electronics, and expanded applications in RF and millimeter-wave (mmW) power amplifiers for 5G mobile communications, the development of high-volume production techniques derived from CMOS technology for GaN electronic devices has become highly demanded. In this article, we will review the history and principles of each unit process for conventional HEMT technology with Au-based metallization schemes, including epitaxy, ohmic contact, and Schottky metal gate technology. The evolution and status of CMOS-compatible Au-less process technology will then be described and discussed. In particular, novel process techniques such as regrown ohmic layers and metal–insulator–semiconductor (MIS) gates are illustrated. New enhancement-mode device technology based on the p-GaN gate is also reviewed. The vertical GaN device is a new direction of development for devices used in high-power applications, and we will also highlight the key features of such kind of device technology. [ABSTRACT FROM AUTHOR]

This paper attempts to disclose a new GaN-based device, called the P-Cascode GaN HEMT, which uses only a single gate driver to control both the D-mode GaN and PMOS transistors. The merit of this synchronous buck converter is that it can reduce the circuit complexity of the synchronous buck converter, which is widely used to provide non-isolated power for low-voltage and high-current supply to system chips; therefore, the power conversion efficiency of the converter can be improved. In addition, the high side switch using a single D-mode GaN HEMT, which has no body diode, can prevent the bi-directional flow and thus reduce the power loss and cost compared to a design based on a series of two opposite MOSFETs. The experiment shows that the proposed P-Cascode GaN HEMT efficiency is above 98% when it operates at 500 kHz with 6 W output. With the input voltage at 12 V, the synchronous buck converter provides an adjustable regulated output voltage from 1.2 V to 10 V while delivering a maximum output current of 2 A. [ABSTRACT FROM AUTHOR]

In this study, we investigate the degradation characteristics of E-mode GaN High Electron Mobility Transistors (HEMTs) with a p-GaN gate by designed pulsed and prolonged negative gate (VGS) bias stress. Device transfer and transconductance, output, and gate-leakage characteristics were studied in detail, before and after each pulsed and prolonged negative VGS bias stress. We found that the gradual degradation of electrical parameters, such as threshold voltage (VTH) shift, on-state resistance (RDS-ON) increase, transconductance max (Gm, max) decrease, and gate leakage current (IGS-Leakage) increase, is caused by negative VGS bias stress time evolution and magnitude of stress voltage. The significance of electron trapping effects was revealed from the VTH shift or instability and other parameter degradation under different stress voltages. The degradation mechanism behind the DC characteristics could be assigned to the formation of hole deficiency at p-GaN region and trapping process at the p-GaN/AlGaN hetero-interface, which induces a change in the electric potential distribution at the gate region. The design and application of E-mode GaN with p-GaN gate power devices still need such a reliability investigation for significant credibility. [ABSTRACT FROM AUTHOR]

In this study, we investigated the resonance mechanism of 6.78 MHz resonant wireless power transfer (WPT) systems. The depletion mode of a gallium nitride high-electron-mobility transistor (GaN HEMT) was used to switch the states in a class-E amplifier circuit in this high frequency. The D-mode GaN HEMT without a body diode prevented current leakage from the resonant capacitor when the drain-source voltage became negative. The zero-voltage switching control was derived according to the waveform of the resonant voltage across the D-mode GaN HEMT without the use of body diode conduction. In this study, the effect of the resonant frequency and the duty cycle on the resonance mechanism was derived to achieve the highest WPT efficiency. The result shows that the power transfer efficiency (PTE) is higher than 80% in a range of 40 cm transfer distance, and the power delivered to load (PDL) is measured for different distances. It is also possible to cover different applications related to battery charging and others using the proposed design. [ABSTRACT FROM AUTHOR]