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III-nitride heterostructure-based metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs), compared with Schottky and p-GaN gate HEMTs, have demonstrated significant potential in the next-generation high-power electronic devices due to their exceptional gate reliability. This study presents a comprehensive investigation of threshold voltage (VTH) instability in III-nitride heterostructure-based MIS-HEMTs, with a specific emphasis on the interfaces of the multi-heterostructures. Two widely studied amorphous materials, namely, Al2O3 and SiNx, have been extensively examined as primary gate insulators in GaN-based MIS-HEMTs. To efficiently remove native oxides from the (Al)GaN surface, a novel in situ high-temperature remote plasma pretreatment (RPP) technique has been developed. This technique involves sequential application of NH3/N2 plasmas on the (Al)GaN surface before depositing the gate insulators using plasma-enhanced atomic layer deposition. The remarkable RPP process has proven to be a highly effective method for revealing atomic steps on the GaN surface, irrespective of whether the surface has undergone oxidation or etching processes. To further enhance the interface quality and potentially reduce bulk traps in the gate insulator, optimization of deposition temperature and post-deposition annealing conditions have been explored. Additionally, an electron-blocking layer, such as SiON, is incorporated into the MIS-HEMTs to prevent electron injection into bulk traps within the insulator. Novel characterization techniques including constant-capacitance and isothermal-mode deep-level transient spectroscopy have also been developed to explore the failure mechanisms in MIS-HEMTs. These techniques allow for the differentiation between bulk traps in the GaN epitaxy and those present within the gate insulators. This in-depth physical understanding provides valuable insights into the sources of failure in GaN-based MIS-HEMTs. [ABSTRACT FROM AUTHOR]

In this work, we manifest that the epitaxial structure for p-GaN gate high-electron-mobility transistor is a versatile platform to develop electronics for operating in an extremely wide temperature range (X-WTR) from 2 to 675 K, with comprehensive X-WTR studies on device operation and circuit behaviors. The key enabler for the high-temperature operation is the wide bandgap that substantially suppresses the thermal excitation of the intrinsic carrier. However, for the low-temperature side, the two-dimensional electron and hole gas (2DEG and 2DHG) channels at the heterojunctions are formed by the temperature-insensitive polarization fields, which free the carriers from freezing out. The monolithically integrated GaN n-FET, p-FET, and the resultant complementary circuits are, therefore, shown to operate in X-WTR. [ABSTRACT FROM AUTHOR]

In this work, AlN polarization-enhancement interlayer (AlN-PEL) is adopted to enhance two-dimensional hole gas (2DHG) density in a p-GaN/AlN-PEL(∼2 nm)/AlGaN(<6 nm)/GaN heterostructure, aiming at monolithic integration of p/n-channel field effect transistors (p-FETs) on GaN-on-Si substrate. Owing to the strong built-in polarization of the AlN-PEL, high density 2DHG over 2.3 × 1013 cm−2 with good immunity to thermal freeze out effect is realized. Assisted by a two-step gate trench etching process, enhancement-mode (E-mode) buried-channel GaN p-FETs with temperature independent ON-resistance RON, and ON/OFF current ratio ION/IOFF (>108), have been fabricated. The fabricated p-FETs also deliver thermally stable subthreshold swing as well as threshold voltage Vth, and smaller Vth shift than that of p-FETs without the AlN-PEL, which is primarily due to enhanced 2DHG confinement by the AlN-PEL. The proposed structure is an attractive platform for monolithic integration of GaN-based logic and power devices for cryogenic applications as low as 10 K. [ABSTRACT FROM AUTHOR]

A normally-off SiC-JFET/GaN-HEMT cascode device is recently proposed, featuring a cascode configuration that incorporates a high-voltage (HV, e.g., 1200 V) silicon-carbide (SiC) junction field effect transistor (JFET) delivering the HV blocking capability and a low-voltage (LV) enhanced-mode (E-mode) gallium-nitride (GaN) high electron mobility transistor (HEMT) providing the normally-off gate control. This all-wide-bandgap device exhibits superior thermal stability and high switching speed compared to its counterparts, e.g., the SiC/Si cascode device. In this article, we study short-circuit (SC) characteristics of a 1200-V/80-mΩ SiC/GaN cascode device by considering the interactions between the SiC and GaN devices. The single-event SC withstand time (SCWT) of the demonstrated SiC/GaN cascode device is tested to be ∼3 μs under a dc-bus voltage (Vdc) of 600 V when the turn-on gate voltage (VG-on) is set to 5 V. The SC failure occurs due to the formation of hot spots induced by thermal-runaway leakage currents inside the SiC JFET. The SCWT of the cascode device could be improved by applying a moderate VG-on (e.g., SCWT = ∼6.5 μs at VG-on = 3.5 V under v dc = 600 V) without affecting the on-state resistance (r on). However, it is identified that current oscillations tend to be triggered during the SC period when VG-on is too low (e.g., lower than 3 V). The oscillation behavior is attributed to the energy stored in the interconnection parts between the SiC and GaN devices. The harmful oscillations could be suppressed by applying higher VG-on, which drives the SiC JFET into low-transconductance mode by modulating the drain-to-source voltage (VDS-G) of the LV GaN HEMT. [ABSTRACT FROM AUTHOR]

Hot electrons with high kinetic energy could be generated in the channel of GaN high-electron-mobility transistors (HEMTs) during hard switching operation. Those “lucky” hot electrons scattered to the vulnerable interface between the passivation and barrier layers could bombard the interface region and create new defects that would lead to degradation of the dynamic on-resistance (${R}_{ON}$) after long-term operations. In this work, we propose a solution to the hot-electron induced device degradation through channel engineering, i.e., deploying a double-channel structure in place of the conventional single-channel structure in ${p}$ -GaN gate HEMTs. It is revealed that hot electrons are mostly generated in the lower channel and thus the additional scattering interface can effectively deter the hot electrons from reaching the vulnerable surface. Dynamic ${R}_{ON}$ degradation induced by long-term stresses at “semi-on” states is shown to be substantially suppressed in the ${p}$ -GaN gate HEMT with the double-channel structure. [ABSTRACT FROM AUTHOR]

With a switching drain bias, the gate reliability of Schottky-type ${p}$ -GaN gate high-electron-mobility transistors (HEMTs) under AC positive gate bias stress has been systematically investigated. The mean-time-to-failure (MTTF) under such application-relevant stress is found to be prolonged compared to that extracted from static and AC gate bias stress tests with the absence of a switching drain bias and exhibits positive coefficients with frequency and OFF-state drain bias (${V} _{\text {DSQ}}$). Such results can be explained by the drain-induced hole insufficiency in the gate stack at large ${V} _{\text {DSQ}}$ , a physical mechanism that results in elevated energy band at ON-state when ${V} _{\text {DSQ}}$ is just switched to low voltage. This non-equilibrium transient status could suppress injection of electrons from the 2DEG channel to the ${p}$ -GaN gate, which in turn substantially weakens the hot-electron’s generation in the depleted ${p}$ -GaN layer and the subsequent bombardment to the gate-metal/ ${p}$ -GaN interface, and thus prolongs the gate lifetime. [ABSTRACT FROM AUTHOR]

A 650-V/84-mΩ normally-off GaN/SiC cascode device is demonstrated with systematic static and dynamic characterizations. The cascode device features a low-voltage enhancement-mode (E-mode) gallium nitride (GaN) high electron mobility transistor (HEMT) to enable normally-off gate control, and a 650-V normally-on silicon carbide (SiC) JFET provides the high-voltage blocking capability. The GaN/SiC cascode device exhibits many application-desired behaviors, including reverse conduction capability, avalanche breakdown capability, thermally stable threshold voltage, low input/output capacitances, no dynamic RON degradation, and negligible dynamic VTH shift. In addition, a customized double-pulse test board is built to evaluate the switching performance of this cascode device and other 650-V SiC-based power devices. This cascode device adopting low-voltage GaN HEMT has low QG×RON and exhibits excellent switching performance. The switching losses (EON and EOFF) are much smaller than that of the 650-V Si/SiC cascode device and comparable to that of the state-of-the-art 650-V SiC MOSFETs. [ABSTRACT FROM AUTHOR]

A cost-effective engineered bulk silicon (EBUS) substrate technology is presented, featuring p-n junction implemented on bulk Si substrates using mainstream ion implantation and thermal annealing processes. Standard p-GaN/AlGaN/GaN heterostructures are successfully grown on the EBUS substrate and used to fabricate 200-V enhancement-mode p-GaN gate HEMTs. By creating deep trenches in the EBUS substrate to isolate the local P+ silicon regions underneath the high-side (HS) and low-side (LS) power switches, adverse effects (e.g., back-gating and dynamic ON-resistance degradation) in the use of conventional bulk Si substrate are all eliminated. The mechanism of crosstalk suppression in the GaN-on-EBUS platform is revealed in comparison with conventional GaN-on-Si platform and verified by a series of designed tests. [ABSTRACT FROM AUTHOR]

Although the gallium nitride (GaN) high-electron-mobility transistor/silicon carbide (SiC) junction field-effect transistor (JFET) cascode device exhibits certain performance advantages over the SiC metal–oxide–semiconductor field-effect transistor (MOSFET), its robustness in harsh operating conditions is unknown. In this work, the short-circuit (SC) robustness of 650-V GaN/SiC cascode devices is characterized, analyzed, and compared with that of several mainstream SiC MOSFETs. GaN/SiC cascode devices exhibit competitive SC capability. Different failure modes are observed: thermal runaway in the SiC JFET of the cascode device and gate-to-source breakdown in the SiC MOSFET. The difference is associated with the configuration of the dielectric above SiC in the JFET and the source pad layout. According to the waveforms in SC events, failure spots, and thermal simulation results, a thermal runaway in the SiC JFET is attributed to the positive feedback between the drain leakage current and the junction temperature in the local area (hotspot) without the source pad on the top. The SC withstand time of the cascode device can be extended by reduction of the active area without the source pad. [ABSTRACT FROM AUTHOR]

In this article, the impacts of the off-state gate bias (VGS,OFF) on dynamic on-resistance (RON) are systematically investigated in commercial Schottky-type p-GaN Gate high-electron-mobility transistors. Double-pulse tester and pulsed I–V system are adopted to evaluate the dynamic RON with various off-state gate and drain bias (VDS,OFF) under hard- and soft-switching conditions. More negative VGS,OFF can aggravate the dynamic RON degradation under both soft- and hard-switching, especially when switching with a high VDS,OFF. The impacts of VGS,OFF on switching transients and off-state stress are investigated separately to reveal the underlying mechanisms, which are found to be associated with the generation and movements of holes. The off-state gate bias of the voltage-driving scheme needs to be carefully considered according to the identified mechanisms in terms of dynamic on-resistance degradation. [ABSTRACT FROM AUTHOR]

Enhancement-mode (E-mode) p-channel gallium nitride (GaN) field-effect transistors (p-FETs) are essential components for GaN-based complementary logic circuits. For the ease of integration with n-FETs, they could be fabricated on the commercial p-GaN gate high-electron-mobility-transistor (HEMT) platform, on which the two-dimensional electron gas at the AlGaN/GaN hetero-interface is completely depleted in as-grown epi-structures. However, under the gated region where p-GaN is recessed and depleted at thermal equilibrium, a parasitic electron channel (PEC) could appear at the AlGaN/GaN interface. This Letter reports experimental investigations on the PEC with specifically designed structures, confirming that the PEC does exist but imposes limited impacts on electrical characteristics of p-FETs. When connected with an external contact, the PEC could act as a back gate to modulate the overlaying p-channel. If isolated from external contacts, which is the case of p-FETs under normal operations, electrons in the PEC would redistribute under the active region of p-FETs in the horizontal direction (i.e., parallel to the surface) under different biases but are mostly confined near the AlGaN/GaN interface in the vertical direction (i.e., perpendicular to the surface). [ABSTRACT FROM AUTHOR]

We report GaN high-electron-mobility transistors (HEMTs) with a thick (7.7 µm) GaN buffer on a Czochralski low resistivity Si (LRS) substrate. The GaN HEMTs exhibit high performance for both radio-frequency (RF) amplification and power conversion. The thick GaN buffer was grown by means of vacancy engineering, delivering a low dislocation density of ∼1.6 × 108 cm−2, contributing to suppressed RF signal coupling to the lossy Si substrate and a high vertical voltage blocking capability. For RF performance, GaN HEMTs with a 650 nm gate exhibit an fT/fMAX value of 25.1/32.3 GHz and a maximum output power POUT of 2.2 W/mm at 4 GHz with a drain voltage VDS of 20 V, which is comparable with the performance of RF GaN HEMTs on a high-resistivity silicon substrate without the existence of the field plate. For power performance, the vertical breakdown voltage of the wafer is 1160 V, and the three-terminal lateral breakdown voltage is 885 V in a GaN HEMT with a gate-to-drain distance of 8 µm. The thick GaN layer on the LRS substrate scheme thus provides a compelling platform for monolithic integration of high-performance RF devices and high-voltage power devices. [ABSTRACT FROM AUTHOR]

This article was originally published online on 27 December 2023 with an error in the affiliation. The affiliation appears correct above. All online versions of this article were corrected on 4 April 2024.By Yingjie Wang; Sen Huang; Qimeng Jiang; Xinhua Wang; Zhongchen Ji; Jie Fan; Haibo Yin; Ke Wei; Xinyu Liu; Qian Sun and Kevin J. ChenReported by Author; Author; Author; Author; Author; Author; Author; Author; Author; Author; Author [Extracted from the article]

The short circuit (SC) behavior of a newly developed GaN-HEMT/SiC-JFET cascode device is investigated in this work. The reverse leakage current through the drain-side PN junction (IR) of the SiC JFET under SC conditions is measured simultaneously during the SC event. IR of the SiC JFET increases to 6.4 A at the end of a 5-μs nondestructive SC pulse. The increase of IR during the SC pulse induces a turning point in the SC current waveform and a dramatic negative threshold voltage shift (>30 V) in the SiC JEFT, resulting in high transient drain-to-source voltage in the GaN HEMT. The underlying mechanism is associated with the increase of local potential in the P-type layer of the SiC JFET with inevitable spread resistance (RSPD). It is suggested that reducing RSPD can improve SC capability and high IR should be considered in SC failure mechanism analysis and the design of external gate resistance of the SiC JFET. [ABSTRACT FROM AUTHOR]

Short circuit (SC) capability of 650-V Schottky-type p-GaN gate high-electron-mobility transistors (HEMTs) under single and repetitive tests is characterized in this article. The investigated devices exhibit strong capability under a single SC test, but weak capability under repetitive SC tests with a bus voltage of 400 V and a gate drive voltage of 6 V. The failure mechanism under repetitive SC tests is revealed through electrothermal simulation and microscale failure spot analysis. Thermal fatigue cracks are formed due to the high temperature spike and local temperature fluctuations in the narrow GaN channel and buffer layers, leading to weak repetitive SC capability. The unique heat confinement effect in the GaN layer plays an important role in the formation of high temperature spike and fatigue cracks. The withstand time in a single SC test is several hundred microseconds due to fast drain current drop that results from high temperature and, to a lesser degree, the dynamic threshold voltage shift during the SC transient. The device failure in a single-event SC test is related to heat diffusion to a wider region. Some guidelines are proposed for handling and improving the repetitive SC capability. [ABSTRACT FROM AUTHOR]

The surface of the p-GaN layer in Schottky-type p-GaN gate high-electron-mobility transistors (HEMTs) can be reinforced with enhanced immunity to hot electron bombardment by reconstructing the surface region of p-GaN into GaON. The surface region of p-GaN is treated by remote oxygen plasma and subsequently annealed at 800 °C, thereby becoming a thin crystalline gallium oxynitride (GaON) layer that will be in direct contact with the Schottky metal. The GaON exhibits a lower valence band maximum energy than that of the p-GaN, which leads to a higher Schottky barrier at the metal/GaON interface to holes and, thus, greatly suppresses the forward gate leakage. More importantly, with higher thermodynamic stability and a larger bandgap of ∼4.1 eV, the GaON reinforces the susceptible metal/p-GaN interface against the hot electrons and, thus, substantially enhances the long-term gate reliability of p-GaN gate HEMTs under forward bias stress. The high-temperature thermal process is indispensable for the surface reconstruction, without which the plasma oxidation only reduces the gate leakage but fails to prolong the time-dependent gate breakdown lifetime. [ABSTRACT FROM AUTHOR]

A closely coupled double-channel (DC) structure realized on an 8-inch GaN-on-Si wafer is utilized to fabricate GaN high-electron-mobility-transistors (HEMTs) with enhanced RF linearity. The strong channel-to-channel coupling from this DC structure enables efficient transport of electrons between the two parallel channels to accommodate balanced carrier concentration and current density between the two channels. Consequently, the nonlinearity of source resistance under high current operations and the external bias dependence of cut-off frequencies could be substantially mitigated, leading to device linearity enhancement. Meanwhile, the DC structure enables simple top gate control that favors a high-yield planar process. Two-tone measurements at 4 GHz show that the DC GaN HEMT delivers an output 3rd-order intermodulation point (OIP3) that is improved by 5.2 dB over a single-channel GaN HEMT. [ABSTRACT FROM AUTHOR]

In a ${p}$ -channel field-effect-transistor (${p}$ -FET) bridge HEMT device recently realized on a commercial ${p}$ -GaN/AlGaN/GaN-on-Si power HEMT epi-wafer, it is revealed that the device’s reverse-conduction turn-on voltage (${V}_{\text {RT}}$) can be effectively decoupled from the forward threshold voltage (${V}_{\text {TH}}$) of Schottky-type ${p}$ -GaN gate HEMTs. Unlike the conventional Schottky-type ${p}$ -GaN gate HEMTs, of which ${V}_{\text {RT}}$ is closely linked to ${V}_{\text {TH}}$ , the ${p}$ -FET-bridge HEMT enables separate designs of ${V}_{\text {RT}}$ and $V_{\text {TH}}$ so that low-loss reverse conduction and high threshold voltage can be simultaneously realized. In addition, ${V}_{\text {RT}}$ can be further reduced by engineering the AlGaN barrier layer, which will also benefit a lower channel sheet resistance without lowering ${V}_{\text {TH}}$. [ABSTRACT FROM AUTHOR]

The threshold voltage (VTH) of an enhancement-mode Schottky-type p-GaN gate high-electron-mobility transistor (HEMT) is found to have a special dependence on the drain bias. The device commonly requires higher gate voltage to switch on the transistor from a high-drain-voltage off-state than what is expected from the static device characteristics. The reason behind the dynamic VTH has been proved to be the floating p-GaN layer, where charges could be stored and further influence VTH under different drain bias. In this article, a SPICE-compatible equivalent circuit model is presented according to the structure of Schottky-type p-GaN gate HEMTs. It features a floating node to imitate the charge storage process within the gate stack. Compared to conventional models, the proposed model could accurately predict the dynamic VTH characteristics and switching behaviors of power electronics circuits, where Schottky-type p-GaN gate HEMTs are deployed as power transistors. The phenomena related to the dynamic VTH, including the disappearance of Miller plateau, the overestimated false-turn-on problem, and the higher reverse conduction loss are evaluated with a half-bridge circuit and the merits of the proposed model are verified. [ABSTRACT FROM AUTHOR]

In this work, we study the gate leakage mechanisms of E-mode p-n junction/AlGaN/GaN (PNJ) high electron mobility transistors (HEMTs), which have been shown to deliver low gate leakage and wide safe operating gate-bias range. The intrinsic gate leakage through the PNJ-gate was found to be limited by the transport of holes through the p-GaN layer, which occurs via Poole-Frenkel emission and phonon-assisted tunneling in low and high gate bias region, respectively. In addition, lateral leakage current and the role of variable hopping process (VRH) are also discussed. Gate leakage current models based on the revealed mechanisms can quantitatively reproduce the gate-leakage behavior in the entire relevant range of gate biases and temperatures. [ABSTRACT FROM AUTHOR]

In this work, the impact ionization-induced OFF-state breakdown is revealed and systematically investigated in 100 V Schottky-type p-GaN gate high-electron-mobility transistors. Impact ionization is found to occur in the peak electric-field region at the source-terminated field-plate edge and is initiated by electrons injected from the source-side two-dimensional electron gas channel through the buffer layer. Hot electrons generated from impact ionization, when being captured by surface traps, could lead to redistribution and peak value reduction of electric-field. Consequently, the sudden rise in the OFF-state leakage current by impact ionization could be self-clamped temporally to avoid catastrophic rupture of the device. The impact ionization is further verified by the increase in dynamic OFF-state leakage current and a negative shift in threshold voltage, both of which result from positively charged holes (generated from impact ionization) drifting toward the gated channel. [ABSTRACT FROM AUTHOR]

A normally-off SiC-JFET/GaN-HEMT cascode device is recently proposed, featuring a cascode configuration that incorporates a high-voltage (i.e., 1200 V) SiC junction field effect transistor (JFET) and a low-voltage GaN high electron mobility transistor (HEMT). This cascode device exhibits superior thermal stability and switching performance compared to the SiC MOSFETs, but also inevitably presents challenge in dv/dt-control as the input gate does not directly control the high-voltage JFET. Since dv/dt-control is of great importance to the management and suppression of electromagnetic interference in power electronics systems, methods of controlling the dv/dt rates of SiC/GaN cascode devices need to be developed. In this article, we conduct systematic investigation on different dv/dt control schemes with theoretical analysis and experimental evaluation. A dv/dt-control method based on diode-clamped external JFET gate resistor is proposed and evaluated by comparing it with other more conventional methods. The proposed dv/dt-control method is verified to provide a balanced dv/dt-control on the device turn-on and turn-off. [ABSTRACT FROM AUTHOR]

GaN power transistors are attracting increasing attention for next-generation power switching applications. However, GaN power transistors should not be regarded as a direct replacement for silicon power metal–oxide–semiconductor field-effect transistors, since the unique properties of GaN power transistors require special treatment. In this work we review a unique and critical property of GaN power transistors, the dynamic threshold voltage (Vth). GaN power transistors feature a p-GaN gate structure, while the gate metal contacts the p-GaN layer through a Schottky junction. This unique p-GaN gate structure is distinctively different from the silicon metal–oxide–semiconductor gate structure in that the p-GaN layer is actually a floating region without direct electrical contact to the external electrodes of the device. The floating p-GaN layer enables a charge storage process during the switching operation of a p-GaN gate high-electron-mobility transistor. Through carefully designed experiments, the strong correlation between charge storage and the dynamic Vth shift has been revealed; this also serves as an approach to accurately predict the dynamic Vth during switching operation. The influence of the dynamic Vth on power switching applications is discussed. For example, with a positive dynamic Vth shift, the reverse conduction voltage of a GaN power transistor is significantly increased, adding to the power loss. This positive Vth shift also demands a sufficiently high gate drive voltage to ensure that the GaN power transistor can be fully turned on, which reduces the safety margin for the gate reliability of the device. To design a GaN based power switching circuit, accurate simulation of the device/circuit characteristics is necessary. Circuit simulations that do not consider the dynamic Vth lead to a large deviation from the measurement circuit characteristics. We demonstrate that by incorporating the dynamic Vth into the circuit model this deviation can be effectively eliminated. [ABSTRACT FROM AUTHOR]

A gallium nitride (GaN) ring oscillator based on high-performance one-chip complementary logic (CL) inverters is demonstrated on a conventional ${p}$ -GaN gate power HEMT (high-electron-mobility transistor) platform. It manifests the feasibility of the multiple-stage monolithic integration of GaN CL gates, the most energy-efficient digital circuit configuration, and consequently the potential of deploying CL circuits in the all-GaN power integration as peripheral circuits with higher energy efficiency. Thanks to the successful monolithic integration of enhancement-mode ${p}$ -channel and ${n}$ -channel field-effect transistors, the integrated CL inverters in this work present remarkable performances, including stringent rail-to-rail operations, substantially suppressed static power dissipation at both logic ‘low’ and ‘high’ states, suitable transition threshold voltages of ~2.0 V (40% of the common 5-V supply) and wide noise margins above 1.8 V (36% of 5 V). [ABSTRACT FROM AUTHOR]

By deploying a surface reinforcement layer (SRL) at the interface between Schottky metal and ${p}$ -GaN in the gate stack, a ${p}$ -GaN gate high-electron-mobility transistor (HEMT) with enhanced gate reliability is demonstrated. Prior to the gate metal deposition, the SRL is formed by an oxygen-plasma treatment and a subsequent high-temperature annealing process (at 800 °C) that enables surface reconstruction. Such a process converts several nanometers of ${p}$ -GaN near the surface into a crystalline GaON layer, which exhibits stronger immunity to hot electron bombardment. With nearly identical threshold voltage and ON -resistance, the ${p}$ -GaN gate HEMT with SRL yields two orders of magnitude reduction in gate leakage current at ON -state and an increase from 10.5 V to 12.7 V in forward gate breakdown voltage. Time-dependent gate breakdown measurement reveals an increase from 5.9 V to 7.8 V in the maximum ON -state gate drive voltage for a 10-year lifetime with a 1 % gate failure rate, which effectively expands the operating voltage margin of the ${p}$ -GaN gate power HEMT. [ABSTRACT FROM AUTHOR]

Systematic characterizations of a cascode device with a low-voltage enhance-mode (E-mode) p-GaN gate high-electron mobility transistor as the control device and a high-voltage (HV) depletion-mode (D-mode) silicon carbide junction field effect transistor (JFET) as the voltage blocking device are presented in this article. The demonstrated device with a breakdown voltage rating of 1200 V and a static on-resistance (RON) of 100 mΩ features small device capacitances with fast switching speed, avalanche breakdown capability, thermally stable threshold voltage (VTH), and no dynamic RON degradation. To identify its safe operation in the off-state with a high drain bias, the off-state middle point voltage (VM) between the E-mode device drain and D-mode device source is investigated. A relatively low off-state VM is achieved under both static and dynamic modes. In addition to the device-level characterization, a custom-designed double-pulse test circuit is built to evaluate the transient switching performance of the cascode device. Optimal gate drive conditions are proposed to 1) overcome the drain bias induced positive dynamic VTH shift; and 2) suppress the increased dynamic off-state leakage current (IOFF) induced by on-state hole injection. Under 800 V/16 A testing conditions, high switching speed with the drain voltage peak slew rates of 72 V/ns during turn-on and 121 V/ns during turn-off is achieved. [ABSTRACT FROM AUTHOR]

In this article, an ultrathin-barrier (UTB) AlGaN/GaN diode featuring metal–insulator–semiconductor (MIS)-gated hybrid anode (MG-HAD) and in situ Si3N4 cap passivation is demonstrated. The intrinsic turn-on voltage (VON) as low as 0.31 V determined by the as-grown AlGaN-barrier thickness (4.9 nm) is obtained and the VON exhibits excellent uniformity. More importantly, benefit from the MIS-gated hybrid anode structure, the UTB MG-HAD features good thermal stability in reverse blocking capability. The device delivers a substantially low leakage less than 1~μm/mm at −300 V at high temperature (HT) up to 200 °C, which is more than 100 × lower than that in the reference device w/o gate dielectric. Besides, the device exhibits respectably improved dynamic characteristics due to the incorporation of in situ Si3N4-cap passivation layer and remote plasma pretreatment (RPP) prior to Al2O3 gate dielectric deposition. The UTB MG-HAD featuring precisely VON modulation and low reverse leakage is of great interest for power electronic applications. [ABSTRACT FROM AUTHOR]

A 1200-V/100-mΩ silicon carbide (SiC) junction field-effect-transistor (JFET)/ gallium nitride (GaN) high-electron-mobility-transistor (HEMT) hybrid power switch is demonstrated, which features a flip-chip copackaged cascode configuration incorporating a vertical SiC JFET and a lateral GaN-HEMT. The high-voltage SiC-JFET provides the high-voltage blocking capability while the low-voltage GaN-HEMT enables the normally-off gate control with superior switching characteristics. Compared to conventional SiC-JFET/Si-mosfet cascode devices, the SiC-JFET/GaN-HEMT cascode device exhibits fast switching speed, which has been validated by systematic characterizations including static/dynamic device-level measurements and board-level hard-switching tests. Meanwhile, the device is free from several notorious issues of high-voltage GaN power transistors such as dynamic on-state resistance degradation and threshold voltage shift. Such a wide-bandgap semiconductor-based hybrid switch is suitable to be deployed in high-power and high-efficiency power conversion systems. [ABSTRACT FROM AUTHOR]

When the gated channel region of a GaN high-electron-mobility transistor (HEMT) is configured into multiple sub-channels in parallel and separated by embedded isolating patterns, the effective resistance of the access regions could be reduced, and consequently, the knee voltage (${V}_{{\text {K}}}$) of the transistor could be lowered. In this work, each sub-channel is defined as a convergent funnel-like shape, with its width gradually shrunk from the source side to the drain side. Different from conventional channels with uniform width under the entire gate, the funnel-shaped channel could converge electrons as they transport from source side to drain side, which facilitates electrons’ acceleration toward saturation velocity under a smaller drain-to-source bias, leading to a reduced intrinsic ${V}_{{\text {K}}}$ in the gated channel. Thus, more desirable ${I}$ - ${V}$ characteristics and more balanced performance enhancement in RF linearity and power added efficiency are achieved at a low supply voltage, making the convergent-channel HEMT attractive for power amplifiers in mobile terminals. [ABSTRACT FROM AUTHOR]

The low-temperature gate reliability of Schottky-type p-GaN gate AlGaN/GaN heterojunction field-effect transistors under forward gate voltage stress is investigated. Both temperature-accelerated and voltage-accelerated time-dependent gate breakdown stress experiments are performed. The p-GaN gate exhibits a shorter time-to-failure at a lower temperature. It is found that the time-to-failure at "use conditions" predicted by acceleration tests at high gate bias stress could be overestimated at low temperatures. Such a discrepancy stems from the distinct dominant gate leakage mechanisms at high/low gate bias stress conditions. The dominant physical mechanism of the low-temperature gate leakage current is identified to be Poole–Frenkel emission at low gate bias and Fowler–Nordheim tunneling at high bias. From the physical model, a more accurate lifetime projection can be obtained for given use conditions. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a GaN-based $\textit {p-n}$ junction gate (PNJ) HEMT featuring an ${n}$ -GaN/ ${p}$ -GaN/AlGaN/GaN gate stack. Compared to the more conventional ${p}$ -GaN gate HEMT with a Schottky junction between the gate metal and ${p}$ -GaN layer, the $\textit {p-n}$ junction can withstand higher reverse bias at the same peak electric-field as the depletion region extends to both the ${n}$ -side and ${p}$ -side, while exhibiting lower leakage current. The PNJ-HEMT shows a positive threshold voltage (${V}_{\text {TH}}$) of 1.78 V, a small gate leakage $(\sim 10^{-3}$ mA/mm @ ${V}_{\text {GS}} = {10}\,\, \text {V}$). In particular, a large forward gate breakdown voltage of 19.35 V at 25 °C and 19.70 V at 200 °C was achieved with the PNJ-gate HEMT. [ABSTRACT FROM AUTHOR]

In this paper, systematic characterization and the corresponding suppression strategies of dynamic OFF-state leakage current (${I}_{ \mathrm{\scriptscriptstyle OFF}}$) in Schottky-type p-GaN gate high-electron-mobility transistors (HEMTs) are presented based on fast pulsed ${I}$ – ${V}$ measurement and consecutive switching measurement. It is found that the high ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ under dynamic pulse mode without hole injection is a result of the reduced voltage blocking capabilities (both lateral and vertical) with weaker trapping effect in the buffer, and the dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ induced by ON-state hole injection is caused by further increased lateral conductivity through the buffer from source to drain. The corresponding behaviors under continuous waveforms with different switching conditions are analyzed to identify effective approaches for the suppression of dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ in practical switching operations. A higher temperature is shown to be beneficial to the reduction of the dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ induced by ON-state hole injection. To completely eliminate the dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ caused by ON-state hole injection and minimize the OFF-state power consumption in practical power switching applications, a sufficiently large negative OFF-state gate bias (e.g., ${V}_{\text {GS}, \mathrm{\scriptscriptstyle OFF}}\le -{3}$ V) is recommended in the gate driver turn-off voltage design for the Schottky-type p-GaN gate HEMTs. [ABSTRACT FROM AUTHOR]

This paper presents a study of the ON-state gate overdrive of enhancement-mode AlGaN/GaN HEMTs fabricated by fluorine plasma implantation technology. A critical gate forward voltage (VGC) is observed, beyond which the channel turn-on voltage (or threshold voltage) of the devices exhibits a persistent and nonrecoverable negative shift. This phenomenon is explained by a proposed physical model based on the impact ionization of the F ions in the barrier layer by hot electron injection. The proposed physical model is further validated by the temperature-dependent characterization of VGC that shows an eventual stabilization at higher temperatures (>125 °C), owing to the efficient relaxation of hot electrons by phonon scattering. The determination of VGC provides valuable guideline for the design of gate drive circuits of GaN power circuits [ABSTRACT FROM AUTHOR]

Commercially available normally-off GaN power high-electron-mobility transistor (HEMT) devices have typically adopted a p-GaN gate structure. In the gate region, there exist a Schottky junction (between gate electrode and the p-GaN layer) and a p-GaN/AlGaN/GaN heterojunction. As the p-GaN layer is not directly shorted to the gate electrode and conducting channel, it can be considered as electrically “floating.” With drain voltage changing during a switching process, the variation of net charge in the floating p-GaN layer would cause instability in threshold voltage. Besides, because of the distinctive features of the p-GaN gate HEMT structure under certain gate/drain bias, gate-related junction capacitances would exhibit behavior different from those of a Si mosfet. Consequently, the switching transient performance of a GaN transistor with a p-GaN gate could be significantly influenced by the aforementioned factors. In this work, the threshold voltage instability associated with the drain-to-gate voltage stress is first analyzed. Despite the difficulty in directly performing capacitance measurements inside the device structure, a hybrid physical-behavior modeling method is proposed. The model is capable of extracting the capacitance bias relationships with regard to the gate region from static terminal measurements. In previous works, the advanced analytical model would make modest change on a Si mosfet's model. As a result, without fully considering the specific features of a GaN transistor with the p-GaN gate, the simulated waveforms would exhibit 20–50% discrepancy from the experiment. In contrast, the proposed switching transient analytical approach would exhibit improved accuracy (<10% disparity). Consequently, the switching transient performance of the GaN transistor with the p-GaN gate could be more accurately evaluated. [ABSTRACT FROM AUTHOR]

An MOS field plate-protected Schottky-drain (gated Schottky-drain) is successfully integrated on a double-channel AlGaN/GaN MOS-HEMT to provide reverse blocking capability. The leakage suppression MOS field plate is deployed on the etched upper GaN channel layer after a barrier fully recess process, leading to a low reverse OFF-state leakage current of −20 nA/mm (at −100 V). The drain metal is deployed adjacent to the MOS field plate, contacting the upper MOS-channel and lower heterojunction channel from the sidewall. A metal-2DEG Schottky contact with a low turn-ON voltage of 0.5 V is achieved. Since the lower channel (below the MOS field plate) is separated from the etched surface of upper GaN channel layer, a high-conductivity MOS-gated channel with a sheet resistance of 806 $\Omega$ /Square is obtained. The device exhibits a threshold voltage of +0.6 V (at $10~\mu \text{A}$ /mm and +1.9 V from linear extrapolation) and an ON-resistance of ~18 $\Omega {\cdot} \text {mm}$. Besides, a high forward (and reverse) breakdown voltage of 790 V (and −656 V, all at $10~\mu \text{A}$ /mm) is achieved. [ABSTRACT FROM AUTHOR]

A systematic analytical investigation of the charge distribution and gate control of the normally-off GaN double-channel MOS-HEMT (DC-MOS-HEMT) is presented in this paper. Compared to conventional GaN MOS-HEMT, the DC-MOS-HEMT features a thin AlN insertion layer (AlN-ISL) below the original two dimensional electron gas (2DEG) channel, thus forming a second channel at the interface between AlN-ISL and the underlying GaN. This paper reveals the impact of the AlN-ISL on the 2DEG distribution and the gate control of the channels. The sensitivity of ${V}_{\text {th}}$ against the recess depth is also analytically studied and is found to be nearly independent of the recess depth as long as the recess is terminated in the upper channel layer. The analytical results are well supported by numerical device simulations, and the physical mechanisms behind these findings are explained along with the analytical investigations. [ABSTRACT FROM AUTHOR]

Ultra-thin-barrier (UTB) AlGaN/GaN heterostructure is utilized for fabrication of normally-OFF GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs). The sheet resistance of 2-D electron gas in the UTB Al0.22Ga0.78N(5-nm)/GaN heterostructure is effectively reduced by SiNx passivation grown by low-pressure chemical vapor deposition, from 2570 to 334~\Omega /□. The fabricated Al2O3/AlGaN/GaN MIS-HEMTs exhibit normally-OFF behavior with good V_{\mathrm {TH}} uniformity and low V_{\mathrm {TH}} -hysteresis. 20 mm-gate-width power devices featuring a low R\sc {\mathrm{ on}} of 0.75~\Omega ( \textI\mathrm {D,MAX} =6.5 A) are also demonstrated on the platform. [ABSTRACT FROM PUBLISHER]

Integration of a photon source into the AlGaN/GaN high election mobility transistor (HEMT) platform will realize the functionality of on-chip optical pumping of deep electron traps which suppress the dynamic performances of power HEMTs. Here, we report a Schottky-on-heterojunction light-emitting diode (SoH-LED) realized on the p-doping-free lateral AlGaN/GaN heterostructure. A physical mode based on hot electron induced surface states impact ionization was proposed to explain the hole generation and injection processes in this p-doping-free SoH-LED. Since the SoH-LED shares identical epitaxial structures with HEMT, integration of SoH-LED and HEMT requires no additional epi-layers during the wafer growth and minimum process modification during device fabrication. The SoH-LED structure was seamlessly integrated into the HEMT platform as an on-chip photon source. Experiment results showed that the SoH-LED photons can effectively assist the electron de-trapping processes from both of the surface and bulk deep traps, demonstrating the feasibility of using on-chip generated photons to improve the dynamic performances of AlGaN/GaN power HEMTs. [ABSTRACT FROM AUTHOR]

In this letter, we demonstrate an integrated process that illustrates the compatibility of AlN/SiNx passivation with high-performance (i.e. low leakage and high breakdown) low-pressure chemical vapor deposition (LPCVD) SiNx gate dielectric for GaN-based MIS-HEMT. It is shown that the AlN/SiNx passivation structure maintains its superior capability of suppressing the current collapse after enduring high temperature of 780 °C during the LPCVD-SiNx deposition. The AlN/SiNx passivation is shown to be significantly better than the LPCVD-SiNx passivation by delivering small dynamic $R_{{\mathrm{\scriptscriptstyle ON}}}$ degradation, especially under high drain bias switching with $V_{{\mathrm{DS}}} > 100$ V. [ABSTRACT FROM PUBLISHER]

Very fast transients of V {\text {TH}} shift and their impact on R\mathrm{\scriptscriptstyle ON} under dynamic AC (1 k–1 MHz) positive gate stress in depletion-mode (D-mode) metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) are revealed. We achieve data acquisition within 120 ns right after each stress pulse throughout the entire stress time range from 10^ -7 up to 10^ 3 s, by virtue of a short stress-to-sense delay of \sim 100 ns and high sampling rate up to 50 MSa/s. Despite the considerable V_{\mathrm {\mathbf {TH}}} shift, its impact on R\mathrm{\scriptscriptstyle ON} in D-mode MIS-HEMT is modest, if the device is under sufficient gate overdrive. Furthermore, V_{\mathrm {\mathbf {TH}}}$ shift and the consequent R\mathrm{\scriptscriptstyle ON} increase under dynamic stress, which are more relevant to high-frequency switching operation, exhibits frequency dependence within 1 k–1 MHz and is always smaller than that under conventionally used static (constant) stress. [ABSTRACT FROM PUBLISHER]

The temperature dependence of current collapse (CC) in AlGaN/GaN high-electron mobility transistors on silicon substrate is studied in this paper. Devices without and with Si3N4 passivation are used to investigate the behavior of surface- and buffer-induced CC, respectively. It is found that the degree of surface-induced CC in unpassivated devices has a weak temperature dependence, which is induced by the cancelling out between enhanced carrier injection based on surface hopping and enhanced emission when the temperature is increased. On the other hand, the degree of buffer-induced CC in the Si3N4 passivated devices is reduced at higher temperature since the energy of hot electrons is reduced due to the phonon scattering and the trapping of hot electrons in the buffer is mitigated. Temperature-dependent transient measurement is also carried out to investigate the recovery process for these two type of CC. Two types of trap levels are identified in the unpassivated and Si3N4 passivated devices, respectively. The trap level $E1$ with an activation energy of 0.08 eV is supposed to be related to the surface trapping, while $E2$ with an activation energy of 0.22 eV is located in the buffer layer. [ABSTRACT FROM AUTHOR]

In this work, by using an on-chip integrated Schottky-on-heterojunction light-emitting diode (SoHLED) which is seamlessly integrated with the AlGaN/GaN high electron mobility transistor (HEMT), we studied the effect of on-chip light illumination on the de-trapping processes of electrons from both surface and bulk traps. Surface trapping was generated by applying OFF-state drain bias stress, while bulk trapping was generated by applying positive substrate bias stress. The detrapping processes of surface and/or bulk traps were monitored by measuring the recovery of dynamic on-resistance Ron and/or threshold voltage Vth of the HEMT. The results show that the recovery processes of both dynamic Ron and threshold voltage Vth of the HEMT can be accelerated by the on-chip SoH-LED light illumination, demonstrating the potentiality of on-chip hybrid opto-HEMTs to minimize the influences of traps during dynamic operation of AlGaN/GaN power HEMTs. [ABSTRACT FROM AUTHOR]

Al2O3–AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) featuring indium tin oxide transparent gate electrode are fabricated for the purpose of evaluating the gate reliability. The transparent gate electrode allows electroluminescence (EL) observation in the region directly under the gate, which cannot be performed in conventional MIS-HEMTs with opaque gate electrode. Normally OFF transparent-gate MIS-HEMTs were realized by implanting fluorine ions into the AlGaN barrier prior to the deposition of Al2O3. The dependence of EL on bias condition was investigated with the color of EL resolved by filters. Gate dielectric degradation in MIS-HEMTs was induced by OFF state drain-bias stress, as detected by the increased gate leakage current. Location of the gate degradation could be clearly identified by EL imaging through the transparent gate. From the EL image, the passivation layer under the overhang of T-shaped gate is found to be vulnerable to OFF-state stress, which is confirmed by simulation. [ABSTRACT FROM AUTHOR]

High quality Al2O3 film grown by atomic layer deposition (ALD), with ozone (O3) as oxygen source, is demonstrated for fabrication of normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Significant suppression of Al-O-H and Al-Al bonds in ALD-Al2O3 has been realized by substituting conventional H2O source with O3. A high dielectric breakdown E-field of 8.5 MV/cm and good TDDB behavior are achieved in a gate dielectric stack consisting of 13-nm O3-Al2O3 and 2-nm H2O-Al2O3 interfacial layer on recessed GaN. By using this 15-nm gate dielectric and a high-temperature gate-recess technique, the density of positive bulk/interface charges in normally-off AlGaN/GaN MIS-HEMTs is remarkably suppressed to as low as 0.9 x 1012cm-2, contributing to the realization of normally-off operation with a high threshold voltage of +1.6 V and a low specific ON-resistance RON,sp of 0.49 mΩ cm². [ABSTRACT FROM AUTHOR]

We investigate in detail the influence of AlN passivation on dynamic ON-resistance ( \(R_{\mathrm{{\scriptstyle ON}}}\) ) and electric field ( \(E\) -field) distribution in high-voltage AlGaN/GaN high electron mobility transistors (HEMTs) on a Si substrate based on pulsed \(I\) – \(V\) measurements, electroluminescence (EL) microscopy, and 2-D physics-based numerical device simulations. It is found that the dynamic \(R_{\mathrm{{\scriptstyle ON}}}\) increase has been significantly suppressed to below 10% at various temperatures ranging from −50 to 200 °C owing to the effective and robust compensation of deep acceptor-like trap states at the AlN/GaN (passivation/cap) interface by the additional positive polarization charges induced in the epitaxial AlN thin passivation layer grown in a plasma-enhanced atomic layer deposition system. To the best of our knowledge, this is the first time that highly suppressed current collapse in an AlGaN/GaN HEMT on a Si substrate within a wide temperature range is ever reported. By monitoring the dynamic \(R_{\mathrm{{\scriptstyle ON}}}\) for 100 consecutive 133-kHz switching cycles, its variation is observed to be less than 2.5%, indicating excellent stability of the passivation effectiveness. The electric field in an AlN-passivated device is found to be well confined at the drain-side gate edge, as shown in EL measurement and numerical simulation results. This phenomenon directly suggests that the virtual gate effect arising from surface trap charging has been effectively alleviated by the AlN passivation technique. [ABSTRACT FROM AUTHOR]

The degradation of transient OFF-state leakage current in AlGaN/GaN HEMTs induced by ON-state gate overdrive was observed and studied. The OFF-state leakage current increases significantly with a pre- ON-state gate overdrive. It takes more than 30 s for the leakage current decay to its equilibrium level in dark. The decay processes follow a stretched-exponential law. We propose that this degradation corresponds to the persistent photoconductivity of GaN buffer induced by the electroluminescence during the Schottky gate overdrive. Both the ON-state gate overdrive bias and duration dependences of the OFF-states leakage degradation were studied. This work indicates that gate-protection is necessary to preserve low OFF-state leakage current and improve device reliability. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In this paper, we investigate the enhancement-mode (E-mode) LaLuO3 (LLO)-AlGaN/GaN metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs) fabricated using fluorine (F) plasma ion implantation with a gate-dielectric-first planar process. The E-mode MIS-HEMTs exhibit a threshold voltage (VTH) of 0.6 V, a peak transconductance of \sim193~mS/mm, a small hysteresis of 0.04 V in linear region characterized by a pulse-mode current-voltage measurement, and significantly suppressed current collapse under high-drain-bias switching conditions. X-ray photoelectron spectroscopy and secondary ion mass spectrometry analyses manifest that the negatively charged F ions penetrating into the (Al)GaN barrier layer serve as the primary VTH modulation mechanism, whereas the F ions in the fluorinated LLO film form chemical bonds with La/Lu atoms and become charge-neutral. The suppressed current collapse is verified as an advantageous byproduct of the F plasma ion implantation that also fluorinated the SiNx sidewalls in the vicinity of the gate electrode, and therefore, suppress electron injection to the gate-drain access region. [ABSTRACT FROM AUTHOR]

We present the detailed dc and radio-frequency characteristics of an Al0.3Ga0.7N/GaN/In0.1Ga0.9N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In0.1Ga0.9N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In0.1Ga0.9N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF4-based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1 × 100 μm E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs. [ABSTRACT FROM AUTHOR]

Fabrication and characterization of AlGaN/GaN HEMT inverters and ring oscillators utilizing integrated enhancement/depletion-mode (E/D-mode) AlGaN/GaN HEMTs are presented. The core technique is a CF4 plasma treatment that can effectively convert a D-mode AlGaN/GaN heterostructure to an E-mode heterostructure. A significant advantage of the plasma-treated E-mode HEMTs is that the gate current is reduced in both reverse- and forward-bias regions due to the effectively enhanced barrier height induced by the negatively charged fluorine ions in the AlGaN barrier. As a result, the input voltage swing is expanded by about 1 V for the E-mode HEMT, enabling convenient input/output logic level matching for multistage logic circuits such as ring oscillators. The fabricated 17-stage direct-coupled field-effect transistor logic ring oscillator using the 1-μm-gate technology can operate properly at a larger supply voltage of 3.5 V, and a minimum propagation delay of 130 ps/stage is achieved. [ABSTRACT FROM AUTHOR]

This paper presents a method with an accurate control of threshold voltages (Vth) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from 4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 °C. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results confirm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs. [ABSTRACT FROM AUTHOR]