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${p}$ -channel GaN field-effect transistors (FETs) with a SiNx/GaON gate stack have been demonstrated with enhanced stability within a wide range of voltage bias and temperature. In this letter, the gate leakage characteristics and reliability of this unconventional staggered gate stack are investigated. At relatively low gate voltages, the gate current is suppressed owing to the buried-channel structure and the presence of GaON that presents an effective hole barrier. With more negative gate biases, the gate leakage was found to be dominated by the transport of holes that spillover from the ${p}$ -channel through the SiNx via Poole-Frenkel emission. Based on this gate leakage mechanism at large gate bias, $\sqrt {{\mathrm {E}}} $ model was used for gate lifetime prediction. The maximum applicable ON-state gate voltage of −7.3 V is obtained for a 10-year lifetime with a 1% gate failure rate. [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 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]

The high-speed superiority of GaN power devices with silicon-based peripheral circuits is not yet fully leveraged, mainly due to the parasitic inductance of interconnections. In this article, we demonstrate a GaN-based gate driver with an overcurrent (OC) protection circuit and an undervoltage lockout (UVLO) circuit on a p-GaN gate power HEMT platform. The gate driver features a rail-to-rail output voltage, suppressed gate ringing, and tunable driving speed, all of which are highly desired in high-efficiency and high-speed GaN power systems. To offer timely but reliable protections, the OC protection and UVLO circuit are designed with reference to the switching speed and the threshold voltage of GaN power devices. The OC protection is implemented with a separated sensing branch and blanking time controller, and the response time to an OC event is reduced to 10 ns after the blanking time. The UVLO circuit has a fixed hysteresis of 0.5 V, and its threshold voltage is especially tailored for the GaN integrated circuits. [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]

A tunnel-oxide-free GaN-based non-volatile memory (NVM) device is proposed to untangle the trilemma among speed, retention, and endurance in the implementation of conventional NVM. The program and erase (P/E) are based on bipolar charge injection controlled by individual junction barriers, whereas the data retention relies on bipolar charge trapping in an interfacial charge storage layer. The wide bandgap energy of GaN allows the formation of deep-level traps at a dielectric/GaN interface and high junction barriers in GaN, both of which benefit long-term retention. As such, the GaN-based NVM device could get rid of the tunnel oxide, thereby enabling faster P/E processes and facilitating to decouple the enhancement of endurance from that of speed and retention. The proposed device demonstrates a P/E time of 100 ns, endurance over 106 cycles, and long retention time. [ABSTRACT FROM AUTHOR]

Recently, a GaN reverse-conducting high-electron-mobility transistor (RC-HEMT) has been demonstrated. The RC-HEMT features built-in distributive Schottky contacts, which provide a low-loss freewheeling path. The RC-HEMT offers the same functionality as an HEMT/Schottky barrier diode (SBD) pair, but consumes much less chip area, thus its ${R}_{ \mathrm{\scriptscriptstyle ON}}$ (ON-resistance $\times $ total device width) is significantly reduced. Theoretically, ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of the RC-HEMT can approach that of an HEMT. In practice, due to certain geometry limitations, ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of an RC-HEMT lies between an HEMT/SBD pair and a single HEMT. This work investigates the factors that influence ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of the RC-HEMT using both experimental measurements and numerical simulations. It is found that ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of the RC-HEMT is strongly affected by the geometry of the source and channel regions, where HEMT sections and SBD sections are interdigitally distributed. With coarse patterning of these regions, ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of the RC-HEMT is close to the HEMT/SBD pair. Through proper geometry scaling, ${R}_{ \mathrm{\scriptscriptstyle ON}}$ of RC-HEMT is reduced and approaches that of HEMT. [ABSTRACT FROM AUTHOR]

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]

GaN ${p}$ -channel field-effect transistors (${p}$ -FETs) are essential components for implementing the energy-efficient complementary logic circuitry for monolithic GaN power integration. This letter reports the threshold voltage (${V}_{\mathrm {TH}}$) instability of ${p}$ -channel GaN MOSFETs with Al2O3 as the gate dielectric. When the gate-to-source bias sweeps toward the negative direction, the ${V}_{\mathrm {TH}}$ exhibits a shift toward more negative values. The impacts of such ${V}_{\mathrm {TH}}$ instability on GaN complementary logic circuits are characterized using logic inverters and ring oscillators. It is shown that impacts on circuit functions are limited with an operating voltage swing below 5 V. However, the propagation delay could be gradually prolonged, as a result of the drifting ${V}_{\mathrm {TH}}$. [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]

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]

The dynamics of the intrinsic threshold voltage (VTH) shift in 650-V Schottky-type p-GaN gate HEMT under high-frequency switching is investigated by a bootstrap voltage clamping circuit in which the GaN HEMT serves as the key bootstrapping device. This new testing setup covers a wide range of OFF-state drain bias (VDSQ) up to 400V, a short OFF-to-ON (or stress-to-sense) delay (Tdelay) of 60ns, and a high switching frequency (ƒsw) up to 2MHz. The dynamic VTH’s dependences on VDSQ and ƒsw are characterized by this testing setup. A higher VDSQ (up to 400V) results in a larger dynamic VTH shift (Δ VTH). The Δ VTH exhibits a gradually increasing trend under the continuous high-frequency switching mode. In addition, after continuous switching operation for 200 μs, the maximum Δ VTH shows no ƒsw dependence with a wide range of frequency from 50kHz to 2MHz. [ABSTRACT FROM AUTHOR]

We report a processing technique to form a surface reinforcement layer (SRL) in GaN metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs) with the aim to suppress device dynamic on-resistance (RON) degradation caused by long-term hot-electron stress. The SRL is a crystalline (Al)GaON layer formed by reconstruction of the heterojunction surface through plasma oxidation and high temperature annealing. MIS-HEMTs with SRL exhibit substantially suppressed dynamic RON degradation than the conventional devices without SRL, after long-term hot-electron stress. The (Al)GaON SRL exhibits substantially enhanced thermal stability and strong immunity to energetic carriers. The SRL-enabled suppression of hot-electron-induced degradation is further verified by the electroluminescence characterizations. [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]

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]

In this work, the deep-level transient spectroscopy (DLTS) is conducted to investigate the gate stack of the p-GaN gate HEMT with Schottky gate contact. A metal/p-GaN/AlGaN/GaN heterojunction capacitor is prepared for the study. The DLTS characterization captures the transient capacitance change in the stack, from which the capacitance of the metal/p-GaN Schottky junction can be extracted. By proper selection of the rate window, the impacts of the hole insufficiency effect are avoided during trap states evaluation. Thus, the information of deep energy levels in the p-GaN layer is revealed, which consists of an electron trap state with activation energy of 0.85 eV and a hole trap state with activation energy of 0.49 eV. The identification of these trap states in the p-GaN layer provides a physical foundation for understanding the threshold voltage instability in Schottky-type p-GaN gate power HEMTs. [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]

The OFF-state drain leakage characteristics in 600-V p-GaN HEMTs with an ohmic gate contact are investigated under dynamic switching conditions instead of commonly used quasi-static measurement setup. It is found that fast dynamic OFF-state leakage current (dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$) is substantially higher than the slow-ramping quasi-static ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ due to the weaker trapping effect in the buffer layer. With sufficiently large positive ON-state gate bias, further increase in dynamic ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ is observed and is attributed to ON-state hole injection that leads to energy band lowering in the buffer. The underlying physical processes are explained by the dynamic behavior of the traps in the buffer layer. This letter indicates that the ${I}_{ \mathrm{\scriptscriptstyle OFF}}$ under practical switching operations is much higher than the static measurement results and should be used to evaluate dynamic OFF-state power consumption in the p-GaN HEMTs with an ohmic gate contact. [ABSTRACT FROM AUTHOR]