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In this study, metal–silicide-based contacts to GaN-cap/AlGaN/AlN-spacer/GaN-on-Si heterostructure were investigated. Planar Schottky diodes with Cu-covered anodes comprising silicide layers of various metal–silicon (M–Si) compositions were fabricated and characterized in terms of their electrical parameters and thermal stability. The investigated contacts included Ti–Si, Ta–Si, Co–Si, Ni–Si, Pd–Si, Ir–Si, and Pt–Si layers. Reference diodes with pure Cu or Au/Ni anodes were also examined. To test the thermal stability, selected devices were subjected to subsequent annealing steps in vacuum at incremental temperatures up to 900 °C. The Cu/M–Si anodes showed significantly better thermal stability than the single-layer Cu contact, and in most cases exceeded the stability of the reference Au/Ni contact. The work functions of the sputtered thin layers were determined to support the discussion of the formation mechanism of the Schottky barrier. It was concluded that the barrier heights were dependent on the M–Si composition, although they were not dependent on the work function of the layers. An extended, unified Schottky barrier formation model served as the basis for explaining the complex electrical behavior of the devices under investigation. [ABSTRACT FROM AUTHOR]

GaN integrated circuit technologies have dramatically progressed over the recent years. The prominent feature of GaN high-electron mobility transistors (HEMTs), unparalleled output power densities, has created a paradigm shift in the established and emerging high-power applications. In this article, we present a review on the developments and prospects of GaN integrated circuit power amplifiers (PAs). The progress of GaN transistors including improvements in their important features, i.e., supply voltage, substrate material, transistor scaling approach, and device modeling are elaborated and the current state-of-the-art processes with 20-nm gate length, 450 GHz cut-off frequency, and over 600 V supply voltage are discussed. We also investigate developments in the GaN integrated circuit PA architectures and their implementation challenges including the reactive matching PAs capable of delivering over 100 W output power and operating up to 200 GHz, PA linearity, back-off efficiency enhancement, reconfigurable PAs, and distributed PA architectures. Finally, we discuss the prospects of GaN technology and possible future improvements, in transistor and circuit levels, which can advance performance and functionality of GaN integrated circuits.

Threshold voltage (VTh) drift of Schottky-type p-GaN gate high-electron mobility transistor (HEMT) was analyzed under high-voltage and hard switching conditions with a resistive load (R-Load). To overcome the distortion in the drain current (ID) – gate voltage (VG) curve caused by the internal drain capacitance (IDisp) charging current during the switching transients, a method is presented to accurately measure dynamic VTh through appropriate correction of IDisp. The dynamic variation of VTh was tracked with a gate-drive rise/fall times of 200 ns. During the switching operation, VTh increases rapidly at the beginning and then saturates at a stable value. This is attributed to the gate forward current during turn-on that increases the hole depletion of the p-GaN gate through the AlGaN barrier. The dynamic VTh increases linearly at a rate of 40% with the increase of gate turn-on voltage. VTh is independent of duty cycle but minorly increases with frequency at high voltages, suggesting that channel hot electrons be injected into the AlGaN barrier and p-GaN. [ABSTRACT FROM AUTHOR]

With a reverse p-n junction in the gate stack design, this work demonstrates a 1.2 kV/25 A normally off p-n junction/AlGaN/GaN high-electron mobility transistor (PNJ-HEMT). Benefiting from the robust gate terminal, the PNJ-HEMT exhibits a large gate breakdown voltage of 18.2 V and a positive threshold voltage of 1.7 V, enabling a wide gate-bias window. Thereafter, with an applicable VGS of 10 V, the transient switching characteristics in nanosecond timescale (11.7-ns rise time and 10.5-ns fall time) and notable immunity to dynamic Ron degradation, as well as record-high dynamic breakdown voltage (1.62 kV) under transient overvoltage have been demonstrated. In particular, rugged reliability is validated after over 1-million times dynamic breakdown with a 1.5-kV peak overvoltage. To the best of our knowledge, this is the first demonstration of high-VGS (10 V) GaN HEMT's circuit-level operating capability with considerable reliability, and has well exceeded the VGS limit of 5–7 V in conventional p-GaN gate-terminal devices, thus possessing great potentials in high-power, high-frequency, and high-reliability applications. [ABSTRACT FROM AUTHOR]

In this study, we investigated the heat dissipation characteristics of gallium nitride (GaN) high-electron mobility transistors (HEMTs) fabricated on a freestanding GaN substrate (GaN-on-GaN HEMTs) by simulation. Although the GaN substrate has a lower thermal conductivity than the conventional silicon carbide (SiC) substrate, it was demonstrated that the GaN-on-GaN HEMT has a thermal resistance comparable to the conventional GaN-on-SiC HEMT in a simulated structure composed of a 100-μm-thick substrate and a 2-μm-thick GaN epitaxial layer. This is because GaN-on-GaN has a higher thermal conductivity of the GaN epitaxial layer and a lower thermal boundary resistance than GaN-on-SiC. Next, the thermal design of a 400-W/eight-channel selective heating microwave oven, which consists of eight pairs of patch antennas and power amplifier (PA) modules, involving a 50-W-class GaN-on-GaN HEMT PA, a driving amplifier, an oscillator, and several circuits, was carried out. The fan and fin specifications and enclosure configuration have been optimized to keep the temperature of the metal base on which the PA modules are mounted below 40 °C during full operation. Excellent agreement between the oven thermal simulation and the prototype thermocouple measurements was confirmed for the temperature difference between the metal base and the ambient. Finally, device dimensions for reliable operation were determined by simulating the channel temperature of the GaN-on-GaN HEMT PA during microwave heating. To keep the maximum channel temperature below 200 °C, the simulated device layout with a unit gate width (Wgu) of 100–300 μm and a gate-to-gate spacing (Lgg) of 30–70 μm was acceptable at a drain efficiency (ηD) of 75%. However, appropriate Wgu and Lgg were found to be required with a ηD of 65%. [ABSTRACT FROM AUTHOR]

We report a high maximum frequency of oscillation (${f} _{\text {max}}$) and a current-gain cutoff frequency (${f} _{\text {T}}$) of 800 and 260 GHz, respectively, with pseudomorphic high-electron mobility transistor (PHEMT), using an InGaAs/InAs composite channel and an asymmetric gate recess. This result was achieved with long gate length ${L} _{\text {G}} = {75}$ nm. The RF small signal equivalent circuit (SSEC) was extracted up to 110 GHz. Moreover, noise parameters extraction gives a minimum noise figure (NFmin) of 0.8 dB (with associated gain ${G} _{\text {ass}} = {16}$ dB) and 1.8 dB (with associated gain ${G} _{\text {ass}} = {11.6}$ dB) at 40 and 94 GHz, respectively. In this study, the gate leakage current was considered in the SSEC of the transistor (${g} _{\text {gf}}$ and ${g} _{\text {df}}$) and for extraction of the noise parameters. [ABSTRACT FROM AUTHOR]

Single event burnout (SEB) of enhancement mode GaN high-electron mobility transistors (HEMTs) under heavy ion irradiation is systematically studied based on simulations in this work. Susceptibility to SEB of different particle incidence environments is investigated to determine the device’s sensitive position. Under these conditions, the mechanism of SEB in enhancement mode HEMTs is mainly associated with the charge congestion and subsequent continuous impact ionization. A hardened HEMT structure with the gate and drain double field plates is proposed correspondingly. Apart from better output characteristics and almost 25% improvement of breakdown voltage, the hardened structure also demonstrates better tolerance of SEB. This hardened structure can modify the electric field in the high field region so that charge accumulation is reduced significantly, thus effectively decreasing the possibility of SEB’s occurrence. With a vertical heavy ion strike of 63.8 MeV $\cdot $ cm2/mg linear energy transfer (LET), SEB threshold voltage of the conventional structure HEMT is 160 V, while that of the hardened structure HEMT is much higher, reaching 249 V. Besides, the hardened device exhibits better SEB tolerance than the conventional device even at different higher LET conditions. Furthermore, the contribution to SEB tolerance of each field plate is also discussed. [ABSTRACT FROM AUTHOR]

Anew single event burnout (SEB) hardened AlGaN/GaN structure with a thin barrier interlayer (IL) is presented in this work. The proposed hardened structure is compared with the simulation results of the conventional structure. The comparative analysis demonstrates that the IL lifts the conduction band energy in buffer layer and a new quantum well is formed. The quantum well limits the electrons induced by heavy ion below the second channel into the first channel. Thus, better SEB performance is achieved compared with the conventional structure. Moreover, the breakdown characteristic of the new structure is significantly improved, while the output characteristic is slightly reduced. Under the condition that the two structures are irradiated vertically by heavy ion having the linear energy transfer (LET) value of 0.6 pC/μm, the SEB threshold voltage of the conventional structure is 280 V, while the hardened structure can achieve 375 V.

In this letter, we successfully demonstrated a AlGaN/GaN high-electron mobility transistor on silicon substrate with high product of maximum oscillation frequency (fmax) and gate length (LG) by reducing the gate resistance (Rg) using a thick, high aspect ratio rectangular gate (R-gate) structure with an LG of 265 nm and thickness of 315 nm which was fabricated using a thick polymethyl methacrylate lift-off process. The maximum drain current is over 1 A/mm, and the peak transconductance is 291 mS/mm. The values of cutoff frequency and fmax are 43.7 GHz and 126.5 GHz at a drain voltage (Vd) of 12 V, respectively. Rg is extracted through the small-signal model, and the value is given as 0.21 Ω-mm which is comparable to devices with the T-gate structure. This low Rg results in a high fmax and high fmax × LG product of 33.52 GHz-μm, comparable to previously reported GaN-on-Si transistors for both R-gate and T-gate structures.

This article reports a systematic study focused on the mechanical stress effect of field-plate dielectric film on the electric characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMTs were fabricated on SiC substrates, where the stress of a SiN field-plate dielectric film ranged from −252 (compressive) to +26.5 (tensile) MPa. Si-rich and compressive SiN films exhibited a significant increase in the isolation leakage. On the other hand, relatively N-rich and tensile SiN films showed a significant increase in the gate leakage current of HEMTs. In addition, pulsed I – V measurements showed the suppression in the current collapse by increasing the tensile stress. Consequently, small current collapse and small gate leakage current were obtained simultaneously with good isolation, as the film stress was optimized. [ABSTRACT FROM AUTHOR]

In this work, an evaluation of the performance of discrete elements intended for an L-band high-temperature GaN-based LC-oscillator is carried out between room temperature and 300 °C. GaN high-electron mobility transistors (HEMTs) on sapphire substrate, metal–insulator–metal (MIM) capacitors, thin-film inductors, and resistors on sapphire and quartz substrates are fabricated and characterized through dc and S-parameter measurements up to 20 GHz. The GaN HEMTs on sapphire achieved ƒt of 18 GHz and ƒmax of 30 GHz at room temperature and ƒt of 11 GHz and ƒmax of 21 GHz at 300 °C while maintaining dc gate leakage below 0.1 mA/mm at 300 °C. MIM capacitors are characterized at temperatures up to 300 °C and at frequencies up to 10 MHz, showing stable capacitance values with changes as small as 1.37% over the entire temperature range. Planar spiral inductors are characterized using S-parameter measurements up to 20 GHz and 300 °C. While a temperature stable maximum oscillation frequency above 1 GHz is achieved, the quality factor degrades by up to 48% at 300 °C. A detailed analysis of the temperature dependence of the inductor is given. [ABSTRACT FROM AUTHOR]

This article describes the thermal resistance “junction-to-case” measurement results for GaN high-electron mobility transistors (HEMTs). The measurements were taken by using the apparatus which includes two methods of thermal resistance measurements. For the first one that uses standard MIL-STD-750-3, a sequence of heating current pulses passes through the transistor channel and then the junction temperature is measured. The second method uses modulated heating power and measures the response which is a variable component of the junction temperature. To exclude the influence of the heating pulses duration, which is typical for the standard method, preliminary measurements of the transient response of thermal impedance were taken. The analysis of this characteristic allows determining the optimal values of the pulse duration. To reduce the influence of the delay time caused by transient electrical processes in the transistor when it switches from heating mode to the measurement of the thermal sensitive parameter (TSP) mode, the TSP signal was extrapolated to the end of the heating pulse. A comparative analysis shows that the measurement results obtained by the standard and modulation methods differ by less than 2%. The results of a study of the influence of the heating current pulses magnitude on the thermal resistance measurements results are presented. It was found that, as heating current increased, the thermal resistance measured values increased, which indicates the nonlinear nature of the dependence of the temperature in the transistor channel on the power dissipated in it. [ABSTRACT FROM AUTHOR]

This article reports on the experimental and analytical determination of the optimum carbon concentration in GaN to achieve enhanced breakdown in AlGaN/GaN high-electron mobility transistors (HEMTs). The lateral breakdown voltage increases when carbon doping is increased from 3 × 1018 to 1019 cm−3 beyond which it decreases, whereas there is no substantial enhancement in the vertical breakdown voltage with carbon doping. We invoke carrier statistics in a compensated semiconductor vis-à-vis the formation energy of carbon-occupying Ga (or N) vacancies to explain the observed buffer leakage. Temperature-dependent data indicate that the buffer leakage current is due to hopping transport, the activation energy of which yields the positions of the defect states within the bandgap. The increase in buffer leakage beyond optimum C concentration is attributed to the formation of shallow donor traps by carbon atoms occupying Ga vacancies (CGa). The observations correlated with the relative intensities of the defect-mediated peaks in the cathodoluminescence (CL) data of the samples. Based on our findings, a C doping beyond 1019 cm−3 is not recommended for GaN buffers in order to achieve high breakdown voltages. [ABSTRACT FROM AUTHOR]

In this article, we investigate the impact of hard-switching on the dynamic ON-resistance (RON) in the AlGaN/GaN high-electron mobility transistors (HEMTs). The pulsed measurements were taken on a set of GaN-on-Si wafers, showing a significant RON increase after hard-switching compared with soft-switching. The impact of hard-switching was found to be strongly dependent on the surface passivation stoichiometry. Both hot electrons and self-heating are generated during hard-switching and they were investigated separately. For the self-heating effect, we found that the heating energy dissipated during hard-switching followed a different trend to the dynamic RON, showing that self-heating was not responsible for the dynamic RON. Following hard-switching, we found that the recovery of the RON occurred on a time scale of microseconds, far too fast to be explained by buffer trapping. Consequently, we suggest that the hard-switching-induced hot electrons are trapped on the surface and result in the dynamic RON. To support these conclusions, we undertook 2-D-TCAD simulations. Self-heating was found to be incompatible with the measurements, and surface-trapped hot electrons during hard-switching were shown to be consistent with the experimental observation. Based on the analysis, we find that modifying the field plates and stoichiometries of SiNx can be the possible solutions to suppress dynamic RON after hard-switching. [ABSTRACT FROM AUTHOR]

We report on the fabrication and high-frequency characterization of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). In devices with a gate length of 160 nm, a record power density of 10.5 W/mm with 34% power added efficiency (PAE) has been measured at 40 GHz in MOCVD-grown HEMTs biased at V-DS = 30 V. Under similar bias conditions, more than 8.6 W/mm, with 32% PAE, were obtained on the MBE-grown sample. The dependence of output power, gain, and PAE on gate and drain voltages, and frequency have also been analyzed.

The N-polar GaN high-electron mobility transistors (HEMTs) have demonstrated a powerful performance as the Ga-polar GaN HEMTs. This investigation aims to show the direct current performance and cutoff frequency (${f}_{\text {T}}$) of the planar N-polar GaN HEMTs with gate lengths downscaling from $4~\mu \text{m}$ to 50 nm by 2-D device simulation. The impacts of traps and gate dielectrics and the roles of the field-dependent mobility and the source and drain series resistances are investigated. For our central-gated device with a 10-nm top GaN channel layer, a 30-nm Al0.3Ga0.7N barrier layer, and a 3-nm Al2O3 gate dielectric, the gate length (${L}_{\text {G}}$) as the transition point from the long-channel behavior to the short-channel one is found to be 200 nm. For ${L}_{\text {G}} < {200}$ nm, a linear ${f}_{\text {T}}^{-{1}}$ versus ${L}_{\text {G}}$ relation shows up, and notable short-channel effects appear, i.e., the negative shift of the threshold voltage, the increase of the drain-induced barrier lowering, and the almost ${L}_{\text {G}}$ -independent constant maximum transconductance in the saturation region. The degradation of the ${f}_{\text {T}}\times {L}_{\text {G}}$ product with the decrease in the aspect ratio between ${L}_{\text {G}}$ and the equivalent gate-channel distance (${t}_{t}$) is shown to be quite small. The ${L}_{\text {G}}/{t}_{t}$ ratio for 15% degradation of the ${f}_{\text {T}}\times {L}_{\text {G}}$ product from its upper limit ($19.23~\text {GHz}\cdot \mu \text {m}$) is 5.5. This small degradation of the frequency characteristics in short-channel devices is attributed to the quite small fringing capacitance of the gate. [ABSTRACT FROM AUTHOR]

A high-electron mobility transistor (HEMT) with a GaN channel is simulated using a deterministic multi-subband Boltzmann transport equation solver. A structure that includes an InGaN back barrier is considered. The electron mobility is calculated for the 1-D heterostructure to identify the contribution of each scattering mechanism to the electron mobility. For GaN-based HEMTs, electron transport in a non-equilibrium state is determined by solving the multi-subband Boltzmann equation expanded with Fourier harmonics. The polar optical phonon and the deformation potential phonon are considered to explain the scattering of the device while considering the Pauli principle. The results of a simulation conducted to assess the mobility and ${I}$ – ${V}$ characteristics are compared with experimental results from the AlInN/AlN/GaN/InGaN HEMT structure. The cutoff frequency of the HEMT is also estimated under the quasi-static approximation. [ABSTRACT FROM AUTHOR]

We report the improvement in the electrostatic discharge (ESD) characteristics of an AlGaN/GaN high‐electron mobility transistor (HEMT) with a metal–insulator–metal (MIM) capacitor structure on an aluminum nitride (AlN) flip‐chip (FC) submount. Compared with an HEMT without an FC, the measured results for the HEMT with an FC revealed improvements of 25% and 150% under drain‐to‐source and gate‐to‐source ESD stress, respectively. This improvement can be attributed to an additional stress‐bypassing path formed in the MIM structure on the AlN FC submount that allowed the flow of ESD current and supported the charge by using additional capacitance. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

A recessed barrier layer (RBL) structure is proposed in themicrometer-sized AlGaN/GaN high-electron mobility transistor (HEMT) for terahertz applications. It is found by using numerical simulation that the properly designed RBL structure can trigger the formation and propagation of electron domains in the 2-D electron gas (2-DEG) channel. As a result, the fundamental frequency can be extended even up to terahertz regime, and also it can be tuned in a certain range by the bias voltage of Schottky contact gate that acts as a hot-electron injector just like the notch-dopedGunn diode. Our simulations showthat the 0.3- $\mu \text{m}$ -gate HEMT with the RBL structure having the depth of 10–15 nm and the length of 200–400 nm can generate the stable and tunable oscillations in the range of around 0.8–1.6 THz under the normally gate bias of −0.2–0.4 V, as well as the ratio of RF-to-dc current component ranging from 3.04% to 7.2%. These available characteristics come from the redistribution of electric field in the 2-DEG channel. Compared with the ungated and the top-gated planar Gunn diodes ever reported, the proposed RBL structural AlGaN/GaN HEMT can be easily fabricated and operated under normal conditions, demonstrating a great potential application in terahertz radiation sources. [ABSTRACT FROM AUTHOR]

We report on the demonstration and investigation of Ta2O5 as high-k dielectric for InAlN/GaN-MOS high-electron mobility transistor (HEMT)-on-Si. Ta2O5 of thickness 24 nm and dielectric constant ~30 was sputter deposited on InAlN/GaN HEMT and was investigated for different post deposition anneal (PDA) conditions. The gate leakage was 16 nA/mm at −15 V which was ~five orders of magnitude lower compared to reference HEMT. The 2-D electron gas (2-DEG) density was found to vary with annealing temperature suggesting the presence of net charge at the Ta2O5/InAlN interface. Dispersion in the capacitance–voltage (${C}$ – ${V}$) characteristics was used to estimate the frequency-dependent interface charge, while energy band diagrams under flat band conditions were investigated to estimate fixed charge. The optimum anneal condition was found to be 500 °C which has resulted into a flat band voltage spread ($\Delta {V}_{\text {FB}}$) of 0.4 V and interface fix charge (${Q} _{\text {f}}$) of ${3.98} \times {10}^{{13}}$ cm $^{-2}$. X-ray photoelectron spectroscopy spectra of as-deposited and annealed Ta2O5film were analyzed for Ta and O compositions in the film. The sample annealed at 500 °C has shown Ta:O ratio of 0.41. X-ray diffraction analysis was done to check the crystallization of amorphous Ta2O5 film at higher annealing temperatures. [ABSTRACT FROM AUTHOR]

This paper theoretically explores photocontrolled terahertz amplified modulator in electron plasma wave optically switched resonant tunneling diode (ORTD) gate high-electron mobility transistor. We present a developed distributed circuit model based on the Khmyrova model. Photoexcitation causes ORTD operating state between negative differential conductivity and positive differential conductivity, which power gain of the device can be control by photoexcitation. Numerical and analytical results show that photoexcitation enabling amplified modulator can realize much larger modulation depth (>95%) than what has been reported in photocontrolled modulator. Our results show the potential of this device in several fields of terahertz technology, such as photocontrolled modulator, mixer, and other two port networks.

This study proposes AlGaN/GaN/silicon high-electron mobility transistors (HEMTs) grown by a metallorganic chemical vapor deposition (MOCVD) system. The large-signal linearity and high-frequency noise of HEMTs without and with different passivation layers are compared. The experimental data show that the addition of a TiO2 passivation layer to undoped AlGaN/GaN HEMT’s increases the value of the third-order intercept point (OIP3) by up to 70% at 2.4 GHz. Furthermore, the minimum noise figure (NFmin) of the HEMT with TiO2 passivation is significantly reduced.

Through implementation of the 3-D tri-gate topology, GaN-based high-electron mobility transistors (HEMTs) have been fabricated and high-frequency performances as well as the short-channel effects are investigated. The designed tri-gate transistors are highly-scaled having 100 nm of gate length, which introduces the condition of a short channel. It is demonstrated that higher sub-threshold slopes, reduced drain-induced barrier lowering and better overall off-state performances have been achieved by the nano-channel tri-gate HEMTs with an AlGaN barrier. A lattice-matched InAlGaN barrier with the help of the fin-shaped nano-channels provide improved gate control, increasing current densities, and transconductance gm. In a direct comparison, very high drain current densities (~3.8 A/mm) and gm (~550 mS/mm) have further been obtained by employing a pure AlN barrier.

Significant threshold voltage $V_{\mathrm {th}}$ shifts are observed during 10-keV X-ray irradiation of AlGaN/GaN high-electron mobility transistors (HEMTs). Shifts are much smaller for lower dose-rate Cs-137 irradiation than that for higher dose-rate X-ray irradiation. This occurs because hydrogen transport and interactions with defects and impurities in these crystalline wide bandgap semiconductor materials differ fundamentally from those in amorphous SiO2. The electric field is also higher in GaN-based HEMTs than in bipolar base oxides. The absence of enhanced low-dose-rate sensitivity in these devices simplifies testing of GaN-based HEMTs for space applications. No significant X-ray-induced $V_{\mathrm {th}}$ shifts are observed in development stage InAlN/GaN HEMTs. [ABSTRACT FROM AUTHOR]

We investigate the vertical leakage mechanism in metal–organic chemical vapor deposition-grown carbon (C)-doped AlGaN/GaN High Electron Mobility Transistors (HEMTs) on 6-in silicon wafer. Substrate bias polarity-dependent ${I}$ – ${V}_{s}$ , temperature-dependent fitting, and band diagram analysis pointed to the Poole–Frenkel (P–F) type of conduction mechanism for vertical transport in the devices with breakdown as high as 580 V for a buffer of $\textsf {4}~\mu \text{m}$. Trap activation energy of 0.61 eV was estimated from the P–F fitting which matches well with values reported in the literature. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. The variation in leakage as a function of dislocation density at a given field is predicted and is found to be the most abrupt in the range from $\sim 10^{\textsf {7}}$ to $\sim 10^{\textsf {9}}$ cm $^{-\textsf {2}}$ of dislocation density. This can be attributed to a sharp decrease in trap activation energy in the above range of dislocation density, possibly due to complex formation between point defects and dislocations. [ABSTRACT FROM AUTHOR]

In0.08Al0.92N/GaN fin-type high-electron mobility transistors (fin-HEMTs) with different gate-to-drain lengths (${L} _{\text {gd}}$) are fabricated and characterized by dc and low-frequency noise (LFN) measurements. The fabricated device with the largest ${L} _{\text {gd}}$ exhibits the degradation of the maximum drain current and transconductance with a positive shift of the threshold voltage. LFN measurements of the In0.08Al0.92N/GaN fin-HEMTs reveal clear 1/ ${f}$ behavior of the noise spectra, and the minimum value is observed in the device at ${L} _{\text {gd}} = {20}\,\,\mu \text{m}$. The devices with smaller ${L} _{\text {gd}}$ follow a carrier number fluctuation noise model owing to electron trapping/detrapping into the In0.08Al0.92N barrier layer from the 2-D electron gas (2DEG) channel. In contrast, the device with the largest ${L} _{\text {gd}}$ shows correlated mobility fluctuations due to the large 2DEG mobility fluctuations in the large access area. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron mobility transistors (HEMTs) and metal–oxide–semiconductor (MOS)-HEMTs using HfO2 as a gate dielectric have been analyzed at room temperature, after short thermal annealing (STA) and thermal cycle tests, during off-state electrical step stress, high-temperature reverse bias (HTRB), and positive bias temperature instability (PBTI) tests. The results showed that the leakage current in as-fabricated MOS-HEMTs decreased by 106 and the on/off ratio increased by over 104 than the HEMTs. Moreover, it was even higher after an STA test, up to 108, in the MOS-HEMTs, and the surface trapping effects were mitigated, especially if a KOH cleaning was used before HfO2 deposition. The MOS-HEMTs also showed higher electrical stability after off-state step electrical stress, HTRB, and PBTI tests. [ABSTRACT FROM AUTHOR]

This paper presents an extensive investigation of the impact of the resistivity of the silicon substrate on the vertical leakage and charge trapping in 200 V GaN-on-Si enhancement-mode high-electron mobility transistors. Three wafers having different substrate resistivities were submitted to combined DC characterization, step-stress experiments, and electroluminescence (EL) analysis. The results described within this paper demonstrate that: 1) the use of a highly resistive silicon substrate can increase the vertical breakdown voltage of the transistors, due to the fact that the voltage drop on the GaN buffer is mitigated by the partial depletion of the substrate (this latter causes a plateau region in the drain to substrate I–V characteristic) and 2) highly resistive substrate results in stronger trapping effects, due to the capacitance of the depleted substrate and the resulting backgating effects. The results described within this paper indicate that the choice of the resistivity of the substrate is the result of a tradeoff between high breakdown voltage (that could be in principle achieved through a highly resistive substrate) and the minimization of trapping processes (which can be hardly obtained with a resistive substrate). [ABSTRACT FROM AUTHOR]

The stoichiometry of low-pressure chemical vapor deposition SiNx surface passivation is shown to change vertical conductivity at the top of the epitaxial stack in GaN-on-Si power high-electron mobility transistors (HEMTs). This changes the charge stored in the carbon-doped GaN layer during high-voltage operation, and allows direct control of buffer-related current collapse in HEMTs. Substrate bias ramps are used to identify the changes in C:GaN charge trapping and vertical leakage. Channel length dependence indicates a lateral conductivity in the C:GaN with a localized increase in vertical conductivity under the ohmic contacts. An optimum SiNx recipe is identified which simultaneously delivers low current collapse and low drain leakage. [ABSTRACT FROM PUBLISHER]

We have demonstrated highly scaled, self-aligned AlGaN/GaN fin-shaped field-effect transistors (FinFETs), which were fabricated using e-beam lithography and a regrown n+ GaN ohmic process with a sacrificial dummy gate. Our devices were very aggressively scaled, with fin widths, gate length, and source drain spacing as small as 50, 60, and 200nm, respectively. DC characteristics, when normalized to active device periphery (number of fins times fin width), showed peak transconductance of 1.5 mS/ \mu \textm and ON resistance of $390~\Omega $ - \mu \textm . Enhancement mode device operation was observed for fin widths below 100 nm. [ABSTRACT FROM AUTHOR]

Current collapse is a phenomenon that increases the channel resistance during the switching process when the high gate and drain voltages are applied. The effect can be found from GaN-based high-electron mobility transistors (HEMTs) and is an obstacle for their applications to power electronics. There have been numerous reports on suppressing current collapse using semiconductor process technologies. In this paper, an enhancement mode (E-mode) AlGaN/GaN/AlGaN double heterostructure was proposed. The current collapse phenomena were studied on devices of E-mode MIS and Schottky gate HEMTs. The results indicate the suppression of current collapse for devices with double heterostructure. [ABSTRACT FROM AUTHOR]

A high maximum frequency of oscillation ( f\mathrm {{max}} ) of 910 GHz was achieved at InAlAs/InGaAs high-electron mobility transistors (HEMTs) with a relatively long gate length ( LG ) of 75 nm by adopting an asymmetric gate recess and a double-side-doped structure. The f\mathrm {{max}} improved significantly by extending the drain-side gate recess length ( L\mathrm {{RD}} ) to 250 nm; meanwhile, the source-side gate-recess length ( L\mathrm {{RS}} ) was kept to 70 nm. The improvement in f\mathrm {{max}} was due to a decrease in the drain output conductance ( gd ) and drain-to-gate capacitance ( \textC\mathrm {{GD}} ) after the extension of L\mathrm {{RD}} . gd was further suppressed by applying a double-side-doped structure to the InP-based HEMTs. A reduction in gd resulted in a drastic improvement in f\mathrm {{max}} even though LG was a longer value. [ABSTRACT FROM AUTHOR]

A stacked high-electron mobility transistor (HEMT) power amplifier (PA) has been designed and implemented in a commercial 0.1- \mu \textm InGaAs pHEMT process to increase gain and output power at millimeter-wave frequencies. The stability problem of the stacked HEMT has been analyzed. A new layout of the stacked HEMT for improving the high frequency stability is proposed and used in the PA design. Measurements on the three-stage PA with parallel devices verify the saturated output power of 25 dBm and the maximum power added efficiency of 15% at 61 GHz, which is the highest reported output power of stacked HEMT PAs. The chip size measures 3.2 mm2 which makes this the most power dense $V$ -band amplifier reported from GaAs with 100 mW/mm2. [ABSTRACT FROM PUBLISHER]

We report an electroluminescence (EL) microscopy study of operating cross-sectioned AlGaN/GaN high-electron mobility transistors. By examining devices in a cross-sectional view, the distribution and intensity of photons emitted from underneath the optically opaque metal of the gate and drain structures can be studied. The location and the shape of EL bright spots were quantitatively compared with simulated device behavior, revealing a strong correlation between the measured EL intensity and the expected distribution of hot electrons in the channel. Under constant low-power conditions, the bulk of the EL signal migrates from the drain edge of the gate field plate to the drain edge of the source-connected field plate (SCFP) as the drain bias is increased. Hot electrons have been cited as a dominant contributor to device degradation for some devices, so quantifying their location and bias dependence is critical to understanding how this degradation rate might scale with bias and device design. In addition, devices both with and without an SCFP were imaged to quantitatively investigate the influence of the field plate on the EL signal. Finally, a measurement of the spectra of EL signals is used to estimate the temperature of hot electrons in the device. [ABSTRACT FROM AUTHOR]

This paper presents two structures of wide band gap high electron mobility transistor (HEMT). One structure is made-up of a stack of AlGaN layer over GaN layer. This structure is characterized by two-dimensional (2-D) electron gas layer formed at the interface of the AlGaN and GaN layers. The 2-D electron gas plays an important role in determining the carrier-mobility $$(\upmu )$$ and hence drain-to-source current $$(I_\mathrm{DS})$$ of HEMT. The other structure introduces an AlN spacer layer between the AlGaN and GaN layers to improve these characteristics. This paper compares the output characteristics curves $$(I_\mathrm{DS}-V_\mathrm{DS})$$ and transconductance characteristics curves $$(I_\mathrm{DS}-V_\mathrm{GS})$$ obtained from simulations performed using Silvaco $$\hbox {ATLAS}^\mathrm{TM}$$ . The modified structure with spacer layer shows improvements in carrier-mobility and hence drain-to-source current. This paper estimates and compares the subthreshold slope (SS) of the two devices. AlGaN/AlN/GaN HEMT offers an SS of 80 mV/decade whereas AlGaN/GaN HEMT offers an SS of 95 mV/decade. Thus, an improvement in SS of about 18.75 % is achieved in AlGaN/AlN/GaN HEMT compared to AlGaN/GaN HEMT. HEMT with spacer layer also offers 10 $$\times $$ improvement in $$I_{DS}$$ as compared to HEMT without spacer layer. The proposed HEMTs achieve $$\approx $$ 3.19 $$\times $$ improvement in breakdown voltage, $$>$$ 1.3 $$\times $$ improvement in SS compared to HEMTs previously proposed in the literature. [ABSTRACT FROM AUTHOR]

High operating voltage and high power are important developing goals of GaN-based high-electron mobility transistor (HEMT) at present. In this letter, we demonstrated a GaN-based HEMT exhibiting a breakdown voltage of greater than 300 V, an operating voltage of up to 80 V, an output power of up to 750 W, a power-added efficiency (PAE) of 80% and an associated power gain of 16.8 dB at 1.3 GHz. The device simultaneously realizes high voltage, high power, and high PAE, and is suitable for L -band high-power applications. [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]

This paper reports the experimental demonstration of time-dependent dielectric breakdown in GaN-based high-electron mobility transistors (HEMTs) submitted to OFF-state stress. Based on combined breakdown measurements, constant voltage stress tests, and 2-D simulations, we demonstrate the following relevant results. First, GaN-based HEMTs with a breakdown voltage higher than 1000 V (evaluated by dc measurements) may show time-dependent failure when exposed to OFF-state stress with V\mathrm {DS} in the range 600–700 V. Second, time-to-failure (TTF) is Weibull-distributed, and has an exponential dependence on the stress voltage level. Third, time-dependent breakdown is ascribed to the failure of the SiN dielectric at the edge of the gate overhang, on the drain side. Fourth, 2-D simulations confirm that—in this region—the electric field exceeds 6 MV/cm, i.e., the dielectric strength of SiN. Finally, we demonstrate that by limiting the electric field in the nitride through epitaxy and process improvements, it is possible to increase the TTF by three orders of magnitude. [ABSTRACT FROM AUTHOR]

Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal–organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude ( 10^-4 A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic $I$ – $V$ conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV. [ABSTRACT FROM PUBLISHER]

The proposed hybrid Schottky–ohmic drain structure is analyzed in detail for AlGaN/GaN power high-electron mobility transistors on the Si substrate. Without any additional photomasks and process steps, the hybrid drain design can alter the electric field distribution to improve the breakdown voltage VBK . In addition, it provides an additional current path to achieve zero onset voltage and reduce the ON-resistance. It was found that the Schottky extension Lext is critical to VBK , $R_{{\mathrm{{\scriptstyle ON}}}} , and also the current collapse phenomena of the transistors. The extended Schottky electrodes for optimized transistor characteristics are investigated, and the physics behind are discussed. With an Lext\sim 2 –3 \mu m, VBK can be improved up to 60% with an $R_{{\mathrm{{\scriptstyle ON}}}}$ degradation below 3%. [ABSTRACT FROM AUTHOR]

This paper discusses the impact of the back-gate bias on the dc, low-frequency noise, and dynamic behavior characteristics of a p-GaN gate high-electron mobility transistor on silicon substrate. This paper is investigated to understand the physical mechanisms of the back-gate terminal modulation of normally OFF GaN power devices. When a negative back-gate bias VB voltage is applied, the 2-D electron gas channel will get closer to AlGaN/GaN heterointerface and interface scattering, such as interface roughness and alloy-disorder scattering will increases significantly, which may be responsible for the increased ON-state resistance ( R\mathrm{{\scriptstyle ON}} ). Meanwhile, the opportunity for the capture of carriers by deep-level traps is reduced and the low-frequency noise is thereby suppressed. Under positive VB bias, R\mathrm{{\scriptstyle ON}} can be reduced but, according to capacitance-voltage measurements and carrier fluctuations extracted from the low-frequency noise spectra, the transported carriers are obviously trapped by the deep-level. [ABSTRACT FROM AUTHOR]

This paper describes recent technological advances on III-nitride-based transistors for power switching applications. Focuses are placed on the progress toward enhancing the breakdown voltage, lowering the ON-resistance, suppressing current collapse, and reducing the leakage current in AlGaN/GaN high-electron mobility transistors (HEMTs). Recent publications revealed that the tradeoff relation between ON-resistance and breakdown voltage in AlGaN/GaN HEMTs exceeded the SiC limit and was getting close to the GaN limit; however, the breakdown voltage achieved was still lower than the theoretical impact ionization limit. A novel process featuring strain-controlled annealing with a metal stack, including Al gave rise to significant reduction in the sheet resistance in AlGaN/GaN heterostructures, suggesting the possibility of dramatic reduction in ON-resistance of GaN-based power devices. Some of the interesting approaches to suppress current collapse indicated that surface trapping effects must be controlled by the optimization of surface processing as well as by the reduction of bulk traps in the epitaxial layers. Close correlation between the local gate leakage current and point defects exposed on the free-standing GaN substrate demonstrated that further reduction of defects on bulk GaN substrates is truly required as future challenges. [ABSTRACT FROM AUTHOR]

DC and transient self-heating effects are investigated in normally off AlGaN/GaN transistors designed for a high-power operation. Electrical and optical methods are combined with thermal simulations; 2- \(\mu \) s-long voltage pulses dissipating about 4.5 W/mm are applied on four different transistor structures combining GaN or AlGaN buffer on an n-type SiC substrate with or without Ar implantation. Transistors with only 5% Al mass fraction in the buffer show almost a threefold increase in the transient self-heating if compared with devices on the GaN buffer. On the other hand, 2- \(\mu \) s-long pulses were found not to be long enough for the Ar-implanted SiC substrate to influence the device self-heating unless AlGaN composition changes. In the dc mode, however, both the buffer composition and Ar implantation significantly influence the self-heating effect with the highest temperature rise for the transistor having the AlGaN buffer grown on the Ar-implanted SiC. We point on possible tradeoffs between the transistor high-power design and the device thermal resistance. [ABSTRACT FROM PUBLISHER]

A novel simple approach to extract parasitic source and drain resistances of high electron mobility transistors (HEMT) is presented. This method could obtain the parasitic resistances by determining the portion of channel resistance involved in the measured end-resistance based on the identification of the channel position corresponding to the measured floating-gate voltage with the floating-gate, drain- and source-current-injection configurations on a single device. The technique is demonstrated on AlGaN/GaN HEMTs. It is found that the ratio of the channel resistance involving in the end-resistance to the total channel resistance approaches to a constant independent on the gate length, which could simplify the practical application of this novel method. The experimental results show that the source and drain resistances extracted by this method coincide with series resistance extracted by traditional Transmission Line Model (TLM) measurement. [ABSTRACT FROM AUTHOR]

The direct-die-attached cooling solution with a diamond heat spreader and hybrid Si heat sink has been developed for hotspot cooling of a GaN-on-Si device. The hybrid heat sink combines the benefits of microchannel flow and microjet impingement. In the fabricated test chip, the small hotspot is used to represent one unit of a GaN transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the thermal and fluidic performances. Two types of simulation models have been constructed using the commercial Finite Element Method software COMSOL, using the multiphysics features and temperature-dependent material properties. A submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. Various heating powers 10–150 W are loaded on eight tiny hotspots of size \(450 \times 300~\mu \) m (heat flux on each hotspot 0.93–13.89 kW/cm \(^{2})\) . An overall spatially averaged heat transfer coefficient of \(11.53 \times 10^{4}\) W/m \(^{2}\) K has been achieved in the microjet-based hybrid heat sink. Consistent results from the experimental and simulation studies have verified the high heat dissipation capability of the designed cooling solution. Several simulations have been conducted to investigate the effects of the heat sink structure and dimensions on the performances for hotspot thermal management. [ABSTRACT FROM AUTHOR]