Schottky-gate High-electron-mobility Transistors Research Articles

Examination of Trapping Effects on Single-Event Transients in GaN HEMTs

The effect of various trapping centers on single event-transients (SETs) in GaN high-electron-mobility transistors (HEMTs) are examined via calibrated technology computer aided design (TCAD) simulations. A single-channel, Schottky-gate HEMT computational model is developed and validated with static characteristics and single-photon absorption laser data. Simulations of SETs show that a two-dimensional electron gas (2DEG) enhancement effect occurs, which leads to additional collected charge in excess of the generated charge. The origins of this effect are examined and shown to result from an increase in occupation rate of the surface donor traps as the influx of charge is developed in the charge track. Variations in trap characteristics are then introduced and their effect on internal charge collection processes examined. It is further shown the SET response is dominated by the properties of the surface donor traps (i.e., energy level and density) such that the SET drain current closely follows the transient decay of occupation rate back to the original DC level.

Effect of Double Insulators on the Performance Improvement of 3 MeV Proton-Irradiated AlGaN/GaN MIS-HEMTs

To improve the performance of AlGaN/GaN high electron mobility transistors (HEMTs) after exposure to high energy proton irradiation, a MIS-HEMT with Al2O3/SiNx (SiNx next to semiconductor) double insulators is proposed and fabricated. Besides, the common Schottky gate HEMT and MISHEMTs with single SiNx layer and single Al2O3 layer are also fabricated as a control for comparison. After exposed to 3 MeV proton irradiation with a fluence of 1 × 1014cm−2, the MIS-HEMT with Al2O3/SiNx double insulators shows the smallest drain saturation current and breakdown voltage degradation, the smallest voltage drift and interface charge change through IV and CV tests. The smallest degradation of the carrier density and mobility contributes to its better saturation current degradation performance. Besides, experimental results of voltage drift are in accordance with simulation results. What’s more, the MIS-HEMT with Al2O3/SiNx double insulators exhibits the lowest drain current degradation and the quickest response during hard switching tests. A relatively large displacement threshold energy of Al2O3 and better passivation effect of SiNx contribute to the reliability improvement of AlGaN/GaN HEMTs after the proton-irradiation.

Remarkable Reduction in IG with an Explicit Investigation of the Leakage Conduction Mechanisms in a Dual Surface-Modified Al2O3/SiO2 Stack Layer AlGaN/GaN MOS-HEMT.

We demonstrated the performance of an Al2O3/SiO2 stack layer AlGaN/GaN metal-oxide semiconductor (MOS) high-electron-mobility transistor (HEMT) combined with a dual surface treatment that used tetramethylammonium hydroxide (TMAH) and hydrochloric acid (HCl) with post-gate annealing (PGA) modulation at 400 °C for 10 min. A remarkable reduction in the reverse gate leakage current (IG) up to 1.5×10-12 A/mm (@ VG = -12 V) was observed in the stack layer MOS-HEMT due to the combined treatment. The performance of the dual surface-treated MOS-HEMT was significantly improved, particularly in terms of hysteresis, gate leakage, and subthreshold characteristics, with optimized gate annealing treatment. In addition, an organized gate leakage conduction mechanism in the AlGaN/GaN MOS-HEMT with the Al2O3/SiO2 stack gate dielectric layer was investigated before and after gate annealing treatment and compared with the conventional Schottky gate. The conduction mechanism in the reverse gate bias was Poole-Frankel emission for the Schottky-gate HEMT and the MOS-HEMT before annealing. The dominant conduction mechanism was ohmic/Poole-Frankel at low/medium forward bias. Meanwhile, gate leakage was governed by the hopping conduction mechanism in the MOS-HEMT without gate annealing modulation at a higher forward bias. After post-gate annealing (PGA) treatment, however, the leakage conduction mechanism was dominated by trap-assisted tunneling at the low to medium forward bias region and by Fowler-Nordheim tunneling at the higher forward bias region. Moreover, a decent product of maximum oscillation frequency and gate length (fmax × LG) was found to reach 27.16 GHz∙µm for the stack layer MOS-HEMT with PGA modulation. The dual surface-treated Al2O3/SiO2 stack layer MOS-HEMT with PGA modulation exhibited decent performance with an IDMAX of 720 mA/mm, a peak extrinsic transconductance (GMMAX) of 120 mS/mm, a threshold voltage (VTH) of -4.8 V, a higher ION/IOFF ratio of approximately 1.2×109, a subthreshold swing of 82 mV/dec, and a cutoff frequency(ft)/maximum frequency of (fmax) of 7.5/13.58 GHz.

Positive Shifting of Vth with Enhanced DC Performance in AlGaN/GaN Schottky-Gate HEMT through Optimized UV/O3 Treated Gate Interface and Thermal Engineering

The surface condition under the gate is critically important to improve the device characteristics and thus reliability of the AlGaN/GaN high electron mobility transistor (HEMT). In this report, we demonstrated the reduction of leakage current (IG) with betterment of current collapse in Al0.25Ga0.75N/GaN Schottky gate (SG)-HEMT by improving metal/GaN interface using optimized ultraviolet-ozone (UV/O3) plasma treatment with post metal annealing modulation at 450 °C. The x-ray photoelectron spectroscopy (XPS) was used to verify the formation of GaOXNY layer at the metal/GaN interface. Owing to, the screening of the internal/external polarization charges by 10 min. UV/O3 surface plasma treatment and incremental increase of Schottky barrier height (Φ B) with post metal annealing modulation, resulted in the positive shifting of threshold voltage (VTH) in SG-HEMT. Due to the combined effects of the UV/O3 plasma treatment and metal annealing modulation, the magnitude of the interface trap density were effectively reduced to one order of magnitude, which further improved the current collapse significantly in SG-HEMT to 2.5% from 12.2%. The 10 min. UV/O3 treated SG-HEMT with PMA modulation exhibited decent performance with an IDMAX of 620 mA mm−1, a GMMAX of 135 mS mm−1, a threshold voltage (VTH) of −1.8 V, a higher ION/IOFF ratio of approximately a subthreshold swing (SS) of 83 mV dec−1, with a low gate leakage current of

AlGaN/GaN Schottky-Gate HEMTs With UV/O₃-Treated Gate Interface

The surface condition under gate of AlGaN/GaN heterostructure plays a critical role in high-electron mobility transistor (HEMT). In this study, the effects of ultraviolet/ozone (UV/O3) treatment applied to Al0.3 Ga0.7 N/GaN heterostructure on the electrical performance of AlGaN/GaN Schottky-gate HEMT were investigated. The reverse-bias leakage current of Schottky diode was reduced by three orders after the treatment. X-ray photoelectron spectroscopy confirms the formation of the Ga2O x layer which serves as an interface passivation layer and thus suppresses trap-assisted electron tunneling. Capacitance–voltage measurements of AlGaN/GaN HEMT show shifts of threshold and on-set voltages, indicating decreased surface states as a result of the treatment. The electrical characteristics of AlGaN/GaN HEMT exhibit improved transconductance and subthreshold swing values after the treatment.

Low-Damage Etching for AlGaN/GaN HEMTs Using Photo-Electrochemical Reactions

This paper describes our recent efforts to optimize photo-electrochemical (PEC) etching for fabricating recessed-gate aluminum gallium nitride/gallium nitride (AlGaN/GaN) high-electron-mobility transistors (HEMTs). Selecting the proper light wavelength and voltage conditions enabled PEC etching on AlGaN/GaN heterostructures to produce smooth and flat surfaces. Self-termination phenomena observed under optimal PEC condition were useful for precisely controlling the etching depth in the AlGaN layer. Two types of HEMTs, i.e., Schottky-gate and metal-insulator-semiconductor (MIS)-gate, were fabricated. A recessed-gate AlGaN/GaN structure fabricated with PEC etching showed positive threshold voltage, and its variation was very small with a standard deviation of only 0.019 V for the Schottky-gate HEMTs and 0.032 V for the MIS-gate HEMTs. A recessed-gate structure with PEC etching showed better current transport controllability with a small subthreshold-slope than that of planar-gate and dry-etched-gate AlGaN/GaN structures.

Lateral Charge Distribution and Recovery of Dynamic <inline-formula> <tex-math notation="LaTeX">$R_{\mathrm{\scriptscriptstyle ON}}$ </tex-math> </inline-formula> in AlGaN/GaN HEMTs

Lateral distribution of buffer charge trapping is investigated in AlGaN/GaN high-electron-mobility transistors (HEMTs) using a capacitance-based sense-node technique. Devices which are a Schottky-gate HEMT with an additional Schottky sense contact positioned between gate and drain can be used to monitor a lateral variation in 2-D electron gas (2-DEG) depletion as the device recovers from trapping. This paper reveals three distinct trapping types and subsequent recovery behavior in devices with varying passivation nitride stoichiometry. The results demonstrate that the two time-constant recoveries often seen in GaN HEMTs is in part due to a laterally localized charge and in part a distributed charge in the buffer. In addition, the observation of a shoulder in the capacitance–voltage curves during device recovery indicates a pinchoff shift for only a fraction of the area under the sense contact. This effect suggests a submicrometer lateral variation of trapping in the device.

Ka-Band MMIC Using AlGaN/GaN-on-Si With Recessed High- Dual MIS Structure

This letter reports the excellent radio frequency output characteristics of AlGaN/GaN-on-Si power amplifier (PA) monolithic microwave integrated circuits (MMICs) operating at the Ka-band, employing a recessed metal–insulator–semiconductor (MIS) structure with SiN x /HfON dual dielectric layers. The fabricated 0.15- $\mu \text{m}$ AlGaN/GaN-on-Si high electron-mobility transistor (HEMT) exhibited a low OFF-state current and reduced current collapse in comparison with the conventional Schottky gate HEMT; therefore, an enhanced output power of the PA MMIC is achieved. The fabricated two-stage PA MMIC exhibited a continuous-wave output power of >29 dBm with a power-added efficiency (PAE) of 14.5% at the drain voltage of 20 V, at 26.5 GHz.

Low-temperature characteristics of normally-off AlGaN/GaN-on-Si gate-recessed MOSHFETs

We investigated the low-temperature operation of normally-off AlGaN/GaN heterostructure field-effect-transistors (HFETs) with gate-recessed metal-oxidesemiconductor (MOS) structure and normally-on Schottky-gate high-electron-mobility-transistors (HEMTs). Device characteristics measured from 100 to 300 K exhibited the distinct temperature-dependence between two types of devices. While the increase of on-current was observed from normally-on AlGaN/GaN Schottky-gate HEMTs due to the enhanced mobility in the two-dimensional electron gas (2-DEG) channel, normally-off AlGaN/GaN gate-recessed MOSHFETs demonstrated distinctive characteristics at 100 K including the decrease of on-current and the positive shift of threshold voltage (Vth). The temperature-dependence observed in gate-recessed MOSHFETs is attributed to the different characteristics of the recessed channel from the 2-DEG channel, in which the sheet carrier density (nsh) and mobility (μ) were reduced as the temperature approaches the cryogenic regime.

Effects of Gd2O3 Gate Dielectric on Proton-Irradiated AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) and MOS-HEMTs using Gd2O3 as gate dielectric were irradiated with 2-MeV protons up to fluence of $1 \times 10^{15}$ cm−2. Results showed that proton irradiation causes a strong degradation in the Schottky gate devices, featured by more than three orders of magnitude increase in reverse leakage current, a 30% decrease in maximum drain current, and the same percentage of increase in ON-resistance, respectively. Scanning transmission electron microscopy showed that radiation induced a diffusion of Ni into Au in the gate and void formation, degrading the transistors’ characteristics. The Gd2O3 gate dielectric layer prevented this diffusion and void formation. MOS-HEMTs with Gd2O3 gate dielectric show 50% less decrease of performance under proton irradiation than Schottky gate HEMTs (conventional HEMTs). The trapping effects of Gd2O3 gate layer before and after irradiation are also discussed.

Suppression of Current Collapse in Enhancement Mode GaN-Based HEMTs Using an AlGaN/GaN/AlGaN Double Heterostructure

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.

High-performance AlGaN/GaN MIS-HEMT device based on in situ plasma nitriding and low power chemical vapor deposition Si3N4 gate dielectrics

Gallium nitride (GaN)-based high electron mobility transistor (HEMT) power devices have demonstrated great potential applications due to high current density, high switching speed, and low ON-resistance in comparison to the established silicon (Si)-based semiconductor devices. These superior characteristics make GaN HEMT a promising candidate for next-generation power converters. Many of the early GaN HEMTs are devices with Schottky gate, which suffer a high gate leakage and a small gate swing. By inserting an insulator under gate metal, the MIS-HEMT is highly preferred over the Schottky-gate HEMT for high-voltage power switche, owing to the suppressed gate leakage and enlarged gate swing. However, the insertion of the gate dielectric creates an additional dielectric/(Al) GaN interface that presents some great challenges to AlGaN/GaN MIS-HEMT, such as the threshold voltage (Vth) hysteresis, current collapse and the reliability of the devices. It has been reported that the poor-quality native oxide (GaOx) is detrimental to the dielectric/(Al) GaN interface quality that accounted for the Vth instability issue in the GaN based device. Meanwhile, it has been proved that in-situ plasma pretreatment is capable of removing the surface native oxide. On the other hand, low power chemical vapor deposition (LPCVD)-Si3N4 with free of plasma-induced damage, high film quality, and high thermal stability, shows great potential applications and advantages as a choice for the GaN MIS-HEMTs gate dielectric and the passivation layer. In this work, an in-situ pre-deposition plasma nitridation process is adopted to remove the native oxide and reduce surface dangling bonds prior to LPCVD-Si3N4 deposition. The LPCVD-Si3N4/GaN/AlGaN/GaN MIS-HEMT with a high-quality LPCVD-Si3N4/GaN interface is demonstrated. The fabricated MIS-HEMT exhibits a very-low Vth hysteresis of 186 mV at VG-sweep=(-30 V, +24 V), a high breakdown voltage of 881 V, with the substrate grounded. The hysteresis of our device at a higher positive end of gate sweep voltage (VG +20 V) is the best to our knowledge. Switched off after an off-state VDS stress of 400 V, the device has a dynamic on-resistance Ron only 36% larger than the static Ron.

AlGaN/GaN high electron mobility transistors with selective area grown p-GaN gates**Project supported by the National Natural Sciences Foundation of China (Nos. 61376090, 61306008) and the National High Technology Program of China (No. 2014AA032606).

We report a selective area growth (SAG) method to define the p-GaN gate of AlGaN/GaN high electron mobility transistors (HEMTs) by metal-organic chemical vapor deposition. Compared with Schottky gate HEMTs, the SAG p-GaN gate HEMTs show more positive threshold voltage (Vth) and better gate control ability. The influence of Cp2Mg flux of SAG p-GaN gate on the AlGaN/GaN HEMTs has also been studied. With the increasing Cp2Mg from 0.16 μmol/min to 0.20 μmol/min, the Vth raises from −0.67 V to −0.37 V. The maximum transconductance of the SAG HEMT at a drain voltage of 10 V is 113.9 mS/mm while that value of the Schottky HEMT is 51.6 mS/mm. The SAG method paves a promising way for achieving p-GaN gate normally-off AlGaN/GaN HEMTs without dry etching damage.

Al2O3 surface passivation and MOS-gate fabrication on AlGaN/GaN high-electron-mobility transistors without Al2O3 etching process

Al2O3 passivation by thermal oxidation of aluminium on AlGaN/GaN high-electron-mobility transistors (HEMTs) without Al2O3 etching is proposed. The deposition of a 5-nm-thick Al film was carried out, followed by a lift-off process to remove Al from the ohmic and contact pad area. Subsequently, the Al film was annealed under O2 ambient. When the gate bias was −7 V, the gate leakage currents of a conventional nonpassivated HEMT, a surface-passivated Schottky-gate HEMT, and a surface-passivated MOS-HEMT were determined as 140, 96, and 4.1 µA/mm, respectively. The current collapse phenomenon in the Al2O3-surface-passivated devices was evidently suppressed compared with that in the nonpassivated HEMT.

Investigation of trap states in high Al content AlGaN/GaN high electron mobility transistors by frequency dependent capacitance and conductance analysis

Trap states in Al0.55Ga0.45N/GaN Schottky-gate high-electron-mobility transistors (S-HEMTs) and Al2O3/Al0.55Ga0.45N/GaN metal-oxide-semiconductor HEMTs (MOS-HEMTs) were investigated with conductance method in this paper. Surface states with time constant of (0.09–0.12) μs were found in S-HEMTs, and electron tunneling rather than emission was deemed to be the dominant de-trapping mechanism due to the high electric field in high Al content barrier. The density of surface states evaluated in S-HEMTs was (1.02–4.67)×1013 eV−1·cm−2. Al2O3 gate insulator slightly reduced the surface states, but introduced low density of new traps with time constant of (0.65–1.29) μs into MOS-HEMTs.

Thermally robust RuOx Schottky diodes and HEMTs on III‐nitrides

AbstractThe potential usage of thermally robust RuOx Schottky contacts in III‐nitride‐based Schottky diodes and high electron mobility transistors (HEMTs) has been investigated. RuOx is deposited on nitride surface by sputtering Ru in Ar/O2 ambient. The influence of post sputtering annealing ambient and temperature on the characteristics of RuOx Schottky diodes on n‐type GaN on Si(111) substrate is addressed. We also report the comparative study of thermal stability between RuOx and Ni/Au Schottky diodes on InxAl1‐xN/GaN on Si(111) substrate in terms of the reverse bias leakage current and Schottky barrier height (SBH). RuOx contact shows a higher SBH of 1.24 eV and a lower leakage current of ∼10‐5 Acm‐2 (∼four orders of magnitude lower than that of Ni/Au) on InxAl1‐xN/GaN at –40 V after annealing at 800 oC in N2. In addition, RuOx Schottky gate HEMT on InxAl1‐xN/GaN on Si(111) substrate has shown a low gate leakage current of ∼6×10‐8 A/mm at ‐8 V after annealing at 700 oC in N2 for 1 minute. The lower gate leakage current has resulted in a transistor ON/OFF current ratio of ∼105. The maximum transconductance and ON‐resistance achieved by such HEMTs are about 0.18 S/mm and 8.33 Ωmm, respectively, for a gate length of 1.5 µm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved Device Performance of GaN/AlGaN High-Electron-Mobility Transistor Using PdO Gate Interlayer

We demonstrate significant improvements of GaN/AlGaN high-electron-mobility transistors (HEMTs) by employing a PdO gate interlayer, which exhibit device performance superior to that of Pd Schottky gate HEMTs. The PdO gate interlayer effectively reduces the gate leakage current by four orders of magnitude, and it also increases the ION/IOFF ratio to four orders of magnitude. The improved AlGaN/GaN/PdO HEMT shows a nearly ideal subthreshold slope of 66 mV/dec. The flicker noise characteristic is also observed to be lower in PdO-gate HEMTs than in Pd-Gate HEMTs. The high-work-function PdO layer and associated barrier height enhancement are the origins of the improved device performance.

Enhanced AlGaN/GaN MOS-HEMT Performance by Using Hydrogen Peroxide Oxidation Technique

This paper investigates enhanced device characteristics of AlGaN/GaN metal-oxide-semiconductor high electron mobility transistor (HEMT) (MOS-HEMT) fabricated by using hydrogen peroxide (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) oxidation technique which demonstrates the advantages of simplicity and cost effectiveness. A 13-nm-thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> oxide was grown upon the surface of AlGaN barrier layer and served as the gate dielectric layer and the surface passivation layer at the same time to effectively decrease gate leakage current and prevent RF current collapse, which are the critical issues of nitride HEMTs. Enhanced device performances of dc, RF, power, and reliability of the present MOS-HEMT are comprehensively investigated as compared with a conventional Schottky-gate HEMT.

Impact of Gate and Passivation Structures on Current Collapse of AlGaN/GaN High-Electron-Mobility Transistors under Off-State-Bias Stress

Using a dual-gate structure, we have investigated the impact of gate-stress position on the current collapse behavior of AlGaN/GaN high-electron-mobility transistors (HEMTs) without surface passivation. When the gate-bias stress under the off state was applied to the additional gate between the main gate and the drain electrode, we observed a marked increase in on-resistance (RON). On the other hand, the off-state stress on the main gate itself caused a decrease in drain saturation current as well as an increase in RON. The calculation of electric field at the AlGaN surface showed that the field peaks existed at the gate edges on both the drain and source sides, probably causing electron charging at the AlGaN surface near both gate-edge areas. These results indicated that the off-state gate stress induces “virtual gates” in the gate edges expanding in both the drain and source directions. The impacts of device structures on the current collapse have been characterized, using Schottky-gate HEMTs with and without surface passivation and metal–oxide–semiconductor (MOS) gate HEMTs. The surface passivation and MOS-gate structure was effective in mitigating current collapse, which was explained in terms of surface state density, electric field strength, and gate leakage current.

Temperature-Dependent Microwave Noise Characteristics in ALD $\hbox{Al}_{2}\hbox{O}_{3}$/AlGaN/GaN MISHEMTs on Silicon Substrate

Temperature-dependent microwave noise characteristics are presented in an atomic-layer-deposited /AlGaN/GaN metal-insulator-semiconductor high-electron mobility transistor (HEMT) (MISHEMT) on a Si substrate over a wide temperature range from -40 to 200 C. Typical noise parameters, including minimum noise figure , noise equivalent resistance , and associate gain , are measured over the whole temperature range. The conventional Schottky-gate HEMT with the same epistructure is also compared. The temperature dependences of and for the MISHEMT are found to be similar to those for the conventional HEMT, respectively, whereas less temperature dependence of is found in the GaN MISHEMT. The degradation rate of the noise performance of MISHEMT is found to be comparable to that of the other reported GaN HEMTs on SiC and sapphire substrates and also comparable to that of GaAs HEMTs.