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This study investigates and compares the impact of different etching techniques on the fabrication of GaN high electron mobility transistors (HEMTs) between the inductively coupled plasma reactive ion etching (ICP-RIE) and the neutral beam etching (NBE) for the gate recess. By conducting direct current analysis, it was found that devices manufactured using the NBE exhibited superior electrical performance as compared with those produced using the ICP-RIE. These enhanced electrical characteristics include a transconductance of up to 100.4 mS/mm, a threshold voltage (Vth) of −2.3 V, an on/off current ratio of 1.1 × 109, a subthreshold swing (S.S.) of 99.63 mV/dec, and a remarkably low gate leakage current. Additionally, we noted varying degrees of hysteresis in the I–V characteristics were related to process disparities possibly leading to interface defects. Multi-frequency capacitance-voltage (C–V) measurements were used to identify the interface defects at the oxide/AlGaN interface of the gate. The results revealed that devices fabricated using the NBE exhibited a lower interface defect density as compared with those fabricated using the ICP-RIE, thereby elucidating the reduced hysteresis observed in the I–V characteristics. These findings indicated the significant advantages of the NBE process in the fabrication of GaN HEMTs.

This study presents a modified version of MIS-HEMT via incorporation of dielectric pocket with a dual metal enabled gate (DP-DMG) architecture to achieve an enhancement in the electrical performance of AlGaN/GaN HEMT. A noteworthy improvement in DC/RF characteristics is observed with DP-DMG HEMT in comparison to conventionally used single metal gated Schottky (SMG) HEMT including gate leakage. Moreover, DP-DMG is further studied with different dielectrics to tune its electrical performance along with the depth of the dielectric pocket. The maximum rise in drain current and transconductance achieved with different dielectric DP-DMG HEMT is 13% and 13.5%, respectively. Apart from this, a significant positive shift of ∼0.6 V is recorded with HfO2 with a dielectric thickness of 5 nm. RF simulations showed significant improvement (∼2×) in the cut-off frequency of DP-DMG HEMT in comparison to SMG HEMT. [ABSTRACT FROM AUTHOR]

A novel lattice matched double barrier Al0.72In0.16Ga0.12N/Al0.18In0.04Ga0.78N/GaN normally-off high electron mobility transistor (HEMT) is designed and simulated by solving a set of thermodynamic transport equations. Using the experimentally calibrated physical models with bearing mobility degradation by surface roughness in account, the recess gate and double barrier of the proposed device achieves a maximum drain current density (ID,max) of 1149 mA/mm and a maximum transconductance (gD,max) of 358 mS/mm with a positive threshold voltage (Vth) of 0.2 V. The small polarization charge of first barrier is responsible for positive Vth. IDS,max in the double barrier HEMT at high gate bias condition is due to injection of electrons from upper 2DEG which is almost impossible at lower gate voltage because of insufficient energy to cross the barrier. The injection of electrons is further supported by the second peak in the gm curve at low gate bias VG = 1V. The outcome of this study suggests that the proposed device will be beneficial for high-frequency and high-power electronic applications.

Based on our proposed precision two-step gate recess technique, a suspended gate-recessed Pt/AlGaN/GaN heterostructure gas sensor integrated with a micro-heater is fabricated and characterized. The controllable two-step gate recess etching method, which includes O2 plasma oxidation of nitride and wet etching, improves gas sensing performance. The sensitivity and current change of the AlGaN/GaN heterostructure to 1–200 ppm NO2/air are increased up to about 20 and 12 times compared to conventional gate device, respectively. The response time is also reduced to only about 25 % of value for conventional device. The sensor has a suspended circular membrane structure and an integrated micro-hotplate for adjusting the optimum working temperature. The sensitivity (response time) increases from 0.75 % (1250 s) to 3.5 % (75 s) toward 40 ppm NO2/air when temperature increase from 60°C to 300°C. The repeatability and cross-sensitivity of the sensor are also demonstrated. These results support the practicability of a high accuracy and fast response gas sensor based on the suspended gate recessed AlGaN/GaN heterostructure with an integrated micro-heater. [ABSTRACT FROM AUTHOR]

In this study, the device characteristics of dual-gate GaN high-electron-mobility transistors (HEMTs) were determined. The research investigated an enhancement-mode (E-mode) GaN HEMT with a second gate connected to the source and located between the main gate and drain. Two dual-gate GaN HEMTs with different second-gate designs, using Schottky or metal–insulator–semiconductor (MIS) contacts, were simulated and fabricated, and the direct current of the devices was investigated. In device simulation, p-GaN gates were used to achieve E-mode operation in HEMTs. Technology computer-aided design (TCAD) simulation indicated that the saturation drain current (ID, sat) of devices with Schottky and MIS second gates was 88% and 38% lower than that of a single gate structure, while increasing on resistance (Ron) by 31% and 8%, respectively. In device fabrication, a Schottky and MIS second gate were respectively added to an E-mode p-GaN gate HEMT and an E-mode recesses-gate GaN MIS-HEMT. Compared with single-gate structures, the devices with a Schottky and MIS second gates reduced ID, sat by 75% and 32%, respectively, while increasing Ron by 25% and 6%, respectively. The measured electrical characteristics indicate the same trend obtained from TCAD simulation: The dual-gate design can improve the short-circuit capability of GaN HEMTs by reducing the ID, sat under on-state and high-current conditions. [ABSTRACT FROM AUTHOR]

Fabrication of gate-recessed SrSnO3 (SSO) metal-semiconductor field-effect transistors (MESFETs) with Ni Schottky gates is reported on bi-layer epitaxial SSO films with a thin heavily-doped cap layer. Devices with 0.5- $\mu \text{m}$ gate length showed enhancement-mode behavior with a saturation drain current, ${I}_{\text {DSAT}}$ , of 33 mA/mm and peak transconductance, ${g}_{\text {m,max}}$ , of 65 mS/mm. The ${g}_{\text {m,max}}$ value is a roughly $2\times $ improvement over control devices fabricated on single-layer films. Gate-recessed SSO MESFETs with Pt Schottky gates were also explored on the bi-layer films. Devices with 1- $\mu \text{m}$ gate length displayed ${I}_{\text {DSAT}} =133$ mA/mm and $g_{\text {m,max}} =73$ mS/mm, after thermal annealing. [ABSTRACT FROM AUTHOR]

The present work reports on the hydrogen gas detection properties of Pt-AlGaN/GaN high electron mobility transistor (HEMT) sensors with recessed gate structure. Devices with gate recess depths from 5 to 15 nm were fabricated using a precision cyclic etching method, examined with AFM, STEM and EDS, and tested towards H2 response at high temperature. With increasing recess depth, the threshold voltage (${V}_{\textit {TH}}$) shifted from −1.57 to 1.49 V. A shallow recess (5 nm) resulted in a 1.03 mA increase in signal variation ($\Delta {I}_{\textit {DS}}$), while a deep recess (15 nm) resulted in the highest sensing response (${S}$) of 145.8% towards 300 ppm H2 as compared to reference sensors without gate recess. Transient measurements demonstrated reversible H2 response for all tested devices. The response and recovery time towards 250 ppm gradually decreased from 7.3 to 2.5 min and from 29.2 to 8.85 min going from 0 nm to 15 nm recess depth. The power consumption of the sensors reduced with increasing recess depth from 146.6 to 2.95 mW. [ABSTRACT FROM AUTHOR]

A new Pd|HfO2|AlGaN|GaN metal-oxide-semiconductor (MOS) enhancement-mode high electron mobility transistor (HEMT) is fabricated with low-temperature sensitization, activation, electroless-plating, and two-step gate-recess approaches. Experimentally, a high positive threshold voltage Vth of 1.96 V, a very low gate leakage IG of 6.3 × 10–8 mA/mm, a high maximum extrinsic transconductance gm, max of 75.3 mS/mm, a high maximum drain saturation current ID, max of 266.9 mA/mm, and a high ON/OFF current ratio of 7.6 × 107 are obtained at 300 K. Moreover, the related temperature-dependent characteristics, over temperature ranges from 300 to 500 K, are comprehensively studied. The very low temperature coefficients on gate current, drain saturation current, transconductance, and threshold voltage confirm the thermal-stable capability of the studied device. Therefore, based on these advantages, the studied Pd|HfO2|AlGaN|GaN MOS structure is suitable for the development of high-performance HEMTs. [ABSTRACT FROM AUTHOR]

Low damage atomic layer etching (ALE) gate recess is developed for fabrication of millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). Plasma ion induced bombardments to the AlGaN barrier is effectively suppressed by the ALE recess, contributing to a well-controlled recessed surface morphology. The suppressed lattice damage to AlGaN/GaN heterostructure is also reflected by a significantly reduced gate leakage as well as an invisible threshold voltage shift associated with damage induced traps. With a 0.15-μm T-gate fabrication technology, a high power-gain cutoff frequency fMAX of 205 GHz has been achieved. The ALE-recessed AlGaN/GaN HEMTs exhibits a record high power-added-efficiency (PAE) of 43.6% at 40 GHz in a continuous-wave mode. The associated gain and output power density are also remarkably improved compared with controlled HEMTs with conventional gate recess process. [ABSTRACT FROM AUTHOR]

In this letter, we present normally-off GaN-on-Si MOSFETs based on the combination of tri-gate with a short barrier recess to yield a large positive threshold voltage (${V}_{\text {TH}}$) while maintaining a low specific on resistance (R $_{ON, \text {SP}}$) and high current density (${I}_{\text {D}}$). The tri-gate structure offered excellent channel control, enhancing ${V}_{\text {TH}}$ from +0.3 V for the recessed to +1.4 V for the recessed tri-gate, along with a much reduced hysteresis in ${V}_{\text {TH}}$ , and a significantly increased transconductance (${g} _{\text {m}}$). Additional conduction channels at the sidewalls of the tri-gate trenches compensated the degradation in ON-resistance (${R} _{ON}$) from the gate recess, resulting in a small ${R} _{ON}$ of $7.32~\pm ~0.26~\Omega \cdot $ mm for ${L} _{\text {GD}}$ of $15~ {\mu }\text{m}$ and an increase in the maximum drain current (${I}_{\text {D}}^{\text {max}}$). In addition, the tri-gate inherently integrates a gate-connected field-plate (FP), which improved the breakdown voltage (${V} _{\text {BR}}$) and reduced the degradation in dynamic ${R} _{ON}$. With proper passivation techniques, these devices could be very promising as high-performance power switches for future power applications. [ABSTRACT FROM AUTHOR]

This study investigated the effects of a thin aluminum oxynitride (AlOxNy) gate insulator on the electrical characteristics of AlGaN/GaN-on-SiC metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). The fabricated AlGaN/GaN-on-SiC MIS-HEMTs exhibited a significant reduction in gate leakage and off-state drain currents in comparison with the conventional Schottky-gate HEMTs, thus enhancing the breakdown voltage. The effects of gate recess were also investigated while using recessed MIS-HEMT configuration. The Johnson’s figures of merit (= fT × BVgd) for the fabricated MIS-HEMTs were found to be in the range of 5.57 to 10.76 THz·V, which is the state-of-the-art values for GaN-based HEMTs without a field plate. Various characterization methods were used to investigate the quality of the MIS and the recessed MIS interface.

In this letter, normally-OFF AlGaN/GaN metal–oxide–semiconductor high-electron-mobility transistors with a threshold voltage of 2.2 V have been achieved by an atomic layer etching technique. Combined with surface passivation by atomic layer deposition of composite HfSiO high- $\kappa $ gate dielectric, a well-controlled gate-recess process with minimized surface damage results in improved interface properties with a low interface trap density of $2.8\times 10^{11}$ eV−1cm−2 and suppressed gate leakage current with a high current on/off ratio over 1011. A maximum current density of 518 mA/mm with an ON-resistance of 10.1 $\Omega \cdot \textsf {mm}$ and a high breakdown voltage of 1456 V at an OFF-state current density of $1~\mu \text{A}$ /mm are also achieved. In the meantime, the dynamic ${\text {R}}_{\mathrm {on}}$ is only 1.2 times the static ${\text {R}}_{\mathrm {on}}$ after OFF-state drain voltage stress of 120 V and 2.6 times after 300-V stress. [ABSTRACT FROM AUTHOR]

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

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 SiNx/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. [ABSTRACT FROM AUTHOR]

Gallium oxide (Ga 2 O 3 ) metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated with gate recess depths of 110 nm and 220 nm, respectively. The gate recess was formed by dry plasma etching with Cr metal as the mask. The fabricated devices with a 25-nm HfO 2 gate dielectric both showed a low off-state drain current of about 1.8 × 10 −10 A/mm. The effects of recess depth on the electronic characteristics of Ga 2 O 3 MOSFETs were investigated. Upon increasing the recess depth from 110 nm to 220 nm, the saturated drain current decreased from 20.7 mA/mm to 2.6 mA/mm, while the threshold voltage moved increased to +3 V. Moreover, the breakdown voltage increased from 122 V to 190 V. This is mainly because the inverted-trapezoidal gate played the role of a gate-field plate, which suppressed the peak electric field close to the gate. [ABSTRACT FROM AUTHOR]

In this paper, we report the device performance of a high-voltage enhancement-mode (E-mode) GaN MOSHEMT on silicon substrate. Normally off operation is realized by a self-terminated precision gate recess process on an optimized high-electron mobility transistor structure. The GaN MOSHEMT is fully pinched off at zero gate bias, suggesting a "true" normally off operation. The threshold voltage is 0.4 V with a drain current density of 1 μA/mm as the criteria. The device with 15-μm gate-drain distance and 100-μm gate width exhibits a maximum drain current of 356 mA/mm at 8-V gate bias. The on/off current ratio of the device is larger than 1010 with a subthreshold slope of 80 mV/dec. The gate leakage current is below 10-7 mA/mm up to 9-V gate bias. The off-state breakdown voltage (BV) is as high as 1528 V (880 V) measured with floating (grounded) silicon substrate at a drain leakage current criterion of 5 μA/mm. The specific on-resistance (RON,SP) of the device is 2.79 mΩ⋅cm², and the power figure of merit (BV²/RON,SP) is 277 MW/cm². High-voltage pulsed I-V measurement indicates that the dynamic on-resistance is only 1.6 times the static one with a pulse width of 10 μsat 400-V off-state quiescent drain bias. The high performance of the normally off GaN MOSHEMT is supposed to benefit from the high quality low pressure chemical vapor deposition Si3N4 passivation layer and the advanced E-mode device fabrication process. [ABSTRACT FROM AUTHOR]

Normally-off AlGaN/GaN-based MOS-HEMT has been fabricated by utilizing damage-free self-terminating tetramethyl ammonium hydroxide (TMAH) recess etching. The device exhibited a threshold voltage of +2.0 V with good uniformity, extremely small hysteresis of ∼20 mV, and maximum drain current of 210 mA/mm. The device also exhibited excellent off-state performances, such as breakdown voltage of ∼800 V with off-state leakage current as low as ∼10 −12  A and high on/off current ratio (I on /I off ) of 10 10 . These excellent device performances are believed to be due to the high quality recessed surface, provided by the simple self-terminating TMAH etching. [ABSTRACT FROM AUTHOR]

In this paper, we report a promising approach for the gate recess process with a suppressed current collapse in GaN-based high electron mobility transistors (HEMTs) by means of neutral beam (NB). A recessed gate structure has been widely studied as a way to realize normally-off operation in GaN, InP, and GaAs-based HEMTs. Since GaN-based materials are usually etched by dry process, plasma-induced damage is a serious concern. NB is free from electrical charges and UV photons, resulting in an accurate control of etching depth and less plasma-induced damages. In this work, we introduce NB irradiation to the gate interface and measured DC and gate-pulsed output characteristics. The results suggest that NB is applicable for the gate recess etching with suppressing the current collapse. [ABSTRACT FROM AUTHOR]

In this letter, a gate recessed normally-off GaN metal–oxide–semiconductor high-electron-mobility transistor on silicon substrate is fabricated using AlN/Si3N4 as the passivation layer. The thin AlN layer serves the dual role of protecting the gate channel region from direct plasma bombardment during the RIE Si3N4 removal and passivating the surface states in the access region. As a result, the effective carrier mobility in the normally-off channel is found to improve from the 568 cm2/ \text V\cdot \text s in conventional Si3N4 passivation process to a high value of 1154 cm2/ \text V\cdot \text s . A saturated output current density of 603 mA/mm and an ON-resistance of 5.3~\Omega \cdot \text mm was obtained for devices with \textL\text {G}/\textL\text {GS}/\textL\text {GD}/\text{W}\text {G} = 1.5 /1.5/3/ 20~\mu \textm . Meanwhile, the degradation of dynamic ON-resistance is significantly suppressed due to the effective passivation of surface states by the AlN layer grown by plasma-enhanced atomic layer deposition. [ABSTRACT FROM AUTHOR]

Developing effective technique to protect the etched- GaN surface from the degradation in a high-temperature (i.e., at ~ 780°C) process, such as low-pressure chemical vapor deposition (LPCVD), is essential for fabricating normally-off GaN MIS-FETs with high-quality dielectric/GaN interface and highly reliable gate dielectric. In this letter, we developed an approach of obtaining such a protection layer using oxygen-plasma treatment followed by in situ annealing prior to the LPCVD-SiNx deposition. A sharp and stable crystalline oxidation interlayer (COIL) between the LPCVD-SiNx and etched-GaN was successfully formed. The LPCVD-SiNx/GaN MIS-FETs with COIL deliver normally-off operation with a VTH of 1.15 V, small on-resistance, small hysteresis, and thermally stable VTH . [ABSTRACT FROM AUTHOR]

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

We demonstrate a \beta -Ga2O3 MOSFET with record-high transconductance ( {g}_{m} ) of 21 mS/mm and extrinsic cutoff frequency ( {f}_{T} ) and maximum oscillating frequency ( f\max ) of 3.3 and 12.9 GHz, respectively, enabled by implementing a new highly doped ohmic cap layer with a sub-micron gate recess process. RF performance was further verified by CW Class-A power measurements with passive source and load tuning at 800 MHz, resulting in {P}{OUT} , power gain, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%, respectively. These preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using $\beta $ -Ga2O3. [ABSTRACT FROM PUBLISHER]

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated using a gate recess process and a surface treatment with tetramethylammonium hydroxide (TMAH) prior to gate metal deposition. Electrical characterizations show improved extrinsic transconductance and saturation current, as well as more uniform off-state behavior with reduced off-current by a factor of 3.5 and gate leakage current by a factor of 4.2 in the devices with TMAH treatment. The analyses based on atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the TMAH treatment effectively reduces the roughness of the recess-etched AlGaN surface and removes the native oxide layer on the AlGaN surface, suggesting a simple and viable route towards the fabrication of gate-recessed HEMTs based on AlGaN/GaN heterostructure with improved controllability and uniformity. [ABSTRACT FROM AUTHOR]

A method for highly controllable etching of AlGaN/GaN for the fabrication of high sensitivity HEMT based sensors is developed. The process consists of cyclic oxidation of nitride with O 2 plasma using ICP-RIE etcher followed by wet etching of the oxidized layer. Previously reported cyclic oxidation-based GaN etching obtained very slow etching rate (∼0.38nm/cycle), limited by oxidation depth. The proposed approach allows fine control of the oxidation enabling the formation of accurately controlled recess of very thin (20∼30nm) barrier layers. With optimized power settings, etch rates from ∼0.6 to ∼11nm/cycle were obtained. AFM results did not show any increase in surface roughness after etching, indicating that surface quality of the etched layer was not affected by the etching process. [ABSTRACT FROM AUTHOR]

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

In this letter, a plasma-free etch stop structure is developed for GaN HEMT toward enhancement-mode operation. The self-terminated precision gate recess is realized by inserting a thin AlN/GaN bilayer in the AlGaN barrier layer. The gate recess is stopped automatically at the GaN insertion layer after high-temperature oxidation and wet etch, leaving a thin AlGaN barrier to maintain a quantum well channel that is normally pinched off. With addition of an Al2O3 gate dielectric, quasi normally OFF GaN MOSHEMTs have been fabricated with high threshold uniformity and low ON-resistance comparable with the normally ON devices on the same wafer. A high channel mobility of 1400 cm ^2/\textrm V\cdot \textrm s was obtained due to the preservation of the high electron mobility in the quantum-well channel under the gate. [ABSTRACT FROM AUTHOR]

A low on-resistance normally-off GaN double-channel metal–oxide–semiconductor high-electron-mobility transistor (DC-MOS-HEMT) is proposed and demonstrated in this letter, which features a 1.5-nm AlN insertion layer (ISL) located 6 nm below the conventional barrier/GaN interface, forming a second channel at the interface between the AlN-ISL and the underlying GaN. With gate recess terminated at the upper channel, normally-off operation was obtained with V\mathrm {th} of +0.5 V at I\mathrm {DS}= 10 ~\mu \text{A} /mm or +1.4 V from the linear extrapolation of the transfer curve. The lower heterojunction channel is separated from the etched surface in the gate region, thereby maintaining its high field-effect mobility with a peak value of 1801 cm2/( \textV\cdot \texts ). The on-resistance is as small as 6.9 \Omega \cdot \text mm for a DC-MOS-HEMT with LG/L\mathrm {GS}/L\mathrm {GD} = 1.5/2/15~\mu \text{m} , and the maximum drain current is 836 mA/mm. A high breakdown voltage (>700 V) and a steep subthreshold swing of 72 mV/decade are also obtained. [ABSTRACT FROM AUTHOR]

In this article, a high-performance AlGaN/AlN/GaN high electron mobility transistor (HEMT) with broad gate-to-source operation voltages is fabricated and demonstrated. The experimental results exhibit a maximum extrinsic transconductance of 94.2 mS/mm at VGS = -2.58 V and a threshold voltage of -3.68 V at drain output current of 1 mA/mm.The insertion of an AlN interfacial layer can enhance the carrier confinement ability in channel and extend the gate operation voltage to +5 V. Furthermore, the effect of gate recess on device performance is investigated by two-dimensional simulation. The simulated results show that the AlGaN/AlN/GaN HEMT with gate recess structure has higher equivalent Schottky barrier height, larger electron concentration in channel, larger drain output current and transconductance as compared with the device without gate recess structure. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In this paper, the partial gate recess for performance improvement of enhancement-mode (E-mode) GaN power devices is experimentally demonstrated. The gate recess with a careful control of the recess depth was performed with an optimized recessed barrier thickness of \sim 1.5 nm that is thin enough to completely deplete the 2-D electron gas channel in the gate region. Meanwhile, the remaining barrier preserves the as-grown quantum well of the heterostructure physically intact and thus, effectively mitigates the lattice damage caused by gate recess. The fabricated E-mode Al2O3/AlGaN/GaN MISFETs deliver a threshold voltage ( V_{\rm TH} ) of +1.5 V. The maximum drain current density ( I_{D,{\rm max}} ) and transconductance ( Gm,max ) are 693 mA/mm and 166 mS/mm, respectively. The MISFETs with an LGD of $10 ~\mu $ m feature an OFF-state breakdown voltage of 860 V at a leakage current of $1 ~\mu $ A/mm. The corresponding specific ON-resistance ( $R_{{\rm {\mathrm{{\scriptstyle ON}}},sp}} ) is as low as 1.18 m \Omega \cdot \mathrm{cm}^{2} yielding a high-power figure of merit of 626 MW/ \mathrm{cm}^{2}$ . In comparison with the reference MOSFETs by fully gate recess, the respectably improved device performance of the MISFETs attributes to the enhanced electron mobility achieved by the partial gate recess. [ABSTRACT FROM AUTHOR]

Highlights: [•] Si3N4 and SiO2 double passivation is deposited top and bottom of T gate. [•] 0.1μm gate length realisation using double exposure e-beam lithography. [•] Wide gate recess process increases DC properties. [•] High power density of 2.4W/mm for Ka-band high frequency application. [ABSTRACT FROM AUTHOR]

In this article, a gate recessed AlGaN/GaN high electron mobility transistor (HEMT) was developed using 4-in. compound semiconductor process. We designed and fabricated four types of AlGaN/GaN HEMT to characterize the performance of devices. The effects of the gate recess depth and the device dimension of AlGaN/GaN HEMT are demonstrated. We present the scaling effect of the source-gate and the drain-gate enhancing the drain current and the transconductance. In addition, the current collapse was measured to estimate the trap effect in the surface and the buffer layer. Moreover, the comparison of RF performance of HEMT with different structure designs was performed. Based on these results, the optimization and the improvement of gate recessed GaN devices can be defined. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2103-2106, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27036 [ABSTRACT FROM AUTHOR]

Design considerations and performance of n++ GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 x 1020 cm-3. To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n++ GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the OFF-state breakdown. HEMTs with a gate length of 0.25 µm and a 4-&um;m source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of ~0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation. [ABSTRACT FROM AUTHOR]

Abstract: This work compares AlGaN/GaN high-electron-mobility transistors (HEMT) with different gate recess depths. Small signal analysis showed that the best device performance was achieved at an appropriate recess depth primarily that was associated with maximized intrinsic transconductance and minimized source resistance. An f T × L g product of as high as 25.6GHz-μm was obtained in a device with a gate recess depth of 10nm. Further increasing the recess depth lowered the intrinsic transconductance and thereby worsened the performance. This effect was explained by the degradation of transport properties by the epitaxial damage that was itself caused by plasma-assisted dry etching processes. [Copyright &y& Elsevier]

Scaled AlGaN/GaN heterostructure field‐effect transistors (HFETs) with high unity current gain cut‐off frequency (fT) and maximum oscillation frequency (fmax) were fabricated and characterised on SiC substrate. In the device, scaled source‐to‐drain distance (Lsd) of 600 nm was realised by employing non‐alloyed regrown n+‐GaN ohmic contacts. A 60 nm T‐shaped AlGaN/GaN HFETs showed excellent DC and RF performance after gate recess. A record extrinsic transconductance (gm) of 764 mS/mm was obtained in the AlGaN/GaN HFETs. Moreover, the maximum fT and fmax of the fabricated device reach to 149 and 263 GHz at the same bias, exhibiting a record‐high value of fT*fmax. This indicates that the AlGaN/GaN HFETs still have the potential for D‐band (110–170 GHz) power‐amplifier application with further optimisation. [ABSTRACT FROM AUTHOR]

The gate-recess technology for Si δ-doped InAs/AISb high-electron-mobility transistors (HEMTs) has been investigate by combining atomic force microscopy (AFM) inspection of the gate-recess versus time with electrical device characterization Deposition of the gate metal on the In0.5A10.5As protection layer or on the underlying AISb Schottky layer resulted in device suffering from high gate-leakage current. Superior dc and high frequency device performance were obtained for HEMTs with insulating layer between the gate and the Schottky layer resulting in a reduction of the gate leakage current IG by more than orders of magnitude at a drain-to-source voltage VDS of 0.1 V The existence of this intermediate insulating layer was evident from the electrical measurements. AFM measurements suggested that the insulating layer was due to a native oxidation of t AISb Schottky layer. The insulated-gate HEMT with a gate length of 225 nm exhibited a maximum drain current ID higher than 500 mA/mm with good pinchoff characteristics, a dc transconductance gm of 1300 mS/mm, and extrinsic values for cutoff frequency fT and maximum frequency of oscillation fmax of 160 and 120 GHz, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we present an enhancement of punchthrough voltage in AlGaN/GaN high-electron-mobility- transistor devices by increasing the electron confinement in the transistor channel using an AlGaN buffer-layer structure. An optimized electron confinement results in a scaling of punchthrough voltage with device geometry and a significantly reduced subthreshold drain leakage current. These beneficial properties are pronounced even further if gate-recess technology is applied for device fabrication. Physical-based device simulations give insight in the respective electronic mechanisms. [ABSTRACT FROM AUTHOR]

The effect of gate-recess length on DC and RF performance of AlGaN/GaN gate-recessed high electron mobility transistors (HEMTs) was studied. 0.15 μm gate-length AlGaN/GaN HEMTs with varying gate-recess length from 0.5 to 1.0 μm were fabricated. DC and microwave performance did not exhibit significant dependence on the gate-recess length. The devices showed a maximum drain current density as high as 1.1 A/mm, a unity gain cutoff frequency of 79 GHz, and a maximum frequency of oscillation of 125 GHz. The measured series source resistances were essentially identical for all devices. Therefore, all the gate-recessed HEMTs exhibited similar DC and RF performance. This conclusion is derived from the fact that the values of ohmic contact resistances dominated over the values of channel resistances. The nature of the breakdown in the HEMTs was also studied. The results of temperature-dependent breakdown voltage measurements suggest that the breakdown mechanism was mainly due to tunneling gate leakage currents via shallow traps. [Copyright &y& Elsevier]

The fabrication and characterization of GaN MESFETs grown on sapphire recessed using inductively-coupled-plasma reactive ion etching (ICP-RIE) are reported for the first time. The MESFETs were recessed using a Cl2/Ar plasma in ICP-RIE, and the drain current was monitored during recess. The devices with a gate length of 0.25 μm exhibited a peak extrinsic transconductance of 36 mS/mm and a threshold voltage of −3.7 V. The unity current gain cutoff frequency fT and maximum frequency of oscillation fmax were measured to be 28 and 55 GHz, respectively at room temperature. This recessed-gate process produced high extrinsic transconductances and lower threshold voltages compared with those of unrecessed GaN MESFETs. Also, the values of fT and fmax are at least twice the highest frequency data ever reported for GaN MESFETs. The influence of substrate temperature on the DC and microwave characteristics is also reported. [Copyright &y& Elsevier]

Based on our proposed self-terminating gate recess etching technique, normally-off recess-gated AlGaN/GaN MOSFET has been demonstrated with a novel method using GaN cap layer (CL) as recess mask, which, as a result, simplifies the device fabrication process and lowers the fabrication cost. The GaN CL is capable of acting as an effective recess mask for the gate recess process, which includes a thermal oxidation for 45 min at 650 °C followed by 4-min etching in potassium hydroxide (KOH) at 70 °C. After gate recess process, no obvious change is observed in terms of the surface morphology of the GaN CL, the contact resistance of the Ohmic contact formed directly on the GaN CL as well as the sheet resistance of the two-dimensional electron gas (2-DEG) channel layer under the GaN CL. The fabricated device exhibits a threshold voltage ( \(V_{\mathrm {th}})\) as high as 5 V, a maximum drain current ( \(I_{\mathrm {\rm dmax}})\) of \(\sim 200\) mA/mm, a high on/off current ratio of \(\sim 10^{10}\) together with a low forward gate leakage current of \(\sim 10^{\mathrm {-5}}\) mA/mm. Meanwhile, the OFF-state breakdown voltage ( \(V_{\mathrm {br}})\) of the device with gate-drain distance of 6 \(\mu \) m is 450 V. [ABSTRACT FROM AUTHOR]