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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]

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, 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]

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]

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]

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 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]

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]

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]

Monolithic integration of enhancement/depletion (E/D)-mode AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) are demonstrated for mixed signal applications. The gate length for the E-mode and D-mode devices are 0.2 \mum and 0.35 \mum, respectively, and drain current of 496 (Vgs=+5~\rm V) and 720 mA/mm (Vgs=0~V) with a peak transconductance of 147 and 131 mS/mm are measured. Small-signal measurements result matched current-gain cutoff frequency (fT)/maximum oscillation frequency (f\max) of 29.9/55.2 GHz for the E-mode and 27.5/47.5 GHz for the D-mode devices. Direct-coupled FET logic E/D MIS-HEMT inverter and 51-stage ring oscillator are fabricated. The incorporation of gate dielectric enables the inverter a large logic voltage swing of 3.71 V at a supply voltage VDD of 5 V. The 51-stage ring oscillator implemented with 106 transistors shows an oscillation frequency of 427.6 MHz at VDD=5~\rm V, corresponding to a stage delay of 23 ps. [ABSTRACT FROM AUTHOR]

We present an etch-stop barrier structure for GaN high-electron-mobility transistors. With this new etch-stop structure, gate recess can be precisely controlled while maintaining a low surface defect density. Normally off GaN metal–insulator–semiconductor FETs fabricated using the new technology demonstrated a record effective channel mobility of 1131 \cm^2\cdot\V^-1\cdot\s^-1 with a subthreshold slope of 62 mV/decade. Due to the low interface density of states, these devices have very low hysteresis and negligible frequency dispersion in the capacitance–voltage ( C–V) measurements. [ABSTRACT FROM PUBLISHER]