Yingkui Zheng

High-

This letter reports a 0.2- μm gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on an Si substrate passivated with an AlN/SiN_x (4/20 nm) stack layer. The 4-nm-thick AlN was grown by plasma-enhanced atomic-layer-deposition. The AlN/SiN_x-passivated HEMTs exhibit a high maximum drain current of 930 mA/mm, an three-terminal OFF-state breakdown voltage (BV_DS) of 119 V, and a small threshold voltage shift of 130 mV in a wide drain bias range (V_DS=3-24 V). Owing to the additional positive polarization charge in the AlN passivation layer, the access resistance R_s in the GaN-on-Si HEMTs is significantly reduced while maintaining small parasitic gate-drain capacitance C_gd, contributing to a high power-gain cutoff frequency f_MAX of 182 GHz and a high Johnsons figure of merit of BV_DS × f T of 6.43 × 10¹² V/s simultaneously. The accuracy of the RF performance is verified by a small signal modeling based on measured S-parameters.

f_{{\rm MAX}}

In this letter, we report high-performance enhancement-mode (E-mode) Al2O3/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) fabricated with high-temperature low-damage gate recess technique. The high-temperature gate recess is implemented by increasing the substrate temperature to 180 °C to enhance the desorption of chlorine-based etching residues during the dry etching of AlGaN barrier. High-crystal-quality Al2O3 gate dielectric was grown by atomic-layer deposition using O3 as the oxygen source to suppress hydrogen-induced weak bonds. The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of 1.6 V, a pulsed drive current of 1.13 A/mm, and very low OFF-state standby power of

High Johnson's Figure-of-Merit 0.2-

6.8 \times 10-8 W/mm at VGS = 0 V and VDS = 30 V. At 4 GHz and in pulse-mode operation, the output power density and power-added efficiency were measured to be 5.76 W/mm and 57%, both of which are the highest for GaN-based E-mode MIS-HEMTs reported to date.

\mu{\rm m}

The effects of GaN channel layer thickness on dc and RF performance of AlGaN/GaN high-electron mobility transistors (HEMTs) with a state-of-the-art composite AlGaN/GaN (1/1 μm) buffer were systematically investigated. Although HEMTs with a thick GaN channel layer exhibit slight degraded dc and RF small-signal performance associated with short-channel effects, they demonstrate significantly enhanced OFF-state breakdown voltage and RF large-signal performance. The 1-mm HEMTs with a 150-nm-thick GaN channel layer feature a 1.4 dB higher saturated POUT and about 10% higher PAE than that with a 50-nm-thick GaN channel layer, in both Classes AB and B operation conditions. Pulse I-V characterization reveals that the buffer-related current collapse is also suppressed in the thick GaN channel sample as compared with the thin one, suggesting that a thick GaN channel layer will not only reduces the deep traps in the channel, but also reduces the electron capture probability by deep traps in the composite AlGaN/GaN buffer. The selection of a proper GaN channel layer thickness is thus of great importance to the designation of GaN-based power amplifiers for various applications.

Gate AlGaN/GaN HEMTs on Silicon Substrate With

Low-pressure chemical vapor deposition (LPCVD) technique is utilized for SiNx passivation of AlGaN/GaN high-electron-mobility transistors (HEMTs). A robust SiNx/ AlGaN interface featuring high thermal stability and well-ordered crystalline structure is achieved by a processing strategy of “passivation-prior-to-ohmic” in HEMTs fabrication. Effective suppression of surface-trap-induced current collapse and lateral interface leakage current are demonstrated in the LPCVD-SiNx passivated HEMTs, as compared with conventional plasma-enhanced chemical vapor deposition-SiNx passivated ones. Energy dispersive X-ray spectroscopy mapping analysis of SiNx/AlGaN interfaces suggests the interface traps are likely to stem from amorphous oxide/oxynitride interfacial layer.

{\rm AlN}/{\rm SiN}_{{\rm x}}

A high-temperature (180 °C) gate recess technique featuring low damage and in-situ self-clean capability, in combination with O₃-assisted atomic-layer-deposition (ALD) of Al₂O₃ gate dielectric, is developed for fabrication of high performance normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs), which exhibit a threshold voltage of +1.6 V, a pulsed drive current of 1.1 A/mm, and low dynamic ON-resistance under hard-switching operation. Chlorine-based dry-etching residues (e.g. AlCl₃ and GaCl₃) are significantly reduced by increasing the wafer temperature during the gate recess to their characteristic desorption temperature, while defective bonds like Al-O-H and positive fixed charges in ALD-Al₂O₃ are significantly suppressed by substitution of H₂O with O₃ precursor.

Passivation

The effects of interface oxidation on the transport behavior of the 2-D electron gas (2DEG) in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN (PEALD-AlN) passivation were investigated using temperature-dependent Hall-effect and X-ray photoelectron spectroscopy (XPS) characterizations. AlGaN/GaN heterostructure with a 4-nm-thick PEALD-AlN passivation exhibits good 2DEG transport behavior and stability at moderately high temperature (e.g., 275 °C). However, serious oxidation of the AlN/GaN (cap layer) interface occurs as the sample is heated up to 400 °C in low-pressure atmosphere, as verified by an increased Ga-O bond in Ga 3d core-level spectra. The oxidation leads to a significant reduction of 2.47 × 1012 cm−2 in the 2DEG density in the channel. A modified AlN passivation structure with Al2O3/AlN (10/4 nm) stack is shown to be able to effectively suppress the oxidation of the AlN/GaN interface, demonstrating an enhanced 2DEG density and high-temperature stability even when the s...

High RF Performance Enhancement-Mode Al 2 O 3 /AlGaN/GaN MIS-HEMTs Fabricated With High-Temperature Gate-Recess Technique

The thermal stability and electrical characteristics of GaN high-electron-mobility transistors (HEMTs) were investigated. Storage tests were carried out at 400°C for 48 h to study the ohmic-contact stability by means of the transmission line model. It was found that Ti/Al/Ni/Au ohmic contacts were stable and had superior thermal performance, but the Schottky contact may be more sensitive to the temperature. After thermal storage for 48 h at 400°C, the Schottky barrier height was increased, and the ideality factor decreased. Two types of isolation structures were also investigated under the same condition. DC tests were implemented to study the phenomenon and provide feedback for potential process improvements.

Effect of GaN Channel Layer Thickness on DC and RF Performance of GaN HEMTs With Composite AlGaN/GaN Buffer

The physical mechanism of low-thermal-budget Au-free ohmic contacts to AlGaN/GaN heterostructures is systematically investigated with current-voltage, high-resolution transmission electron microscopy, and temperature-dependent contact resistivity characterizations. With a low annealing temperature of 600 °C, pre-ohmic recess etching of the AlGaN barrier down to several nanometers is demonstrated to be an effective method to reduce the contact resistance between Ti/Al/Ti/W ohmic metals and AlGaN/GaN heterostructures. However, further over recess of the AlGaN barrier leads to only sidewall contact to 2D electron gas channel and thus degraded contact performance. It is verified by temperature-dependent contact resistivity measurements that field emission (tunneling) dominates the current transport mechanism in Au-free ohmic contacts with AlGaN barrier partially and over recessed, while both field emission and thermionic emission contribute to traditional Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructur...

Robust SiN x /AlGaN Interface in GaN HEMTs Passivated by Thick LPCVD-Grown SiN x Layer

Silicon nitride (SiNx) film grown by low-pressure chemical vapor deposition (LPCVD) is utilized as a gate dielectric for AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Trap distribution at the gate-dielectric/III-nitrides interface is characterized by a temperature-dependent ac-capacitance technique. The extracted interface state density D it decreases from 2.92 × 1013 to 1.59 × 1012 cm−2 eV−1 as the energy level depth (E C-E T) increases from 0.29 to 0.50 eV, and then levels off to E C-E T = 0.80 eV. Capacitance-mode deep level transient spectroscopy (C-DLTS) and energy band diagram simulations reveal that deep levels with E C-E T > 0. 83 eV are responsible for the dispersion of capacitances at high temperature (>125 °C) and low frequencies (<1 kHz). A high-resolution transmission electron microscope (TEM) reveals that re-oxidation of the RCA-treated AlGaN barrier surface may be responsible for the relatively high density of shallow states at the LPCVD-SiNx/III-nitride interface.