Kevin J. Chen

High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500/spl deg/C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (g/sub m/) was obtained at V/sub gs/=0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At V/sub gs/=0 V, the off-state drain leakage current is 28 /spl mu/A/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 /spl mu/m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak g/sub m/ of 148 mS/mm, a current gain cutoff frequency f/sub T/ of 10.1 GHz and a maximum oscillation frequency f/sub max/ of 34.3 GHz.

Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode

This paper presents a method with an accurate control of threshold voltages (Vth) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from -4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 degC. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results confirm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs

High-performance InP-based enhancement-mode HEMTs using non-alloyed ohmic contacts and Pt-based buried-gate technologies

High performance InP-based InAlAs/InGaAs enhancement-mode HEMTs are demonstrated using two improved approaches to device structure design and fabrication, i.e., nonalloyed ohmic contacts and Pt-based buried-gate technologies, to reduce the source resistance (R/sub S/). With specially designed cap layer structures, nonalloyed ohmic contacts to the device channel were obtained providing contact resistance as low as 0.067 /spl Omega//spl middot/mm. Furthermore, in device fabrication, a Pt-based buried-gate approach is used in which depletion-mode HEMTs are first intentionally fabricated, and then, the Pt-based gate metal is annealed at 250/spl deg/C, causing the Pt-InAlAs reaction to take place under the gate electrode so that Pt sinks into InAlAs and depletes the channel. As a result, the depletion-mode HEMTs are changed to enhancement-mode, while the channel region between the source and gate electrodes remain undepleted, and therefore, the small R/sub S/ of 0.2 /spl Omega//spl middot/mm can be maintained. Excellent maximum transconductance of 1170 mS/mm was obtained for a 0.5-/spl mu/m-gate device. A maximum current-gain cutoff frequency f/sub T/ of 41.2 GHz and maximum unilateral power-gain cutoff frequency f/sub max/ of 61 GHz were demonstrated for a 0.6-/spl mu/m-gate enhancement-mode HEMT.

Effective Passivation of AlGaN/GaN HEMTs by ALD-Grown AlN Thin Film

An effective passivation technique for AlGaN/GaN high-electron-mobility transistors (HEMTs) is presented. This technique features an AlN thin film grown by plasma-enhanced atomic layer deposition (PEALD). With in situ remote plasma pretreatments prior to the AlN deposition, an atomically sharp interface between ALD-AlN and III-nitride has been obtained. Significant current collapse suppression and dynamic ON-resistance reduction are demonstrated in the ALD-AlN-passivated AlGaN/GaN HEMTs under high-drain-bias switching conditions.

InP-based high-performance monostable-bistable transition logic elements (MOBILEs) using integrated multiple-input resonant-tunneling devices

MOBILEs (monostable-bistable transition logic elements), which have the advantages of multiple-input and multiple-function, are demonstrated in InP-based material system using monolithic integration of resonant-tunneling diodes and high electron mobility transistors. The high peak current density, high peak-to-valley ratio, and high transconductance, which are required for high-performance MOBILEs, are demonstrated in this InP-based material system. A fabricated MOBILE with three-input gates having 1:2:4 width ratio can perform weighted-sum threshold logic operation, and has a wide range of applications in new computing architectures, such as neural networks.

High-performance AlGaN∕GaN lateral field-effect rectifiers compatible with high electron mobility transistors

A high electron mobility transistor (HEMT)-compatible power lateral field-effect rectifier (L-FER) with low turn-on voltage is demonstrated using the same fabrication process as that for normally off AlGaN∕GaN HEMT, providing a low-cost solution for GaN power integrated circuits. The power rectifier features a Schottky-gate-controlled two-dimensional electron gas channel between the cathode and anode. By tying up the Schottky gate and anode together, the forward turn-on voltage of the rectifier is determined by the threshold voltage of the channel instead of the Schottky barrier. The L-FER with a drift length of 10μm features a forward turn-on voltage of 0.63V at a current density of 100A∕cm2. This device also exhibits a reverse breakdown voltage (BV) of 390V at a current level of 1mA∕mm and a specific on resistance (RON,sp) of 1.4mΩcm2, yielding a figure of merit (BV2∕RON,sp) of 108MW∕cm2. The excellent device performance, coupled with the lateral device structure and process compatibility with AlGaN∕GaN...

Enhancement-Mode

Enhancement-mode Si₃N₄/AlGaN/GaN metal-insulator-semiconductor HFETs (MISHFETs) with a 1-mum gate footprint are demonstrated by combining CF₄ plasma treatment technique and a two-step Si₃N₄ deposition process. The threshold voltage has been shifted from -4 [for depletion-mode HFET] to 2 V using the techniques. A 15-nm Si₃N₄ layer is inserted under the metal gate to provide additional isolation between the gate Schottky contact and AlGaN surface, which can lead to reduced gate leakage current and higher gate turn-on voltage. The two-step Si ₃N₄ deposition process is developed to reduce the gate coupling capacitances in the source and drain access region, while assuring the plasma-treated gate region being fully covered by the gate electrode. The forward turn-on gate bias of the MISHFETs is as large as 7 V, at which a maximum current density of 420 mA/mm is obtained. The small-signal RF measurements show that the current gain cutoff frequency (f_T) and power gain cutoff frequency (f_max) are 13.3 and 23.3 GHz, respectively

hboxSi_3hboxN_4hbox/AlGaN/GaN

We demonstrate a single-chip switch-mode boost converter that features a monolithically integrated lateral field-effect rectifier (L-FER) and a normally off transistor switch. The circuit was fabricated on a standard AlGaN/GaN HEMT epitaxial wafer grown with GaN-on-Si technology. The fabricated rectifier with a drift length of 15 mum exhibits a breakdown voltage of 470 V, a turn-on voltage of 0.58 V, and a specific on-resistance of 2.04 mOmegaldrcm2. The L-FER exhibits no reverse recovery current associated with the turn-off transient because of its unipolar nature. A prototype of GaN-based boost converter that includes monolithically integrated rectifiers and transistors is demonstrated using conventional GaN-on-Si wafers for the first time to prove the feasibility of the GaN-based power IC technology.

MISHFETs

In this letter, 600-V normally-OFF SiNx/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) is reported. Normally-OFF operation and low OFF-state gate leakage are obtained by using fluorine plasma ion implantation in conjunction with the adoption of a 17-nm SiNx thin film grown by plasma-enhanced chemical vapor deposition as the gate insulator. The normally-OFF MIS-HEMT exhibits a threshold voltage of +3.6 V, a drive current of 430 mA/mm at a gate bias of 14 V, a specific ON-resistance of 2.1 mΩ·cm2 and an OFF-state breakdown voltage of 604 V at a drain leakage current of 1 μA/mm with VGS=0 V, and the substrate grounded. Effective current collapse suppression is obtained by AlN/SiNx passivation as proved by high-speed pulsed I-V and low-speed high-voltage switching measurement results.

Single-Chip Boost Converter Using Monolithically Integrated AlGaN/GaN Lateral Field-Effect Rectifier and Normally Off HEMT

Vertical leakage/breakdown mechanisms in AlGaN/GaN high-electron-mobility transistors grown on low-resistivity p-type (111) Si substrate are studied by temperature-dependent current-voltage ( I-V) measurements. It is found that the top-to-substrate vertical breakdown voltage (BV) is dominated by the space-charge-limited current conduction involving both acceptor and donor traps in the GaN buffer/transition layer. From the temperature-dependent transient backgating measurements, the acceptor level at EV + 543 meV and the donor level at EC-616 meV were identified.