Kei May Lau

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

Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO2 nanorod-array patterned sapphire template

High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral overgrowth (NELO) method on a SiO2 nanorod-array patterned sapphire substrate (NAPSS). The transmission electron microscopy images suggest that the voids between SiO2 nanorods and the stacking faults introduced during the NELO of GaN can effectively suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED were enhanced by 52% and 56%, respectively, compared to those of a conventional LED. The improvements originated from both the enhanced light extraction assisted by the NAPSS and the reduced dislocation densities using the NELO method.

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 report a low-density drain high-electron mobility transistor (LDD-HEMT) that exhibits enhanced breakdown voltage and reduced current collapse. The LDD region is created by introducing negatively charged fluorine ions in the region between the gate and drain electrodes, effectively modifying the surface field distribution on the drain side of the HEMT without using field plate electrodes. Without changing the device physical dimensions, the breakdown voltage can be improved by 50% in LDD-HEMT, and the current collapse can be reduced. No degradation of current cutoff frequency (ft) and slight improvement in power gain cutoff frequency (fmax) are achieved in the LDD-HEMT, owing to the absence of any additional field plate electrode

MISHFETs

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.

Normally Off AlGaN/GaN Low-Density Drain HEMT (LDD-HEMT) With Enhanced Breakdown Voltage and Reduced Current Collapse

This letter presents the high-temperature performance of AlGaN/GaN HEMT direct-coupled FET logic (DCFL) integrated circuits. At 375 degC, enhancement-mode (E-mode) AlGaN/GaN HEMTs which are used as drivers in DCFL circuits exhibit proper E-mode operation with a threshold voltage (VTH) of 0.24 V and a peak current density of 56 mA/mm. The monolithically integrated E/D-mode AlGaN/GaN HEMTs DCFL circuits deliver stable operations at 375 degC: An E/D-HEMT inverter with a drive/load ratio of 10 exhibits 0.1 V for logic-low noise margin (NML) and 0.3 V for logic-high-noise margin (NMH) at a supply voltage (VDD) of 3.0 V; a 17-stage ring oscillator exhibits a maximum oscillation frequency of 66 MHz, corresponding to a minimum propagation delay ( taupd) of 446 ps/stage at VDD of 3.0 V

AlGaN/GaN/InGaN/GaN DH-HEMTs with an InGaN notch for enhanced carrier confinement

We present the design, fabrication, and characterization of AlGaN-GaN double-channel HEMTs. Two carrier channels are formed in an AlGaN-GaN-AlGaN-GaN multilayer structure grown on a sapphire substrate. Polarization field in the lower AlGaN layer fosters formation of a second carrier channel at the lower AlGaN-GaN interface, without creating any parasitic conduction path in the AlGaN barrier layer. Unambiguous double-channel behaviors are observed at both dc and RF. Bias dependent RF small-signal characterization and parameter extraction were performed. Gain compression at a high current level was attributed to electron velocity degradation induced by interface scattering. Dynamic IV measurement was carried out to analyze large-signal behaviors of the double-channel high-electron mobility transistors. It was found that current collapse mainly occurs in the channel closer to device surface, while the lower channel suffers minimal current collapse, suggesting that trapping/detrapping of surface states is mainly responsible for current collapse. This argument is supported by RF large-signal measurement results.

High-Temperature Operation of AlGaN/GaN HEMTs Direct-Coupled FET Logic (DCFL) Integrated Circuits

Fabrication and characterization of AlGaN/GaN HEMT inverters and ring oscillators utilizing integrated enhancement/depletion-mode (E/D-mode) AlGaN/GaN HEMTs are presented. The core technique is a CF4 plasma treatment that can effectively convert a D-mode AlGaN/GaN heterostructure to an E-mode heterostructure. A significant advantage of the plasma-treated E-mode HEMTs is that the gate current is reduced in both reverse- and forward-bias regions due to the effectively enhanced barrier height induced by the negatively charged fluorine ions in the AlGaN barrier. As a result, the input voltage swing is expanded by about 1 V for the E-mode HEMT, enabling convenient input/output logic level matching for multistage logic circuits such as ring oscillators. The fabricated 17-stage direct-coupled field-effect transistor logic ring oscillator using the 1-mum-gate technology can operate properly at a larger supply voltage of 3.5 V, and a minimum propagation delay of 130 ps/stage is achieved

AlGaN-GaN double-channel HEMTs

Abstract This article describes a comprehensive technique to determine the shapes of GaAs epitaxial layers grown by organometallic chemical vapor deposition (OMCVD) on patterned substrates. A simple technique called the Borgstrom construction, which can predict epitaxial shapes, is reviewed. However, the assumptions in this model limit its applications and the Wulff construction is needed. The Wulff construction can predict edge shapes for constrained and unconstrained (such as at the edge of a mask) epitaxial growth. This technique is valid for any geometric situation assuming the growth rate versus orientation (growth rate polar diagram) is known. A semi-empirical growth rate polar diagram is given for GaAs at 750°C and atmospheric pressure OMCVD. The predictions made by the Wulff construction compare favorably with experimental results.