Xubo Song

Graphene Amplifier MMIC on SiC Substrate

Low-noise amplifier is one of the most attractive applications of graphene transistors in the RF area. In this letter, a graphene amplifier MMIC is fabricated on the quasi-free-standing bilayer epitaxial graphene grown on SiC (0001) substrate. In order to realize both the high gain and low return loss, Au matching lines are designed as the input and output impedance match networks. The fabricated graphene amplifier MMIC shows a small-signal power gain of 3.4 dB at 14.3 GHz and a minimum noise figure of 6.2 dB. This letter is a significant step forward for graphene electronics in low-noise amplifier and demonstrates graphenes potential in RF applications for future high-speed electronics.

AlGaN/GaN Polarization-Doped Field-Effect Transistors With Graded Heterostructure

The design, fabrication, and characterization of AlGaN/GaN polarization-doped field-effect transistors (PolFETs) with graded heterostructure are presented in this paper. The 3-D profiles of carriers are obtained across the graded AlGaN/GaN heterostructure grown on a sapphire substrate, which brings about some novel features. The dc transfer and frequency characteristics exhibit bias-insensitive throughout the low- and high-voltage operating regions, demonstrating the potential for high linearity applications. In addition, dynamic I-V measurement was carried out to analyze the trapping behaviors. Negligible current collapses were observed in the unpassivated PolFETs with graded heterostructure, which can be explained in the aspect of unique energy band profiles of AlGaN/GaN graded heterostructures.

DC and RF characteristics of enhancement-mode InAlN/GaN HEMT with fluorine treatment

We report an enhancement-mode InAlN/GaN HEMT using a fluorine plasma treatment. The threshold voltage was measured to be +0.86 V by linear extrapolation from the transfer characteristics. The transconductance is 0 mS/mm at VGS = 0 V and VDS = 5 V, which shows a truly normal-off state. The gate leakage current density of the enhancement-mode device shows two orders of magnitude lower than that of the depletion-mode device. The transfer characteristics of the E-mode InAlN/GaN HEMT at room temperature and high temperature are reported. The current gain cut-off frequency (fT) and the maximum oscillation frequency (fmax) of the enhancement-mode device with a gate length of 0.3 μm were 29.4 GHz and 37.6 GHz respectively, which is comparable with the depletion-mode device. A classical 16 elements small-signal model was deduced to describe the parasitic and the intrinsic parameters of the device.

70-nm-gated InAlN/GaN HEMTs grown on SiC substrate with f T / f max > 160 GHz

InAlN/GaN high-electron-mobility transistors (HEMTs) on SiC substrate were fabricated and characterized. Several techniques, consisting of high electron density, 70 nm T-shaped gate, low ohmic contacts and a short drain-source distance, are integrated to gain high device performance. The fabricated InAlN/GaN HEMTs exhibit a maximum drain saturation current density of 1.65 A/mm at V gs = 1 V and a maximum peak transconductance of 382 mS/mm. In addition, a unity current gain cut-off frequency ( f T ) of 162 GHz and a maximum oscillation frequency ( f max ) of 176 GHz are achieved on the devices with the 70 nm gate length.

Excellent-Performance AlGaN/GaN Fin-MOSHEMTs with Self-Aligned Al

AlGaN/GaN fin-shaped metal-oxide-semiconductor high-electron-mobility transistors (fin-MOSHEMTs) with different fin widths (300nm and 100 nm) on sapphire substrates are fabricated and characterized. High-quality self-aligned Al2O3 gate dielectric underneath an 80-nm T-shaped gate is employed by aluminum self-oxidation, which induces 4 orders of magnitude reduction in the gate leakage current. Compared with conventional planar MOSHEMTs, short channel effects of the fabricated fin-MOSHEMTs are significantly suppressed due to the trigate structure, and excellent dc characteristics are obtained, such as extremely flat output curves, smaller drain induced barrier lower, smaller subthreshold swing, more positive threshold voltage, higher transconductance and higher breakdown voltage.

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Analog applications attract increasing interest for graphene field-effect transistors (GFETs). GFET with a cutoff frequency of up to 427 GHz has been reported; however, the device suffered from the large parasitic parameters and poor drain current saturation, which made their maximum oscillation frequency lower than the cutoff frequency. In this letter, quasi-free-standing bilayer graphene transistors with a gate length of 60 nm and ultra-thin gate dielectric are fabricated by an improved, self-aligned, process. Good gate coupling is achieved, and parasitic parameters are suppressed to a significant extent. The as-measured extrinsic cutoff frequency reaches 70 GHz and the maximum oscillation frequency reaches 120 GHz, which are the highest extrinsic frequencies reported for graphene transistors to date. Our results show the application potential of graphene RF devices in future high-speed electronic systems.

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The performance of scaled AlGaN/GaN HFETs was significantly improved by using gate-recess technology and regrown n⁺-GaN Ohmic contacts. Source-to-drain distance (Z<inf>sd</inf>) was scaled to 600 nm by employing regrown n⁺-GaN Ohmic contacts. Low-damage gate recess was taken before 60 nm T-shaped gate metallization. The drain current curve becomes flat over knee voltage, and the short-channel effects were suppressed obviously after gate recessing. The peak transconductance (g<inf>m</inf>) increased from 607 mS/mm to 764 mS/mm. Moreover, after gate recessing, the value of maximum oscillation frequency (f<inf>max</inf>) increases from 192 GHz to 263 GHz, while there is almost no decrease in the value of unity current gain cut-off frequency (f<inf>T</inf>). The devices with gate recess exhibit a record-high value of √f<inf>T</inf>·f<inf>max</inf> in AlGaN/GaN HFETs. This indicates that the AlGaN/GaNHFETs still have the potential for D-band application with further optimization.

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Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features. We report on monolayer and bilayer epitaxial graphene field-effect transistors (GFETs) fabricated on SiC substrates. Compared with monolayer GFETs, the bilayer GFETs exhibit a significant improvement in dc characteristics, including increasing current density IDS, improved transconductance g m , reduced sheet resistance R on , and current saturation. The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs. Furthermore, the improved dc characteristics enhance a better rf performance for bilayer graphene devices, demonstrating that the quasi-free-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.

Gate Dielectric

Pulse transient simulation is very useful for analyzing the mechanism of dynamic current collapse in GaN-based HEMTs. In this paper, the dependency of current collapse on the device structure is analyzed based on the two-dimensional numerical simulation. The turn-on pulse gate-lag transient currents of Al0.3Ga0.7N/GaN HEMTs with different gate lengths and drain-source distances were obtained, and the results show that current collapse is dependent on the device dimensions, i.e. small-scale devices suffer from larger current collapse due to the stronger electric field. The current collapse caused either by surface trapping effect or by bulk trapping effect increases with the gate length decreasing. However, due to the interaction between the bulk and surface trapping effects, the total current collapse may be reduced for the GaN-based devices.

Improvement of the Frequency Characteristics of Graphene Field-Effect Transistors on SiC Substrate

W-Band High Electron Mobility Transistors (HEMTs) with 100nm gate length were fabricated on the AlGaN/GaN heterostuctures with 22 nm and 18 nm AlGaN barrier layer, respectively. Due to the suppressed short-channel effect, the DC and RF characteristics of AlGaN/GaN HEMT with 18 nm barrier layer are much better than the ones with 22 nm barrier layer. The conclusion can be drown that once the ratio of gate length to the barrier layer thickness is smaller than 5:1, the short-channel effect affects the electric characteristics of W-Band AlGaN/GaN HEMTs obviously.