J. S. Moon

Epitaxial-Graphene RF Field-Effect Transistors on Si-Face 6H-SiC Substrates

We report dc and the first-ever measured small-signal radio-frequency (RF) performance of epitaxial-graphene RF field-effect transistors (FETs), where the epitaxial-graphene layer is formed by graphitization of 2-in-diameter Si-face semi-insulating 6H-SiC (0001) substrates. The gate is processed with a metal gate on top of a high-k Al2 O3 gate dielectric deposited via an atomic-layer-deposition method. With a gate length (Lg) of 2 mum and an extrinsic transconductance of 148 mS/mm, the extrinsic current-gain cutoff frequency (fT) is measured as 4.4 GHz, yielding an extrinsic fT ldr Lg of 8.8 GHz middot mum. This is comparable to that of Si NMOS. With graphene FETs fabricated in a layout similar to those of Si n-MOSFETs, on-state current density increases dramatically to as high as 1.18 A/mm at Vds = 1 V and 3 A/mm at Vds = 5 V. The current drive level is the highest ever observed in any semiconductor FETs.

Top-Gated Epitaxial Graphene FETs on Si-Face SiC Wafers With a Peak Transconductance of 600 mS/mm

In this letter, we present state-of-the-art performance of top-gated graphene n-FETs and p-FETs fabricated with epitaxial graphene layers grown on Si-face 6H-SiC substrates on 50-mm wafers. The current-voltage characteristics of these devices show excellent saturation with on-state current densities (I_on) of 0.59 A/mm at V_ds = 1 V and 1.65 A/mm at V_ds = 3 V. I_on/I_off ratios of 12 and 19 were measured at V_ds = 1 and 0.5 V, respectively. A peak extrinsic g_m as high as 600 mS/mm was measured at V_ds = 3.05 V, with a gate length of 2.94 ¿m. The field-effect mobility versus effective electric field (E_eff) was measured for the first time in epitaxial graphene FETs, where record field-effect mobilities of 6000 cm²/V·s for electrons and 3200 cm²/V·s for holes were obtained at E_eff ~ 0.27 MV/cm .

Ultra-low resistance ohmic contacts in graphene field effect transistors

We report on an experimental demonstration of graphene-metal ohmic contacts with contact resistance below 100 Ω µm. These have been fabricated on graphene wafers, both with and without hydrogen intercalation, and measured using the transmission line method. Specific contact resistivities of 3 × 10−7 to 1.2 × 10−8 Ω cm2 have been obtained. The ultra-low contact resistance yielded short-channel (source-drain distance of 0.45 µm) HfO2/graphene field effect transistors (FETs) with a low on-resistance (Ron) of 550 Ω µm and a high current density of 1.7 A/mm at a source-drain voltage of 1 V. These values represent state-of-the-art (SOA) performance in graphene-metal contacts and graphene FETs. This ohmic contact resistance is comparable to that of SOA high-speed III–V high electron mobility transistors.

Low-Phase-Noise Graphene FETs in Ambipolar RF Applications

In this letter, we present both the 1/f noise and phase noise performance of top-gated epitaxial graphene field-effect transistors (FETs) in nonlinear circuit applications for the first time. In the case of frequency doublers, the fundamental signal is suppressed by 25 dB below the second harmonic signal. With a phase noise of -110 dBc/Hz measured at a 10-kHz offset, a carrier-to-noise degradation (ΔCNR) of 6 dB was measured for the frequency doubler. This implies noiseless frequency multiplication without additional 1/f noise upconversion during the nonlinear process. The frequency multiplication was demonstrated above the gigahertz range. The 1/f noise of top-gated epitaxial graphene FETs is comparable or lower than that of exfoliated graphene FETs.

Graphene: Its Fundamentals to Future Applications

Currently, graphene is a topic of very active research fields from science to potential applications. For various RF circuit applications, including low-noise amplifiers, the unique ambipolar nature of graphene field-effect-transistors can be utilized for high-performance frequency multipliers, mixers, and high-speed radiometers. Potential integration of graphene on silicon substrates with CMOS compatibility would also benefit future RF systems. The future success of the RF circuit applications depends on vertical and lateral scaling of graphene MOSFETs to minimize parasitics and improve gate modulation efficiency in the channel with zero or a small bandgap. In this paper, we highlight recent progress in graphene materials, devices, and circuits for RF applications. For passive RF applications, we show its transparent electromagnetic shielding in Ku-band and transparent antennas, where its success depends on the quality of materials. We also attempt to discuss future applications and challenges of graphene.

55% PAE and High Power Ka-Band GaN HEMTs With Linearized Transconductance via

We report small- and large-signal performances of 140-nm gatelength field-plated GaN HEMTs at Ka-band frequencies, in which the GaN HEMTs were fabricated with n+ source contact ledge. The parasitic channel resistance is reduced by ~ 50%, whereas the peak extrinsic transconductance is improved by 20% from 370 to 445 mS/mm. The GaN HEMTs with n+ source ledge exhibit improvement of maximum stable gain by at least 0.7 dB over reference devices without n+ ledge. At 30 GHz, CW output power density of 10 W/mm is measured with peak PAE of 40% and associated gain of 8.4 dB at Vds = 42 V. At Vds = 30 V, the output power density is measured as 7.3 W/mm with peak PAE of 50%, peak DE of 58%, and associated gain of 8.5 dB. The best PAE was measured as 55% at 5 W/mm at 30, 33, and 36 GHz, where the associated gains were 7.9, 7.6, and 8.2 dB, respectively.

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Most recent GaN-based HEMT technology has been focused toward microwave power applications. In this work, we report DC and RF characteristics of the first E-mode AlGaN/GaN HEMTs fabricated down to 0.2 /spl mu/m gatelength, and having an f/sub t/ reaching 25 GHz. Further improvement of E-mode GaN HEMTs could open potential applications for mixed-signal ICs with a high dynamic range.

GaN Source Contact Ledge

In this letter, we report the first experimental demonstration of wafer-scale ambipolar field-effect transistor (FET) on Si (111) substrates by synthesizing a graphene layer on top of 3C-SiC(111)/Si(111) substrates. With lateral scaling of the source-drain distance to 1 μm in a top-gated layout, the ON-state current of 225 μA/μm and peak transconductance of > 40 μS/μm were obtained at Vds = 2 V, which is the highest performance of graphene-on-Si FETs. The peak field-effect mobilities of 285 cm2 /Vs for holes and 175 cm2 /Vs for electrons were demonstrated, which is higher than that of ultra-thin-body SOI (n, p) MOSFETs.

Submicron enhancement-mode AlGaN/GaN HEMTs

We have demonstrated 8.5-11.5 GHz class-E MMIC high-power amplifiers (HPAs) with a peak power-added-efficiency (PAE) of 61% and drain efficiency (DE) of 70% with an output power of 3.7 W in a continuous-mode operation. At 5 W output power, PAE and DE of 58% and 67% are measured, respectively, which implies MMIC power density of 5 W/mm at Vds = 30 V. The peak gain is 11 dB, with an associated gain of 9 dB at the peak PAE. At an output power of 9 W, DE and PAE of 59% and 51 % were measured, respectively. In order to improve the linearity, we have designed and simulated X-band class-E MMIC PAs similar to a Doherty configuration. The Doherty-based class-E amplifiers show an excellent cancellation of a third-order intermodulation product (IM3), which improved the simulated two-tone linearity C/IM3 to >; 50 dBc.

Top-Gated Graphene Field-Effect Transistors Using Graphene on Si (111) Wafers

We report the state-of-the-art performance of deep-submicrometer gate length dual-gate GaN HEMTs and cascode GaN HEMTs with 10× reduced gate-to-drain feedback capacitance compared with single-gate GaN HEMTs. With 150-nm gate length field-plated gate structures, these GaN HEMTs demonstrated improvement of small-signal gain by 10 dB, compared with single-gate GaN HEMTs. Large-signal load-pull measurements showed peak power-added-efficiency (PAE) of 71%-74% without harmonic tuning at 10 GHz, up to a measured continuous-wave output power level of 2.3-2.5 W. The 74% PAE is very close to a theoretical maximum PAE of 78.5% without harmonic tuning. Compared with single-gate GaN HEMTs, both the dual-gate and cascode GaN HEMTs offer~10% improvement in peak PAE at the output power of 2.3-2.5 W.