Hwa-Chang Seo

Graphene FETs for Zero-Bias Linear Resistive FET Mixers

In this letter, we present the first graphene FET operation for zero-bias resistive FET mixers, utilizing modulation of graphene channel resistance rather than ambipolar mixer operations, up to 20 GHz. The graphene FETs with a gate length of 0.25 μm have an extrinsic cutoff frequency fT of 40 GHz and a maximum oscillation frequency fMAX of 37 GHz. At 2 GHz, the graphene FETs show a conversion loss of 14 dB with gate-pumped resistive FET mixing, with at least > 10-dB improvement over reported graphene mixers. The input third-order intercept points (IIP3s) of 27 dBm are demonstrated at a local oscillator (LO) power of 2.6 dBm. The excellent linearity demonstrated by graphene FETs at low LO power offers the potential for high-quality linear mixers.

Graphene FET-Based Zero-Bias RF to Millimeter-Wave Detection

We report direct radio-frequency (RF) and millimeter-wave detection of epitaxial graphene field-effect transistors (FETs) up to 110 GHz with no dc biases applied, leveraging the nonlinearity of the channel resistance. A linear dynamic range of >; 40 dB was measured, providing at least 20-dB greater linear dynamic range compared to conventional CMOS detectors at transistor level. The measured noise power of the graphene FETs was ~7.5 × 10-18 V2/Hz at zero bias and without 1/f noise. At a 50-Ω load, measured detection responsivity was 71 V/W at 2 GHz to 33 V/W at 110 GHz. The noise-equivalent power at 110 GHz was estimated to be ~80 pW/Hz0.5. For the first time, we demonstrated graphene FETs as zero-bias ultrawideband direct RF detectors with comparable or better performance than state-of-the-art FET-based detectors without dc biases applied.

Lateral Graphene Heterostructure Field-Effect Transistor

We report the first experimental demonstration of a lateral graphene heterostructure field-effect transistor (HFET) at wafer scale, where the graphene heterostructure channel consists of epitaxial graphene (Gr)/fluorographene (GrF)/graphene (Gr). GrF is a widebandgap material, providing a potential barrier to lateral carrier transport. Gate bias modulation of the Gr/GrF/Gr barrier via an electric field effect results in normally-off enhancement-mode graphene HFETs with an ON-OFF switching ratio of 105 at room temperature. These devices also demonstrate excellent current-voltage saturation, providing a potential path for active RF applications.

Graphene and Lateral Heterostructure for THz Imaging

We report millimeter-wave and sub-terahertz detection using graphene FETs up to 220 GHz at zero-bias to reduce 1/f noise. Detection leveraged the nonlinearity of the channel resistance through resistive field-effect transistor mixing for high-dynamic range. At a 50-Ω load, measured detection responsivity was 70 V/W at 2 GHz to 33 V/W at 110 GHz. The measured noise power of the graphene FETs was ~7.5 ×10-18 V2/Hz at zero-bias. Noise equivalent power at 110 GHz was estimated to be ~80 pW/Hz0.5. A linear dynamic range of > 40 dB was measured, providing 15-20 dB greater linear dynamic range compared to conventional CMOS detectors at the transistor level. The emerging graphene heterostructure diodes offer the RC limited cutoff frequency (fc) of 2.9 THz with the noise equivalent power of ~ 8 pW/Hz0.5 at 200 GHz due to its small junction-capacitance and diode nonlinearity.

Zero-bias THz detection using graphene transistors

We report zero-bias millimeter-wave and sub-THz detection using graphene FETs up to 220 GHz and graphene heterostructure diodes with reduced 1/f noise in direct detection. This detection leveraged the nonlinearity of the channel resistance through resistive field-effect transistor mixing. At a 50 ohm load, measured device responsivity was 70 V/W at 2 GHz to 33 V/W at 110 GHz. The measured noise power of the graphene FETs was ~7.5 × 10-18 V2/Hz at zero-bias. The NEP at 110 GHz was estimated to be ~80 pW/Hz0.5. A linear dynamic range of >40 dB was measured, providing 15 - 20 dB greater linear dynamic range compared to conventional CMOS detectors at the transistor level.

11 THz figure-of-merit phase-change RF switches for reconfigurable wireless front-ends

We report on GeTe-based, phase-change RF switches in a series configuration with an embedded micro-heater for thermal switching. With heater parasitics reduced, these GeTe RF switches show on-state resistance of 0.12 ohm*mm and off-state capacitance of 0.12 pF/mm. The RF switch figure-of-merit is estimated to be 11 THz, which is about 15 times better than state-of-the-art silicon-on-insulator switches. With 50-μm-wide GeTe switches, RF insertion loss was 0.25 dB and isolation was 24 dB at 20 GHz. Harmonic powers were suppressed >90 dBc at 35 dBm, meeting wireless requirements. The GeTe switches were characterized under W-CDMA signals without spectral regrowth up to 25 dBm.

Graphene based active and passive component development on transparent substrates

Antennas collect radio waves and channel them into radio frequency (RF) transmission lines which direct the signals to circuits from which information can be demodulated and decoded. Glass, the most common portal between outside and inside environments, is clear at the visible part of the electromagnetic spectrum, and it is also relatively transparent to a large portion of the electromagnetic spectrum useful for radio wave communications. Since glass as a building material is used everywhere, it could be a readily accessible substrate upon which to mount or fabricate the antennas and RF electronics, but only if these circuit components are also transparent. In this paper, we present our development to date of glass RF circuits along two tracks: 1) transparent antennas and 2) graphene based active and passive circuit elements. Along the first track we have demonstrated antennas made from nanowire films capable of an optical transparency of 72% and sheet resistance of 4-5Ω/sq. Along the second track, we have in so far demonstrated graphene on glass field effect transistors with an fmax of 7 GHz, varactors with 1.4:1 tuning range, resistors with 3-70 kΩ, and capacitors from 13-860 pF. This is just the start; our plans are to increase the frequency and tuning ranges of the active and passive devices. Since graphene is inherently transparent at visible wavelengths, we ultimately would like to merge these two tracks to integrate active and passive RF circuitry with the antenna either directly on glass or as an applique put on glass, circuits which we’ve termed RF Glass®.

Graphene transistors for RF applications: Opportunities and challenges

Graphene offers new opportunities for the optimization of high frequency FETs by virtue of high carrier velocity, excellent scaling properties, configurability as electron or hole channel devices, and limited scattering. The saturation velocity (vsat) of graphene has not been determined clearly yet, but it is estimated to be ∼5 times greater than that for Si MOSFETs [1]. With large on-state current density and transconductance per gate capacitance compared to Si, [2] graphene has the potential to offer excellent switching characteristics and short-circuit current gain cut-off frequency. With observed constant device transconductance over the gate voltages [3], graphene FETs could potentially offer low-noise amplifiers (LNAs) with higher dynamic range per given DC power (OIP3/Pdc) beyond the antimony-based low-bandgap devices. With graphene FETs biased near the ambipolar point, graphene FETs behave close to ideal “square-law” devices near the ambipolar point i.e., I(Vg) ∝ Vg2, it would greatly suppress odd-order harmonics and/or third-order intermodulation products and improve dynamic range in communications [4]. Graphene-on-Si FETs [5] could potentially be further developed and processed in a manner compatible with Si CMOS with desirable integration density for system-on-chip applications. It also presents unique problems, such as limitations in voltage and poor pinchoff characteristics. The future success of the RF circuit applications depends on high-quality material growth on large-wafer scale, vertical and lateral scaling of graphene MOSFETs to minimize parasitics and improve gate modulation efficiency in the channel, a bandgap engineering of graphene channels in the MOSFETs, and innovative circuit concepts. [6]

Phase-change materials for reconfigurable RF applications

In this talk, we report on GeTe RF switches on silicon substrate with state-of-the-art switch figure-of-merit of ~14 femtosecond or 11 THz, ~20x greater than all of current FET switches. This was accomplished using an embedded refractory micro-heater with reduced parasitics. The spectral responses of the GeTe-based RF switches were tested for the first time under W-CDMA signals. With a 15 dBm interferer, we did not see spectral regrowth of the switches. Under single tone, the harmonic powers were at 90 dBc at 35 dBm with GeTe width of 150 μm. While at a very early development stage, we report that GeTe PCM RF switches are a promising technology upon improved reliability for future wireless RF front-ends.

10.6 THz figure-of-merit phase-change RF switches with embedded micro-heater

We report on GeTe-based phase-change RF switches with embedded micro-heater for thermal switching. With heater parasitics reduced, GeTe RF switches show onstate resistance of 0.05 ohm*mm and off-state capacitance of 0.3 pF/mm. The RF switch figure-of-merit is estimated to be 10.6 THz, which is about 15 times better than state-of-the-art silicon-on-insulator switches. With on-state resistance of 1 ohm and off-state capacitance of 15 fF, RF insertion loss was measured at <;0.2 dB, and isolation was >25 dB at 20 GHz, respectively. RF power handling was >5.6 W for both onand off-state of GeTe.