Kangmu Lee

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 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.

Projected Performance of Heterostructure Tunneling FETs in Low Power Microwave and mm-Wave Applications

Characteristics of heterostructure tunneling FETs (HTFETs) at microwave and mm-wave frequencies are reviewed, and their simulated performance in a variety of prototype circuits is presented. The results illustrate that HTFETs provide substantial benefits in low power, high frequency circuits, related to their high nonlinearity at low voltages (critical to rectifiers and mixers), as well as to their high transconductance and gain at low current and low power levels. Parasitic capacitance and noise models of HTFETs are summarized. mm-wave low noise amplifiers, oscillators, and mixers with simulated operation at power levels below 1 mW are described. High responsivity passive mm-wave detectors which could provide noise-equivalent temperature differences below 1 °C are presented.

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]

Graphene: Status and prospects as a microwave material

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. It also offers unique features such as quadratic variation of current with gate voltage, and problems, such as limitations in voltage and poor pinchoff characteristics. This paper reviews graphene material properties, and status and prospects of research efforts to demonstrate graphene-based microwave and mm-wave transistors.