Luke Nyakiti | Texas A&M University (original) (raw)

Papers by Luke Nyakiti

Optical Materials Express, Aug 15, 2019

Vanadium dioxide is a material that undergoes phase changes and switches between metallic and ins... more Vanadium dioxide is a material that undergoes phase changes and switches between metallic and insulating states, thereby producing dramatic changes in optical properties. This transition is a reversible but hysteretic process, which is investigated here as a function of atomic layer deposited film thickness. Increasing the thickness of vanadium dioxide films from 8.6 to 57 nm lead to an increase in hysteresis width. Thicker films develop two different slopes (steep and gradual) when cooling through the metal-insulator transition, where the steep transition matches that of the heating cycle of the transition. This asymmetry in the hysteresis is apparent and similar in both the electrical and optical measurements. Temperature-dependent Raman spectroscopy and temperature-dependent x-ray diffraction confirm the same anomaly suggesting a structural dependence on hysteretic shape. Transmission electron microscopy identifies texturing and faceting in-plane, especially along the surface of these films, and confirms the x-ray diffraction data. Identifying this facet texturing is valuable for film growth as well as for applications, such as logic and memory systems, that utilize the hysteretic nature of vanadium dioxide.

Journal of vacuum science and technology, Apr 17, 2012

A novel method for creating bilayer graphene is described where single-layer CVD graphene grown o... more A novel method for creating bilayer graphene is described where single-layer CVD graphene grown on Cu is bonded to single-layer epitaxial graphene grown on Si-face SiC. Raman microscopy and x ray photoelectron spectroscopy demonstrate the uniqueness of this bilayer, as compared to a naturally formed bilayer, in that a Bernal stack is not formed with each layer being strained differently yet being closely coupled. Electrical characterization of Hall devices fabricated on the unusual bilayer show higher mobilities, and lower carrier concentrations, than the individual CVD graphene or epitaxial graphene layers.

Carbon, 2015

ABSTRACT Controlling the uniformity and morphology of graphene grown on the C-face of SiC is more... more ABSTRACT Controlling the uniformity and morphology of graphene grown on the C-face of SiC is more difficult than on the Si-face. To improve graphene grown on the C-face, a continuous growth process was developed in a conventional tube furnace that included in situ surface preparation by annealing in H2 followed by an Ar-mediated growth, which was done at a variety of different temperatures and pressures. Optimized H2 etch conditions for the C-face were developed to improve the starting substrate morphology and reduce the effect of substrate defects on growth. The resulting graphene film, however, had non-uniform thickness due to intrinsic bulk defects within the SiC substrate and an interfacial oxide. Differences between substrate properties, such as polytype, are shown to have a significant effect on growth, with a 4H substrate displaying faster in-plane graphene growth than a 6H substrate. A primarily 2-domain graphene film with significant rotational disorder was found regardless of the starting substrate and growth conditions. Ultra-high vacuum desorption of the interfacial oxide caused the graphene to reorder into a single preferred rotational orientation, suggesting trace oxygen impurities in the growth chamber can play an important role in graphene growth on the C-face of SiC.

Materials Letters, 2007

ABSTRACT The structure and size of molybdenum nitride nanoparticles were investigated using high-... more ABSTRACT The structure and size of molybdenum nitride nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM). Typical sizes of the particles were between 3 and 5 nm and they were mostly clustered together. High-resolution lattice imaging shows that the particles are single crystalline in nature and defect free. Two different phases of molybdenum nitride, γ-Mo2N (cubic) and δ-MoN (hexagonal) were identified. In addition, body centered cubic molybdenum phase was also present. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN suggests the existence of a temperature gradient in the chamber leading to formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C).

The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk alumi... more The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal's orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.

ABSTRACT A three-dimensional numerical finite element modeling method is applied to compare inter... more ABSTRACT A three-dimensional numerical finite element modeling method is applied to compare interfacial residual thermal stress distribution in AlN single crystals grown by using different substrates such as silicon carbide, boron nitride, tungsten, tantalum carbide, and niobium carbide. A dimensionless coordinate system is used which reduces the numbers of computations and hence simplifies the stress analysis. All components of the stress distribution, both in the film and in the substrate, including the normal stress along the growth direction as well as in-plane normal stresses and shear stresses are fully investigated. This information about the stress distribution provides insight into understanding and controlling the AlN single crystal growth by the sublimation technique. The normal stress in the film at the interface along the growth direction and the shear stresses are zero except at the edges, whereas in-plane stresses are nonzero. The in-plane stresses are compressive when TaC and NbC substrates are used. A small compressive stress might be beneficial in prohibiting crack growth in the film. The compressive stress in the AlN is lower for the TaC substrate than that for the NbC. Tensile in-plane stresses are formed in the AlN for 6H -SiC , BN, and W substrates. This tensile stress in the film is detrimental as it will assist in the crack growth. The stress concentration at the edges of the AlN film at the interface is compressive in nature when TaC and NbC are used as a substrate. This causes the film to bend downward (i.e., convex shape) and assist it to adhere to the substrate. The AlN film curves upward or in a concave shape when SiC, BN, and W substrates are used since the stress concentration at the edges of the AlN film is tensile at the interface and this may cause detachment of the film from the substrate.

ABSTRACT We report the realization of top-gated graphene nanoribbon field effect transistors (GNR... more ABSTRACT We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

ABSTRACT With a wide band gap of greater than 3.0 eV and the ability to self-heal from radiation ... more ABSTRACT With a wide band gap of greater than 3.0 eV and the ability to self-heal from radiation damage, icosahedral boron arsenide (B12As2) is an apt candidate for use in next-generation betavoltaics. By capturing and converting high energy electrons from radioisotopes into usable electricity, “nuclear batteries” made from B12As2 could potentially power devices for decades. Compared to bulk crystals or epitaxial films, B12As2 nanowires may have lower defect densities or may even be defect-free, leading to better electrical properties and device performance. In our study, B12As2 nanowires were synthesized via vapor-liquid-solid (VLS) growth using platinum powder and nickel powder on silicon carbide and 20 nm thick nickel film on silicon substrates from 700 °C to 1200 °C. Platinum yielded the highest quality nanowires from 900 °C to 950 °C, resulting in platinum particles densely covered with wires formed by straight segments connected by sharp angular kinks. At these growth temperatures, diameters ranged from less than 30 nm to about 300 nm as determined by scanning electron microscopy and transmission electron microscopy. Growth temperatures of 850 °C or less produced curled wires 200-1000 nm in diameter. Transmission electron microscopy and selected area electron diffraction revealed excellent crystallinity in wires grown above 850 °C, while wires grown at or below 850 °C were partially amorphous. Wires grown from the 20 nm nickel film displayed similar morphologies at temperatures up to 850 °C; from 900 °C to 950 °C, straight, isolated wires were grown with diameters of 200-400 nm. Nickel powder only produced wires larger than 1 μm in diameter. The comparative quality and growth of B12As2nanowires will be discussed.

ABSTRACT Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-fac... more ABSTRACT Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

Plan-view and cross-sectional transmission electron microscopy have been used for a detailed stud... more Plan-view and cross-sectional transmission electron microscopy have been used for a detailed study of the defects formed in capped InAs/ GaAs quantum dot ͑QD͒ samples. Three main types of defects, V-shaped defects, single stacking faults, and stacking fault pyramids, were found to form under growth conditions that led to either very large, indium enriched, or coalesced islands. All three types of defects originate at the buried quantum dot layer and then travel through the GaAs cap to the surface on the ͕111͖ planes. The V-shaped defects were the most common and typically consisted of two pairs of closely spaced 60°Shockley partials with a ͗211͘ line direction. The two pairs originate together at the buried QD layer and then travel in "opposite" directions on different ͕111͖ planes. The second type of defect is the single stacking fault which consists of a single pair of partial dislocations separated by an Ϸ50 nm wide stacking fault. Finally, both complete and incomplete stacking fault pyramids were observed. In the case of the complete stacking fault pyramid the bounding dislocations along the ͓110͔, ͓110͔, ͓101͔, and ͓101͔ directions were identified as stair rods. A possible mechanism for the stacking fault pyramid formation, which can also account for the creation of incomplete stacking fault pyramids, is presented.

ABSTRACT Residual thermal stress distribution in AlN single crystal, grown on tungsten as a cruci... more ABSTRACT Residual thermal stress distribution in AlN single crystal, grown on tungsten as a crucible material, was investigated using a numerical study. It has been demonstrated that a three-dimensional, instead of a two-dimensional, formulation predicts significantly greater values of stress. Dimensionless coordinates were used to essentially simplify the stress analysis and reduce the number of calculations. In addition, thermoelasticity approach simplifies the study of stresses for a nonstationary temperature field. The stress in the AlN film along the thickness or [0001] growth direction is essentially zero but the in-plane stress is large. The stress at the corner of the film is much higher due to stress concentration and could cause formation of microcracks. The stress in the film is tensile while that in the substrate is compressive, which causes a reversal of the stress across the interface. Separation or delamination of the film from the substrate could occur due to this reversal of the stress at the interface. The stress decreases as the thickness of the film increases or the thickness of the substrate decreases. Thus, formation of microscopic cracks in the film could be avoided by using a thinner substrate. The analysis on interaction of neighboring islands in order to simulate coalescence of island growth indicates stress concentration at the boundaries of the islands, which could produce threading dislocations and hence polycrystalline growth. The analysis of the effect of misorientation of the neighboring grains on the residual thermal stress in the film has shown that a large stress can develop at the grain boundary and can lead to grain boundary cracking.

ABSTRACT In this letter, we present the first graphene FET operation for zero-bias resistive FET ... more ABSTRACT 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.

Optical Materials Express, Aug 15, 2019

Vanadium dioxide is a material that undergoes phase changes and switches between metallic and ins... more Vanadium dioxide is a material that undergoes phase changes and switches between metallic and insulating states, thereby producing dramatic changes in optical properties. This transition is a reversible but hysteretic process, which is investigated here as a function of atomic layer deposited film thickness. Increasing the thickness of vanadium dioxide films from 8.6 to 57 nm lead to an increase in hysteresis width. Thicker films develop two different slopes (steep and gradual) when cooling through the metal-insulator transition, where the steep transition matches that of the heating cycle of the transition. This asymmetry in the hysteresis is apparent and similar in both the electrical and optical measurements. Temperature-dependent Raman spectroscopy and temperature-dependent x-ray diffraction confirm the same anomaly suggesting a structural dependence on hysteretic shape. Transmission electron microscopy identifies texturing and faceting in-plane, especially along the surface of these films, and confirms the x-ray diffraction data. Identifying this facet texturing is valuable for film growth as well as for applications, such as logic and memory systems, that utilize the hysteretic nature of vanadium dioxide.

Journal of vacuum science and technology, Apr 17, 2012

A novel method for creating bilayer graphene is described where single-layer CVD graphene grown o... more A novel method for creating bilayer graphene is described where single-layer CVD graphene grown on Cu is bonded to single-layer epitaxial graphene grown on Si-face SiC. Raman microscopy and x ray photoelectron spectroscopy demonstrate the uniqueness of this bilayer, as compared to a naturally formed bilayer, in that a Bernal stack is not formed with each layer being strained differently yet being closely coupled. Electrical characterization of Hall devices fabricated on the unusual bilayer show higher mobilities, and lower carrier concentrations, than the individual CVD graphene or epitaxial graphene layers.

Carbon, 2015

ABSTRACT Controlling the uniformity and morphology of graphene grown on the C-face of SiC is more... more ABSTRACT Controlling the uniformity and morphology of graphene grown on the C-face of SiC is more difficult than on the Si-face. To improve graphene grown on the C-face, a continuous growth process was developed in a conventional tube furnace that included in situ surface preparation by annealing in H2 followed by an Ar-mediated growth, which was done at a variety of different temperatures and pressures. Optimized H2 etch conditions for the C-face were developed to improve the starting substrate morphology and reduce the effect of substrate defects on growth. The resulting graphene film, however, had non-uniform thickness due to intrinsic bulk defects within the SiC substrate and an interfacial oxide. Differences between substrate properties, such as polytype, are shown to have a significant effect on growth, with a 4H substrate displaying faster in-plane graphene growth than a 6H substrate. A primarily 2-domain graphene film with significant rotational disorder was found regardless of the starting substrate and growth conditions. Ultra-high vacuum desorption of the interfacial oxide caused the graphene to reorder into a single preferred rotational orientation, suggesting trace oxygen impurities in the growth chamber can play an important role in graphene growth on the C-face of SiC.

Materials Letters, 2007

ABSTRACT The structure and size of molybdenum nitride nanoparticles were investigated using high-... more ABSTRACT The structure and size of molybdenum nitride nanoparticles were investigated using high-resolution transmission electron microscopy (HRTEM). Typical sizes of the particles were between 3 and 5 nm and they were mostly clustered together. High-resolution lattice imaging shows that the particles are single crystalline in nature and defect free. Two different phases of molybdenum nitride, γ-Mo2N (cubic) and δ-MoN (hexagonal) were identified. In addition, body centered cubic molybdenum phase was also present. It is anticipated that molybdenum is formed because of insufficient N2 supply or slow reaction rate. A mixture of γ-Mo2N and δ-MoN suggests the existence of a temperature gradient in the chamber leading to formation of γ-Mo2N at lower temperature (500 to 700 °C) and δ-MoN at higher temperature (850 °C).

The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk alumi... more The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal's orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.

ABSTRACT A three-dimensional numerical finite element modeling method is applied to compare inter... more ABSTRACT A three-dimensional numerical finite element modeling method is applied to compare interfacial residual thermal stress distribution in AlN single crystals grown by using different substrates such as silicon carbide, boron nitride, tungsten, tantalum carbide, and niobium carbide. A dimensionless coordinate system is used which reduces the numbers of computations and hence simplifies the stress analysis. All components of the stress distribution, both in the film and in the substrate, including the normal stress along the growth direction as well as in-plane normal stresses and shear stresses are fully investigated. This information about the stress distribution provides insight into understanding and controlling the AlN single crystal growth by the sublimation technique. The normal stress in the film at the interface along the growth direction and the shear stresses are zero except at the edges, whereas in-plane stresses are nonzero. The in-plane stresses are compressive when TaC and NbC substrates are used. A small compressive stress might be beneficial in prohibiting crack growth in the film. The compressive stress in the AlN is lower for the TaC substrate than that for the NbC. Tensile in-plane stresses are formed in the AlN for 6H -SiC , BN, and W substrates. This tensile stress in the film is detrimental as it will assist in the crack growth. The stress concentration at the edges of the AlN film at the interface is compressive in nature when TaC and NbC are used as a substrate. This causes the film to bend downward (i.e., convex shape) and assist it to adhere to the substrate. The AlN film curves upward or in a concave shape when SiC, BN, and W substrates are used since the stress concentration at the edges of the AlN film is tensile at the interface and this may cause detachment of the film from the substrate.

ABSTRACT We report the realization of top-gated graphene nanoribbon field effect transistors (GNR... more ABSTRACT We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

ABSTRACT With a wide band gap of greater than 3.0 eV and the ability to self-heal from radiation ... more ABSTRACT With a wide band gap of greater than 3.0 eV and the ability to self-heal from radiation damage, icosahedral boron arsenide (B12As2) is an apt candidate for use in next-generation betavoltaics. By capturing and converting high energy electrons from radioisotopes into usable electricity, “nuclear batteries” made from B12As2 could potentially power devices for decades. Compared to bulk crystals or epitaxial films, B12As2 nanowires may have lower defect densities or may even be defect-free, leading to better electrical properties and device performance. In our study, B12As2 nanowires were synthesized via vapor-liquid-solid (VLS) growth using platinum powder and nickel powder on silicon carbide and 20 nm thick nickel film on silicon substrates from 700 °C to 1200 °C. Platinum yielded the highest quality nanowires from 900 °C to 950 °C, resulting in platinum particles densely covered with wires formed by straight segments connected by sharp angular kinks. At these growth temperatures, diameters ranged from less than 30 nm to about 300 nm as determined by scanning electron microscopy and transmission electron microscopy. Growth temperatures of 850 °C or less produced curled wires 200-1000 nm in diameter. Transmission electron microscopy and selected area electron diffraction revealed excellent crystallinity in wires grown above 850 °C, while wires grown at or below 850 °C were partially amorphous. Wires grown from the 20 nm nickel film displayed similar morphologies at temperatures up to 850 °C; from 900 °C to 950 °C, straight, isolated wires were grown with diameters of 200-400 nm. Nickel powder only produced wires larger than 1 μm in diameter. The comparative quality and growth of B12As2nanowires will be discussed.

ABSTRACT Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-fac... more ABSTRACT Vertical rectifying contacts of epitaxial graphene grown by Si sublimation on the Si-face of 4H-SiC epilayers were investigated. Forward bias preferential conduction through the step edges was correlated by linear current density normalization. This phenomenon was observed on samples with 2.7–5.8 monolayers of epitaxial graphene as determined by X-ray photoelectron spectroscopy. A modified Richardson plot was implemented to extract the barrier height (0.81 eV at 290 K, 0.99 eV at 30 K) and the electrically dominant SiC step length of a Ti/Al contact overlapping a known region of approximately 0.52 μm wide SiC terraces.

Plan-view and cross-sectional transmission electron microscopy have been used for a detailed stud... more Plan-view and cross-sectional transmission electron microscopy have been used for a detailed study of the defects formed in capped InAs/ GaAs quantum dot ͑QD͒ samples. Three main types of defects, V-shaped defects, single stacking faults, and stacking fault pyramids, were found to form under growth conditions that led to either very large, indium enriched, or coalesced islands. All three types of defects originate at the buried quantum dot layer and then travel through the GaAs cap to the surface on the ͕111͖ planes. The V-shaped defects were the most common and typically consisted of two pairs of closely spaced 60°Shockley partials with a ͗211͘ line direction. The two pairs originate together at the buried QD layer and then travel in "opposite" directions on different ͕111͖ planes. The second type of defect is the single stacking fault which consists of a single pair of partial dislocations separated by an Ϸ50 nm wide stacking fault. Finally, both complete and incomplete stacking fault pyramids were observed. In the case of the complete stacking fault pyramid the bounding dislocations along the ͓110͔, ͓110͔, ͓101͔, and ͓101͔ directions were identified as stair rods. A possible mechanism for the stacking fault pyramid formation, which can also account for the creation of incomplete stacking fault pyramids, is presented.

ABSTRACT Residual thermal stress distribution in AlN single crystal, grown on tungsten as a cruci... more ABSTRACT Residual thermal stress distribution in AlN single crystal, grown on tungsten as a crucible material, was investigated using a numerical study. It has been demonstrated that a three-dimensional, instead of a two-dimensional, formulation predicts significantly greater values of stress. Dimensionless coordinates were used to essentially simplify the stress analysis and reduce the number of calculations. In addition, thermoelasticity approach simplifies the study of stresses for a nonstationary temperature field. The stress in the AlN film along the thickness or [0001] growth direction is essentially zero but the in-plane stress is large. The stress at the corner of the film is much higher due to stress concentration and could cause formation of microcracks. The stress in the film is tensile while that in the substrate is compressive, which causes a reversal of the stress across the interface. Separation or delamination of the film from the substrate could occur due to this reversal of the stress at the interface. The stress decreases as the thickness of the film increases or the thickness of the substrate decreases. Thus, formation of microscopic cracks in the film could be avoided by using a thinner substrate. The analysis on interaction of neighboring islands in order to simulate coalescence of island growth indicates stress concentration at the boundaries of the islands, which could produce threading dislocations and hence polycrystalline growth. The analysis of the effect of misorientation of the neighboring grains on the residual thermal stress in the film has shown that a large stress can develop at the grain boundary and can lead to grain boundary cracking.

ABSTRACT In this letter, we present the first graphene FET operation for zero-bias resistive FET ... more ABSTRACT 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.