Bottom-gated Graphene Research Articles

High-Frequency Limits of Graphene Field-Effect Transistors with Velocity Saturation

The current understanding of physical principles governing electronic transport in graphene field effect transistors (GFETs) has reached a level where we can model quite accurately device operation and predict intrinsic frequency limits of performance. In this work, we use this knowledge to analyze DC and RF transport properties of bottom-gated graphene on boron nitride field effect transistors exhibiting pronounced velocity saturation by substrate hyperbolic phonon polariton scattering, including Dirac pinch-off effect. We predict and demonstrate a maximum oscillation frequency exceeding 20 GHz . We discuss the intrinsic 0.1 THz limit of GFETs and envision plasma resonance transistors as an alternative for sub-THz narrow-band detection.

Poly (ethylene imine)-modulated transport behaviors of graphene field effect transistors with double Dirac points

The bottom-gated graphene field-effect transistors (GFETs) with HfO2 as the dielectric layer are fabricated and the transport behaviors with double Dirac points defined as the right conductance minima (VDirac+) and the left one (VDirac-) are obtained, which exhibit the unique W-shaped transform characteristics rather than the typical V-shape. This observation indicates that the graphene in the channel region shows the different doping behaviors from that underlying the metal contact region. The W-shaped characteristics of GFETs are affected by the ambient environment and the time-dependence of double Dirac points has been also found. After being treated by Poly (ethylene imine) (PEI), the transport behaviors of GFETs could be modulated, especially for VDirac+ showing more obvious negative shift, indicating that PEI has a remarkable n-doping effect on the graphene in the channel region. PEI also screens the environmental influence and could protect the graphene from being p-doped to some extent. At the same time, the annealing effect has been also studied and it is found that the proper annealing process could contribute to the recovery of the graphene properties.

Flexible bottom-gate graphene transistors on Parylene C substrate and the effect of current annealing.

Flexible graphene transistors built on a biocompatible Parylene C substrate would enable active circuitry to be integrated into flexible implantable biomedical devices. An annealing method to improve the performance of a flexible transistor without damaging the flexible substrate is also desirable. Here, we present a fabrication method of a flexible graphene transistor with a bottom-gate coplanar structure on a Parylene C substrate. Also, a current annealing method and its effect on the device performance have been studied. The localized heat generated by the current annealing method improves the drain current, which is attributed to the decreased contact resistance between graphene and S/D electrodes. A maximum current annealing power in the Parylene C-based graphene transistor has been extracted to provide a guideline for an appropriate current annealing. The fabricated flexible graphene transistor shows a field-effect mobility, maximum transconductance, and a Ion/Ioff ratio of 533.5 cm2/V s, 58.1 μS, and 1.76, respectively. The low temperature process and the current annealing method presented here would be useful to fabricate two-dimensional materials-based flexible electronics.

Mechanically robust 39 GHz cut-off frequency graphene field effect transistors on flexible substrates.

Graphene has been regarded as a promising candidate channel material for flexible devices operating at radio-frequency (RF). In this work we fabricated and fully characterized double bottom-gate graphene field effect transistors on flexible polymer substrates for high frequency applications. We report a record high as-measured current gain cut-off frequency (ft) of 39 GHz. The corresponding maximum oscillation frequency (fmax) is 13.5 GHz. These state of the art high frequency performances are stable against bending, with a typical variation of around 10%, for a bending radius of up to 12 mm. To demonstrate the reliability of our devices, we performed a fatigue stress test for RF-GFETs which were dynamically bend tested 1000 times at 1 Hz. The devices are mechanically robust, and performances are stable with typical variations of 15%. Finally we investigate thermal dissipation, which is a critical parameter for flexible electronics. We show that at the optimum polarization the normalized power dissipated by the GFETs is about 0.35 mW μm(-2) and that the substrate temperature is around 200 degree centigrade. At a higher power, irreversible degradations of the performances are observed. Our study on state of the art flexible GFETs demonstrates mechanical robustness and stability upon heating, two important elements to assess the potential of GFETs for flexible electronics.

Enhancing controllability and stability of bottom-gated graphene thin-film transistors by passivation with methylamine

This paper is intended to aid to bridge the gap between chemistry and electronic engineering. In this work, the fabrication of chemical vapour deposited graphene field-effect transistors employing silicon-nitride (Si3N4) gate dielectric is presented, showing originally p-type channel conduction due to ambient impurities yielding uncontrollable behaviour. Vacuum annealing has been performed to balance off hole and electron conduction in the channel, leading to the observation of the Dirac point and therefore improving controllability. Non-covalent functionalisation by methylamine has been performed for passivation and stability reasons yielding electron mobility of 4800 cm2/V s and hole mobility of 3800 cm2/V s as well as stabilised controllable behaviour of a bottom-gated transistor. The introduction of interface charge following the non-covalent functionalisation as well as the charge balance have been discussed and analysed.

Ambipolar/unipolar conversion in graphene transistors by surface doping

An ambipolar/unipolar conversion of conduction polarity in bottom-gate graphene field-effect transistor (FET) was realized by intended/unintended surface doping. Exposing the graphene FET in air made it fully p-type while covering graphene with Al nanofilm or poly(ethylene imine) (PEI) layer yielded a recovery of ambipolar conduction. The alteration of the conduction polarity in graphene FET was due to hole or electron-doping effect on graphene. Distinct changes in carrier mobility and current-voltage relationship were discussed between graphene with Al and PEI doping, and the dielectric screening by PEI was proposed as the possible mechanism.

Theory of remote phonon scattering in top-gated single-layer graphene

We extend the theory of interfacial plasmon-phonon scattering to top-gated single-layer graphene. As with bottom-gated graphene, interfacial plasmon-phonon (IPP) modes are formed from the coupling between the graphene plasmon and the surface polar phonon modes in the top and bottom oxides. We study the effect of the top oxide thickness on dynamic screening and electron-IPP coupling. The remote phonon-limited electron mobility ${\ensuremath{\mu}}_{\mathrm{RP}}$ and electron scattering rates in a HfO${}_{2}$-covered, SiO${}_{2}$-supported single-layer graphene are computed at various electron densities $n$ for different dielectric thickness ${t}_{\mathrm{ox}}$. We find that ${\ensuremath{\mu}}_{\mathrm{RP}}$ is much more dependent on ${t}_{\mathrm{ox}}$ at low $n$. They also agree with the experimentally estimated room temperature ${\ensuremath{\mu}}_{\mathrm{RP}}$ from Zou et al. [Phys. Rev. Lett. 105, 126601 (2010)]. The electron density dependent ${\ensuremath{\mu}}_{\mathrm{RP}}$ is predicted to be between 5500 and $24\phantom{\rule{0.16em}{0ex}}200$ cm${}^{2}$V${}^{\ensuremath{-}1}$s${}^{\ensuremath{-}1}$ from $n={10}^{12}$ to ${10}^{13}$ cm${}^{\ensuremath{-}2}$ at 300 K.

Effect of Temperature and Humidity on $\hbox{NO}_{2}$ and $\hbox{NH}_{3}$ Gas Sensitivity of Bottom-Gate Graphene FETs Prepared by ICP-CVD

Graphene was used for a sensing part of bottom-gate field-effect transistor (FET)-type gas sensors, and the sensing performance of the sensors was studied. We investigated the effect of temperature <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(T)$</tex></formula> and relative humidity on the gas sensitivity of bottom-gate graphene FETs prepared by the inductively coupled plasma chemical vapor deposition method. The conductivity change of graphene exposed to nitrogen oxide <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{NO}_{2})$</tex></formula> and ammonia <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{NH}_{3})$</tex></formula> was increased with increasing temperature and humidity. For <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{NO}_{2}$</tex></formula> gas, the graphene FET shows increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I_{D}$</tex></formula> with increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T$</tex></formula> and also increasing <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $I_{D}$</tex></formula> with increasing humidity. However, the FET shows an opposite trend with the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$T$</tex></formula> and humidity for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ \hbox{NH}_{3}$</tex></formula> gas.

Bandgap Tailoring and Synchronous Microdevices Patterning of Graphene Oxides

Reported here is femtosecond laser mediated bandgap tailoring of graphene oxides (GOs) for direct fabrication of graphene-based microdevices. When femtosecond laser pulses were used to reduce and pattern GO, oxygen contents in the reduced region could be modulated by varying the laser power. In this way, the bandgap of reduced GO was precisely modulated from 2.4 to 0.9 eV by tuning the femtosecond laser power from 0 to 23 mW. Through the first-principle study, the essence of GO bandgap tailoring is proved to be femtosecond laser reduction induced oxygen-content modulation. As representative illustrations, bottom-gate graphene FETs were fabricated in situ by using femtosecond laser reduced GO as the channel material, and an optimized room temperature on–off ratio of 56 is obtained. The controlled reduction of GO by femtosecond laser contributes great potential for bandgap tailoring and microdevices patterning of graphene toward future electronics.

High-Efficiency Graphene Photo Sensor Using a Transparent Electrode

We report the first implementation of transparent electrodes in bottom-gate graphene transistors used for photo detection. Compared to conventional nontransparent electrodes, the transparent electrodes allow photons to transmit through to the graphene beneath, providing an enlarged absorption area and thereby giving rise to an enhancement of photocurrent generation. The devices are fabricated with an asymmetric metallization scheme and the experimental results show that the maximum photocurrent density using the transparent electrodes (ITO and Pd/ITO) is over two times higher than that using the nontransparent electrodes (Ti and Pd), indicating a significant enhancement in the performance of graphene photo sensors.

Reliability of bottom-gate graphene field-effect transistors prepared by using inductively coupled plasma-chemical vapor deposition

Stability degradation in bottom-gate graphene field-effect transistors prepared by using inductively coupled plasma-chemical vapor deposition method are investigated under bias, temperature and illumination stress. The stretched-exponential time dependence model, which can be derived based on the trapping/detrapping of charges to/from existing traps and continuous redistribution of charges in bulk dielectrics, is well applied in fitting the time dependence of the transfer curve shifts in all stress conditions. The stress wavelength is considered as important factors as well as the bias and temperature stress in the transfer characteristic instabilities of graphene FEFs.