Sk. Fahad Chowdhury

Three-Gigahertz Graphene Frequency Doubler on Quartz Operating Beyond the Transit Frequency

We demonstrate a 500-nm graphene frequency doubler with a record 3-GHz bandwidth, exceeding the device transit frequency by 50%, a previously unobserved result in graphene, indicating that graphene multiplier devices might be useful beyond their transit frequency. The maximum conversion gain of graphene ambipolar frequency doublers is determined to approach a near lossless value in the quantum capacitance limit. In addition, the experimental performance of graphene transistor frequency detectors is demonstrated, showing responsivity of 25.2 μA/μW. The high-frequency performance of these gigahertz devices is enabled by top-gate device fabrication using synthesized graphene transferred onto low capacitance, atomically smooth quartz substrates, affording carrier mobilities as high as 5000 cm2/V ·s.

High-Performance Current Saturating Graphene Field-Effect Transistor With Hexagonal Boron Nitride Dielectric on Flexible Polymeric Substrates

Graphene transistors using hexagonal boron nitride as the gate dielectric are implemented on mechanically flexible polyimide films. Current saturation is observed for the first time in graphene transistors on a plastic substrate. An atomically smooth insulating surface is achieved with the proposed capture-release process and two-step annealing process, resulting in subnanometer surface roughness. The device shows strong electrical performance: Extracted mobility exceeds 2300 cm2/V·s for both electron and hole transport, and drive current is over 300 μS/μm. This transport symmetry affords frequency doublers with high spectral purity and a conversion gain of - 29.5 dB and output power of -22.2 dBm, representing the highest performance for graphene transistors on flexible substrates.

Transformation of the Electrical Characteristics of Graphene Field-Effect Transistors with Fluoropolymer

We report on the improvement of the electronic characteristics of monolayer graphene field-effect transistors (FETs) by an interacting capping layer of a suitable fluoropolymer. Capping of monolayer graphene FETs with CYTOP improved the on-off current ratio from 5 to 10 as well as increased the field-effect mobility by as much as a factor of 2 compared to plain graphene FETs. Favorable shifts in the Dirac voltage toward zero with shift magnitudes in excess of 60 V are observed. The residual carrier concentration is reduced to ~2.8 × 10(11) cm(-2). Removal of the fluoropolymer from graphene FETs results in a return to the initial electronic properties before depositing CYTOP. This suggests that weak, reversible electronic perturbation of graphene by the fluoropolymer favorably tune the electrical characteristics of graphene, and we hypothesize that the origin of this improvement is in the strongly polar nature of the C-F chemical bonds that self-organize upon heat treatment. We demonstrate a general method to favorably restore or transform the electrical characteristics of graphene FETs, which will open up new applications.

Enhanced sensitivity of graphene ammonia gas sensors using molecular doping

We report on employing molecular doping to enhance the sensitivity of graphene sensors synthesized via chemical vapor deposition to NH3 molecules at room temperature. We experimentally show that doping an as-fabricated graphene sensor with NO2 gas improves sensitivity of its electrical resistance to adsorption of NH3 molecules by about an order of magnitude. The detection limit of our NO2-doped graphene sensor is found to be ∼200 parts per billion (ppb), compared to ∼1400 ppb before doping. Electrical characterization and Raman spectroscopy measurements on graphene field-effect transistors show that adsorption of NO2 molecules significantly increases hole concentration in graphene, which results in the observed sensitivity enhancement.

Self-aligned graphene field-effect transistors with polyethyleneimine doped source/drain access regions

We report a method of fabricating self-aligned, top-gated graphene field-effect transistors (GFETs) employing polyethyleneimine spin-on-doped source/drain access regions, resulting in a 2X reduction of access resistance and a 2.5X improvement in device electrical characteristics, over undoped devices. The GFETs on Si/SiO2 substrates have high carrier mobilities of up to 6300 cm2/Vs. Self-aligned spin-on-doping is applicable to GFETs on arbitrary substrates, as demonstrated by a 3X enhancement in performance for GFETs on insulating quartz substrates, which are better suited for radio frequency applications.

Improvement of graphene field-effect transistors by hexamethyldisilazane surface treatment

We report the improvement of the electrical characteristics of graphene field-effect transistors (FETs) by hexamethyldisilazane (HMDS) treatment. Both electron and hole field-effect mobilities are increased by 1.5 × –2×, accompanied by effective residual carrier concentration reduction. Dirac point also moves closer to zero Volt. Time evolution of mobility data shows that mobility improvement saturates after a few hours of HMDS treatment. Temperature-dependent transport measurements show small mobility variation between 77 K and room temperature (295 K) before HMDS application. But mobility at 77 K is almost 2 times higher than mobility at 295 K after HMDS application, indicating reduced carrier scattering. Performance improvement is also observed for FETs made on hydrophobic substrate—an HMDS-graphene-HMDS sandwich structure. Raman spectroscopic analysis shows that G peak width is increased, G peak position is down shifted, and intensity ratio between 2D and G peaks is increased after HMDS application. We attribute the improvements in electronic transport mainly to enhanced screening and mitigation of adsorbed impurities from graphene surface upon HMDS treatment.

State-of-the-art graphene transistors on hexagonal boron nitride, high-k, and polymeric films for GHz flexible analog nanoelectronics

We report graphene field-effect transistors on hexagonal boron nitride, high-k, and polymeric films featuring state-of-the-art electrical and mechanical properties on flexible substrates. The record electrical performance includes the highest ON current (~0.3mA/μm), the first demonstration of current saturation on flexible films and intrinsic gain, and the highest conversion gain flexible graphene frequency doubler. Extrinsic transit frequency of 2.23GHz, and maximum frequency of 1.15GHz are also achieved. In addition, robust electrical response down to 0.7mm mechanical bending radius is realized.

Black Phosphorous Thin-Film Transistor and RF Circuit Applications

In this letter, we discuss the design, fabrication, and high-frequency characterization of black phosphorous (BP)based field-effect transistors (FETs) and their circuit applications. We demonstrate BP radio frequency (RF) FETs with an extrinsic transit frequency ~3 GHz and an extrinsic maximum oscillation frequency ~7 GHz. We also demonstrate various BP FET-based RF circuits working in the megahertz range for the first time. We show the design and simulation of BP-based RF amplifier using experimentally obtained scattering parameters, operating at gigahertz frequency with substantial gain. The experimental and simulation results reveal the major performance bottlenecks of these circuits and place BP FET as a promising device candidate for future thin-film nanoelectronic RF systems.

Enhancement of graphene field-effect transistor by surface treatment

We report the enhancement of electrostatic transfer characteristics of fabricated graphene field-effect transistor (FET) by hexamethyldisilazane (HMDS) surface treatment. Charge carrier mobility increases by over 50% on average for both electron and hole. Impurity concentration also reduces by over 50% on average and charge neutrality point usually moves towards zero gate voltage. Electrostatic transfer characteristics of as fabricated FET show small variation with temperature resulting in weakly temperature sensitive carrier mobility. However, there is significant variation in characteristics between room temperature and 77 K after HMDS treatment and charge carrier mobility at 77K is almost two times the mobility at room temperature. We attribute the performance enhancements upon HMDS treatment to removal or modification of various contaminants present on graphene surface after device fabrication, which results in reduced carrier scattering. Dielectric screening of charged impurities may also contribute towards performance enhancements.

Impact of contact and access resistances in graphene field-effect transistors on quartz substrates for radio frequency applications

High-frequency performance of graphene field-effect transistors (GFETs) has been limited largely by parasitic resistances, including contact resistance (RC) and access resistance (RA). Measurement of short-channel (500 nm) GFETs with short (200 nm) spin-on-doped source/drain access regions reveals negligible change in transit frequency (fT) after doping, as compared to ∼23% fT improvement for similarly sized undoped GFETs measured at low temperature, underscoring the impact of RC on high-frequency performance. DC measurements of undoped/doped short and long-channel GFETs highlight the increasing impact of RA for larger GFETs. Additionally, parasitic capacitances were minimized by device fabrication using graphene transferred onto low-capacitance quartz substrates.