Low-frequency noise in nanoscale ballistic transistors

Low-frequency "1/f" noise is a major issue for nanoscale devices such at carbon nanotube transistors. We show that nanoscale ballistic transistors give voltage-dependent sensitivity to the intrinsic potential fluctuations from nearby charge traps. A distinctive dependence on gate voltage is predicted, without reference to the number of carriers. This dependence is confirmed by comparison with recent measurements of nanotube transistors. Possible ways of decreasing the noise are discussed.     

Flexible gigahertz transistors derived from solution-based single-layer graphene

Flexible electronics mostly relies on organic semiconductors but the limited carrier velocity in polymers and molecular films prevents their use at frequencies above a few megahertz. Conversely, the high potential of graphene for high-frequency electronics on rigid substrates was recently demonstrated. We conducted the first study of solution-based graphene transistors at gigahertz frequencies, and we show that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates. Our graphene flexible transistors have current gain cutoff frequencies of 2.2 GHz and power gain cutoff frequencies of 550 MHz. Radio frequency measurements directly performed on bent samples show remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene field-effect transistors over other types of flexible transistors based on organic materials.     

Logic circuits based on individual semiconducting and metallic carbon-nanotube devices

Nanoscale transistors employing an individual semiconducting carbon nanotube as the channel hold great potential for logic circuits with large integration densities that can be manufactured on glass or plastic substrates. Carbon nanotubes are usually produced as a mixture of semiconducting and metallic nanotubes. Since only semiconducting nanotubes yield transistors, the metallic nanotubes are typically not utilized. However, integrated circuits often require not only transistors, but also resistive load devices. Here we show that many of the metallic carbon nanotubes that are deposited on the substrate along with the semiconducting nanotubes can be conveniently utilized as load resistors with favorable characteristics for the design of integrated circuits. We also demonstrate the fabrication of arrays of transistors and resistors, each based on an individual semiconducting or metallic carbon nanotube, and their integration on glass substrates into logic circuits with switching frequencies of up to 500 kHz using a custom-designed metal interconnect layer.     

Liquid-crystalline processing of highly oriented carbon nanotube arrays for thin-film transistors

We introduce a simple solution-based method for the fabrication of highly oriented carbon nanotube (CNT) arrays to be used for thin-film transistors. We exploit the liquid-crystalline behavior of a CNT solution near the receding contact line during tilted-drop casting and produced long-range nematic-like ordering of carbon nanotube stripes caused by confined micropatterned geometry. We further demonstrate that the performance of thin-film transistors based on these densely packed and uniformly oriented CNT arrays is largely improved compared to random CNTs. This approach has great potential in low-cost, large-scale processing of high-performance electronic devices based on high-density oriented CNT films with record electrical characteristics such as high conductance, low resistivity, and high career mobility.     

Stretchable Transistor-Structured Artificial Synapses for Neuromorphic Electronics

Stretchable synaptic transistors, a core technology in neuromorphic electronics, have functions and structures similar to biological synapses and can concurrently transmit signals and learn. Stretchable synaptic transistors are usually soft and stretchy and can accommodate various mechanical deformations, which presents significant prospects in soft machines, electronic skin, human–brain interfaces, and wearable electronics. Considerable efforts have been devoted to developing stretchable synaptic transistors to implement electronic device neuromorphic functions, and remarkable advances have been achieved. Here, this review introduces the basic concept of artificial synaptic transistors and summarizes the recent progress in device structures, functional-layer materials, and fabrication processes. Classical stretchable synaptic transistors, including electric double-layer synaptic transistors, electrochemical synaptic transistors, and optoelectronic synaptic transistors, as well as the applications of stretchable synaptic transistors in light-sensory systems, tactile-sensory systems, and multisensory artificial-nerves systems, are discussed. Finally, the current challenges and potential directions of stretchable synaptic transistors are analyzed. This review presents a detailed introduction to the recent progress in stretchable synaptic transistors from basic concept to applications, providing a reference for the development of stretchable synaptic transistors in the future.     

Multiscale modeling of nanowire-based Schottky-barrier field-effect transistors for sensor applications

We present a theoretical framework for the calculation of charge transport through nanowire-based Schottky-barrier field-effect transistors that is conceptually simple but still captures the relevant physical mechanisms of the transport process. Our approach combines two approaches on different length scales: (1) the finite element method is used to model realistic device geometries and to calculate the electrostatic potential across the Schottky barrier by solving the Poisson equation, and (2) the Landauer-Büttiker approach combined with the method of non-equilibrium Green's functions is employed to calculate the charge transport through the device. Our model correctly reproduces typical I-V characteristics of field-effect transistors, and the dependence of the saturated drain current on the gate field and the device geometry are in good agreement with experiments. Our approach is suitable for one-dimensional Schottky-barrier field-effect transistors of arbitrary device geometry and it is intended to be a simulation platform for the development of nanowire-based sensors.     

Self-Aligned, Extremely High Frequency III-V Metal-Oxide-Semiconductor Field-Effect Transistors on Rigid and Flexible Substrates

This paper reports the radio frequency (RF) performance of InAs nanomembrane transistors on both mechanically rigid and flexible substrates. We have employed a self-aligned device architecture by using a T-shaped gate structure to fabricate high performance InAs metal-oxide-semiconductor field-effect transistors (MOSFETs) with channel lengths down to 75 nm. RF measurements reveal that the InAs devices made on a silicon substrate exhibit a cutoff frequency (f(t)) of ～165 GHz, which is one of the best results achieved in III-V MOSFETs on silicon. Similarly, the devices fabricated on a bendable polyimide substrate provide a f(t) of ～105 GHz, representing the best performance achieved for transistors fabricated directly on mechanically flexible substrates. The results demonstrate the potential of III-V-on-insulator platform for extremely high-frequency (EHF) electronics on both conventional silicon and flexible substrates.     

Probing electrostatic potentials in solution with carbon nanotube transistors

We have used single-walled carbon nanotube transistors to measure changes in the chemical potential of a solution due to redox-active transition-metal complexes. The interaction of the molecules with a gold electrolyte-gate wire changes the electrostatic potential sensed by the nanotube, which in turn shifts the gate-voltage dependence of the nanotube conductance. As predicted by the Nernst equation, this shift depends logarithmically on the ratio of oxidized to reduced molecules.     

Fabrication of field-effect transistors from hexathiapentacene single-crystal nanowires

This paper describes a simple, solution-phase route to the synthesis of bulk quantities of hexathiapentacene (HTP) single-crystal nanowires. These nanowires have also been successfully incorporated as the semiconducting material in field-effect transistors (FETs). For devices based on single nanowires, the carrier mobilities and current on/off ratios could be as high as 0.27 cm2/Vs and >103, respectively. For transistors fabricated from a network of nanowires, the mobilities and current on/off ratios could reach 0.057 cm2/Vs and >104, respectively. We have further demonstrated the use of nanowire networks in fabricating transistors on mechanically flexible substrates. Preliminary results show that these devices could withstand mechanical strain and still remain functional. The results from this study demonstrate the potential of utilizing solution-dispersible, nanostructured organic materials for use in low-cost, flexible electronic applications.     

Supported lipid bilayer/carbon nanotube hybrids

Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube--with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.     

Scalable fabrication of self-aligned graphene transistors and circuits on glass

Graphene transistors are of considerable interest for radio frequency (rf) applications. High-frequency graphene transistors with the intrinsic cutoff frequency up to 300 GHz have been demonstrated. However, the graphene transistors reported to date only exhibit a limited extrinsic cutoff frequency up to about 10 GHz, and functional graphene circuits demonstrated so far can merely operate in the tens of megahertz regime, far from the potential the graphene transistors could offer. Here we report a scalable approach to fabricate self-aligned graphene transistors with the extrinsic cutoff frequency exceeding 50 GHz and graphene circuits that can operate in the 1-10 GHz regime. The devices are fabricated on a glass substrate through a self-aligned process by using chemical vapor deposition (CVD) grown graphene and a dielectrophoretic assembled nanowire gate array. The self-aligned process allows the achievement of unprecedented performance in CVD graphene transistors with a highest transconductance of 0.36 mS/μm. The use of an insulating substrate minimizes the parasitic capacitance and has therefore enabled graphene transistors with a record-high extrinsic cutoff frequency (> 50 GHz) achieved to date. The excellent extrinsic cutoff frequency readily allows configuring the graphene transistors into frequency doubling or mixing circuits functioning in the 1-10 GHz regime, a significant advancement over previous reports (～20 MHz). The studies open a pathway to scalable fabrication of high-speed graphene transistors and functional circuits and represent a significant step forward to graphene based radio frequency devices.     

Flexible organic transistors and circuits with extreme bending stability

Flexible electronic circuits are an essential prerequisite for the development of rollable displays, conformable sensors, biodegradable electronics and other applications with unconventional form factors. The smallest radius into which a circuit can be bent is typically several millimetres, limited by strain-induced damage to the active circuit elements. Bending-induced damage can be avoided by placing the circuit elements on rigid islands connected by stretchable wires, but the presence of rigid areas within the substrate plane limits the bending radius. Here we demonstrate organic transistors and complementary circuits that continue to operate without degradation while being folded into a radius of 100 μm. This enormous flexibility and bending stability is enabled by a very thin plastic substrate (12.5 μm), an atomically smooth planarization coating and a hybrid encapsulation stack that places the transistors in the neutral strain position. We demonstrate a potential application as a catheter with a sheet of transistors and sensors wrapped around it that enables the spatially resolved measurement of physical or chemical properties inside long, narrow tubes.     

High-performance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have many exceptional electronic properties. Realizing the full potential of SWNTs in realistic electronic systems requires a scalable approach to device and circuit integration. We report the use of dense, perfectly aligned arrays of long, perfectly linear SWNTs as an effective thin-film semiconductor suitable for integration into transistors and other classes of electronic devices. The large number of SWNTs enable excellent device-level performance characteristics and good device-to-device uniformity, even with SWNTs that are electronically heterogeneous. Measurements on p- and n-channel transistors that involve as many as approximately 2,100 SWNTs reveal device-level mobilities and scaled transconductances approaching approximately 1,000 cm(2) V(-1) s(-1) and approximately 3,000 S m(-1), respectively, and with current outputs of up to approximately 1 A in devices that use interdigitated electrodes. PMOS and CMOS logic gates and mechanically flexible transistors on plastic provide examples of devices that can be formed with this approach. Collectively, these results may represent a route to large-scale integrated nanotube electronics.     

How good can monolayer MoS₂ transistors be?

Monolayer molybdenum disulfide (MoS(2)), unlike its bulk form, is a direct band gap semiconductor with a band gap of 1.8 eV. Recently, field-effect transistors have been demonstrated experimentally using a mechanically exfoliated MoS(2) monolayer, showing promising potential for next generation electronics. Here we project the ultimate performance limit of MoS(2) transistors by using nonequilibrium Green's function based quantum transport simulations. Our simulation results show that the strength of MoS(2) transistors lies in large ON-OFF current ratio (>10(10)), immunity to short channel effects (drain-induced barrier lowering ～10 mV/V), and abrupt switching (subthreshold swing as low as 60 mV/decade). Our comparison of monolayer MoS(2) transistors to the state-of-the-art III-V materials based transistors, reveals that while MoS(2) transistors may not be ideal for high-performance applications due to heavier electron effective mass (m = 0.45 m(0)) and a lower mobility, they can be an attractive alternative for low power applications thanks to the large band gap and the excellent electrostatic integrity inherent in a two-dimensional system.     

Evaluation of the radiation tolerance of several generations of SiGe heterojunction bipolar transistors under radiation exposure

For the potential use in future high luminosity applications in high energy physics (HEP) (e.g., the large hadron collider (LHC) upgrade), we evaluated the radiation tolerance of several candidate technologies for the front-end of the readout application-specific integrated circuit (ASIC) for silicon strip detectors. The devices investigated were first, second and third-generation silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs).The DC current gain as a function of collector current was measured before and after irradiation with 24 GeV protons up to fluences of 10¹⁶ p/cm² and with a ⁶⁰Co gamma source up to 100 Mrad. The analog section of an amplifier for silicon strip detectors typically has a special front transistor, chosen carefully to minimize noise and usually requiring a larger current than the other transistors, and a large number of additional transistors used in shaping sections and for signal-level discrimination. We discuss the behavior of the three generations of transistors under proton and gamma exposure, with a particular focus on issues of noise, power and radiation limitations.     

High frequency performance of individual and arrays of single-walled carbon nanotubes

We have studied the high frequency performance limits of single-walled carbon nanotube (SWNT) transistors in the diffusive transport regime limited by the acoustic phonon scattering. The relativistic band structure of single-walled carbon nanotubes combined with the acoustic phonon scattering provides an analytical model for the charge transport of the radio frequency transistors. We were able to obtain the intrinsic high frequency performance such as the cut-off frequency and the linearity of the SWNT transistors. We have extended our model to include transistors based on arrays of SWNTs. The effect of electrostatic screening in a dense array of SWNTs on the cut-off frequency is studied.     

The effect of irradiation on the characteristics of MOS transistors

We have experimentally studied the effect of X-ray radiation on the parameters of MOS transistors. An analysis showed that correct evaluation of the density of surface states and the gate insulator charging by method of subthreshold current-voltage characteristics requires taking into account the planar inhomogeneity of a transistor. Some complication of the method is compensated by the increasing accuracy of determination of the surface parameters and the additional possibility of determining fluctuations of the surface potential.     

Air-stable ambipolar field-effect transistors and complementary logic circuits from solution-processed n/p polymer heterojunctions

We demonstrate the use of n/p polymer/polymer heterojunctions deposited by sequential solution processing to fabricate ambipolar field-effect transistors and complementary logic circuits. Electron and hole mobilities in the transistors were ～0.001-0.01 cm(2)/(V s) in air without encapsulation. Complementary circuits integrating multiple ambipolar transistors into NOT, NAND, and NOR gates were fabricated and shown to exhibit sharp signal switching with a high voltage gain.     

Self-assembled molecular nanowires of 6,13-Bis(methylthio)pentacene: growth, electrical properties, and applications

We demonstrate a comprehensive study of self-assembled molecular nanowire, including molecular design, one-dimensional crystal growth, resistivity measurement of individual wire, and application to a field-effect transistor. Appropriate molecular design and control of interfacial interactions lead to single crystalline wire growth with an extensive pi-stacking motif. Resistivity measurements of an individual molecular wire indicate that these structural features are advantageous for electrical transport. Finally, field-effect transistors with single- and double-wire channels were fabricated to give some indication of the potential application of the molecular wires.     

Spin Injection Efficiency at the Source/Channel Interface of Spin Transistors

Almost all spintronic transistors (e.g., spin field-effect transistors, spin bipolar transistors, and spin-enhanced MOSFETs) require high efficiency of spin injection from a ferromagnetic contact into a semiconductor channel for proper operation. In this paper, we calculate the efficiency of spin injection from a realistic nonideal ferromagnetic contact into the semiconductor quantum wire channel of a spintronic transistor, taking into account the presence of an axial magnetic field (caused by either the ferromagnetic contact or external agents) and spin orbit interaction. In our calculations, the temperature is assumed to be low enough that phonon scattering is weak and transport is phase-coherent, although not ballistic because of elastic scattering caused by impurities and defects. We consider a single impurity in the channel and show that the conductance depends strongly on the exact location of this impurity because of quantum mechanical interference effects. This is a nuisance since it exacerbates device variability. The ldquosignrdquo of the impurity potential, i.e., whether it is attractive or repulsive, also influences the channel conductance. Surprisingly, at absolute zero temperature, the spin injection efficiency can reach 100% at certain gate biases, even though the ferromagnetic injector is nonideal. However, this efficiency drops rapidly with increasing temperature.