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Epitaxial graphene (EG) on cubic silicon carbide (3C-SiC) on silicon holds the promise of tunable nanoelectronic and nanophotonic devices, some uniquely unlocked by the graphene/cubic silicon carbide combination, directly integrated with the current well-established silicon technologies. Yet, the development of graphene field-effect devices based on the 3C-SiC/Si substrate system has been historically hindered by poor graphene quality and coverage, as well as substantial leakage issues of the heteroepitaxial system. We address these issues by growing EG on 3C-SiC on highly resistive silicon substrates using an alloy-mediated approach. In this work, we demonstrate a field-effect transistor based on EG/3C-SiC/Si with gate leakage current 6 orders of magnitude lower than the drain current at room temperature, which is a vast improvement on the current literature, opening the possibility for dynamically tunable nanoelectronic and nanophotonic devices on silicon at the wafer level. [ABSTRACT FROM AUTHOR]

The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance. [ABSTRACT FROM AUTHOR]

The high capacitance from an electrolyte dielectric layer used in a thin film transistor architecture results in the development of electrolyte-gated organic field effect transistors (EGOFETs), which are able to operate at lower voltages compared with the commonly organic field effect transistors. When ions diffuse into the semiconductor, changing its conductive properties, the resulting devices are named organic electrochemical transistors (OECTs). In this case, an increase in channel conductivity occurs due to ionic doping. Depending on the gate voltage range operation, it is possible to obtain a de-doping process, suitable for achieving a low off-current. In this work, a simple and easy method is proposed based on an additional dielectric layer deposition of poly(methyl methacrylate) at the electrolyte dielectric/semiconductor interface, which provides: (i) good efficiency to block the diffusion of anions into the channel, resulting in an EGOFET for a gate voltage range up to − 0.6 V and (ii) an improvement in the on–off ratio in approximately three orders of magnitude for OECTs due to the low off-current obtained without promoting ionic de-doping in the channel, for a gate voltage range up to − 1.1 V. Both modes of operation were obtained in the same transistor structure. In addition, parameters such as field effect mobility, on–off ratio, and threshold voltage were also estimated for different transistor structures and gate voltage ranges. [ABSTRACT FROM AUTHOR]

Transistor-based artificial synapses are expected to tackle the inherent limitations of traditional von Neumann architecture for neuromorphic computing paradigm. Organic electronic materials are promising components of future neuromorphic systems, but mimicking the functions of biological synapses for symmetric weight update and desired variation margin still remains challenging. Here, we propose a synaptic transistor based on pentacene/poly(α-methylstyrene) (PαMS) architecture capable of exhibiting the main behavior of a biological spiking synapse. The ambipolar charge trapping of the transistor enables symmetric variation of the channel conductivity with desirable margin. Comprehensive synaptic functions, including the postsynaptic current with different pulse amplitudes, short-term to long-term plasticity transition, reversible channel conductance potentiation and depression, and repetitive and symmetrical learning processes, are emulated. The realization of essential synaptic functions based on the cumulative charge trapping of pentacene/PαMS structure provides a feasible device structure toward the future demand of neuromorphic computing. [ABSTRACT FROM AUTHOR]

We report transport measurements of ionic liquid-gated field-effect transistors with multilayer MoS2 channels. Ambipolar characteristics with high ON/OFF ratios (>105) are observed. The current–voltage characteristics agree well with those calculated using a simple model assuming a high recombination probability and negligible contact resistance, except when holes are doped in the vicinity of the source contact. Electron and hole mobilities are estimated using the current–voltage characteristics in both the unipolar and ambipolar modes, wherein a p–n junction is formed in the latter. The electron mobility depends significantly on the carrier distribution in a channel that is controlled by the gate voltage. This result is attributed to the significant field dependence of the electron mobility. [ABSTRACT FROM AUTHOR]

It is shown that the dynamic response of a thin-film transistor (TFT) comprises a polymer or organic semiconductor and can be directly correlated with charge transport parameters. Charge transport is assumed to take place in accordance with the often employed multiple trap and release model. We can simulate the non-quasistatic response of TFTs for devices in which the mobility is not constant, but depends on carrier density and other parameters. The previous model of Burns has been significantly improved and made applicable to organic and polymer transistors. Our approach works for an arbitrary density of states including both a relatively shallow exponential distribution of tail states and deeper Gaussian trap states. We set the gate voltage, VG, in our TFTs to be zero. The source voltage, VS, is varied and, thus, the value of gate-to-source voltage is varied. The predictive power of the model that is presented in this paper will be very useful for accurate device modeling and for charge transport studies. [ABSTRACT FROM AUTHOR]

Luminescence quenching in the presence of polarons is one of the major challenges in organic light emitting devices. In this work, exciton quenching in the presence of polarons is studied using phase sensitive photocurrent measurements on pentacene field effect transistors. The enhancement of conduction in the organic field effect transistors on light illumination is studied using photocurrent spectral response measurements and corresponding optical simulations. The photocurrent is shown to be governed by the polaron mobility and the exciton quenching efficiency, both of which depend on the polaron density in the channel. Two models are proposed on the exciton dynamics in the presence of gate induced polarons in the transistor channel. The first model simulates the steady-state exciton concentration profile in the presence of exciton-polaron interaction. The second one is a three-dimensional steady state exciton-polaron interaction model, which supports the findings from the first model. It is shown that the excitons quench by transferring its energy to polarons, thereby promoting the latter to high energy states in the density of states manifold. The polarons move in the higher energy states with greater microscopic mobility before thermalizing, thereby leading to an enhancement of conduction. It is observed that for the present system, where charge carrier transport is by hopping, all polarons interact with excitons. This implies that for low mobility systems, the interaction is not limited to deep trapped polarons. [ABSTRACT FROM AUTHOR]

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances Rc to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that Rc decreases from 105-106 Ω cm for 15 min air exposure to 3×10³ Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of Rc to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V0. In channel-dominated (low Rc) FETs, the mobility is in the range of 10-17 cm²/(Vs); in contrast, in contact-limited (high Rc) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ - Rc dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts beyond traditional metal electrodes. [ABSTRACT FROM AUTHOR]

Field-effect transistors with organic-inorganic heterojunctions of molybdenum trioxide (MoO3)/pentacene and MoO3/copper-phthalocyanine (CuPc) as active layers were prepared and analyzed. These transistors showed normally-on operation mode and a shift of threshold voltage comparing to the corresponding single-layer organic device. The interfacial electronic structures of MoO3/pentacene and MoO3/CuPc heterojunctions were investigated by ultraviolet photoemission spectroscopy. Significant electron energy level bending and space charge regions of high conductivity were observed at the heterojunction. Formation of the organic-inorganic heterojunctions and characteristics of the corresponding field-effect transistors were analyzed by considering charge exchange at heterojunction interfaces. [ABSTRACT FROM AUTHOR]

Paramount to the rational design of electronic materials is the accurate characterization of their intrinsic properties. In particular, many applications of conducting and semiconducting soft materials have been driven by the development of materials with high, bias-stable field-effect mobility. Here, we demonstrate the effect of parasitic resistance and bias-dependent mobility on device electrical characteristics. Specifically, we analyze two of the most commonly employed test algorithms—the output and transfer curves—via a closed-form analysis. The analysis exhibits characteristics endemic to those published in literature, such as effective mobilities with maxima with respect to gate voltage that may lead to overstatements of mobility by manyfold. Furthermore, analysis reveals that common overestimation relative to intrinsic and output-estimated mobilities is caused solely by gate-bias-dependent mobility, and parasitic resistance can only lead to an underestimation of the effective mobility. We introduce a method for accurate mobility estimation and a dimensionless variable for evaluating the importance of contact resistance. [ABSTRACT FROM AUTHOR]

Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene) with different channel lengths were fabricated under different substrate pretreatment process conditions. These OFET devices were characterized using low frequency noise (LFN) spectroscopy, and the device performance parameters were correlated with the level of LFN. It is observed that the devices with higher noise levels showed poorer device properties when compared with the devices operated at same Ids of the same channel length. It is also observed that the noise level increased with the increase in channel length for devices with the same pretreatment conditions, which is due to increased trapping and detrapping in the channel material interface domain. The OFET device operating around the threshold voltage Vth will have a 1/f noise slope that is flatter, having a gradient that is smaller in magnitude. The threshold voltage of a device can thus be observed to be at the gate voltage in which 1/f noise intensity is the highest at low frequencies. [ABSTRACT FROM AUTHOR]