Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

Phospholipid film in electrolyte-gated organic field-effect transistors

A totally innovative electrolyte-gated field effect transistor, embedding a phospholipid film at the interface between the organic semiconductor and the gating solution, is described. The electronic properties of OFETs including a phospholipid film are studied in both pure water and in an electrolyte solution and compared to those of an OFET with the organic semiconductor directly in contact with the gating solution. In addition, to investigate the role of the lipid layers in the charge polarization process and quantify the field-effect mobility, impedance spectroscopy was employed. The results indicate that the integration of the biological film minimizes the penetration of ions into the organic semiconductor thus leading to a capacitive operational mode as opposed to an electrochemical one. The OFETs operate at low voltages with a field-effect mobility in the 10⁻³ cm² V⁻¹ s⁻¹ range and an on/off current ratio of 10³. This achievement opens perspectives to the development of FET biosensors potentially capable to operate in direct contact with physiological fluids.     

Theory of negative resistance of junction field-effect transistors

A general theory of the voltage-controlled negative resistance in a field-effect transistor is presented. The prerequisite to the negative resistance is mathematically determined as a relation among the source-gate voltage, the source-drain voltage, and the pinchoff voltage. It is suggested that there will be a variety of circuit configurations with the positive feedback function applied to the gate of the field-effect transistor. Three basic examples of the practical application of the theory are also given.     

A lithographic process for integrated organic field-effect transistors

This paper reports a photolithographic process for fabricating organic field-effect transistors which provides two layers of metal with arbitrary via placement, and optionally allows for subtractive lithographic patterning of the transistor active layer. The demonstrated pentacene transistors have a field-effect mobility of 0.1/spl plusmn/0.05 cm/sup 2//(V/spl middot/s). Parylene-C is used both as the gate dielectric and an encapsulation layer which allows for subtractive lithographic patterning. Also demonstrated is a PMOS inverter without level shifting circuitry and level-restoring V/sub High/ and V/sub Low/. This work demonstrates a high definition, multilayer, integrated photolithographic process which creates organic field effect transistors suitable for use in integrated circuit applications such as a display backplanes.     

Tunable contact resistance in double-gate organic field-effect transistors

A study of the contact resistance (R_sd) in pentacene-based double-gate transistors is presented. In top-contact transistors, as the negative bias of the additional top-gate bias is increased, R_sd decreases by over five orders of magnitude for small bottom-gate voltages. In bottom-contact transistors, R_sd is reduced by about ten times for all bias values, implying improved charge transport in all operating regimes. The different tunability of R_sd in top/bottom-contact transistors is attributed to different charge injection modulation by the coplanar/staggered top gate. Therefore, double-gate architecture offers a novel and effective approach to limit R_sd and its relevant impacts on organic transistor.     

High-mobility tetrathiafulvalene organic field-effect transistors from solution processing

We report on mobilities up to 3.6 cm²/V s in organic field-effect transistors (OFETs) with solution-processed dithiophene- and dibenzo-tetrathiafulvalene (DT- and DB-TTF) single crystals as active materials. In the devices, the channel length varies from 100 μm down to sub 100 nm, and the SiO₂ thickness is either 100 nm, 50 nm, or 20 nm. The devices exhibit excellent operation characteristics with an on/off-ratio exceeding 10⁶. Temperature dependent measurements between 50 and 400 K reveal a thermally activated transport with increased activation above 200 K. The mobility exhibits exponential activation with two distinct exponents.     

High-performance organic field-effect transistors with binary ionic liquids

High-performance rubrene single-crystal field-effect transistors are developed with binary ionic liquid electrolytes used for gating. Inclusion of small amount of inorganic salts in the ionic liquids enhances the degree of dissociation for the organic ions and accelerates formation of the electric-double-layers in response to the gate voltage. High carrier mobility of 2.9 cm²/Vs is achieved in the rubrene single-crystal transistors with the mixture ionic liquid. In addition to the advantage of the low-voltage operation due to concentrated field in ultra-thin electric-double-layers, drastically increased capacitance at above 100 Hz makes the technique of the ionic liquid gating more attractive for fast-switching devices.     

Gate-bias assisted charge injection in organic field-effect transistors

The charge injection barriers in organic field-effect transistors (OFETs) seem to be far less critical as compared to organic light-emitting diodes (OLEDs). Counter intuitively, we show that the origin is image-force lowering of the barrier due to the gate bias at the source contact, although the corresponding gate field is perpendicular to the channel current. In coplanar OFETs, injection barriers up to 1 eV can be surmounted by increasing the gate bias, enabling extraction of bulk transport parameters in this regime. For staggered transistors, however, the injection is gate-assisted only until the gate bias is screened by the accumulation channel opposite to the source contact. The gate-assisted injection is supported by two-dimensional numerical charge transport simulations that reproduce the gate-bias dependence of the contact resistance and the typical S-shaped output curves as observed for OFETs with high injection barriers.     

n-Type self-assembled monolayer field-effect transistors for flexible organic electronics

Within this work we present n-type self-assembled monolayer field-effect transistors (SAMFETs) based on a novel perylene bisimide. The molecule spontaneously forms a covalently fixed monolayer on top of an aluminium oxide dielectric via a phosphonic acid anchor group. Detailed studies revealed an amorphous, two-dimensional semiconducting sheet on top of the dielectric. Reliable transistors with electron mobilities on the order of 10^?3 cm²/V s with limited hysteresis were achieved on rigid as well on flexible substrates. Furthermore, a flexible NMOS-bias inverter based on SAMFETs is demonstrated for the first time.     

Small-molecule vacuum processed melamine-C60, organic field-effect transistors

The transfer of benchtop knowledge into large scale industrial production processes represents a challenge in the field of organic electronics. Large scale industrial production of organic electronics is envisioned as roll to roll (R2R) processing which nowadays comprises usually solution-based large area printing steps. The search for a fast and reliable fabrication process able to accommodate the deposition of both insulator and semiconductor layers in a single step is still under way. Here we report on the fabrication of organic field effect transistors comprising only evaporable small molecules. Moreover, both the gate dielectric (melamine) and the semiconductor (C₆₀) are deposited in successive steps without breaking the vacuum in the evaporation chamber. The material characteristics of evaporated melamine thin films as well as their dielectric properties are investigated, suggesting the applicability of vacuum processed melamine for gate dielectric layer in OFETs. The transistor fabrication and its transfer and output characteristics are presented along with observations that lead to the fabrication of stable and virtually hysteresis-free transistors. The extremely low price of precursor materials and the ease of fabrication recommend the evaporation processes as alternative methods for a large scale, R2R production of organic field effect transistors.     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

Four-terminal field-effect transistors

A derivation is presented for the transconductances of a four-terminal transistor of arbitrary channel doping profile. The derivation takes into account finite barrier potential and is specifically applied to the symmetrical, abrupt-junction, four-terminal, field-effect transistor. Curves are presented showing variations of normalized transconductance with applied gate-source EMF.     

Scaling behaviors of transient noise current in organic field-effect transistors

Top-contact and bottom-gate organic field-effect transistors (OFETs) based on poly(3-hexylthiophene), P3HT polymer has been fabricated with thermal treatment condition. Transient noise currents of the OFETs are measured at various source–drain voltages ranging from 0 V to ?60 V with respect to a fixed gate voltage of ?60 V. The results from conventional power spectral density method are compared with the more robust Detrended Fluctuation Analysis. The latter has been proven to be reliable for fractal signals particularly in the presence of nonstationary effects. Interesting transitions between multiscaling and monoscaling behaviors are observed in the power spectral density as well as the Detrended Fluctuation Analysis plots for different applied source–drain voltage V_ds. Uncorrelated white noise characteristics are observed for noise current measured at low V_ds, meanwhile 1/f noise-like scaling behaviors are observed at intermediate V_ds. At higher V_ds, the noise characteristics appeared to be close to Brownian-like power-law behavior. The scaling characteristics of the transient noise current can be related to the charge carrier dynamics. It is also found that large numbers of trap centers are induced when the device is stressed at high applied V_ds. The existence of these trap centers would disperse charge carriers, leading to 1/f type noise that could diminish the presence of Brownian noise in a very short time.     

Improved ferroelectric field-effect devices

Postfabrication heat treatments in dry helium are shown to produce greater than tenfold improvement in the range of adaptability of the transfer characteristics of adaptive thin-film (Te or CdSe) resistors and transistors. Improved device performance is attributed to removal of water vapor from the ferroelectric (TGS) material.     

Medium-power GaAs field-effect transistors

Results on medium-power GaAs m.e.s.f.e.t.s. are described. Output powers as high as 300 mW at 9 GHz at 1 dB gain compression with a linear gain of 5.2 dB and drain efficiency of 30% have been obtained with single-cell m.e.s.f.e.t.s. At 4 GHz, a power output of 665 mW at 1 dB gain compression, a linear gain of 8 dB and a drain efficiency of 44.5% were realised with a 3-cell m.e.s.f.e.t. Two-tone intermodulation characteristics at 4 GHz are also described. A major innovation has been the use of a high-resistivity chromium-doped epitaxial GaAs buffer layer to isolate the device active region from the bulk-grown substrate.     

Electrical characterization of mica as an insulator for organic field-effect transistors

Electrical properties of conjugated polymer films, including poly(3-hexylthiophene)-2,5-diyl (P3HT), poly(3,3-didodecylquarterthiophene) (PQT-12), and poly(triarylamine) (PTAA), on mica substrates have been studied. The test structure was similar to a standard organic field-effect transistor but with a 150-μm-thick commercially available mica gate insulator/substrate, which allowed to obtain a field-effect mobility of P3HT as high as 0.08 cm²/Vs in the linear regime in ambient air. The influence of interface treatment, thermal annealing, and measurement conditions on the electrical properties of the P3HT films has been characterized and analyzed. We also studied the time dependence of the carrier concentration and mobility before and after a thermal annealing process. The results indicate that mica is a promising insulator for organic field-effect transistors, apart from already being one of the common thin-film materials widely used in electric capacitors.     

Non-functionalized soluble diketopyrrolopyrrole: Simplest p-channel core for organic field-effect transistors

We report the synthesis, characterization and behavior in field-effect transistors of non-functionalized soluble diketopyrrolopyrrole (DPP) core with only a solubilizing alkyl chain (i.e. –C₁₆H₃₃ or –C₁₈H₃₇) as the simplest p-channel semiconductor. The characteristics were evaluated by UV–vis and fluorescence spectroscopy, X-ray diffraction, cyclic voltammetry (CV), thermal analysis, atomic force microscopy (AFM) and density functional theory (DFT) calculation. For top-contact field-effect transistors, two types of active layers were prepared either by a solution process (as a 1D-microwire) or thermal vacuum deposition (as a thin-film) on a cross-linked poly(4-vinylphenol) gate dielectric. All the devices showed typical p-channel behavior with dominant hole transports. The device made with 1D-microwiress of DPP-R18 showed field-effect mobility in the saturation region of 1.42 × 10^?2 cm²/V s with I_ON/I_OFF of 1.82 × 10³. These findings suggest that the non-functionalized soluble DPP core itself without any further functionalization could also be used as a p-channel semiconductor for low-cost organic electronic devices.     

InP Schottky-gate field-effect transistors

MESFET's with gates 1 µm long were fabricated in LPE layers of InP on Cr-doped InP substrates. The quality of the LPE material is characterized by an electron concentration of 4 × 10¹⁵cm^-3with a mobility of 2.6 × 10⁴cm²V^-1s^-1at 77 K for growths from undoped melts. The devices have current gain cutoff frequencies of 20 GHZ or somewhat larger. This value is greater than that of the best analogous GaAs MESFET bya factor of 1.5. A factor of 1.3 is predicted on the basis of a simple theory. The highest power gain cutoff frequency, fmax, for the InP device is 33 GHz which is somewhat smaller than that of the best analogous GaAs device. The lowest minimum noise figure at 10 GHz for these first InP devices is 3.9 dB with an associated gain of 4.8 dB. The best result for the GaAs counterpart is 3.2 dB with an associated gain of 7.8 dB. The power gain of the InP device suffers compared to the GaAs device because of degenerative feedback resulting from a large gate-to-drain capacitance and because of a small output resistance. If the magnitudes of these two equivalent circuit elements were the same for MESFET's in the two materials, fmaxfor the InP device would be more than double its present value.     

Amorphous-silicon silicon-nitride field-effect transistors

n-channel and p-channel amorphous-silicon field-effect transistors have been fabricated on a glassy substrate using undoped and impurity-doped a-Si films as the semiconductor and silicon nitride deposited from an SiH4-N2 mixture as the gate insulator. A change in the source-drain conductance of greater than four orders of magnitude is realised by changing the gate potential from 0 to 5 V.     

Photoinduced negative magnetoresistance in 6,13-bis(triisopropylsilylethynyl)-pentacene field-effect transistors

We report on photoinduced negative organic magnetoresistance in low external magnetic-fields (<100 mT) in 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pentacene) field-effect transistors. An external magnetic field does not influence the dark current of our device. In contrast, there is a significant increase in photocurrent when magnetic field is applied to the irradiated device, which leads to negative magnetoresistance. The magnetoresistance and photoresponse values are strongly correlated and both are influenced by applied voltages and irradiation intensity. We attribute the observed photoinduced negative magnetoresistance in TIPS-Pentacene field-effect transistors to the presence of electron-hole pairs under irradiation. The overall dissociation probability of electron-hole pairs rises under the influence of an external magnetic field, which leads to a higher number of free charge carriers.