Fast-switching insulated gate transistors

Insulated gate transistors (IGT's) with high-speed gate turn-off capability have been developed by using electron irradiation to reduce the minority-carrier lifetime in the drift region. Gate turnoff times as low as 200 ns have been achieved. These devices have been found to offer a unique advantage in the ability to tradeoff conduction and switching losses which allows optimization of device characteristics for each application.     

On the transient response of organic electrochemical transistors

We present a universal model for the transient drain current response in organic electrochemical transistors (OECTs). Using equivalent circuits and charge injection physics, we are able to predict the drain current in OECT devices upon application of a gate voltage input. The model is applicable to both plain and membrane-functionalized devices, and allows us to extract useful physical quantities such as resistances and capacitances, which are related to functional properties of the system. We are also able to use the model to reconstruct the magnitude and shape in time of an applied voltage source based on the observed drain current response. This was experimentally demonstrated for drain current measurements under an applied action potential.     

Stretchable organic electrochemical transistors with micro-/nano-structures

<正>Organic electrochemical transistors(OECTs) have attracted attention due to their unique function of converting ionic and biological signals into electronic signals, high transconductance, low energy consumption(below 1 V), stable operation in aqueous media, good biocompatibility^{[1, 2]}.However, most OECTs are usually built on brittle and stiff substrates, and inappropriate to be adhered to or contacted with delicate human skin, thus impeding their use in wearable electronics. ...     

Maskless metal patterning by meniscus-confined electrochemical etching and its application in organic field-effect transistors

Conductive micropatterns is an essential part for operation of electronic devices in both industrial and academic fields. Conventional mask-based photolithography and vacuum deposition are inadequate to meet the demands of convenience and simplicity due to their complicated operation, costly instrumentations and relatively low resolution (for vacuum deposition). Development of simple and efficient mask-less fabrication techniques of conductive micropatterns is highly expected. Here we report a facile meniscus-confined electrochemical etching (MCEE) approach to fabricate metal micropatterns with resolution down to at least 1.0 μm. Both the applied bias and the moving velocity directly influence the patterning resolution. MCEE process is developed to fabricate source and drain electrodes in organic transistors on both rigid and flexible substrates. Being a maskless direct writing method, the width and morphology of the etched channel can be easily modulated by the bias and the velocity. The organic transistor with top-contact configuration presents better electrical performance with device on/off ratio of 1.1 × 10⁵ and maximum carrier mobility of 1.07 cm²V⁻¹s⁻¹, which implies that MCEE operation doesn't result in the degradation of the already deposited semiconducting film. This mask-less MCEE approach provides a potential complementary to conventional mask-based techniques for the fabrication of microscale metal patterns.     

All-printed low-voltage organic transistors

All-printed low-voltage organic transistors were manufactured on a plastic substrate and measured in normal room atmosphere. All electrodes were ink-jet printed, while the homogeneous semiconductor and insulator layers were subsequently applied by the reverse gravure coating technique. No destructive effects from the rough plastic substrate or the printed silver source and drain electrodes were observed. Changes in the transistor characteristics after 48 days of ageing in room atmosphere were mainly attributed to a reduced conductivity of the polymer gate electrode.     

Water-gated organic nanowire transistors

We gated both p-type, and n-type, organic nanowire (NW) films with an aqueous electric double layer (EDL) in thin-film transistor (TFT) architectures. For p-type NWs, we used poly(3-hexylthiophene) (P3HT) NWs grown via two different routes. Both can be gated with water, resulting in TFTs with threshold lower than for conventionally cast P3HT films under the same gating conditions. However, TFT drain currents are lower for NWs than for conventional P3HT films, which agrees with similar observations for ‘dry’ gated TFTs. For n-type NWs, we have grown ‘nanobelts’ of poly(benzimidazobenzophenanthroline) (BBL) by a solvent/non-solvent mixing route with later displacement of the solvent, and dispersion in a non-solvent. Water-gating such films initially failed to give an observable drain current. However, BBL nanobelts can be gated with the aprotic solvent acetonitrile, giving high n-type drain currents, which are further increased by adding salt. Remarkably, after first gating BBL NW films with acetonitrile, they can then be gated by water, giving very high drain currents. This behaviour is transient on a timescale of minutes. We believe this observation is caused by a thin protective acetonitrile film remaining on the nanobelt surface.     

Pentacene-based organic thin-film transistors

Organic thin-film transistors using the fused-ring polycyclic aromatic hydrocarbon pentacene as the active electronic material have shown mobility as large as 0.7 cm²/V-s and on/off current ratio larger than 10⁸; both values are comparable to hydrogenated amorphous silicon devices. On the other hand, these and most other organic TFT's have an undesirably large subthreshold slope. We show here that the large subthreshold slope typically observed is not an intrinsic property of the organic semiconducting material and that devices with subthreshold slope similar to amorphous silicon devices are possible     

A theoretical model for the time varying current in organic electrochemical transistors in a dynamic regime

The dynamic interaction between cations and doped conductive polymer is at the basis of the working principles of organic electrochemical transistor devices. In this letter, we describe a theoretical model for the transport of saline ions in an electrolyte under the influence of an external voltage in a dynamic regime. We show how this scheme can be used to derive the time varying response and current generated by a conductive PEDOT:PSS polymer based OECT device interacting with those ions. The simulated output of the system displays a very high sensitivity on the parameters of the process including charge, size and concentration of the ions, and the frequency of operation of the device. The proposed model can be used to analyze the activity of an OECT device to derive the physical characteristics of individual species in a solution.     

A two-dimensional simulation of organic transistors

In this paper, we analyze the operation of organic thin-film transistors (TFT's) using two-dimensional (2-B) numerical simulation to: (1) validate the use of simple MOSFET theory to describe the above-threshold behavior; (2) clarify the subthreshold characteristics, and short-channel effects; and (3) illustrate the operation of organic bilayer devices. Our analysis clarifies a number of issues that can help in device design. We also point out differences between the material parameters used in Si-MOSFET and organic FET simulation, and discuss the circumstances under which a semiconductor device simulator can be used for the simulation of organic transistors     

Theory of organic field effect transistors

Field effect transistors with an organic material as active layer are at present essentially used to determine the mobilities in these materials. Until now, in analysing the measured current characteristics, only the simplest (Shockley) model has been used which accounts neither for this type of thin film transistor (TFT), which operates in depletion and accumulation, nor for the nature of the carriers. Starting from two-dimensional simulations for the analogous silicon TFT, we have developed an analytical model for the TFT that accounts for several peculiarities of the current characteristics of this type of transistor. In addition, a first modification has been developed which describes the situation when the charged states are polarons and bipolarons, as is the case in organic materials. Applications to published experimental current characteristics show that a general reanalysis is needed. Copyright © 1998 John Wiley & Sons, Ltd.     

High-performance bottom electrode organic thin-film transistors

Pentacene-based organic field effect transistors (FETs) exhibit enormous potential as active elements in a number of applications. One significant obstacle to commercial application remains: no completely lithographic process exists for forming high-performance devices. Processing constraints prevent electrodes from being lithographically patterned once the semiconductor is deposited, but depositing the electrodes before the semiconductor leads to low-performance transistors. By using self-assembled monolayers (SAMs) to change the surface energy of the metal electrodes and morphology of the pentacene subsequently grown on the electrodes, high-performance transistors may be formed using a process compatible with lithographic definition of the source and drain electrodes     

Gate-planarized organic polymer thin film transistors

We report in this paper the fabrication and characterization of a new gate-planarized organic polymer thin-film transistor (GP OP-TFT). We describe in detail the effects of the measurement procedure on the GP OP-TFT electrical characteristics and extracted parameters and show that it is extremely critical to carefully control the electrical measurement conditions to obtain accurate and meaningful results, before any material optimization is undertaken. We also describe the importance of normalization of electrical characteristics and extracted parameters for a proper comparison of different devices. Finally, we report and analyze the gate voltage and channel length dependence of the TFT field-effect mobility.     

Multicolor nanofiber based organic light-emitting transistors

Self-assembled, molecular crystalline nanofibers form microscale light-emitters for future nanophotonic applications. Such organic nanofibers exhibit many interesting optoelectronic properties, including polarized photo- and electroluminescence, waveguiding, and emission color tunability. Surface-grown nanofibers from two different molecules are implemented in an organic field-effect transistor platform by a double roll-printing scheme. Roll-printing multiple types of nanofibers onto the same device provides a fast and simple alternative to multilayer devices. The combination of nanofibers made from para-hexaphenylene and 5,5′-di-4-biphenyl-2,2′-bithiophene results in a nanofiber based organic light-emitting transistor (OLET) which emits both blue and green light. A comparison of measured electrical transport and electroluminescence (EL) properties results in a correlation between the threshold voltage for transport and the onset voltage for EL emission.     

Printing-induced improvements of organic thin-film transistors

To understand the observation of improved pentacene (Pn) thin-film transistor mobility in flexible printed devices, a method for performing electrical measurements of organic thin-film transistors (OTFT) during the process of transfer printing has been developed. Different sample configurations were designed to test two aspects of the printing process: (1) the formation of the source/drain contacts a Pn thin-film, and (2) the formation of the transfer printed Pn/dielectric interface. In situ measurements show that pressure-induced contacts of gold (Au) electrodes result in a factor of seven mobility improvement compared with evaporation of top Au electrodes on an otherwise identical device configuration. Annealing the laminated device up to 90 °C caused no further improvement, and heating above 90 °C degraded performance. The mobility of a transfer printed device with the rough, as-grown top surface of the Pn in contact with the dielectric was found to increase dramatically with subsequent annealing for a sample temperature up to 120 °C. This is attributed to annealing-induced structural changes in the Pn film at elevated temperatures, consistent with X-ray bulk measurements showing enhanced crystal morphology in transfer printed Pn thin-films.     

High-performance solution-processed organic transistors with electroless-plated electrodes

Electroless-plated gold and platinum films are used as source and drain electrodes in high-performance solution-processed organic field-effect transistors (OFETs), representing a promising large-area, near-room-temperature and vacuum-free technique to form low-resistance metal-to-semiconductor interfaces in ambient atmosphere. Developing non-displacement conditions using a Pt-colloidal catalyst for soft electroless plating, the electrodes are deposited on crystallized thin films of 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) without significant damage to the semiconductor material. The top-contact OFETs show remarkable performance, with a mobility of 6.0 cm² V^?1 s^?1. The method represents a practical fabrication technique to mass-produce circuitry arrays of nearly best-performing OFETs for the printed electronics industry.     

Printed organic transistors for ultra-low-cost RFID applications

Printed electronics provides a potential pathway toward the realization of ultra-low-cost radio frequency identification (RFID) tags for item-level tracking of consumer goods. Here, we report on our progress in developing materials and processes for the realization of printed transistors for low-cost RFID applications. Using inkjet printing of novel conductors, dielectrics, and organic semiconductors, we have realized printed transistors with mobilities >10/sup -1/cm/sup 2//V-s. AC performance of these devices is adequate for 135-kHz RFID, and, with further optimization, 13.56-MHz RFID appears to be within reach. We review the performance of these devices, and discuss optimization strategies for achieving the ultimate performance goals requisite for realizing ultra-low-cost printed RFID.     

Ambipolar organic transistors based on isoindigo derivatives

Structural and transistor properties of isoindigo derivatives are investigated. The unsubstituted isoindigo affords two polymorphs in addition to the reported brickwork structure; one has a stacking structure analogous to indigo, and another consists of nonplanar molecules. The unsubstituted isoindigo exhibits ambipolar transistor properties with the hole and electron mobilities more than 0.01 cm²/Vs, and 6.6′-diphenylisoindigo shows ambipolar transistor properties with the hole/electron mobilities of 0.037/0.027 cm²/Vs. Isoindigo derivatives with electron withdrawing groups show only electron transport, indicating that the lower limit of the HOMO level showing the hole transport is −5.7 eV.     

Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm²/Vs similar to that obtained from a SiO₂ gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.     

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.     

In-plane electroluminescence from microcavity organic light-emitting transistors

We report the observation of in-plane emission beneath the drain electrode in multilayer heterostructure organic light-emitting transistors (OLETs). A novel modification method for the interface between the hole transport layer and the emission layer has been proposed, which brought a great enhancement for the light power and external quantum efficiency. Further, distributed Bragg reflector was incorporated to the in-plane-emitted OLETs, which combined with the top thin layer of Au, forming a vertical microcavity. The electroluminescence spectra were significantly altered by the microcavity and much narrower linewidth was obtained. The results will help to develop high color purity and white OLETs with high performance, which would be useful for multifunctional displays.