Organic Field-effect Transistor Structure Research Articles

Nonvolatile memory based on pentacene organic field-effect transistors with polystyrene para-substituted oligofluorene pendent moieties as polymer electrets

We report the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFET) using polystyrenepara-substituted with π-conjugated oligofluorenes (P(St-Fl)n (n = 1–3)) as chargeable polymer electrets. Effects of fluorene conjugated length on the surface structure and memory characteristics of pentacene OFET were investigated. Among these polymer electrets, the device with the P(St-Fl) exhibited the highest field-effect mobility of 0.47 cm2 V−1 s−1 due to the largest grain size of pentacene growth. The device with P(St-Fl)3 revealed the largest hysteresis window of 76 V due to it having the longest fluorene conjugation length among the studied electrets. The smallest difference of the HOMO energy level between pentacene and P(St-Fl)3 facilitated the charge transfer from pentacene to the polymer electret. The shifts on the transfer curves in both positive and negative directions could be reversibly controlled when applied an external gate bias of ±100 V for a short time (1 μs), indicating the fast trapping-detrapping ability of the polymer electrets. The devices showed excellent nonvolatile behaviors for bistable switching. The ON and OFF states were maintained over 104 s with the Ion/Ioff current ratio of 105–106. The write-read-erase-read (WRER) cycles could be operated over 100 cycles. This study suggested that surface characteristics, charge transport, and memory characteristics of pentacene-based OFET can be manipulated by polymer electrets with different pendent conjugation length.

Charge carrier extraction dynamics for organic field effect transistor structures

We present experimental data and a model for charge carrier extraction from the channel of a device structure resembling an organic field effect transistor. The initially accumulated channel is depleted by a sudden change of the gate voltage. The measured discharge current transient decreases either as a power law or exponentially if the final state of the channel is completely or partially depleted, respectively. The extraction process is modeled with a capacitor/resistor circuit with a time-dependent resistance that increases with decreasing channel carrier density. Analytical and numerical results are discussed and compared with the experimental data.

In situ Electrical Characterization of the Thickness Dependence of Organic Field-Effect Transistors with 1−20 Molecular Monolayer of Pentacene

Field-effect mobility (μ) of pentacene-based organic field-effect transistors (OFETs) is studied as a function of the number of molecular monolayer (ML) by in situ electrical characterization, which greatly improves the accuracy and reproducibility of electrical characteristics of OFETs. The hole μ of pentacene OFET with an average 1 ML (∼1.57 nm) thickness has been observed under a vacuum. The μ of pentacene OFET rapidly increases with increasing surface coverage in the region of film thickness less than saturation thickness (d0), which is about 3.2 ML for pentacene OFETs studied herein. We have observed that pentacene molecular layers beyond d0 have little contribution to the carrier transport in the semiconducting channel. The threshold voltage (VT) of pentacene OFETs has a variable thickness dependence having minimum ∼17 V at pentacene thickness around 3.2 ML. Similar d0 was verified for both drain current and on/off current ratio of pentacene FETs. The atomic force microscopy (AFM) images of the pentacene layer confirm the layer-plus-island (Stranski-Krastanov mode) pentacene growth mechanism on SiO2 substrate. Terrace-like stacking structure begins to be disernable around 3.2 ML of pentacene thin film. From AFM images, top few layers of pentacene terrace stacking become smaller in size after reaching the largest domain size around 10 ML. Our experimental results have demonstrated that the growth quality of the first few pentacene MLs on substrate strongly influences the morphology of the thicker film, packing structure, and electrical characteristics of OFET, including change-carrier mobility.

High quality shadow masks for top contact organic field effect transistors using deep reactive ion etching

In this paper, we demonstrate the fabrication of top-contact silicon shadow masks for organic field effect transistors (OFETs) using plasma deep reactive ion etching (DRIE). Over 50 parallel and interdigitated finger contact masks of 30 µm thickness have been created on a single silicon wafer, with lengths spanning from 6.5 to 60 µm and channel widths varying from 1000 to 50 000 µm. Unlike all other mask fabrication techniques to date, these shadow masks are inexpensive, reusable, have nanoscopically sharp edges and can be made with precise (nanoscale) control over various sizes and shapes. Because a large number of these masks can be made at the same time, they can act as a platform for researchers studying new organic materials and OFET structures. Top contact OFETs have been successfully fabricated using these masks with performances comparable if not superior to those made with standard lithography.

Transient effects controlling the charge carrier population of organic field effect transistor channels

We present device simulations exploring the effects of traps during transient processes in the conducting channel of organic field effect transistors (OFETs). The device structure explored resembles a typical organic thin-film transistor with one of the channel contacts removed. However, the channel length is much longer than in typical OFETs in order to increase the transit time. By measuring the displacement current in these long-channel capacitors, transient effects in the carrier transport in organic semiconductors may be studied. When carriers are injected into the device, a conducting channel is established while traps, which are initially empty, are being populated. The filling of the traps then modifies the transport characteristics of the injected charge carriers. In contrast, dc experiments as they are typically performed to characterize the transport properties of organic semiconductor channels investigate a steady state with traps partially filled. Numerical and approximate analytical models for the formation of the conducting channel and the resulting displacement current are discussed here. The temperature dependence of the effective mobility arising from the temperature dependence of the trap emission rate is explored, and calculated results are compared with experimental data. We show that displacement current measurements on OFET structures provide unique opportunities for the study of trap dynamics involving a wide range of time scales.

Organic Electronics: Relaxation Time Controlled Devices

In present work the charge transport through the metal–semiconductor–metal (MIM), organic field-effect transistor (OFET) and organic static induction transistor (OSIT) is discussed on the basis of dielectric physics. The propagation of injected carriers is evaluated by the transmission line model and discussed for two- and three-electrodes systems individually. On the basis of this analysis we find different carrier transport mechanism: (i) the drift in electric field for MIM structure, (ii) interface charge propagation on the organic semiconductor–gate insulator interface for the OFET structure, and (iii) drift or interface limited charge transport in the OSIT structure. We show that the charge transport mechanism depends on the relaxation time on the interface.

Observation of Carrier Behavior in Organic Field-Effect Transistors with Electroluminescence under AC Electric Field

The carrier behavior in a tetracene thin film under AC electric field was examined by electroluminescence (EL) using the structure of organic field-effect transistors (OFETs). A square wave voltage was applied to the source electrode, whereas the drain and gate electrodes were connected to the ground. Electrons and holes were injected from the Au source electrode alternately, and they recombined at the interface between the tetracene thin film and the source electrode. A series of EL pulses was observed in accordance with the applied AC voltage, i.e., a transient EL was observed in a half cycle of AC. This result indicates that alternating carrier injection causes EL. Furthermore, EL intensity increased with the frequency and amplitude of AC voltage. It was suggested that the amount of electrons and holes injected from the source electrode in a unit time is controlled by the frequency and amplitude of AC voltage.

Organic field-effect transistors based on tetrathiafulvalene derivatives

Abstract In recent years, tetrathiafulvalene (TTF) and its derivatives have been used as semiconducting materials for organic field-effect transistors (OFETs). In this review, we summarize the recent progress in the field of TTF-based OFETs. We introduce the structure and operation of OFETs, and focus on TTF derivatives used in OFETs. TTF derivatives used in OFETs can be divided into three parts by the semiconductor's morphology and the device fabrication technique: (1) TTF derivatives used for single-crystal OFETs, (2) TTF derivatives used for vacuum-deposited thin-film OFETs, and (3) TTF derivatives used for solution-processed thin-film OFETs. The single-crystal OFETs based on TTF derivatives were fabricated by drop-casting method and showed high performance, with the mobility up to 1.4 cm2/Vs. The vacuum-deposited thin-film OFETs based on TTF derivatives were well developed, some of which have shown high performance comparable to that of amorphous silicon, with good air-stability. Although the mobilities of most solution-processed OFETs based on TTF derivatives are limited at 10-2 cm2/Vs, the study on solution-processable TTF derivatives and their devices are promising, because of their low-cost, large-area-coverage virtues. The use of organic charge-transfer (OCT) compounds containing TTF or its derivatives in OFETs is also included in this review.

Conduction Mechanism Based Model of Organic Field Effect Transistor Structure

Carriers mobility model of olygomer and polymer semiconductor based OFET (Organic Field Effect Transistor) structures is presented in this paper. Starting from the conduction mechanism in the mentioned organic materials, a carrier mobility dependence on temperature, electric field and trap density μ(T,E,NT) was investigated, inspiring directly the current-voltage I(V) model of OFET structures. Subsequent simulations were also performed and the obtained results compared with the data available in the literature.

Semiconductors for organic transistors

Organic molecules/polymers with a π-conjugated (hetero)aromatic backbone are capable of transporting charge and interact efficiently with light. Therefore, these systems can act as semiconductors in opto-electronic devices similar to inorganic materials. However, organic chemistry offers tools for tailoring materials' functional properties via modifications of the molecular/monomeric units, opening new possibilities for inexpensive device manufacturing. This article reviews the fundamental aspects behind the structural design/realization of p- (hole transporting) and n-channel (electron-transporting) semiconductors for organic field-effect transistors (OFETs). An introduction to OFET principles and history, as well as of the state-of-the-art organic semiconductor structure and performance of OFETs is provided.

Radiotracer Diffusion Measurements of Noble Metal Atoms in Semiconducting Organic Films.

AbstractThe application of organic field effect transistors (OFETs) for large scale low-cost electronic devices has lead to intense research. Diindenoperylene (DIP) thin films on SiO2 are a prominent system due to their high structural out-of-plane order. While bottom contact OFET structures can be realized easily, preparation of top contacts might cause diffusion of metal atoms (typically Ag or Au) deep into the organic film changing the injection properties at the interface. These properties are of great importance for device fabrication. Therefore, only by understanding the diffusion behaviour of metals into the organic layer, formation of well defined interfaces and control of their properties will become possible. For a better understanding of the diffusion of noble metal atoms into crystalline organic films, a radiotracer technique has been used to obtain diffusion profiles for Ag and Au diffusion in crystalline DIP films. For Ag diffusion in DIP, the decrease in Ag concentration of four orders of magnitude within the first few nanometers indicates that most of the metal atoms remain near the surface while small amounts can penetrate deep into the thin film and can even accumulate at the interface between organic film and the silicon substrate. A comparison with diffusion profiles obtained for polymers indicates that the interplay between diffusion and immobilization by aggregation also determine the diffusion behaviour of metals in organic crystalline materials. Latest experiments support this interpretation of the diffusion profiles. Single atoms are highly mobile in the organic crystalline material due to the weak interaction between the metal and the organic material. Therefore, most of the single atoms that penetrate into the material do so during the initial phase of the deposition. When more and more atoms arrive at the surface, cluster formation sets in. Due to the high cohesive energy of the metal the atoms can not leave the cluster and become immobilized. After deposition of a closed surface layer no further metal diffusion should be observed. With the knowledge about the diffusion processes gained by the radiotracer measurements control of process parameters and development of barrier layers in sub-monolayer range should be possible.

High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds

We report high ON/OFF ratios for organic field-effect transistors (OFETs) based on amorphous thin films of spiro-linked compounds. Bottom-contact OFET structures are fabricated, using 2,2′,7,7′-tetra-( m-tolyl-phenylamino)-9,9′-spirobifluorene (Spiro-TPD) and 2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD) as active materials. The field-effect mobility of holes in Spiro-TPD and Spiro-TAD thin films is 7 × 10 −5 cm 2/Vs. We obtained ON/OFF ratios up to 3.6 × 10 6 with low OFF currents in the pA range. Time-dependent measurements show that the transistor characteristics do not change significantly over 9 months.

Ambipolar organic field-effect transistor based on an organic heterostructure

Ambipolar charge injection and transport are a prerequisite for a light-emitting organic fieldeffect transistor (OFET). Organic materials, however, typically show unipolar charge-carrier transport characteristics. Consequently, organic thin-film field-effect transistors based on a single material as active layer can typically either be operated as p- or as n-channel device. In this article we show that by using a heterostructure with pentacene as hole-transport and N,N′-Ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) as electron-transport material, ambipolar characteristics, i.e., simultaneous p- and n-channel formation, can be observed in a single device. An OFET structure is investigated in which electrons and holes are injected from Mg top and Au bottom contacts into the PTCDI-C13H27 and pentacene layers, respectively. Our device exhibits electron and hole mobilities of 3×10−3 and 1×10−4 cm2/V s, respectively. This device architecture serves as a model structure for ambipolar field-effect transistors, which are a prerequisite for light-emitting field-effect transistors.

Structures of polymer field-effect transistor: Experimental and numerical analyses

We compare two basic organic field-effect transistor structures both experimentally and theoretically. By using time-resolved analysis, we gain insight into the mechanisms affecting the performance of these structures. Using a two-dimensional numerical model, we focus on the top contact structure and analyze the difference between the two structures.