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Organic static induction transistors, which have relatively short vertical channels, are attractive devices for their low operating voltage and high operating speed. However, a gate voltage larger than the Schottky barrier potential usually leads to a large gate leakage current and thus poor device performance. To limit the gate leakage current, we considered adding insulating layers around the gate electrode. The oxidization of aluminum during a physical vapor deposition process was used to form insulating layers around the gate electrode. The results demonstrate that by appending gate insulating layers, gate leakage currents can be effectively reduced and device characteristics, especially the on/off ratio, can be improved.

Grain morphology, crystal structure and transistor characteristics of pentacene films have been studied using molecular beam deposition under various conditions: growth temperature ranging from 5t o 80jC and growth rate from 0.15 to 2 nm/min. Grain morphologies observed with atomic force microscopy are categorized into five groups, which are summarized in a phase diagram. Above the growth temperature of 40 jC, many ‘recessed regions’ which extend independently of molecular steps are observed on the film surfaces. Carrier mobilities of the films estimated from transistor measurements are distributed from 110 5 to 0.20 cm 2 /Vs. A comparative study between the structural and electrical results indicates that the mobility does not explicitly depend either on grain size or on specific grain morphology. Lamellar grains associated with flat-lying molecules among differently oriented grains, gaps between grains and the recessed regions are considered to restrict the overall carrier mobility. D 2004 Elsevier B.V. All rights reserved.

The field-effect transistor (FET) characteristics of isolated copper phthalocyanine (CuPc) nano-crystals have been studied using AFM conductive probes as source electrodes.To extract the intrinsic carrier mobility of a single grain, the characteristics obtained were simulated by assuming a sub-exponential diode connected in series.The proposed model successfully reproduced most of the FET characteristics and the mobility was estimated to be 10 –10 cm yV s.On the other hand, characteristics with high y3 y 22 and low onyoff ratio were sequentially obtained using the same combination of CuPc grain and AFM probe.The characteristics of high onyoff ratio disagreed with the model near its off-state.The high on yoff ratio is explained by the dependence of the carrier injection rate on the gate voltage, which suppresses the off current. 2003 Elsevier Science B.V. All rights reserved.

We have fabricated organic light emitting transistors (OLET) combining static induction transistor with organic light emitting diode and investigated static and dynamic characteristics. The luminance of OLET is controlled by gate voltages as low as 1 V and excellent dynamic operation is obtained at 60 Hz. The results obtained here show that the OLET is a suitable element for flexible sheet displays.

Charge–Transfer (CT) complexes are stabilized by partial transfer of electric charges from donor molecules to acceptor molecules. The conductivity of the film of CT-complexes can be controlled if the degree of charge transfer is varied by applying an external field, for example an electric field. In this study, we fabricated field-effect transistors (FETs) using CT complex Langmuir–Blodgett (LB) films (TMPD-C n TCNQ), and have attempted to control the conductivity of the CT-complex layers by gate voltages. The temperature dependence of conductivity shows that semiconducting and metallic parts coexist in parallel between the electrodes.

Vertical type field effect transistors (FETs) are expected to be used for various organic devices because of their low-voltage, high-current and high-speed operation. We have fabricated lateral and vertical type FETs using copper-phthalocyanine evaporated films and measured the basic static and dynamic characteristics of these FETs. Although both transistors show field effect characteristics, the vertical type FET showed high-frequency and high-current characteristics under relatively low-voltage conditions as compared with those of lateral type FETs. These results demonstrate that the vertical FET operates as a static induction transistor and the short length between the source, drain and gate electrodes in the device structure improves the device characteristics.

We have fabricated planar bottom-contact organic thin-film transistors as a function of the thickness of the pentacene active layer. The highest mobility of the planar bottom-contact transistors is 0.47 cm 2 /Vs with only a 7 nm pentacene active layer. Our planar bottom-contact transistors show much higher mobility than conventional bottom-contact counterparts and even higher than the reported mobility values of top-contact counterparts for each thickness in the range from 2.5 to 10 nm. We find that spike at the edges of source and drain electrodes seriously deteriorates device performance.

Static induction transistors (SITs) using copper phthalocyanine films and Al Schottky gate electrode are fabricated and the basic electrical characteristics are investigated. The electrical characteristics show that the current flows from the source to drain electrodes is controlled by the Al gate voltage and a typical SIT operation with non-saturating properties is examined. Furthermore, a high power and high frequency operation as organic transistors is obtained.

We have investigated the effect of bias voltage applied to an electrically conductive substrate on the formation of adsorption Langmuir–Blodgett (LB) films. In fabricating the adsorption LB films, we used tetracyanoquinodimethane (TCNQ) with a long alkyl chain as a molecule for spreading monolayers on the subphase and tetramethyl- p -phenylenediamine (TMPD) as a material for adsorbing from the subphase to the monolayers. It is well known that TCNQ and TMPD are acceptor-like and donor-like molecules, respectively. In the visible absorption spectra and Fourier transform infrared reflection–absorption spectra of the adsorption LB films fabricated under several bias conditions, the peaks assigned to TCNQ show a clear dependence on the bias voltage. The electrical conductivity corresponding to the quantity of charge-transfer complex was also obtained. These results demonstrate that the bias-voltage LB technique enables us to control the formation of the charge-transfer complex.

Negative photoelectrical properties are observed in adsorption Langmuir–Blodgett films consisting of merocyanine dye and brilliant green under illumination using two lights. These peculiar properties are strongly dependent on the polarized direction of the two lights and their wavelengths corresponding to the absorption peaks of the two dyes. We have analyzed the photoelectric process between the two dyes, comparing the experimental results and the simulation using the model for photogenerated carriers via trap levels formed between the two dyes.

Langmuir-Blodgett (LB) films containing functional dyes were deposited directly onto optical fibers by the adsorption LB method. A merocyanine derivative having a long alkyl chain and rhodamine B were used as spreading and adsorbing molecules from the aqueous subphase, respectively. During the LB film deposition, absorption spectra from the optical interaction with the LB films at the core portion of the fiber were monitored using a multi-channel photo-detector system. Furthermore, the influence of the excitation light, which was guided into the fiber during the deposition, on the film deposition was investigated. The experimental results indicated that the in-situ optical measurement using optical fibers has high sensitivity and this is one of the promising methods to investigate the molecular interactions and the effect of the incident light on the LB film deposition.

IR and inelastic electron tunneling spectroscopy (IETS) techniques are employed to investigate adsorption Langmuir-Blodgett monolayers deposited on silicon substrate with a thin SiO2 film. Merocyanine dye (MC) and crystal violet (CV) are used as a spreading material and an adsorption dye from an aqueous subphase, respectively. IR and IETS data show that the carboxyl group in the MC molecule and the substituent group in the CV molecule are concerned with the adsorption mechanism.

The authors have investigated organic thin films fabricated by a bias-voltage applied Langmuir-Blodgett (LB) technique, which is based on the adsorption LB method. The bias voltage is applied between a conductive indium-tin-oxide coated glass substrate and a subphase through a platinum black counter electrode. Malachite green and 2-dodecyl-7,7,8,8-tetracyanoquinodimethane (C12TCNQ) are used as an adsorbing and a spreading molecule from the aqueous subphase, respectively. Ultraviolet-visible absorption spectra of the LB films demonstrate that the quantity of adsorbing and spreading molecules can be controlled by applying the bias voltage.

Malachite-green (MG) dye which does not contain a long alkyl chain can be adsorbed from aqueous solution onto Langmuir-Blodgett films consisting of arachidic acid (AR) only or a mixture of the acid and merocyanine (MC) dye. A special interaction exists between MC and MG molecules; MG molecules are more selectively adsorbed to MC molecules than to AR molecules. The adsorption Langmuir-Blodgett films show negative photoelectrical properties which are strongly related to the interaction between the two functional dyes.