Ryosuke Tamura

Probing of the electric field distribution in organic field effect transistor channel by microscopic second-harmonic generation

Electric field distribution in channel of pentacene field effect transistor (FET) was successfully probed by microscopic optical second-harmonic generation (SHG) observation. Microspot SHG signals were acquired at various points in the channel with scanning a spot position along source-drain direction. For the FET at off state, enhanced SHG signal was observed, indicating the Laplace field formation reflecting the device geometry. This clearly supports the insulating nature of pentacene layer at off state. After turning on the FET, SHG profile changed drastically, indicating change in the field distribution by the space charge formation in the channel due to the carrier injection.

Maxwell-wagner model analysis for the capacitance-voltage characteristics of pentacene field effect transistor

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of pentacene field effect transistors (FETs) were examined to clarify channel formation in conjunction with the UV/ozone treatment of the source and drain metal (Au) electrodes. Analyzing the I–V and C–V characteristics of FETs using the Maxwell–Wagner model showed that the main charge carriers in the FET channel are holes injected from the source, and that a pentacene FET with a UV/ozone-treated substrate shows a high effective mobility owing to holes smoothly injected into the FET channel. Furthermore, the pentacene film thickness dependence on FET characteristics showed that a channel sheet with a thickness less than 20 nm is formed at the pentacene/SiO2 interface. Finally, the employment of the Maxwell–Wagner model for the analysis of the C–V characteristics was shown to give a good approximation even when taking into account the presence of a charge sheet at the interface.

Modulation in optical second harmonic generation signal from channel of pentacene field effect transistors during device operation

The channel formation process of the pentacene field effect transistor (FET) was shown by the optical second harmonic generation (SHG) ascribed to the electric field induced SHG. The SHG signal probed successfully and nondestructively the off and on states. The enhancement of the SHG signal at the off state was observed with applying the source-drain voltage in the absence of the gate voltage, whereas it remarkably decayed with applying the gate voltage, indicating the channel formation. At the channel formation, holes injected from the source electrode changed the potential profile in pentacene film at the off state and the SHG signal was suppressed.

Analysis of hysteresis behavior of pentacene field effect transistor characteristics with capacitance-voltage and optical second harmonic generation measurements

Analyzing pentacene field effect transistors (FETs) with Au source and drain electrodes as Maxwell-Wagner effect elements, electron and hole injection from the Au electrodes into the FET channel were examined using capacitance-voltage (C−V) and optical second harmonic generation (SHG) measurements. The C−V characteristics show a hysteresis behavior that depends on gate-source (drain) stress biasing, Vgs(Vgd). Charge carriers forming the conducting channel of pentacene FET are mainly holes injected from Au electrodes. Results suggest that this hysteresis behavior is attributable to carriers trapped in the FET channel injected from the Au electrodes, and that hole injection is suppressed after Vgs 0. To further clarify the carrier injection mechanism for a different stress biasing condition, the modulation of the electric field along the FET channel by injected carriers was examined using SHG measurements. At the on state, the SHG signal was well diminished...

Surface morphology and electrical transport properties of polydiacetylene-based organic field-effect transistors

Bottom-contact polymerized 10,12-tricosadiynoic acid (PTDA) field-effect transistors (FETs) were fabricated successfully by the Langmuir–Blodgett (LB) method. The surface morphology of PTDA LB films deposited on a FET substrate was imaged by atomic force microscopy (AFM). The current–voltage (I–V) characteristics of the bottom-contact PTDA FETs were measured in detail. The equation of the FET characteristics was derived with consideration of the carrier transport mechanism of holes conveyed along the channel, and the effective mobility was evaluated. To clarify the carrier injection and succeeding carrier accumulation and carrier transport from a source electrode to a drain electrode across PTDA, a Maxwell–Wagner model was employed to analyze the I–V characteristics of the bottom-contact PTDA FETs. Finally, to further clarify the carrier injection, we measured the capacitance–frequency (C–F) characteristics of the bottom-contact PTDA FETs and the surface potential formed across PTDA LB films on a Au electrode.

Analysis of Threshold Voltage Shift of Pentacene Field Effect Transistor Based on a Maxwell-Wagner Effect

A pentacene field-effect transistor (FET) with ferroelectric gate insulator (PVDF-TeFE) is analyzed as a Maxwell–Wagner effect element. The amount of carriers injected from the source electrode and subsequently accumulated at the pentacene FET channel changes depending on the spontaneous polarization of the gate insulator. The relationship between the spontaneous polarization and the threshold voltage shift is analyzed, and the hysteresis behavior observed in our pentacene FET with a PVDF-TeFE gate insulator is discussed in terms of carrier injection from the source electrode into the FET channel.

Investigation of pentacene field-effect transistor operation by optical second-harmonic generation

Optical second-harmonic generation (SHG) measurement was performed to monitor the operation of an organic field-effect transistor (OFET). An SHG signal could probe the off and on states of the OFET nondestructively. Injected and accumulated charges were also successfully detected by monitoring the SHG signal. SHG process was activated by the external field applied to the OFET at the off state. At the on state of the OFET, accumulated charges formed a space charge field and changed the Laplace field formed at the off state to Poissons electric field. This leads to the decrease in SH intensity strongly activated at the off state. On/off current ratio affected the change in SH intensity with the on/off switching of the OFET. A high on/off ratio resulted in a large change in SH intensity. The gate voltage dependence of SH intensity was successfully explained by taking into account the change in Laplace field and accumulated charges on the basis of a Maxwell–Wagner model.

Optical Second-Harmonic Generation Measurements of Hole Carrier Injection and Trapping in Pentacene Field-Effect Transistor

In order to further clarify the carrier injection into pentacene field-effect transistors (FETs), we focused on the hysteresis behavior of the current–voltage (I–V) and capacitance–voltage (C–V) characteristics, and optical second-harmonic generation (SHG) measurement was employed. It was found that in the on state of pentacene FET, the SHG intensity decreases because of hole injection from the source electrode as well as hole trapping in pentacene. The SHG experiment at the off state of the FET shows that electron injection into pentacene from source and drain electrodes is also probable.

Study of Electron Injection of Pentacene Field Effect Transistor with Au Electrodes by C-V and SHG Measurements

Using pentacene field effect transistors (FETs) with Au source and drain electrodes, electron injection from the Au electrodes into the pentacene was investigated. The capacitance-voltage (C-V) and optical second harmonic generation (SHG) measurements were employed. Electron injection from the Au electrodes was suggested by the hysteresis behavior with the C-V characteristics and slowly decaying SHG signal under DC biasing, A mechanism of hole-injection assisted by trapped electrons is proposed. To confirm electron injection process, light-emitting behavior under the application of AC applied voltage was observed.

Analysis of charge accumulation in pentacene field effect transistor with ferroelectric gate insulator on the basis of Maxwell-Wagner model

Since the discovery of high mobility organic semiconductor materials, many experimental and theoretical efforts have been devoted to investigate organic field effect-transistors (OFETs). As a result, the performance of OFETs has been improved and many important aspects of OFETs’ operation have been revealed. For example, it was found that carriers injected from the source electrode give a significant contribution to the operation of OFETs [1-3]. However, there are still many problems to be solved theoretically and experimentally. Among them are the control of threshold voltage of OFETs, and the analysis of the hysteresis behavior of the FET characteristics caused by carrier trapping, etc [4]. In order to solve these problems, we have to understand the mechanism of charge accumulation at the OFETs channel, and then to establish a method to control it. In the present study, we focus on the effect of the spontaneous polarization of gate insulator, which modulates the amount of accumulated charges at the OFETs’ channel. Then, the characteristics of pentacene field effect transistor (FET) with a ferroelectric gate insulator is examined and analyzed on the basis of Maxwell-Wagner model for characterizing OFETs [4,5].