Eunju Lim

Analysis of pentacene field effect transistor as a Maxwell-Wagner effect element

The pentacene field effect transistor (FET) is analyzed as a Maxwell-Wagner effect element. As a result of the Maxwell-Wagner effect, carriers injected from source electrode are accumulated at the interface between pentacene and SiO2-gate insulator. They are then conveyed along the FET channel by the electric field formed between source and drain electrodes. The drain current Ids shows characteristic behavior depending on the force of the electric field. The transit time and charging time of injected carriers are key parameters to specify FET characteristics ruled by the Maxwell-Wagner effect. Results also show that our pentacene FET characteristics are well explained based on the present theoretical analysis.

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 carrier injection into a pentacene field effect transistor by optical second harmonic generation measurements

Optical second harmonic generation (SHG) measurements were used to clarify carrier injection into pentacene field effect transistors (FETs) with Au source and drain electrodes. The electric field was probed along the pentacene FET channel. Results showed that the SHG was enhanced depending on biasing voltages. In the off state of FET (Vgs>−30 V), the SHG was enhanced in proportion to the Laplace electric field formed in FET. On the other hand, in the on state (Vgs<−30 V), the enhanced SHG in the off state was diminished because of the hole injection from source electrode. Interestingly, in the off state of FET, electron injection from source and drain electrodes was also suggested. Furthermore, this electron injection was eliminated by overcoating the pentacene film by spin coating a polystyrene film. Finally, results show that our FET analysis based on a Maxwell–Wagner model is acceptable.

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...

Probing of electric field in pentacene using microscopic optical second harmonic generation

The electric field distribution in a pentacene field effect transistor (FET) was examined by using microscopic optical second harmonic generation (SHG) measurements. Results showed that the enhanced SHG signal changes depending on biasing conditions. The SHG profile was estimated by analyzing a Laplace field formed in organic FET (OFET) using conformal mapping and other methods. Results showed that the observed SHG profile is in good agreement with the obtained profile, indicating that our microscopic SHG data describe the electric field distribution in OFETs with high accuracy. The decrease in the SHG intensity was also discussed based on a Poisson field formation, caused by carriers injected from a source electrode. These results show that the SHG technique is effective for probing of electric fields formed in organic materials.

Optical second harmonic generation imaging for visualizing in-plane electric field distribution

The electric field distribution in electronic devices, particularly in the organic devices, was visualized by the optical second harmonic generation (SHG) imaging technique on the basis of electric field induced SHG (EFISHG). Two-dimensional SHG images from organic field effect transistor using pentacene were taken with a cooled CCD camera, and the SHG images showed the electric field was successfully visualized with a resolution of 1 mum. The SHG imaging method provides us a novel technique for visualizing the electric field distribution in actual devices under device operation.

Analysis of pentacene field-effect transistor with contact resistance as an element of a Maxwell-Wagner effect system

In organic field-effect transistors (OFETs), the carrier injection from the source and drain electrodes depends on the contact resistance. Results of this study show, based on a dielectric physics analysis of a pentacene OFET as a Maxwell–Wagner (MW) effect element with the contact resistance, that the contact resistance leads to an increase in the time required for charge accumulation at the interface. Considering that background, the carrier injection, transport, and accumulation mechanisms were investigated. Capacitance frequency and time-resolved microscopic optical second harmonic generation (SHG) measurements were discussed, taking into account the usefulness of the MW model analysis, and to clarify the effect of contact resistance. Our experimental results corroborate our analysis, based on the carrier mechanism in OFETs, was supported by optical SHG measurements.

Direct observation of trapped carriers in polydiacetylene films by optical second harmonic generation

Trapped carriers in polydiacetylene (PDA) films were directly observed by the electric field induced second harmonic generation (EFISHG) using field effect transistor (FET) structure. Response of EFISHG signal from PDA-FET with applying voltage depended strongly on the polarity of gate voltage. For negative bias, which promotes hole injection from source electrode, EFISHG signal was not observed during bias application, whereas it was enhanced after turning off the bias. Electric field formed by trapped holes in PDA activated the EFISHG signal for the negative bias condition.