To clarify the carrier transport mechanism of pentacene field-effect transistors (FETs), the FET characteristics was examined in the region where the drain-source voltage Vds is lower than the saturated voltage. The Ids-Vds characteristics shows the ohmic behavior in the low voltage region, whereas it shows the characteristics explained by the Maxwell-Wagner model. This result clearly indicates that carrier injection from source makes a significant contribution to the carrier transport. It was also shown that the change of Ids-Vds characteristics from the ohmic behavior to the Maxwell-Wagner behavior is similar with the I-V characteristics change from the ohmic to the space charge limited current behavior observed in metal-organic film-metal junctions, including metal-pentacene-metal.

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging measurement revealed quantitatively the potential drop at the electrode contact of pentacene field effect transistors (FET). An activation of the SH signal at the edge of Ag-source electrode indicates the presence of large potential drop at pentacene-Ag contact during device operation, whereas negligible potential drop was observed at pentacene-Au contact. These findings agree with the injection characteristics of electrodes owing to the relationship between the work function of the metal and the HOMO level of pentacene.