Studying transient carrier behaviors in pentacene field effect transistors using visualized electric field migration

Time-resolved microscopic optical second harmonic generation (TRM-SHG) imaging was used to study the transient carrier behaviors in pentacene field effect transistor (FET) with Au and Ag electrodes on the basis of the migration of the electric field profile. We found that the time evolution of the electric field distribution in the pentacene FET channel was quite different between the devices using Ag and Au electrodes: enhanced SHG signal representing insufficient carrier accumulation decayed rapidly at the edge of the Au source electrode, while the signal decayed but remained at the edge of the Ag electrode. Interestingly, the peaks of the SHG signal depicting the edge of the carrier sheet in the channel region in both cases migrated in a way of the square root of time. A simple model was proposed to uniformly account for the experimental observations, and a numerical simulation on the transient carrier behaviors with the electric field migration using two-dimensional coupled diffusion-drift and the Poisson equations well-supported this model. Particularly, the results showed that the evaluated carrier mobility in the FET channel based on this model was independent of the metal electrode, indicating identical carrier transport in the bulk between the devices using Au and Ag electrodes.