Probing a two-step channel formation process in injection-type pentacene field-effect transistors by time-resolved electric-field-induced optical second-harmonic generation measurement

► We directly visualized channel formation process of pentacene FETs. ► Poly-4-vinylphenol and polyimide was used as a gate-insulator. ► A two-step channel formation process was observed. ► A local electric field at gate-insulator interface governed the two-step process.