Engineering density of semiconductor-dielectric interface states to modulate threshold voltage in OFETs

Threshold-voltage control is critical to the further development of pentacene organic field-effect transistors (OFETs). In this paper, we demonstrate that the threshold voltage can be tuned through chemical treatment of the gate dielectric layer. We show that oxygen plasma treatment of an organic polymer gate dielectric, parylene, introduces traps at the semiconductor-dielectric interface that strongly affect the OFET performance. Atomic force microscopy, optical microscopy using crossed-polarizers, and current-voltage and capacitance-voltage characterization were performed on treated and untreated devices. A model is presented to account for the effects of trap-introduced charges, both 1) fixed charges (2.0×10^-6 C/cm²) that shift the threshold voltage from -17 to +116 V and 2) mobile charges (1.1×10^-6 C/cm²) that increase the parasitic bulk conductivity. This technique offers a potential method of tuning threshold voltage at the process level.