The hysteresis mechanism is studied in pentacene organic thin-film transistors (OTFTs) with poly(4-vinyl phenol) (PVP) gate insulator by examining OTFTs with an oxide/PVP double layer gate insulator. The oxide thickness affects the direction of the hysteresis as well as its magnitude. This result can be explained on the basis of the charge injection and trapping mechanism rather than slow polarization or ion migration. The hysteresis occurs mainly due to the charges which could be injected from the gate electrode and trapped in the PVP. As the thickness of the oxide layer is increased, the gate charge injection is blocked and the effect of the charges from the channel increases.
Low hysteresis pentacene organic thin-film transistors (OTFTs) were fabricated using plasma-enhanced chemical vapor deposition SiO2/cross-linked poly(vinyl alcohol) gate dielectrics. The hysteresis was considerably reduced by optimizing the thickness of the SiO2 layer. The balancing of the electrons injected from the gate electrode into the gate dielectric and the trapped holes from the channel under the optimum condition is considered to be the mechanism by which the hysteresis is reduced. In addition, the optimized OTFTs with the hybrid gate dielectric show a high dielectric constant and reliable leakage characteristics due to the advantages afforded by each gate dielectric material. The device with W∕L=300∕5 shows a field-effect mobility of 0.12cm2∕Vs, a subthreshold slope of 0.4V/decade, and an on/off ratio of 2.3×106. The bias stress experiments conducted for the OTFTs confirmed the effect of the injected charges and hole traps on the threshold voltage shift.
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