A ring oscillator consisting of pentacene thin-film transistors with controlled threshold voltages

Pentacene thin-film transistors (TFTs) with controlled threshold voltages have been applied to a ring oscillator consisting of enhancement/depletion inverters for evaluation of the dynamic characteristics. The threshold voltage control was demonstrated by using oxygen plasma treatment to the SiO₂ gate dielectric prepared by rf sputtering. The surface roughness of the SiO₂ gate dielectric depended on the sputtering condition. The use of flat SiO₂ gate dielectrics contributed to the improvement of the field-effect mobilities in pentacene TFTs. As a result, the ring oscillator operated at supply voltages of 15-25 V. The oscillation frequency was consistent with the result of circuit simulation for the ring oscillator.