Performance improvement of scaled-down top-contact OTFTs by two-step-deposition of pentacene

When scaling down to the channel length (L_c) of 1.8 μm using a membrane shadow mask, top-contact pentacene thin-film transistors (TFTs) show that grain size dependency on the anomalous leakage current becomes conspicuous as L_c is comparable to the grain size. For scaled-down OTFTs with large and small grain, the obvious difference of off-current in the depletion regime can be attributed to various reasons such as pentacene conductivity, parasitic resistance, locally ill-defined source/drain edge, and Au interdiffusion. To improve mobility as well as I_on/I_off ratio for scaled-down OTFTs, two-step-deposition (TSD) technique that enables us to control the channel conductivity in the depletion and accumulation regime as well as to improve the film continuity was proposed. To the best of our knowledge, the I_on/I_off ratio of 10⁷ and the mobility of 0.20 cm²/V·s for OTFTs with L_c of 1.8 μm deposited by using the TSD technique was one of the best results in the literature.