The performance of a pentacene based organic thin film transistor (OTFT) with polymethylmethacrylate as a dielectric insulator and indium tin oxide based electrical gate is investigated. On the one hand, we showed that the threshold voltage increases with gate voltage, and on the other hand that it decreases with drain voltage. Thus, we noticed that the onset voltage shifts toward positive voltage values with the drain voltage increase. In addition, threshold-onset differential voltage (TODV) is proposed as an original approach to estimate an averaged carrier density in pentacene. Indeed, a value of about 4.5 × 1016 cm−3 is reached at relatively high gate voltage of −50 V; this value is in good agreement with that reported in literature with other technique measurements. However, at a low applied gate voltage, the averaged pentacene carrier density remains two orders of magnitude lower; it is of about 2.8 × 1014 cm−3 and remains similar to that obtained from space charge limited current approach for low applied bias voltage of about 2.2 × 1014 cm−3. Furthermore, high IOn/IOff and IOn/IOnset current ratios of 5 × 106 and 7.5 × 107 are reported for lower drain voltage, respectively. The investigated OTFTs also showed good electrical performance including carrier mobility increasing with gate voltage; mobility values of 4.5 × 10−2 cm2 V−1 s−1 and of 4.25 × 10−2 cm2 V−1 s−1 are reached for linear and saturation regimes, respectively. These results remain enough interesting since current modulation ratio exceeds a value of 107 that is a quite important requirement than high mobility for some particular logic gate applications.
Visualization of trapped charges being ejected from organic thin-film transistor channels by Kelvin-probe force microscopy during gate voltage sweeps