Tuning the Threshold Voltage in Organic Field Effect Transistors by Space Charge Polarization of Gate Dielectric
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1154-B10-09
Tuning the Threshold Voltage in Organic Field Effect Transistors by Space Charge Polarization of Gate Dielectric Heisuke Sakai, Koudai Konno and Hideyuki Murata School of Materials Science, Japan Advanced Institute of Science and Technology, Asahidai, Ishikawa 923-1292, Japan
ABSTRACT We demonstrate a tunable threshold voltage in an organic field effect transistor (OFET) using ion-dispersed gate dielectric. Application of external electric field (Vex) to the gate dielectrics causes the dispersed ions in the gate dielectric to migrate by electrophoresis and form space charge polarization. The threshold voltage (Vth) decreases from -11.3 V to -6.1 V. The shift direction of Vth is easily tuned by the polarity of the external voltage. These shifts are attributed to the ion migration in the ion-dispersed gate dielectric. The UV-VIS differential absorption spectra of the OFETs indicate that the active layer is doped by the migrated anions. This result indicates the active layer was charged not only electrostatically but also electrochemically in OFET without buffer layer. By inserting a buffer layer between the active layer and the ion-dispersed dielectric, the reaction was effectively prevented. INTRODUCTION In recent years, organic field-effect transistors (OFETs) using organic semiconductors have attracted much research interest due to their unique advantages, which include a variety of molecular designs, light weight, low cost of fabrication and mechanical flexibility. In particular, research interest into gate dielectrics is recently increasing, since the choice of the gate dielectrics directly affects the electric properties of OFETs [1,2]. Recently we have reported the effect of gate dielectric polarization on the electric characteristics of OFETs, where the polarization was achieved by the alignment of permanent dipoles in polymer chains [3]. The polarization of gate dielectrics induced the mobile charge carrier in the semiconductor layer at the semiconductor-dielectric interface and as a result, Vth shifted toward a lower voltage. By using polarized gate dielectrics, Vth of OFETs was decreased by 6 V compared to that of OFETs using non-polarized gate dielectrics. However, the decrease of Vth was limited by the small amount of polarization, a limitation which was caused by the difficulty of aligning permanent dipoles in the solid dielectric film. For a further decrease of Vth, novel approaches will be necessary to realize a higher magnitude of polarization in the gate dielectrics. Ion migration in polymer gate dielectric is one of the candidates to realize a higher magnitude of polarization of gate dielectric, because ions in a matrix polymer can easily migrate due to the influence of an external electric field and form a space charge polarization [4,5]. This polarization is easily tunable by using the number of dispersed ions and the ion migration distance. The ion migration distance can be controlled by the time an external electric field is applied. In this study, we report that the reduction of V
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