Jakob Leise1,4, Jakob Pruefer1,4, Ghader Darbandy1, Aristeidis Nikolaou1,4, Michele Giorgio2, Mario Caironi2, Ute Zschieschang3, Hagen Klauk3, Alexander Kloes1, Benjamin Iñiguez4
and James W. Borchert5
Flexible megahertz organic transistors and the critical role of the device geometry on their dynamic performance
Journal of Applied Physics 130, 125501 (2021);
DOI: 10.1063/5.0062146
1NanoP, TH Mittelhessen University of Applied Sciences, Gießen 35390, Germany
2Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Milano 20133, Italy
3Max Planck Institute for Solid State Research, Stuttgart 70569, Germany
4DEEA, Uniersitat Rovira i Virgili, Tarragona 43007, Spain
5Georg August University of Goettingen, Goettingen 37077, Germany
Abstract: The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance.
Fig: Schematic cross-section of a top-contact (TC) organic TFT. Here, the semiconductor layer separates the source and drain contacts from the gate dielectric and thus from the gate-field-induced charge-carrier channel; hence, these transistors are also referred to as staggered TFTs. The overlap regions are assumed as a series connection of two capacitances. However, when the organic semiconductor (OSC) is operated in accumulation, the accumulation charges change the behavior of the series connection. The charge density at the source end of the channel is assumed to be found in the entire gate-to-source overlap region.
Acknowledgments: The authors thankfully acknowledge funding for this project from the German Federal Ministry of Education and Research (“SOMOFLEX,” No. 13FH015IX6) and EU H2020 RISE (“DOMINO,” No. 645760), and the German Research Foundation (DFG) under Grant Nos. KL 1042/9-2, KL 2223/6-1, and KL 2223/6-2 (SPP FFlexCom). The authors would like
