(Invited) Compact Modeling of I-V and C-V Characteristics in OTFTs from 125K to 350K

Abstract

Organic TFTs (OTFs) are very promising devices for flexible, wearable and printed electronics. Nowadays OTFT performances are already comparable to a-Si:H TFTs. In order to design OTFT circuits, physically-based compact models for the OTFT DC, AC and transient characteristics are needed. This requires a correct understanding of the physical effects affecting OTFT behavior. Despite a number of compact I-V models have been presented, only a few C-V models have been published [1-4]. Besides, very few models have considered the temperature dependence, even though temperature analysis is instrumental to identify relevant physical effects. Moreover, most of those temperature studies have been carried out only for I-V characteristics at high temperatures [5] and the very few analysis at low temperatures have not targeted all parameters [6]. In this work, we present a complete study of both current and gate capacitance in OTFTs for temperatures ranging from 150K to 350K. Parameters were extracted using a unified model and extraction method specific to OTFTs, based on assuming an exponential DOS and variable range hopping [7-8]. The dependences of model parameters with the temperature are analyzed. In order to carry out the temperature study of C-V characteristics, we improved an analytical capacitance model [1, 3] for OTFTs in order to be valid from the depletion to the accumulation regime with continuous and smooth regime transitions as well as for the first order gate capacitance derivative. The key parameters used in the capacitance model are the same as those of I-V characteristics and were extracted from them [4, 7]. Furthermore, we show that from the first order gate capacitance model derivative, we can identify the different operating regimes and obtain the threshold voltage () and band-flat voltage ( at different temperatures. We performed measurements on small-molecule OTFTs manufactured in CEA-Liten (Grenoble- France). The OTFTs under study were top gate bottom contact multifingers with and. Our model shows good agreement with the experimental I-V (Fig. 1) and C-V (Fig. 2) characteristics, and the first order capacitance model derivative (Fig. 3) from for temperatures ranging from 120K to 350K, and for frequencies up to 10 kHz. [1] Castro-Carranza, A., et al., (2012). Organic thin-film transistor bias-dependent capacitance compact model in accumulation regime. IET circuits, devices & systems, 6(2), 130-135 [2] Li, L., Marien, et al.,, “Compact Model for Organic Thin-Film Transistors” IEEE Electr Dev Let, vol. 31, no. 3, pp. 210-212, Mar. 2010. [3] Fadlallah, M., et al. “Modeling and characterization of organic thin film transistors for circuit design” J. Appl. Phys, vol. 99, no.104504, May 2006. [4] Estrada, M., et al.,(2013). Frequency and voltage dependence of the capacitance of MIS structures fabricated with polymeric materials. IEEE Transactions on Electron Devices, 60(6), 2057-2063. [5] Estrada, et al., (2010). Modeling the behavior of charge carrier mobility with temperature in thin-film polymeric transistors. Microelectronic Engineering, 87(12), 2565-2570. [6] Haddad, C., et al., Organic Electronics (2018), https://doi.org/10.1016/j.orgel.2018.06.037[7] Estrada, M., at al., (2005). Accurate modeling and parameter extraction method for organic TFTs. Solid-state electronics, 49(6), 1009-1016. [8] Estrada, M.,et al., . (2008). Mobility model for compact device modeling of OTFTs made with different materials. Solid-State Electronics, 52(5), 787-794. Figure 1