Synthetic Metals 156 (2006) 1305–1315 Modeling electrical characteristics of thin-film field-effect transistors I. Trap-free materials P. Stallinga ∗ , H.L. Gomes Universidade do Algarve, FCT, Campus de Gambelas, Faro, Portugal Received 23 June 2006; accepted 20 September 2006 Available online 13 November 2006 Abstract A new analytical model is developed for thin-film field-effect transistors (TFTs). The active layer of the devices is considered purely two- dimensional. In the first part, the basic model is developed for intrinsic materials. It is demonstrated that it accurately describes the electrical characteristics and elucidates on the physical meaning of the device and material parameters, such as threshold voltage and sub-threshold current. It also clarifies the nature of so-called contact effects, often used in literature to explain non-linear I–V curves. Furthermore, ambipolar devices are treated. © 2006 Elsevier B.V. All rights reserved. Keywords: Thin-film field effect transistors; Amorphous silicon; Organic semiconductors; Sexithiophene; Two-dimensional electronics; Contact effects 1. Introduction In spite of the rapid growth of interest in organic elec- tronic materials, the ever-increasing quality of the materials and devices, and the resulting unique devices [1–3] – includ- ing ambipolar devices needed for logic circuits [4] – the debate as to the processes governing and limiting charge conduction is not yet settled. Often, the carrier mobility is bias dependent [5–9], which seems difficult to explain in conventional theo- ries. Another example is the discussion between hopping and band conduction. Apart from this, there is the question of the workings of the device. It is common practice to fully apply the conventional inversion-channel metal-oxide-semiconductor field-effect transistors (MOS-FET) model [10] to the thin-film field-effect transistors (TFT) [11]. This is surprising considering that inversion has not been observed in organic transistors. The majority of the models proposed in the literature are based on the three-dimensional model developed for inorganic MOS-FETs. The Thiais group has added a trap model, known as multi-trap- and-release (MTR) model to simulate the experimental I–V and ∗ Corresponding author. Tel.: +351 969541198; fax: +351 289800030. E-mail address: pjotr@ualg.pt (P. Stallinga). transfer curves [11], while the Bell-Labs group has proposed a model in which a constant mobility is assumed in a trap-free material, but with the concept of contact resistance [12]. In the current work we first derive a simple model for TFTs that can adequately explain the basic I–V and transfer curves. The model is found by removing from existing theories everything that is not needed to explain the TFT behavior. A remarkably simple and yet fully functional theory emerges. Then we introduce per- turbations to the model, such as the effects of the contacts and ambipolar devices. In the second part traps are added to the sys- tem and it is easily shown how they explain the temperature and bias dependence of current and mobility and transient behavior. The device described is a p-channel FET, with organic mate- rials in mind, but the model is equally applicable to n-channel FETs and other materials, such as (amorphous) silicon, with the adequate changes of signs and symbols. 2. Background Fig. 1 shows a cross-section of a thin-film FET with the nomenclature used in the current work. The device consists of a conductor called the gate (made of metal or a highly doped semi- conductor) an insulating layer (which we will call the oxide layer 0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.09.015