2610 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 8, AUGUST 2011 Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors Part I: Long-Channel Devices Fabrizio Torricelli, Juliaan R. Meijboom, Edsger Smits, Ashutosh K. Tripathi, Matteo Ferroni, Stefania Federici, Gerwin H. Gelinck, Luigi Colalongo, Member, IEEE, Zsolt M. Kovacs-Vajna, Senior Member, IEEE, Dago de Leeuw, and Eugenio Cantatore, Member, IEEE Abstract—Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influ- ence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numer- ical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent Manuscript received January 5, 2011; revised April 6, 2011; accepted May 3, 2011. Date of publication June 16, 2011; date of current version July 22, 2011. The work of F. Torricelli and E. Cantatore was supported in part by the Dutch Technology Foundation, which is the applied science division of the Dutch Scientific Foundation, and in part by the Technology Programme of the Ministry of Economic Affairs. The review of this paper was arranged by Editor J. Kanicki. F. Torricelli was with the Department of Information Engineering, Uni- versity of Brescia, 25123 Brescia, Italy. He is now with the Department of Electrical Engineering, Eindhoven University of Technology, 5600 Eindhoven, The Netherlands (e-mail: fabrizio.torricelli@ing.unibs.it; F.Torricelli@tue.nl). J. R. Meijboom and D. de Leeuw are with the Philips Research Laboratories, 5656 Eindhoven, The Netherlands. E. Smits, A. K. Tripathi, and G. H. Gelinck are with the Holst Centre/ Netherlands Organization for Applied Scientific Research, 5656 Eindhoven, The Netherlands. M. Ferroni is with the Department of Physics and Chemistry for Materials and Engineering and National Research Council–National Institute for the Physics of Matter Sensor Laboratory, University of Brescia, 25133 Brescia, Italy. S. Federici is with the Chemistry for Technologies Laboratory and the Consortium for Science and Technology of Materials, University of Brescia, 25123 Brescia, Italy. L. Colalongo and Z. M. Kovacs-Vajna are with the Department of Informa- tion Engineering, University of Brescia, 25123 Brescia, Italy. E. Cantatore is with the Department of Electrical Engineering, Eindhoven University of Technology, 5600 Eindhoven, The Netherlands. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2155910 analysis based on experiments, numerical simulations, and analyt- ical modeling is provided. Index Terms—Field-effect mobility, multiple-trapping-and- release (MTR) transport, numerical simulation, physical-based analytical model, thin-film transistor (TFT), zinc oxide (ZnO). I. I NTRODUCTION T HE INTEREST in electronic devices based on solution- processable semiconducting materials is rapidly increas- ing. While the research in the area of organic materials and devices has been intensifying, a different class of semicon- ducting materials, i.e., metal–oxide–semiconductor (MOxS), is emerging as an alternative technology. Among the known MOxS, zinc oxide (ZnO) has attracted interest because of its nontoxicity, high electron mobility, and high transparency to visible light. Therefore, ZnO field-effect thin-film transistors (TFTs) were investigated for applications in see-through electronics [1]. Furthermore, the ZnO semi- conductor may be deposited with established industrial tech- niques such as sputtering [2], dip coating [3], spin coating [4], thermal evaporation [5], and SP [6]. Its applications range from large-area low-cost integrated circuits, pixel engines in active-matrix displays [7], chemical sensors [8], light-emitting diodes [9], [10], ultraviolet-sensitive photovoltaics [11], and transparent contacts for solar cells [12]. ZnO can be used in hybrid structures with organic materials to fabricate de- vices with higher performance, e.g., high-quality rectifying p-n junctions [13]. The ZnO-TFT structure has been investigated extensively, considering the effect of deposition parameters on the film growth and the transistor performance [2], [14], [15]. It was demonstrated that the ZnO-film morphology, transmittivity, and electrical properties strongly depend on the deposition techniques and processing conditions [16]–[18]. These papers investigate prevalently the correlation between the deposition conditions and the film structure in order to maximize the ZnO mobility. With respect to the study of the charge transport and the modeling of ZnO-TFTs, only few papers can be found in the literature. Hossain et al. [19] modeled a polycrystalline ZnO film as a series of grains with a trap distribution at the grain boundaries. They determined the potential profiles and estimated the trap-state densities at the grain boundaries by 0018-9383/$26.00 © 2011 IEEE