Influence of distributed trap states on the characteristics of top and bottom contact organic field-effect transistors
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S. Scheinert Ilmenau Technical University, Institute of Solid State Electronics and Center of Micro- and Nanotechnologies, PF 100565, D-98684 Ilmenau, Germany (Received 4 November 2003; accepted 6 February 2004)
Numerical simulations of organic field-effect transistors (OFET) of bottom and top contact (BOC, TOC) design with different source/drain contacts were carried out considering an exponential distribution of trap states in the gap of the active layer (a-Si model). For ohmic contacts, the current-voltage characteristics are similar to the trap-free case and there is not much difference between the two designs. However, the currents are lower due to immobile trapped charges, the threshold voltage is shifted, and the inverse subthreshold slope increases due to trap recharging. An analytical approximation for the effective mobility deviates from the simulation up to 20%. For low source/drain work function, there occur particular dependencies of the current on the gate voltage for the two designs, which are explained with the internal concentration and field profiles. A series resistance between source and channel causes in the TOC structure an abrupt transition from the gate voltage independent active region into saturation. In the BOC case, the reverse-biased Schottky-type source contact dominates the current. Through simulation of measured characteristics of prepared OFETs based on a modified poly-(phenylene-vinylene), the observed hysteresis is analyzed.
I. INTRODUCTION
During recent years, there has been an increasing interest in organic field effect transistors (OFETs). Prospects of commercial applications arise from successful experimental demonstrations for their usage in active matrix displays,1,2 smart cards, smart labels, anti-theft stickers, or electronic water-marks. Also logic circuits based on OFET technology have been realized.3–7 Originally, OFETs were prepared to study the charge transport in organic semiconductors and they are still used for this purpose. For both improving the device performance for applications in circuits and the study of transport properties, measured current-voltage characteristics have to be analyzed. Until now, this has been done mainly with the simple Shockley-model (e.g., Refs. 8–11). Thereby, mobilities are extracted from the characteristics with an error up to 50%.12 Organic semiconductors suited for transistors are usually unintentionally p-doped and inversion can be realized only under special conditions which a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0265 2014
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 7, Jul 2004 Downloaded: 15 Mar 2015
have been analyzed in Refs. 13 and 14. Actually, for OFETs the thin-film transistor (TFT) design dominates, the on-state is realized with an accumulation channel and the off-state requires depletion of the layer. An analytical model for the corresponding modification of the Shockley model has been derived in Refs. 15 and 16. It has been shown also that
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