Origin of bias stress induced instability of contact resistance in organic thin film transistors

We report a study on the contact resistance instability induced by the bias stress in staggered pentacene thin film transistors, combining the bias stress measurements with the transfer line method. The contact resistance is increasing with the stress time, and two device parameters are found to contribute to this contact resistance instability: one is the threshold voltage increase due to the charge trapping in the charge accumulation layer; the other is the effective contact length increase due to the charge trapping in the pentacene bulk in the contact region. The gold contact shows lower contact resistance stability compared with the copper contact, which is ascribed to higher density of the deep trap states at the gold contact. This work suggests that the time-dependent charge trapping is responsible for the bias stress effect in organic thin film transistors. (C) 2011 Elsevier B.V. All rights reserved.