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Bias stress instability in top-contact pentacene thin film transistors was observed to be correlated not only to the channel but also to the metal/organic contact. The drain current decay under bias stress results from the combination of the contact resistance change and the threshold voltage shift in the channel. The contact resistance change is contact-metal dependent, though the corresponding channel threshold voltage shifts are similar. The results suggest that the time-dependent charge trapping into the deep trap states in both the contact and channel regions is responsible for the bias stress effect in organic thin film transistors.