The instability of threshold voltage and mobility of pentacene thin film transistors using a poly(4-vinylphenol) gate dielectric have been investigated under constant bias stress. The mobility was very stable in vacuum by exhibiting 2% variation after 6 h stress even under the high gate bias stress of V-GS=-20 V. Meanwhile, we observe a negative shift of threshold voltage under stress in vacuum. This shift is attributed to charges trapped in deep electronic states in pentacene near the gate interface. We propose a model for the negative shift of the threshold voltage and extract the hole concentration, 4.5 x 10(11) cm(-2), needed to avoid the onset of stress effects, resulting in a design rule of the channel width to length ratio larger than 40. (C) 2007 Elsevier B.V. All rights reserved.