Impact of film thickness of organic semiconductor on off-state current of organic thin film transistors

Impact of film thickness on off-state current of bottom contact organic thin film transistor has been investigated using two dimensional numerical simulations and analytical model. Off-state current of the device reduces by six orders of magnitude as film thickness is scaled from 45 nm to 10 nm, with rate of reduction in off-state current being slow first and then significantly higher. An analytical model for off-state current has been developed to offer an insight into above results of off-state current, and the model predictions are found in good agreement with reported experimental results. The developed model is especially important for the device with smaller film thickness as at such film thicknesses, space charge limited current model is inadequate to explain off-state current of such devices. A horizon for scaling device channel length through a reduction in film thickness only has been explored using an analytical model and simulation results. Off-state current of a shorter channel length (L) device can be comparable to a relatively longer channel length (i.e., L + delta L) device if the fractional reduction in film thickness becomes equal to square of the fractional reduction in channel length. Following such reduction in film thickness successively for a number of steps, an expression for film thickness corresponding to the device with a desired value of channel length has been developed and verified with simulation results. Although the device with larger film thickness has a problem of poor subthreshold performance, it, in general, has advantage of better mobility. To alleviate this problem of the device with larger film thickness, a stack contact device has been proposed. An investigation of its subthreshold performance shows that its off-state current can be significantly lower as compared to conventional contact device. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663355]