Device characteristics of polymer dual-gate field-effect transistors

Dual-gate organic field-effect transistors (OFETs) were fabricated by solution processing using different p-type polymer semiconductors and polymer top-dielectric materials on prefabricated substrates with gold source-drain contacts defined by photolithography. The semiconductors and top dielectrics films were processed from solution by spin-coating and the silver top gate was applied by shadow mask evaporation. The dual-gate OFETs were characterized in double gate mode by sweeping the bottom gate bias from 40 to -50 V while fixing the top gate potentials, and vice versa. We demonstrate that the change in the threshold voltage of the bottom gate depends on the top gate bias with two linear relationships for two different regimes. The interpretations of the results, assuming that the mobilities of the top and bottom channels have similar magnitudes, and that the capacitance of the semiconductor layer is not negligible, indicated two distinct regimes for a dual-gate p-type OFET. 1. When one gate is positive, hence the respective channel is in depletion and no gate screening will occur, while the other gate is in accumulation regime, the field of the positive gate will penetrate to such a large extend that the channel in accumulation will be affected by both gates. 2. When both channels are in accumulation, the charges present in the channels will screen the respective gate potentials, hence both channels will operate individually and no mutual influences are observed. For a dual-gate OFET with its top channel in accumulation, we demonstrate a drop in the transconductance when the bottom gate potential becomes negative. This transition regime between both linear regimes is marked by a drop in the transconductance, where the bottom channel depends on the bottom gate only and the top channel will depend on both gates. The transition regime results from the fact that the charges accumulated in the bottom channel will start to screen the influence of the bottom gate potential on the top channel and the change in overall drain current will depend only on the change of the Current of the bottom channel. (C) 2008 Elsevier B.V. All rights reserved.