Advantages of adding a weak second gate in sub-micron bottom-contact organic thin film transistors

Abstract

It is shown that in bottom gate, bottom-contact sub micrometer organic thin-film transistors (TFTs) with Dinaphtho[2,3-b:2′,3′-f]thieno-[3,2-b]thiophene active semiconductors, the addition of a top gate results in substantial improvements in a whole range of electrical performance characteristics. These improvements are not due to additional charge induced by the top gate but originate from the spatial redistribution of the charge induced by the bottom gate by the top gate. In the TFTs described in this work, the bottom gate insulator consists of a bilayer of SiO2 and solution-deposited high-k ZrO2, while the top gate insulator is a relatively thick CYTOP® layer deposited from a solution by spin coating. The capacitances per unit area of the bottom gate insulator and top gate insulator are 87 and 8 nF/cm2, respectively. On account of this large difference, the top gate is referred to as “weak,” as its ability to induce charges in the channel is very much limited. The use of a weak top gate is shown to result in large improvements in the on-state drain current, sub-threshold swing, threshold voltage, and contact resistance compared to single gate operation. Weak top gates are especially useful when processing constraints make it difficult to realize symmetric or close to symmetric dual gate designs. While this demonstration is for a specific materials system, the design principle is more general and is likely to benefit a range of short channel length TFTs including organic/polymer, amorphous metal oxide, and 2D semiconductor-based transistors.