Flexible Self-Aligned Double-Gate IGZO TFT

In this letter, flexible double-gate (DG) thin-film transistors (TFTs) based on InGaZnO₄ and fabricated on free standing plastic foil, using self-alignment (SA) are presented. The usage of transparent indium-tin-oxide instead of opaque metals enables SA of source-, drain-, and top-gate contacts. Hence, all layers, which can cause parasitic capacitances, are structured by SA. Compared with bottom-gate reference TFTs fabricated on the same substrate, DG TFTs exhibit a by 68% increased transconductance and a subthreshold swing as low as 109 mV/dec decade (-37%). The clockwise hysteresis of the DG TFTs is as small as 5 mV. Because of SA, the source/drain to gate overlaps are as small as ≈ 1 μm leading to parasitic overlap capacitances of 5.5 fF μm^-1. Therefore a transit frequency of 5.6 MHz is measured on 7.5 μm long transistors. In addition, the flexible devices stay fully operational when bent to a tensile radius of 6 mm.