Channel Width and Channel Length Dependencies in Amorphous-Oxide-Semiconductor Thin-Film Transistors: From a Device Structure Perspective

Owing to their unique device structures, amorphous oxide semiconductor (AOS)-based thin-film transistors (TFTs) often exhibit non-ideal dependencies on channel width (W) and channel length (L) variations. In particular, two abnormalities are common; 1) the TFT threshold-voltage (V-TH) shifts to the negative gate voltage (V-GS) direction as L decreases for fixed W; and 2) the TFT field-effect mobility (mu(FE)) dramatically increases as W decreases for fixed L. This paper therefore seeks to explain the origin of these non-ideal behaviors and present device topologies that can be utilized to minimize them. The abnormal negative V-TH shift (Delta V-TH) is attributed to unintentional doping of the intrinsic channel region from n(+) doped source/drain regions, whereas the large increase in mu(FE) is due to spreading current, which effectively increases W. It is found that the former can be minimized by reducing the source/drain to active layer contact area, whereas the latter is solved by employing device topologies in which the source and drain electrodes extend over the active layer along the W direction. This paper furthers our knowledge and understanding of the physical topology of TFT designs and its impact to TFT performance.