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

Abstract

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 (VTH) shifts to the negative gate voltage (VGS) direction as L decreases for fixed W; and 2) the TFT field-effect mobility (µFE) 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 VTH shift (ΔVTH) is attributed to unintentional doping of the intrinsic channel region from n+ doped source/drain regions, whereas the large increase in µFE 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.

Analysis on Contact Resistance and Effective Channel Length of Thin Film Transistors Using Composition-Modified In–Ga–Zn-O Active Channels Prepared with Atomic Layer Deposition and Various Electrode Materials

“…Additionally, most previous works dealing with R C and ΔL have been limited to the TFTs using conventional sputter-deposited IGZO channels. 20 However, considering that ALD IGZO is becoming a promising alternative channel for BEOL TFTs, monolithic 3D (M3D), cell transistors for 3D-DRAM, it is very important to clearly understand the physical origins for variations in contact properties between the ALD IGZO and electrode materials. Furthermore, the cationic compositions of IGZO channel layers, which intrinsically determine the R CH , are expected to be closely related to the R C and ΔL for the ALD IGZO TFTs.…”

“…Even though there have been lots of efforts to improve the device performance of ALD IGZO TFTs with variations in the L , there have rarely been systematic studies to investigate the contact behavior and Δ L fluctuation when the composition-modified IGZO channels were prepared by ALD process on the various metal S/D electrode. Additionally, most previous works dealing with R C and Δ L have been limited to the TFTs using conventional sputter-deposited IGZO channels . However, considering that ALD IGZO is becoming a promising alternative channel for BEOL TFTs, monolithic 3D (M3D), cell transistors for 3D-DRAM, it is very important to clearly understand the physical origins for variations in contact properties between the ALD IGZO and electrode materials.…”

24.3: Short‐Channel Indium‐Gallium‐Zinc Oxide Thin‐Film Transistor Enabled by Thermal Dehydrogenation and Oxidizing Defect‐Suppression

“…With the evolution of displays in high-end smartphone and near-eye head-mounted displays, the downscaling of TFT is of paramount importance to enable the required small pixel for high-ppi displays. The "short-channel effect (SCE)" in silicon-based TFT was often ascribed to the drain induced barrier lowering (DIBL), while the effective "SCE" in IGZO TFT was much more serious − thus hard to be completely ascribed to DIBL [2]- [4], but more likely to be caused by the dopant diffusion from the source/drain (S/D) regions [5], [6].…”