High Field-Effect Mobility Two-Channel InGaZnO Thin-Film Transistors for Low-Voltage Operation

In this study, two-channel thin-film transistors (TC TFTs) using sputtered-deposited amorphous indium-gallium-zinc oxide (a-IGZO) as a channel layer and atomic-layer-deposition Al2O3 as gate insulator (GI) are proposed for wearable and portable device application. Symmetric-TC (S-TC) TFT structure consisted of conventional bottom gate (BG) TFT stacked on top of top gate (TG) TFT. Asymmetric-TC (A-TC) TFT contained BG TFT with tandem structure on the TG TFT. It was shown that the TC TFTs exhibited excellent performance such as high field-effect mobility ( $\mu _{\text {FE}}$ ) and ON/ OFF current ratio ( $I_{\scriptstyle{\text{ON/OFF}}}$ ) at low voltages (< 2 V). For instance, the S-TC TFTs gave $\mu _{\text {FE}}$ of 19.67 cm2/Vs and $I_{\scriptstyle{\text{ON/OFF}}}$ of $5.48\times 10^{{8}}$. Furthermore, the A-TC TFTs with tandem structure yielded $\mu _{\text {FE}}$ of 30.15 cm2/Vs, a small threshold voltage of -1.25, a low subthreshold swing of 89 mV/decade, and a high $I_{\scriptstyle{\text{ON/OFF}}}$ of $1.70\times 10^{{9}}$. It was found that the TC TFTs demonstrated better electrical performance than the sum of individual TG and BG TFTs. Under bias stress tests, the TC TFTs experience less ${V}_{\text {th}}$ shift ( $\Delta {V}_{\text {th}}$ ) than the TG and BG TFTs.