Performance Improvement by Double-Layer a-IGZO TFTs With a Top Barrier

The double-layer a-IGZO thin film transistors (DL-TFTs) using a quantum well channel and a top barrier can reduce the subthreshold swing and hysteresis by 0.73<inline-formula> <tex-math notation="LaTeX">times </tex-math></inline-formula> and 0.13<inline-formula> <tex-math notation="LaTeX">times </tex-math></inline-formula>, respectively, in the transfer characteristics using the bottom gate sweep as compared to the single-layer TFTs (SL-TFTs). The wide bandgap barrier on top of the narrow bandgap IGZO channel serves as a protection layer between the IGZO channel and the SiO₂ top gate insulator to prevent plasma-induced damage on the IGZO channel caused by the S/D metal etching and the top gate insulator deposition. As for the mobility using the bottom gate operation with the top gate grounded, the DL-TFTs show higher mobility 1.06<inline-formula> <tex-math notation="LaTeX">$times$ </tex-math></inline-formula> at the room temperature due to less Coulomb scattering caused by the plasma-induced damage for percolation conduction, while the SL-TFTs have higher mobility at low temperatures due to the improved hopping efficiency for thermally activated hopping. The hysteresis is temperature independent down to 160 K, indicating the electrons tunneling between the channel and the top gate insulator is dominant. As for the reliability, DL-TFT has a smaller V_th shift than SL-TFT under both positive bias temperature stress (PBTS) due to less subgap defect in the channel.