(Invited) Heterojunction Channel Engineering for in-Ga-Zn-O Thin-Film Transistors

Abstract

Heterojunction channel engineering approach is presented as a means of enhancing the performance and reliability of In–Ga–Zn–O thin-film transistors (IGZO TFTs). A bottom-gate IGZO TFT with the heterojunction channel was formed by depositing an In-rich IGZO on a typical composition of IGZO111. A potential well of 0.4 eV was expected to form for electrons at the heterojunction interface. Field effect mobilities (μFE) of the TFTs were 12 cm2/Vs and 23 cm2/Vs for homogeneous IGZO111 and high-In IGZO channels, respectively. In addition, threshold voltage stability (ΔVth) of the TFTs showed big a difference between homogeneous IGZO111 and high-In IGZO channels under positive bias and temperature stress (PBTS). The IGZO111 TFT exhibited good Vth stability (ΔVth of +0.4 V) after the PBTS of +20 V for 104 sec at 60 °C. In contrast, although the homogeneous high-In IGZO TFT exhibited a higher μFE than the IGZO111 TFT, the PBTS stability noticeably degraded (ΔVth of +7.0 V) from the IGZO111 TFT.For the heterojunction channel TFTs, when the high-In IGZO layer was deposited on the IGZO layer, μFE increased from the IGZO111 TFT especially in low gate voltage (VGS) region. In addition, the PBTS stability of the heterojunction channel TFT was clearly improved from the high-In IGZO TFT by inserting IGZO111 layer at the interface between HI-IGZO and the GI. A maximum μFE of 24.7 cm2/Vs and improved PBTS stability (ΔVth of +0.3 V) were achieved by the heterojunction channel TFT with a high-In IGZO/IGZO111 thicknesses of 10/5 nm.Device simulation results revealed that quantum confinement effect in the heterojunction channel played an important role in determining carrier transport properties in the TFT, result in improving μFE and PBTS stability. A channel engineering approach will provide a method of overcoming the trade-off between μFE and the PBTS stability of oxide TFTs.