Abnormal Bias Instabilities Induced by Lateral H₂O Diffusion Into Top-Gate Insulator of a-InGaZnO Thin-Film Transistors

Abstract

The environmental stability of self-aligned top-gate (SATG) a-InGaZnO thin-film transistor (TFT) was studied by performing the high-temperature high-humidity (HTHH) test. Despite the maintenance of initial electrical characteristics, the stability under positive bias stress (PBS) was considerably deteriorated, including an abnormal negative ${V} _{\mathrm{ th}}$ shift ( $\boldsymbol{\Delta } {V} _{\mathrm{ th}}$ ), increased off current, and degraded SS. Moreover, the negative ${\Delta } {V} _{\mathrm{ th}}$ was consistently enhanced with the channel length ( ${L}$ ) decreasing. Such ${L}$ dependence was clarified to originate from the lateral diffusion of H2O in TG insulator during HTHH tests, and the PBS instabilities were caused by the ionization and migration of H2O molecules into the a-IGZO channel, as verified by the X-ray photoelectron spectroscopy, C-V characteristics, and recovery behaviors of PBS degradation.

Keywords

• A-InGaZnO
• self-aligned top-gate (SATG)
• H2O
• hydrogen
• positive bias stress (PBS)
• environmental stability