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

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 H
₂
O in TG insulator during HTHH tests, and the PBS instabilities were caused by the ionization and migration of H
₂
O molecules into the a-IGZO channel, as verified by the X-ray photoelectron spectroscopy, C-V characteristics, and recovery behaviors of PBS degradation.