Band Tail State Broadening in IGZO TFTs After pBTI-Induced Negative VT Shift Revealed via DC and 1/f Noise Measurements

TL;DR

This study reveals that positive bias and high temperature stress in IGZO TFTs cause broadening of conduction band tail states rather than creating new traps, with effects being reversible and confirmed by simulations.

Contribution

It demonstrates that threshold voltage shifts are due to tail state broadening, not trap generation, and provides experimental and simulation evidence for this mechanism.

Findings

Stress broadens conduction band tail states in IGZO TFTs.

Threshold voltage and noise degradation are reversible.

Simulations confirm increased hydrogen doping and sub-gap states.

Abstract

We investigate the origin of negative threshold voltage shifts in back-gated amorphous IGZO TFTs under positive bias and high temperature stress. Combined DC and 1/f noise measurements reveal that the stress does not generate new dielectric traps but instead broadens the IGZO conduction band tail states. A recovery experiment confirms that the associated threshold voltage, subthreshold swing, and noise degradation are reversible. Simulations using an in-house Poisson solver confirm the experimental observations that high-temperature stress increases hydrogen doping and the density of sub-gap states.