Fundamental understanding of NBTI degradation mechanism in IGZO channel devices

Negative bias temperature instability (NBTI) of IGZO thin film transistors (TFTs) with different channel thicknesses(tIGZO) and gate lengths is studied. Four main NBTI characteristics are observed: (1) the threshold voltage shift appears to be almost independent of the gate stress voltage (above a given stress voltage determined by the tIGZO), and shows (2) a large activation energy (i.e., a strong temperature dependence) and (3) a steep time kinetics (i.e., a large power-law time exponent), while (4) the NBTI relaxation rate is reduced for increasing temperatures below 200°C. By combining TCAD simulations with experimental data modelling, these peculiar features can be explained by an oxide-field-driven hydrogen release from the gate-dielectric, inducing additional IGZO donor doping and a subsequent negative $V_{\text{th}}$ shift. The latter mechanism is observed to be analogous to the negative $V_{th}$ shift reported during PBTI stress at elevated temperature in IGZO devices [1].