Analysis of Negative Bias Illumination Stress Induced Effect on LTPS and a-IGZO TFT

Abstract

The LTPS and IGZO were the two possible options for next generation displays with its own advantages and disadvantages. In terms of reliability, the LTPS TFT offers the superior threshold voltage stability as compare to the IGZO TFT under the gate bias stress. However, the continuing scaling of the LTPS and IGZO limits the electrical performance at higher voltages. LTPS and IGZO TFT still retains the reliability issues at smaller device dimensions. Therefore, the negative bias stress instability in a-IGZO and LTPS TFT having similar device dimension (width × length) has been investigated and compared. The negative bias stress without illumination shows the good thermal stability and no threshold voltage change in the LTPS TFT. However, under illumination (NBIS), the asymmetrical degradation in the LTPS TFT was observed, shows the degradation in off current (IOFF) almost by two order (10−11 A) due to self-heating effects. Whereas, the NBIS on a-IGZO TFT shows the positive shift in the threshold voltage (ΔVth = 2.71 V), due to the double donor state within a-IGZO and gate insulator. It is noticed that, the LTPS and a-IGZO TFT show different photo-generated carrier behavior under NBIS, which may limit the performance for future CMOS device at smaller dimensions.