Severe hump phenomenon induced by increased charge trapping and suppression of electron capture effect in amorphous In-Ga-Zn-O thin-film transistors under unipolar pulsed drain bias with static positive gate bias stress

This paper demonstrates degradation of amorphous indium gallium zinc oxide thin-film transistors under AC current stress (ACS), which is unipolar AC drain bias applied with positive static gate-bias stress. Under ACS, a severe hump occurs in current-voltage curves; the phenomenon causes a large negative shift similar to degradation caused by positive gate-bias stress (PGS) and a large positive shift by DC-current stress (DCS). In a saturation measurement, the degradation caused by factors in source region had the greatest effect on the hump phenomenon; this result confirms that hump characteristics are caused by charge trapping rather than by defectstate generation by impact ionization. The changes of threshold voltage shifts were extracted at two current levels, then analyzed by comparing with the results of PGS, DCS, and synchronized AC gate and drain bias stress. Positive shift in high-current level was larger after ACS (+ 3.80 V) than after PGS (+3.48 V) because currentcaused Joule-heat thermally energized additional electrons to be trapped in front channel region. Negative shift in low-current level was larger after ACS (- 2.42 V) than after DCS (-1.32 V). Under DCS, the amount of positively-charged species decreased because of neutralization of oxygen vacancies. The flow of current supports electron to be captured in ionized oxygen vacancies and the amount of negative shift is saturated at specific value (-1.32 V) under DCS. The characteristic time constant of stretched exponential curve-fitting reveals clear saturation tendency.