Effect of Rising Edge in Dynamic Stress with Various Duty Ratio in Amorphous Ingazno Thin Film Transistor

Recently, many research explains the positive threshold voltage (V_th) shift of oxide TFT as charge trapping at gate dielectric or interface of gate dielectric and channel layer. But mechanism of degradation of rising edge of dynamic stress has not been explained exactly.

 During all stresses, gate stress of 1 Hz with 50%, 20%, 10% and 1% duty ratio was applied at 120 ℃ and both drain voltage and source voltage were grounded. Gate voltage (V_G) was on-voltage (V_on) of 20 V and off-voltage (V_off) of 0 V in unipolar positive stress, V_on of 0 V and V_off of -20 V in unipolar negative stress, and V_on of 20 V and V_off of -20 V in bipolar stress (Fig.1). During illumination, samples were illuminated using a top LED. We defined V_th as gate voltage at 1 nA of drain current I_D at V_DS of 15V. We defined effective time as sum of V_ontime in all stress including illumination

 Fig. 2 shows the ΔV_th as function of time with 50%, 20% 10% and 1% duty ratio. In dark, positive unipolar gate stress was applied in fig. 2(a) and bipolar gate stress was applied in fig. 2(b). In illumination, positive unipolar gate stress was applied in fig. 2(c) and bipolar gate stress was applied in fig. 2(d). Fig. 3 shows the ΔV_th at effective time of 2000s as reciprocal function of duty ratio for positive unipolar gate stress and bipolar gate stress. At same effective stress time, ΔV_th was increase with decreasing duty ratio and ΔV_th was larger in bipolar gate stress than in unipolar positive gate stress. Fig. 4 shows ΔV_th at effective time of 2000s as reciprocal function of duty ratio under positive unipolar gate stress for illumination and dark. In dark, higher ΔV_th was observed compared with illumination.

 In this study, we investigated relationship of ΔV_th with various bias stress condition including positive unipolar, negative unipolar and bipolar gate stress and ambient including dark and illumination. We will explain positive bias temperature stress based on charge trapping and introduce new mechanism of additional positive ΔV_th on rising edge of dynamic stress.