Effects of electron trapping and interface state generation on bias stress induced in indium-gallium-zinc oxide thin-film transistors

The electrical characteristics of bias temperature stress (BTS) induced in amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) were studied. We analyzed the threshold voltage (V-TH) shift on the basis of the effects of positive bias temperature stress (PBTS) and negative bias temperature stress (NBTS), and applied it to the stretched-exponential model. Both stress temperature and bias are considered as important factors in the electrical instabilities of a-IGZO TFTs, and the stretched-exponential equation is well fitted to the stress condition. V-TH for the drain current-gate voltage (I-DS-V-GS) curve and flat-band voltage (V-FB) for the capacitance-voltage (C-V) curve move in the positive direction when PBTS is induced. However, in the case of NBTS, they move slightly in the negative direction. To clarify the V-TH shift phenomenon by electron and hole injection, the average effective energy barrier (E-t) is extracted, and the extracted values of E-t under PBTS and NBTS are about 1.33 and 2.25 eV, respectively. The oxide trap charges (N-ot) of PBTS and NBTS calculated by C-V measurement are 4.4 x 10(11) and 1.49 x 10(11) cm(-2), respectively. On the other hand, the border trap charges of PBTS and NBTS are 6.7 x 10(8) and 1.7 x 10(9) cm(-2), respectively. This indicates that the increased interface trap charge, after PBTS is induced, captures electrons during detrap processing from the border trap to the conduction band, valence band, and interface trap. (C) 2014 The Japan Society of Applied Physics