We investigated the influence of hafnium doping on negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) in zinc oxide thin film transistors (TFTs). Hafnium zinc oxide (HZO) TFTs exhibited turn-on voltage (V(ON)) shifts of 0 V for negative stress bias and +3 V for positive stress bias, compared with -5 V and +9 V. respectively, for ZnO TFTs. The enhanced improvement of the V(ON) shift may be due to a reduction in interface trap density resulting from the suppression of oxygen vacancy related defects caused by the high binding energy of Hf ions. (C) 2011 Elsevier B.V. All rights reserved.