The stability under high gate bias of the recent high performance n- and p-type unhydrogenated polysilicon TFTs realised with low temperature solid-phase crystallization technology, is studied. The threshold shift Delta V-T is positive for n-type and negative for p-type TFTs whatever the polarity of the stress gate bias. Delta V-T shows a power-law time (t/t(0))(beta) dependence for both polarities. The subthreshold slope increases and the transconductance decreases during these stresses. The overall density of the gap states, determined from the known field-effect analysis, increases during these stresses. The disordered structure of polysilicon is involved to explain this state creation in unhydrogenated TFTs.