Patent ID: 8895976

Claim:
A transistor comprising: an insulating layer; an oxide semiconductor layer to be a channel formation region over the insulating layer; a source electrode layer and a drain electrode layer overlapping with a part of the oxide semiconductor layer; a gate insulating layer overlapping with the oxide semiconductor layer, the source electrode layer, and the drain electrode layer; and a gate electrode overlapping with a part of the oxide semiconductor layer with the gate insulating layer provided therebetween, wherein, after the oxide semiconductor layer is irradiated with light and the light is turned off, a relaxation time of carriers in photoresponse characteristics of the oxide semiconductor layer has at least two kinds of modes: τ 1 and τ 2 , wherein τ 1 <τ 2 is satisfied, wherein τ 2 is 300 seconds or less, and wherein a thickness of the insulating layer is larger than a thickness of the gate insulating layer.