Transistor and semiconductor device

A semiconductor device comprising:

• 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:
• τ1 and τ2,
• wherein τ1<τ2 is satisfied,
• wherein τ2 is 300 seconds or less, and
• wherein a thickness of the insulating layer is larger than a thickness of the gate insulating layer.