Patent ID: 8115201

Claim:
A semiconductor device including a thin film transistor, the thin film transistor comprising: a gate electrode layer; a gate insulating layer over the gate electrode layer; a source electrode layer and a drain electrode layer over the gate insulating layer; a first buffer layer having n-type conductivity over the source electrode layer; a second buffer layer having n-type conductivity over the drain electrode layer; and an oxide semiconductor layer over the first buffer layer and the second buffer layer, wherein the oxide semiconductor layer overlapping with the gate electrode layer is partly over and in contact with the gate insulating layer and is provided between the source electrode layer and the drain electrode layer, wherein a carrier concentration of the first buffer layer and the second buffer layer is higher than a carrier concentration of the oxide semiconductor layer, wherein the oxide semiconductor layer and the source electrode layer are electrically connected to each other with the first buffer layer interposed therebetween, wherein the oxide semiconductor layer and the drain electrode layer are electrically connected to each other with the second buffer layer interposed therebetween, and wherein a part of the first buffer layer and a part of the second buffer layer are in contact with the gate insulating layer in a region between the source electrode and the drain electrode.