Thin film transistor and manufacturing method thereof

A manufacturing method of a thin film transistor, comprising the steps of:

• forming a first wiring layer;
• forming a gate insulating layer so as to cover the first wiring layer;
• forming a first semiconductor layer over the gate insulating layer overlapping with the first wiring layer,
• forming a second semiconductor layer having a lower carrier mobility than the first semiconductor layer over the first semiconductor layer,
• forming an impurity semiconductor layer over the second semiconductor layer;
• forming a first insulating film and a second insulating film so as to cover the first semiconductor layer, the second semiconductor layer and the impurity semiconductor layer;
• anisotropically etching the second insulating film to expose the first insulating film, so that a first sidewall insulating layer is formed;
• etching a portion of the first insulating film, which does not overlap with the first sidewall insulating layer, to expose the impurity semiconductor layer, so that a second sidewall insulating layer which, together with the first sidewall insulating layer, covers at least a side surface of the first semiconductor layer is formed;
• forming a second wiring layer over the impurity semiconductor layer and the second sidewall insulating layer;
• and
• etching part of the second semiconductor layer and part of the impurity semiconductor layer, so that a source region and a drain region are formed.

A thin film transistor comprising:

• a first wiring layer;
• a gate insulating layer over the first wiring layer;
• a first semiconductor layer over the first wiring layer with the gate insulating layer interposed therebetween, wherein an entire portion of the first semiconductor layer is overlapped with the first wiring layer;
• a second semiconductor layer provided over and in contact with the first semiconductor layer and having a lower carrier mobility than the first semiconductor layer;
• an impurity semiconductor layer in contact with the second semiconductor layer;
• a sidewall insulating layer covering at least a side surface of the first semiconductor layer, wherein the sidewall insulating layer contains a silicon oxide layer or a silicon oxynitride layer stacked over a silicon nitride layer;
• and
• a second wiring layer in contact with at least the impurity semiconductor layer.

A thin film transistor comprising:

• a first wiring layer;
• a gate insulating layer over the first wiring layer;
• a first semiconductor layer over the first wiring layer with the gate insulating layer interposed therebetween, wherein an entire portion of the first semiconductor layer is overlapped with the first wiring layer;
• second semiconductor layers provided apart from each other over and in contact with the first semiconductor layer and having a lower carrier mobility than the first semiconductor layer;
• impurity semiconductor layers in contact with the second semiconductor layers;
• a sidewall insulating layer covering at least a side surface of the first semiconductor layer, wherein the sidewall insulating layer contains a silicon oxide layer or a silicon oxynitride layer stacked over a silicon nitride layer;
• and
• second wiring layers in contact with at least the impurity semiconductor layers.