Thin film transistor and display device

A thin film transistor comprising:

• a gate electrode layer;
• a first semiconductor layer;
• a second semiconductor layer provided on and in contact with the first semiconductor layer, wherein carrier mobility of the second semiconductor layer is lower than carrier mobility of the first semiconductor layer;
• a gate insulating layer provided between and in contact with the gate electrode layer and the first semiconductor layer;
• impurity semiconductor layers provided in contact with the second semiconductor layer;
• a source electrode layer provided partially in contact with one of the impurity semiconductor layers, the first semiconductor layer, and the second semiconductor layer;
• and
• a drain electrode layer provided partially in contact with the other of the impurity semiconductor layers, the first semiconductor layer, and the second semiconductor layer;
• wherein an entire surface of the first semiconductor layer on the gate electrode layer overlaps with the gate electrode layer;
• and
• wherein a work function of a material for forming the source electrode layer and the drain electrode layer is φ, a difference between a vacuum level and a bottom of a mobility edge of the first semiconductor layer is χ, a width of a forbidden band of the first semiconductor layer is Eg, and Eg+χ−φ is greater than or equal to 0.5 eV.