Semiconductor device including pair of electrodes and oxide semiconductor film with films of low conductivity therebetween

A display device comprising:

• a first electrode;
• a second electrode;
• a first semiconductor film over the first electrode;
• a second semiconductor film over the second electrode;
• an oxide semiconductor film over the first semiconductor film and the second semiconductor film;
• a first insulating film over the oxide semiconductor film;
• a gate electrode over the first insulating film;
• a second insulating film over the gate electrode;
• and
• a pixel electrode over the second insulating film and electrically connected to the first electrode or the second electrode,
• wherein the oxide semiconductor film comprises indium and zinc,
• wherein the first semiconductor film and the second semiconductor film each have a conductivity less than the oxide semiconductor film,
• wherein the first semiconductor film and the second semiconductor film each have smaller thickness than the oxide semiconductor film, and
• wherein the oxide semiconductor film comprises sodium at a concentration of 5×1018/cm3 or lower.