Patent ID: 8890781

Claim:
A display device comprising: a power supply line; a light-emitting element configured to receive power from the power supply line; a first transistor for electrically connecting the power supply line and the light-emitting element; a signal line for supplying a video signal; a second transistor for electrically connecting the signal line and a gate of the first transistor; wherein a channel formation region in the second transistor comprises an oxide semiconductor layer having a band gap of greater than or equal to 2 eV and a hydrogen concentration of less than or equal to 5×10 19 /cm 3 , and wherein when the second transistor is off state, a drain current of the second transistor is less than or equal to 1×10 −16 A/μm, and the second transistor is configured to hold the first transistor in an on state and connect the power supply line to the light-emitting element to display a still image.