Device substrate, light emitting device and driving method of light emitting device

A device substrate comprising:

• a pixel comprising:
• a pixel electrode;
• a first transistor;
• and
• a second transistor;
• and
• a power supply line overlapping with at least a part of a channel formation region of the first transistor and at least a part of a channel formation region of the second transistor;
• wherein a threshold voltage of the first transistor is lower than a threshold voltage of the second transistor,
• wherein a channel length of the first transistor is longer than a channel width thereof,
• wherein a channel length of the second transistor is equal to or shorter than a channel width thereof,
• wherein gate electrodes of the first transistor and the second transistor are directly connected to each other,
• wherein each polarity of the first transistor and the second transistor is the same, and
• wherein the pixel electrode, the first transistor and the second transistor are connected in series.