Patent ID: 8400067

Claim:
A light emitting device comprising: a first transistor; a second transistor; a third transistor; a fourth transistor; a first line; a second line; a third line; a fourth line; a capacitor; and a light emitting element, wherein: a channel formation region of the second transistor and a channel formation region of the fourth transistor are formed of a same semiconductor island; a channel formation region of the first transistor and a channel formation region of the third transistor are formed of a same semiconductor island; a conductivity type of the first transistor and a conductivity of the third transistor is the same; one of a source and a drain of the first transistor is electrically connected to the third line; the other of the source and the drain of the first transistor is directly connected to one of a source and a drain of the third transistor; a gate of the first transistor is directly connected to one electrode of the capacitor; the gate of the first transistor is directly connected to one of a source and a drain of the fourth transistor; the gate of the first transistor is electrically connected to one of a source and a drain of the second transistor through a first metal; the first metal comprises a region that overlaps with a part of the second line; a first gate of the second transistor is directly connected to the second line; a second gate of the second transistor is electrically connected to the second line; the third line comprises a region that overlaps with a part of the channel formation region of the first transistor; the other of the source and the drain of the third transistor is directly connected to the light emitting element; the other of the source and the drain of the fourth transistor is directly connected to the fourth line; a first gate of the fourth transistor is directly connected to the first line; a second gate of the fourth transistor is electrically connected to the first line; the other electrode of the capacitor is directly connected to the third line; the third line is a power supply line; the light emitting element comprises: a first electrode; a layer comprising an organic light emitting compound over the first electrode; and a second electrode over the layer; a luminescence from the organic light emitting compound is passed through the second electrode; and a ratio of a channel length to a channel width of the first transistor is different from a ratio of a channel length to a channel width of the third transistor.