Element substrate and light emitting device

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:
• the second transistor comprises a channel formation region formed of a first semiconductor island;
• the fourth transistor comprises a channel formation region formed of the first semiconductor island;
• the first transistor comprises a channel formation region formed of a second semiconductor island;
• the third transistor comprises a channel formation region formed of the second 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 one electrode of the capacitor and 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 source/drain metal;
• a part of the source/drain metal overlaps with a part of the second line;
• a gate of the second transistor is directly connected to the second line;
• a part of the third line overlaps with a part of a 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 gate of the fourth transistor is directly 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 second transistor comprises:
• a first semiconductor layer;
• a first gate over the first semiconductor layer;
• and
• a second gate over the first semiconductor layer;
• the fourth transistor comprises:
• a second semiconductor layer;
• a third gate over the second semiconductor layer;
• and
• a fourth gate over the second semiconductor layer;
• the light emitting element is formed over the third transistor, the light emitting element comprises:
• a first electrode;
• a layer containing an organic light emitting compound over the first electrode;
• and
• a second electrode over the layer containing the organic light emitting compound;
• 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.