Element substrate and light emitting device

摘要

A light emitting device and an element substrate which are capable of suppressing variations in the luminance intensity of a light emitting element among pixels due to characteristic variations of a driving transistor without suppressing off-current of a switching transistor low and increasing storage capacity of a capacitor. According to the invention, a depletion mode transistor is used as a driving transistor. The gate of the driving transistor is fixed in its potential or connected to the source or drain thereof to operate in a saturation region with a constant current flow. A current controlling transistor which operates in a linear region is connected in series to the driving transistor, and a video signal for transmitting a light emission or non-emission of a pixel is inputted to the gate of the current controlling transistor through a switching transistor.

主权项

1. 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 fifth line; a sixth 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 one of a source and a drain of the third transistor; a gate of the first transistor is electrically connected to one electrode of the capacitor through a first metal; the gate of the first transistor is electrically connected to one of a source and a drain of the fourth transistor through the first metal and the one electrode of the capacitor; the gate of the first transistor is electrically connected to one of a source and a drain of the second transistor through the first metal and the one electrode of the capacitor; the first metal comprises a region that overlaps with a part of the second line; a first gate of the second transistor is electrically 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 electrically connected to the light emitting element; the other of the source and the drain of the fourth transistor is electrically connected to the fourth line; a first gate of the fourth transistor is electrically 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 electrically connected to the third line; the other electrode of the capacitor is electrically connected to the sixth line; the third line is a power supply line; the sixth 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; anda 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.