Light emitting device

Abstract

The present invention provides a TFT that has a channel length particularly longer than that of an existing one, specifically, several tens to several hundreds times longer than that of the existing one, and thereby allowing turning to an on-state at a gate voltage particularly higher than the existing one and driving, and allowing having a low channel conductance gd. According to the present invention, not only the simple dispersion of on-current but also the normalized dispersion thereof can be reduced, and other than the reduction of the dispersion between the individual TFTs, the dispersion of the OLEDs themselves and the dispersion due to the deterioration of the OLED can be reduced.

1 . (canceled) 
     
     
         2 . A light emitting device comprising:
 a light-emitting element;   a first transistor;   a second transistor;   a first wiring;   a second wiring; and   a third wiring,   wherein one of a source and a drain of the first transistor is electrically connected to the first wiring,   wherein a channel length of the second transistor is longer than a channel length of the first transistor,   wherein one of a source and a drain of the second transistor is electrically connected to one electrode of the light-emitting element,   wherein the second wiring is electrically connected to a gate electrode of the first transistor,   wherein the third wiring is electrically connected to the other of the source and the drain of the second transistor,   wherein end portions of a channel formation region of the second transistor has a first point and a second point that overlap with a gate electrode of the second transistor,   wherein a length connecting the first point and the second point along a channel path is longer than a length connecting the first point and the second point with a straight line, and   wherein the third wiring has at least a region having a line width that is larger than a line width of the first wiring.   
     
     
         3 . The light emitting device comprising according to  claim 2 , wherein the channel formation region of the second transistor comprises at least one corner. 
     
     
         4 . A light emitting device comprising:
 a light-emitting element;   a first transistor;   a second transistor;   a first wiring;   a second wiring; and   a third wiring,   wherein one of a source and a drain of the first transistor is electrically connected to the first wiring,   wherein a channel length of the second transistor is longer than a channel length of the first transistor,   wherein one of a source and a drain of the second transistor is electrically connected to one electrode of the light-emitting element,   wherein the second wiring is electrically connected to a gate electrode of the first transistor,   wherein the third wiring is electrically connected to the other of the source and the drain of the second transistor,   wherein a channel formation region of the second transistor comprises a serpentine shape, and   wherein the third wiring has at least a region having a line width that is larger than a line width of the first wiring.   
     
     
         5 . The light emitting device comprising according to  claim 4 , wherein the channel formation region of the second transistor comprises at least one corner. 
     
     
         6 . A light emitting device comprising:
 a light-emitting element;   a first transistor;   a second transistor;   a first wiring;   a second wiring;   a third wiring; and   a capacitor,   wherein one of a source and a drain of the first transistor is electrically connected to the first wiring,   wherein a channel length of the second transistor is longer than a channel length of the first transistor,   wherein one of a source and a drain of the second transistor is electrically connected to one electrode of the light-emitting element,   wherein the second wiring is electrically connected to a gate electrode of the first transistor,   wherein the third wiring is electrically connected to the other of the source and the drain of the second transistor,   wherein end portions of a channel formation region of the second transistor has a first point and a second point that overlap with a gate electrode of the second transistor,   wherein a length connecting the first point and the second point along a channel path is longer than a length connecting the first point and the second point with a straight line,   wherein the third wiring has at least a region having a line width that is larger than a line width of the first wiring,   wherein the third wiring has a region that functions as one electrode of the capacitor, and   wherein the gate electrode of the second transistor has a region functions as the other electrode of the capacitor.   
     
     
         7 . The light emitting device comprising according to  claim 6 , wherein the channel formation region of the second transistor comprises at least one corner. 
     
     
         8 . A light emitting device comprising:
 a light-emitting element;   a first transistor;   a second transistor;   a first wiring;   a second wiring;   a third wiring; and   a capacitor,   wherein one of a source and a drain of the first transistor is electrically connected to the first wiring,   wherein a channel length of the second transistor is longer than a channel length of the first transistor,   wherein one of a source and a drain of the second transistor is electrically connected to one electrode of the light-emitting element,   wherein the second wiring is electrically connected to a gate electrode of the first transistor,   wherein the third wiring is electrically connected to the other of the source and the drain of the second transistor,   wherein a channel formation region of the second transistor comprises a serpentine shape,   wherein the third wiring has at least a region having a line width that is larger than a line width of the first wiring,   wherein the third wiring has a region that functions as one electrode of the capacitor, and   wherein a gate electrode of the second transistor has a region functions as the other electrode of the capacitor.   
     
     
         9 . The light emitting device comprising according to  claim 8 , wherein the channel formation region of the second transistor comprises at least one corner.