Semiconductor device and driving method thereof

Abstract

A voltage equal to the threshold value of a TFT ( 106 ) is held in capacitor unit ( 109 ). When a video signal is inputted from a source signal line, the voltage held in the capacitor unit is added thereto and a resultant signal is applied to a gate electrode of the TFT ( 106 ). Even when a threshold value is varied for each pixel, each threshold value is held in the capacitor unit ( 109 ) for each pixel. Thus, the influence of a variation in threshold value can be eliminated. Further, holding of the threshold value is conducted by only the capacitor unit ( 109 ) and a charge does not move at writing of a video signal so that a voltage between both electrodes is not changed. Thus, it is not influenced by a variation in capacitance value.

1 . (canceled) 
     
     
         2 . A display device comprising:
 a first gate signal line driver circuit;   a second gate signal line driver circuit; and   a pixel portion between the first gate signal line driver circuit and the second gate signal line driver circuit, the pixel portion comprising a pixel,   wherein the pixel comprises a first transistor, a second transistor, a third transistor, a fourth transistor, a capacitor and a light emitting element,   wherein one of a source and a drain of the first transistor is electrically connected to a first electrode of the capacitor,   wherein a second electrode of the capacitor is electrically connected to a gate of the second transistor,   wherein the gate of the second transistor is electrically connected to one of a source and a drain of the third transistor,   wherein one of a source and a drain of the second transistor is electrically connected to the other of the source and the drain of the third transistor,   wherein the second transistor and the fourth transistor are connected in series between the light emitting element and a current supply line,   wherein a signal from the first gate signal line driver circuit is input to a gate of the first transistor and a gate of the third transistor, and   wherein a signal from the second gate signal line driver circuit is input to a gate of the fourth transistor.   
     
     
         3 . The display device according to  claim 2 ,
 wherein the one of the source and the drain of the second transistor is electrically connected to one of a source and a drain of the fourth transistor, and   wherein the other of the source and the drain of the second transistor is electrically connected to the current supply line.   
     
     
         4 . The display device according to  claim 2 ,
 wherein the other of the source and the drain of the first transistor is electrically connected to a source signal line.   
     
     
         5 . A display device comprising:
 a first gate signal line driver circuit;   a second gate signal line driver circuit; and   a pixel portion between the first gate signal line driver circuit and the second gate signal line driver circuit, the pixel portion comprising a pixel,   wherein the pixel comprises a first transistor, a second transistor, a third transistor, a fourth transistor, a first capacitor, a second capacitor and a light emitting element,   wherein one of a source and a drain of the first transistor is electrically connected to a first electrode of the first capacitor,   wherein a second electrode of the first capacitor is electrically connected to a gate of the second transistor,   wherein a first electrode of the second capacitor is electrically connected to the first electrode of the first capacitor,   wherein the gate of the second transistor is electrically connected to one of a source and a drain of the third transistor,   wherein one of a source and a drain of the second transistor is electrically connected to the other of the source and the drain of the third transistor,   wherein the second transistor and the fourth transistor are connected in series between the light emitting element and a current supply line,   wherein a signal from the first gate signal line driver circuit is input to a gate of the first transistor and a gate of the third transistor, and   wherein a signal from the second gate signal line driver circuit is input to a gate of the fourth transistor.   
     
     
         6 . The display device according to  claim 5 ,
 wherein the one of the source and the drain of the second transistor is electrically connected to one of a source and a drain of the fourth transistor, and   wherein the other of the source and the drain of the second transistor is electrically connected to the current supply line.   
     
     
         7 . The display device according to  claim 5 ,
 wherein the other of the source and the drain of the first transistor is electrically connected to a source signal line.   
     
     
         8 . The display device according to  claim 5 ,
 wherein a second electrode of the second capacitor is electrically connected to the current supply line.