1.  Field of the Invention
The present invention relates to a display device.  In particular, the present invention relates to a display device including a light-emitting element which emits light utilizing electroluminescence. 
2.  Description of the Related Art
A display device including a light-emitting element which emits light utilizing electroluminescence has been developed as an active matrix display device.  Specifically, a display device has been developed in which the light-emitting element is provided in each of pixels arranged in matrix and desired display is performed by appropriately controlling a current which is supplied to each light-emitting element.  Examples of the light-emitting element include an element containing an organic material which emits light utilizing electroluminescence (also referred to as an organic EL element or an organic light-emitting diode). 
For the display device, a means to control the current which is supplied to each light-emitting element are needed.  As the means, a means to control a current which is supplied to a light-emitting element by using a transistor (also referred to as a driving transistor) is known (e.g., see Patent Document 1).  In other words, the means are known in which a transistor whose source and drain are connected to a light-emitting element in series between a wiring for supplying a high power source potential VDD (also referred to as a high power supply potential line) and a wiring for supplying a low power supply potential VSS (also referred to as a low power supply potential line) is provided in each pixel. 
As specific examples of a pixel including a light-emitting element and a driving transistor, structures illustrated in FIGS.  3A to 3D can be given.  Specifically, a pixel illustrated in FIG.  3A includes an n-channel transistor 1001A whose drain is electrically connected to a high power supply potential line, and a light-emitting element 1002A whose anode is electrically connected to a source of the n-channel transistor 1001A and whose cathode is electrically connected to a low power supply potential line.  A pixel illustrated in FIG.  3B includes a p-channel transistor 1001B whose source is electrically connected to a high power supply potential line, and a light-emitting element 1002E whose anode is electrically connected to a drain of the p-channel transistor 1001B and whose cathode is electrically connected to a low power supply potential line.  A pixel illustrated in FIG.  3C includes an n-channel transistor 1001C whose source is electrically connected to a low power supply potential line, and a light-emitting element 1002C whose cathode is electrically connected to a drain of the n-channel transistor 1001C and whose anode is electrically connected to a high power supply potential line.  A pixel illustrated in FIG.  3D includes a p-channel transistor 1001D whose drain is electrically connected to a low power supply potential line, and a light-emitting element 1002D whose cathode is electrically connected to a source of the p-channel transistor 1001D and whose anode is electrically connected to a high power supply potential line. 
Of the pixels illustrated in FIGS.  3A to 3D, the pixel illustrated in FIG.  3B is most commonly used in consideration of the following two technical viewpoints. 
The first viewpoint is a change in potential of a node where a driving transistor and a light-emitting element are electrically connected to each other, which is caused by deterioration of the light-emitting element over time or a change in environment temperature.  Specifically, in the pixels illustrated in FIGS.  3B and 3C, the potential of the source of the driving transistor is kept constant.  In other words, the voltage between the gate and the source of the driving transistor which is included in each of the pixels illustrated in FIGS.  3B and 3C can be kept irrespective of whether or not the light-emitting element deteriorates over time and the environment temperature is changed.  Accordingly, in the pixels illustrated in FIGS.  3B and 3C, a current which is supplied to the light-emitting element at the time when the driving transistor is operated in a saturation region (a current flowing between the source and the drain of the driving transistor) can be kept substantially constant irrespective of whether or not the light-emitting element deteriorates over time and the environment temperature is changed.  On the other hand, the potential of the source of the driving transistor included in each of the pixels illustrated in FIGS.  3A and 3D is changed in response to deterioration of the light-emitting element over time or a change in environment temperature.  In other words, a voltage between the gate and the source of the driving transistor included in each of the pixels illustrated in FIGS.  3A and 3D is changed in response to the deterioration of the light-emitting element over time or the change in the environment temperature.  Accordingly, in the pixels illustrated in FIGS.  3A and 3D, the current which is supplied to the light-emitting element is changed in response to the deterioration of the light-emitting element over time or the change in the environment temperature. 
The second viewpoint is a manufacturing process.  Each of the light-emitting elements included in the pixels illustrated in FIGS.  3A to 3D emits light which is generated by electroluminescence between a pair of electrodes (the anode and the cathode) to the outside.  Therefore, at least one of the pair of electrodes needs to transmit light.  For example, it is necessary that at least one of the pair of electrodes is formed using a light-transmitting material such as indium tin oxide (also referred to as ITO).  Such a material is preferably used for an anode because of its relatively high work function.  Further, such a material is generally formed by a sputtering method.  However, in the case where the light-emitting element is an organic EL element, when the formation of the material is performed by a sputtering method after an organic material is formed, the organic material might be damaged.  Therefore, in the manufacturing process of a driving circuit and a light emitting element, the following order is preferable: a driving transistor and an anode to be included in a light-emitting element are formed, and then an organic material to be included in the light-emitting element is formed.  Here, it is possible to easily form the driving transistors and the light emitting elements in the pixels illustrated in FIGS.  3A and 3B in accordance with the above order. 
In short, the pixel configurations illustrated in FIGS.  3B and 3C are convenient from the first viewpoint, and the pixel configurations illustrated in FIGS.  3A and 3B are convenient from the second viewpoint.  Accordingly, the use of the pixel configuration illustrated in FIG.  3B as the configuration of a pixel in a display device is convenient from the two viewpoints.