1. Field of the Invention
The present invention relates to a display device and, more particularly, to a structure of an active matrix type organic electroluminescent display.
2. Description of the Related Art
Active matrix driven organic electroluminescent displays (referred below to as AMOLED) is expected as flat panel displays of the next generation succeeding conventional liquid crystal displays.
Conventionally, a two-transistor structured circuit, as disclosed in JP-A-2000-163014 (first prior technique), comprising a drive thin-film transistor (referred below to as EL drive TFT) for feeding current to organic electroluminescent elements (referred below simply to as EL element), a holding capacitor connected to a gate electrode of the EL drive TFT for holding a picture signal voltage, and a switch thin-film transistor (referred below to as switch TFT) for feeding a picture signal voltage to the holding capacitor, has been known as a fundamental pixel circuit for a pixel drive circuit of AMOLED.
The two-transistor structured fundamental pixel circuit causes a significant problem that nonuniformity in a picture is caused by dispersion every pixel in a threshold voltage (Vth) and mobility (μ) of the EL drive TFT due to dispersion every location in the crystallizing property of a semiconductor thin film (for which a polycrystal silicon film is ordinarily used) constituting the EL drive TFT.
Since dispersion in threshold voltage and mobility results in dispersion in a drive current value of the EL element, emission intensity disperses to cause minute unevenness to be seen in representation.
Such unevenness in representation becomes particularly problematic when a drive current value is small to represent half tone.
Several measures have been devised in order to suppress that nonuniformity in representation, which is caused by such dispersion in the characteristics of an EL drive TFT.
For example, JP-A-11-219133 discloses a method, in which dispersion in a drive current value is suppressed by making channel length and channel width of an EL drive TFT fairly greater than an average crystal particle size of polycrystal silicon constituting the EL drive TFT (referred below to as second prior technique).
Also, JP-A-2000-3305027 discloses a drive method by a so-called pulse-width modulation, in which an EL drive TFT is driven as a binary switch for effecting a complete OFF state or a complete ON state and tone of a picture is represented by changing a duration of emission (referred below to as third prior technique).
Also, JP-A-11-73158 discloses an area tone system, in which a plurality of EL elements having different luminescent areas are provided in a unit pixel, and an EL drive TFT is connected to each of the plurality of EL elements and driven as a binary switch for effecting a complete OFF state or a complete ON state, whereby tone is represented by changing luminescent areas (referred below to as fourth prior technique).
Also, U.S. Pat. No. 6,229,506B1 discloses a method, in which four TFTs are provided in a pixel to constitute a circuit for cancelling dispersion in a threshold voltage of an EL drive TFT whereby dispersion in drive current is decreased (referred below to as fifth prior technique).
Also, JP-A-8-129359 discloses a method, in which a plurality of EL drive TFTs having different current drive capacities conformed to a plurality of tone currents are connected in parallel to one EL element within each pixel and driven as binary switches for effecting a complete OFF state or a complete ON state, whereby tone representation is controlled by tone currents supplied from the plurality of EL drive TFTs (referred below to as sixth prior technique).
Also, JP-A-2000-221903 discloses a method, in which two EL drive TFTs are provided in a pixel to decrease dispersion in threshold voltages in the EL drive TFTs, thereby reducing dispersion in drive current (referred below to as seventh prior technique).
However, the prior techniques described above involve the following problems.
The second prior technique is directed to averaging dispersion every location in the crystallizing property of the polycrystal silicon by increasing TFT size.
However, even when TFT size is increased, it cannot be made greater than pixel pitch.
Accordingly, since a size of an EL drive TFT for driving an EL element, which constitutes each pixel, is limited within an area of a pixel, and the crystallizing property of a polycrystal silicon film disperses every location, it is not possible to compensate for dispersion between the characteristics of an EL drive TFT in a particular pixel and the characteristics of an EL drive TFT in a pixel adjacent the particular pixel.
It is to be noted that what can be averaged by increasing a TFT size is only dispersion in crystals sized within the TFT size.
Accordingly, it is difficult in the second prior technique to obtain a fairly uniform property of representation.
For the effect of averaging picture representation with the third prior technique, the pulse-width modulation driving is one of valid methods as an AMOLED driving method as having already been proved.
However, known as an essential problem in this driving method is bleeding in a picture generated when animation called pseudo-profile is represented because tone representation is made by luminescence pulse, which is developed on time base.
Also, because of a need for processing a short signal pulse conformed to digital tone, there is caused a problem that the drive circuit is increased in operation frequency and power consumption.
Also, there is also caused a problem that a vertical scanning circuit, which may ordinarily be a simple circuit, becomes complex and a circuit area is increased.
The fourth prior technique is much effective in uniformizing picture representation, but multitone is difficult since it is necessary to form in a unit pixel EL elements having areas conformed to digital tone and to form EL drive TFTs corresponding to the respective EL elements.
Also, it has been known that EL elements are ordinarily decreased in luminescent areas together with operation duration.
In the case of using EL elements having different luminescent areas, deterioration is caused with time beginning with an EL element, which has a small area corresponding to a low-tone bit, thus causing also a problem that normal tone becomes difficult with time.
With the fifth prior technique, the provision of a circuit for canceling dispersion in threshold voltage of an EL drive TFT necessitates a wiring, which is not necessary in a conventional two-transistor configuration, so that a decrease in numerical aperture and yield in manufacture causes a problem.
Also, what can be cancelled is only dispersion in threshold voltage, and dispersion in mobility remains intact. Therefore, there is caused a problem that no fairly uniformizing effect is obtained on drive current.
With the sixth prior technique, a plurality of EL drive TFTs having current drive capacities conformed to digital tone are connected in parallel.
However, it is apparent that normal tone representation is made difficult when the plurality of EL drive TFTs disperse in characteristics.
Also, since the plurality of EL drive TFTs are formed in a single pixel in this method, the technique is in no way effective in decreasing dispersion in representation among a plurality of pixels.
With the seventh prior technique, dispersion in drive current can be decreased in the case where one of two EL drive TFTs connected in parallel is varied in characteristics, but dispersion in drive current cannot be decreased in the case where both the two EL drive TFTs are varied in characteristics, and besides the two EL drive TFTs are formed in a single pixel, so that the technique is in no way effective in decreasing dispersion in representation among a plurality of pixels.