1. Field of the Invention
The present invention relates to an active-matrix type display device having an active element and more particularly to the active-matrix type display device having a self-emissive type device such as an organic EL (Electro-Luminescent) device.
The present application claims priority of Japanese Patent Application No.2000-217907 filed on Jul. 18, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, portable information terminals have become widespread rapidly as typified by i-mode portable cellular phones (i-mode is a trademark of NTT DOCOMO company) and, as a display device for such the portable information terminals, a liquid crystal display is widely used.
When a back light is incorporated into the liquid crystal display, luminance on an entire screen is increased, thus presenting a problem in that the liquid crystal display consumes much power. To solve this problem, a display into which an organic EL device is incorporated as the display device suitably used for portable information terminals (hereinafter referred to as an organic EL display device) is disclosed in Nikkei Electronics (March 15 issue, No. 765, 2000, pages 55-62).
Main contents described in the above literature will be described below.
As the display device using an emissive-type display device which emits light when a current flows, a PDP (Plasma Display Panel) and/or the EL display device are known. The EL display device is classified into an inorganic EL display device and the organic EL display device and is further classified by its structure into a simple-matrix type EL device and an active-matrix type EL device.
FIG. 3 is a schematic conceptual block diagram showing configurations of the conventional simple-matrix type organic EL display device. As shown in FIG. 3, the conventional simple-matrix type organic EL display device includes an EL device 31, a capacitor 32 connected between an anode and a cathode of the EL device 31, a data line 33 connected to the anode and a scanning line 34 connected to the cathode, which are mounted in a matrix form.
The conventional simple-matrix type organic EL display device further has a data line driving circuit 35 and a scanning line driving circuit 36. The data line driving circuit 35 activates one of the data lines 33 and the scanning line driving circuit 36 activates one of the scanning lines 34, thus passing currents through the EL devices 31 each connecting to the data line 33 and scanning line 34 from the data line 33 to the scanning line 34 and causing the EL device 31 to emit light at a value of a luminance corresponding to a value of the current.
Though structure of the simple-matrix type organic EL display device is comparatively simple and its manufacturing costs can be reduced, it is difficult to increase the number of pixels and difficult to achieve a high definition display device. In the simple-matrix type organic EL display device, since the scanning lines 34 are selected one by one to cause the pixels to emit light, emissive time of each of the pixels is 1/the number of scanning lines in one frame period. To maintain the luminance at a specified level within limited time, it is necessary to instantly pass a large electric current through each of the pixels, which presents basic problems in that the luminance becomes low as accumulated emissive time is lengthened and a life of emissive material is shortened due to flowing of such the large electric current as the driving current through the simple-matrix type organic EL display device.
Next, operations and configurations of the conventional active-matrix type organic EL display device will be described by referring to FIG. 4. The conventional active-matrix type organic EL display device includes an EL device 41, a TFT (Thin Film Transistor) 42 connected between an anode of the EL device 41 and a bias line 47, a TFT 43 connected between a gate of the TFT 42 and a data line 45, and a capacitor connected between a gate of the TFT 42, and the bias line 47, which are arranged in a matrix form.
The conventional active-matrix type organic EL display device further has a data line driving circuit 48 and a scanning line driving circuit 49, and bias voltage source 410. When a scanning line 46 is activated by the scanning line driving circuit 49, a TFT 43 connected to the activated scanning line 46 is brought into conduction and a current flows through a data line 45 and through the TFT 43 from the data line driving circuit 48 to a capacitor 44, causing the capacitor 44 to be charged.
When a gate voltage of the TFT 42 becomes higher than a threshold voltage, the TFT 42 becomes conducting, causing currents to be fed through the bias line 47 from a bias voltage source 410 to the El device 41 and causing the EL device 41 to emit light at a value of the luminance corresponding to a value of the current.
As is apparent from the above description, unlike in a case of the simple-matrix type organic EL display device, the active-matrix type organic EL display device has a characteristic that, even if the number of the scanning line is increased, same emissive time as frame period can be secured.
In the comparison of the active-matrix type liquid crystal display device with the active-matrix type organic EL display device, though transmittance (that is, it is equivalent to luminance of the active-matrix type organic EL display device) of the active-matrix type liquid crystal display device is proportional to a voltage applied to the liquid crystal, the luminance of the active-matrix type organic EL display device is proportional to a current and the voltage output from the bias voltage source 410 to the bias line 47 is maintained at a specified level.
Since the organic EL display device is a current-driven type display device, the TFT adapted to simply perform ON/OFF operations such as those used in the active-matrix type liquid crystal display device cannot be used and the TFT having on-resistance being small enough to pass sufficient currents is required.
Such the TFT is difficult to produce using technology to manufacture a general amorphous silicon TFT. To manufacture such the TFT, it is necessary to use a process of manufacturing low-temperature polysilicon TFT being used in some kind of a high definition display device.
If the low-temperature polysilicon TFT is used, it is possible to form the TFT and/or driving circuits on a glass substrate and, when multi-gray shades are generally displayed, almost all circuits on a scanning line side and partial circuits (selection switches) on a data line side are formed on the glass substrate and complicated circuits used to control gray shade displaying are implemented by semiconductor circuits formed on a single crystal substrate.
To achieve full color displaying, in the active-matrix type liquid crystal display device, red, green, and blue color filters are used. In the active-matrix type organic EL display device, the full color displaying is implemented by mounting organic EL devices each emitting light in red, green, or blue. However, this method presents problems in that a life of the organic EL emitting light in red is shorter than that of other organic EL emitting light in other colors and in that the color of the emitted light is not purely red but is nearly orange. Moreover, there is available another method in which the red color, green color, and blue color are mixed to produce white color and pixels each corresponding to each of the red, green, and blue colors are formed by using color filter as in a case of the liquid crystal display device.
However, in the above active-matrix organic EL display device, though the luminance can be controlled by the currents that are passed through the organic EL devices making up each of the pixels, since materials of the organic EL devices each emitting light in red, green, or blue are different, it is difficult to control production processes so that the luminance and life of each of the pixels are made equal.
Furthermore, when such the organic EL device is employed as the display device of portable cellular phones, reduction in power consumption in particular is required. However, in the conventional active-matrix type organic EL display device, time during which displayed contents are not changed exceeds a specified period of time, it is impossible to decrease the luminance for each of pixels, lines or frames, or to lower the luminance for the pixels, the lines or the frames making up an image requiring no more luminous display, thus making it difficult to greatly lower the power consumption.