1. Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to a device for driving pixels of an organic light emitting display device and a method of fabricating the same.
2. Description of the Related Art
Cathode ray tube (CRT) devices have been widely used for various kinds of image display devices. Recently, liquid crystal display (LCD) devices have emerged as an alternative display means for, especially, portable equipments, computer monitors, etc. However, CRT devices are generally heavy and have a big size, and LCD devices also have some unsatisfactory factors such as mediocre brightness, low efficiency, etc. In addition, LCD devices have such a drawback that images may have poor views at the side of an LCD device.
Thus, there have been made various developments for an image display device, as a new generation display means, having a lighter weight, a slimmer size, an affordable price, better efficiency, etc. One of such new generation display devices is an organic light emitting display (OLED) device. The OLED devices utilize the electroluminescence characteristics of certain organic compounds or high polymers, which emit light in response to electric current applied thereto. In the OLED devices, no backlight device is necessary for providing light to a display panel, which is required for the LCD devices. Thus, the OLED devices advantageously have a lighter weight, a smaller (and slimmer) size, a lower cost, etc. and are more readily fabricated compared with the LCD devices. In addition, the OLED devices may have superior brightness and a larger viewing angle.
FIG. 1 is a circuit diagram illustrating a conventional driving circuit for an OLED device, and FIG. 2 is a graphical view of signal waveforms applied to the driving circuit in FIG. 1. Referring to FIGS. 1 and 2, the conventional driving circuit for an OLED device includes a switching transistor QS having a gate and a source connected to a gate line Gq and a data line Dp, respectively, a storage capacitor Cst having one terminal connected to a drain of the switching transistor QS and the other terminal connected to a bias voltage Vdd, and a driving transistor QD having a gate connected to the drain of the switching transistor QS and a source connected to the bias voltage Vdd.
A driving signal is provided from a drain of the driving transistor QD to an organic light emitting diode OLED. The organic light emitting diode OLED has one end connected to a drain of the driving transistor QD and the other end connected to a common electrode voltage VCOM. Generally, the switching transistor QS is an N-type thin film transistor that is turned on by applying a high-level voltage signal to its gate, and the driving transistor QD is a P-type thin film transistor that is turned-off when the high-level voltage signal is applied to its gate.
In the operation the driving circuit in FIG. 1, when the switching transistor QS is turned on by a gate signal provided through the gate line Gq, a data signal from the data line Dp is transferred through the conduction path of the switching transistor QS to the gate of the driving transistor QD as a gate voltage. The gate voltage is maintained for one frame due to the storage capacitor Cst. At this time, channel conductance of the driving transistor QD is determined by the gate voltage applied to the gate and the bias voltage applied to the source of the driving transistor QD. Also, the intensity of a voltage applied between the ends of the organic light emitting diode OLED is determined based on a voltage distribution of the organic light emitting diode OLED with respect to the voltage between the bias voltage Vdd and the common electrode voltage VCOM, where the organic light emitting diode OLED and the driving transistor QD are connected each other in series. The organic light emitting diode OLED emits light in response to current flowing therein, which is corresponding to the intensity of the voltage determined based on the voltage distribution.
Thus, even if the same data signal is applied to the gate of the driving transistor QD SO that the gate-source voltage VGS of the driving transistor QD has an identically value in different driving circuits for different pixels of the OLED device, the voltage distribution may vary depending on characteristics of the driving transistor QD in different pixels so that the intensity of the voltage between the ends of organic light emitting diode may vary as well. As a result, the current flowing the organic light emitting diode OLED may be different in different pixels of the OLED device. Such variation in the current flowing the organic light emitting diode OLED may cause deterioration in brightness of pixels and display quality of the OLED device.
Therefore, it is desired that the driving circuit of an OLED device is improved so that every organic light emitting diode in the respective pixels of the OLED device receives the same driving current in response to the same data signal so as to emit the same amount of light.