1. Field of the Invention
The present invention relates to an active-matrix-type display apparatus wherein pixel circuits each including light-emitting elements, such as organic EL (Electroluminescence) elements, are arranged in a matrix and a fabrication method for the display apparatus.
2. Description of the Related Art
In an image display apparatus, such as a liquid crystal display unit, a great number of pixels are arrayed in a matrix, and in order to display an image, the light intensity is controlled for each of the pixels in response to the information of an image to be displayed.
While this similarly applies to an organic EL display unit or a like display unit, an organic EL display unit is a self-luminous type display unit which has a light emitting element for each of the pixel circuits. The organic EL display unit is advantageous in that, in comparison with a liquid crystal display unit, the visibility of an image is high and a backlight need not be provided, and the speed of response is high.
Further, the organic EL display unit is much different from the liquid crystal display unit in that the gradation of color development is obtained by controlling the luminance of each light-emitting element through the current value applied to the light-emitting element. In short, the light-emitting element is of the current controlled type.
While, in an organic EL display unit, a simple matrix system and an active matrix system can be applied as a driving system therefor, similarly to a liquid crystal display unit, the former system has a problem in that, while the structure is simple, it is difficult to implement a large display unit having high definition. Therefore, development of the active matrix system is carried out energetically, wherein current applied to a light-emitting element in the inside of each pixel circuit is controlled by an active element, usually by a TFT (Thin Film Transistor), provided in the pixel circuit.
FIG. 1 shows a configuration of a general organic EL display apparatus.
Referring to FIG. 1, the display apparatus 1 shown includes a pixel array section 2 wherein pixel circuits (PXLC) 2a are arranged in a m×n matrix, a horizontal selector (HSEL) 3, and a writing scanner (WSCN) 4. The display apparatus further includes signal lines or data lines SGL1 to SGLn selected by the horizontal selector 3 such that a data signal corresponding to luminance information is supplied thereto and scanning lines WSL1 to WSLm selectively driven by the writing scanner 4.
It is to be noted that the horizontal selector 3 and the writing scanner 4 may be formed from a MOSIC or the like on polycrystalline silicon or around the pixels MOSIC.
FIG. 2 shows an example of a configuration of a pixel circuit 2a shown in FIG. 1. It is to be noted that the configuration is disclosed, for example, in U.S. Pat. No. 5,684,365 or Japanese Patent Laid-Open No. Hei 8-234683.
Referring to FIG. 2, the pixel circuit shown has the simplest circuit configuration from among a great number of proposed circuit configurations and is a two-transistor driving type circuit.
The pixel circuit 2a includes a p-channel, thin-film field-effect transistor (hereinafter referred to as TFT) 11 and another TFT 12, a capacitor C11, and an organic EL light emitting element (OLED) 13 which serves as a light-emitting element. Further, in FIG. 2, reference characters SGL and WSL denote a signal line and a scanning line, respectively.
Since an organic EL light-emitting element in most cases has a rectification characteristic, it is sometimes called an OLED (Organic Light Emitting Diode). While a symbol of a diode is used to indicate a light-emitting element in FIG. 2 and so forth, the rectification characteristic is not necessarily required for the OLED in the following description.
The TFT 11 is connected at the source thereof to a power supply potential Vcc, and the light-emitting element 13 is connected at the cathode thereof to the ground potential GND. Operation of the pixel circuit 2a shown in FIG. 2 is described below.
Step ST1:
The scanning line WSL is placed into a selected state (here, into a low-level state), and a writing potential Vdata is applied to the signal line SGL. Consequently, the TFT 12 is rendered conducting to allow the capacitor C11 to be charged or discharged, and the gate potential of the TFT 11 is changed to the potential Vdata.
Step ST2:
The scanning line WSL is placed into a non-selected state (here, into a high-level state). Consequently, the signal line SGL and the TFT 11 are electrically isolated from each other. However, the gate potential of the TFT 11 is retained in stability by the capacitor C11.
Step ST3:
The current to be supplied to the TFT 11 and the light-emitting element 13 is changed to current which has a value corresponding to a gate-source voltage Vgs of the TFT 11, and the light-emitting element 13 continues to emit light with a luminance corresponding to the current value.
The operation for selecting the scanning line WSL to transmit the luminance information applied to a data line to the inside of a pixel, as at the step ST1, is hereinafter referred to as “writing.”
As described above, in the pixel circuit 2a shown in FIG. 2, once writing of the potential Vdata is performed, then the light-emitting element 13 continues to emit light with a fixed luminance until the next rewriting of a potential is performed.
As described above, in the pixel circuit 2a of FIG. 2, the gate application voltage of the TFT 11, which is a driving transistor, is changed to control the value of the current to be supplied to the EL light-emitting element 13.
At this time, the p-channel driving transistor is connected at the source thereof to the power supply potential Vcc, and the TFT 11 always operates in a saturation region. Therefore, the source of the driving transistor serves as a constant current source having a current value calculated in accordance with the following expression 1:Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)where μ, Cox, W, L, Vgs and Vth indicate the mobility of a carrier, the gate capacitance per unit area, the gate width, the gate length, the gate-source voltage of the TFT 11 and the threshold value of the TFT 11, respectively.
In the simple-matrix-type image display apparatus, each of light-emitting elements emits light only at a selected moment, but in the active-matrix-type image display apparatus, each light-emitting element continues to emit light also after the writing comes to an end, as described hereinabove. Therefore, the active-matrix-type image display apparatus is advantageous, particularly for a large display unit having a high definition, in that the peak luminance and the peak current of the light-emitting elements can be decreased in comparison with the simple-matrix-type image display apparatus.
FIG. 3 illustrates the aged deterioration of a current-voltage (I-V) characteristic of an organic EL light-emitting element. Referring to FIG. 3, a curved line indicated by a solid line indicates a characteristic in an initial state, and a curved line indicated by a broken line indicates a characteristic after aged deterioration.
Generally, as times passes, the I-V characteristic of an organic EL light-emitting element deteriorates as seen in FIG. 3.
However, in the two-transistor driving in FIG. 2, constant current is continuously supplied to the organic EL light emitting element in order to perform the constant current driving as described above. Therefore, even if the I-V characteristic of the organic EL light-emitting element deteriorates, the light emission luminance of the EL device does not deteriorate with time.
Incidentally, while the pixel circuit 2a of FIG. 2 is formed from the p-channel TFTs, if the pixel circuit 2a can be formed from n-channel TFTs, then an existing amorphous silicon (a-Si) process can be used for TFT production. Consequently, a reduction of the cost of a TFT substrate can be anticipated.
Now, a basic pixel circuit where each transistor is replaced with a n-channel TFT is described.
FIG. 4 shows a pixel circuit wherein the p-channel TFTs in the circuit shown in FIG. 2 are replaced with n-channel transistors.
The pixel circuit 2b shown in FIG. 4 includes a n-channel TFT 21 and another n-channel TFT 22, a capacitor C21, and an organic EL light-emitting element (OLED) 23 serving as a light-emitting element. Further, in FIG. 4, reference characters SGL and WSL denote a data line and a scanning line, respectively.
In the pixel circuit 2b, the TFT 21 serving as a driving transistor is connected at the drain side thereof to the power supply potential Vcc and at the source thereof to the anode of the EL light-emitting element 23 in such a manner as to form a source follower circuit.
FIG. 5 illustrates an operation point of the TFT 21 as the driving transistor and the EL light-emitting element 23 in an initial state. In FIG. 5, the axis of abscissa indicates the drain-source voltage Vds of the TFT 21, and the axis of ordinate indicates the drain-source current Ids of the TFT 21.
As seen in FIG. 5, the source voltage depends upon an operation point between the TFT 21 serving as the driving transistor and the EL light-emitting element 23, and has a value which differs depending upon the gate voltage.
Since the TFT 21 is driven in a saturation region, the current Ids is supplied which has a current value calculated in accordance with the equation represented by the expression 1 regarding the voltage Vgs corresponding to the source voltage at the operation point.