In recent years, there has been an increasing demand for thin, lightweight, and fast response display devices. Correspondingly, research and development for organic EL (Electro Luminescence) displays and FEDs (Field Emission Displays) have been actively conducted.
Organic EL elements included in an organic EL display emit light at higher luminance for a higher voltage applied thereto and a larger amount of current flowing therethrough. However, the relationship between the luminance and voltage of the organic EL elements easily fluctuates by the influence of drive time, ambient temperature, etc. Hence, when a voltage control type drive scheme is applied to the organic EL display, it becomes very difficult to suppress variations in the luminance of the organic EL elements. On the other hand, the luminance of the organic EL elements is substantially proportional to current and this proportional relationship is less susceptible to external factors such as ambient temperature. Therefore, it is desirable to apply a current control type drive scheme to the organic EL display.
Meanwhile, pixel circuits and drive circuits of a display device are formed using TFTs (Thin Film Transistors) composed of amorphous silicon, low-temperature polycrystal silicon, CG (Continuous Grain) silicon, etc. However, variations easily occur in TFT characteristics (e.g., threshold voltage and mobility). In view of this, a circuit that compensates for variations in TFT characteristics is provided in a pixel circuit of an organic EL display, and by the action of this circuit variations in the luminance of an organic EL element are suppressed.
Schemes to compensate for variations in TFT characteristics in the current control type drive scheme are broadly classified into a current program scheme in which the amount of current flowing through a driving TFT is controlled by a current signal; and a voltage program scheme in which such an amount of current is controlled by a voltage signal. By using the current program scheme variations in threshold voltage and mobility can be compensated for, and by using the voltage program scheme only variations in threshold voltage can be compensated for.
However, the current program scheme has problems. Firstly, since a very small amount of current is handled, it is difficult to design pixel circuits and drive circuits. Secondly, since it is susceptible to parasitic capacitance while a current signal is set, it is difficult to achieve an increase in area. On the other hand, in the voltage program scheme, the influence of parasitic capacitance, etc., is very small and a circuit design is relatively easy. In addition, the influence exerted by variations in mobility on the amount of current is smaller than the influence exerted by variations in threshold voltage on the amount of current, and the variations in mobility can be suppressed to a certain extent in a TFT fabrication process. Accordingly, even in a display device to which the voltage program scheme is applied, sufficient display quality can be obtained.
For an organic EL display adopting the current control type drive method, various pixel circuits are conventionally known (e.g., Non-Patent Documents 1 to 4). FIG. 8 is a circuit diagram of a pixel circuit described in Non-Patent Document 4. A pixel circuit 900 shown in FIG. 8 includes a driving TFT 910, switching TFTs 911 to 913, a capacitor 921, and an organic EL element 930. All of the TFTs included in the pixel circuit 900 are of an n-channel type.
In the pixel circuit 900, the switching TFT 913, the driving TFT 910, and the organic EL element 930 are provided in series between a power supply wiring line Vp having a potential VDD and a cathode CTD of the organic EL element 930. The switching TFT 911 is provided between a source terminal of the driving TFT 910 and a data line Sj, the switching TFT 912 is provided between a gate terminal and a drain terminal of the driving TFT 910, and the capacitor 921 is provided between the gate terminal of the driving TFT 910 and the power supply wiring line Vp. Gate terminals of the respective switching TFTs 911 and 912 are connected to a control wiring line SLT, and a gate terminal of the switching TFT 913 is connected to a control wiring line TNO.
FIG. 9 is a timing chart of the pixel circuit 900. As shown in FIG. 9, first, at time t1, the potential of the control wiring line SLT is changed to a high level. Hence, the switching TFTs 911 and 912 are placed in a conducting state and thus a data potential Vda is applied to the source terminal of the driving TFT 910 from the data line Sj through the switching TFT 911. In addition, at time t1, the potential of the cathode CTD of the organic EL element 930 is also changed to a high level. Hence, a reverse bias voltage is applied between the anode and cathode of the organic EL element 930 and thus the organic EL element 930 is placed in a non-light emitting state. During a period from time t1 to time t2, since both the switching TFTs 912 and 913 are in a conducting state, the gate potential of the driving TFT 910 becomes equal to the potential VDD of the power supply wiring line Vp.
Then, at time t2, the potential of the control wiring line TNO is changed to a low level. Hence, the switching TFT 913 is placed in a non-conducting state and thus a current flows to the data line Sj from the gate terminal (and the drain terminal short-circuited thereto) of the driving TFT 910 through the driving TFT 910 and the switching TFT 911, and the gate potential of the driving TFT 910 gradually falls. When the voltage between the gate and source of the driving TFT 910 becomes equal to a threshold voltage Vth of the driving TFT 910 (i.e., when the gate potential reaches (Vda+Vth)), the driving TFT 910 is placed in a non-conducting state. At this point in time, the potential difference between the electrodes of the capacitor 921 reaches {Vp−(Vda+Vth)}. After this, the capacitor 921 holds this potential difference.
Then, at time t3, the potential of the control wiring line TNO is changed to a high level and the potential of the control wiring line SLT is changed to a low level. Hence, the switching TFTs 911 and 912 are placed in a non-conducting state and the switching TFT 913 is placed in a conducting state. Since the capacitor 921 holds the potential difference {Vp−(Vda+Vth)}, the gate potential of the driving TFT 910 remains at (Vda+Vth) even after time t3. In addition, at time t3, the potential of the cathode CTD of the organic EL element 930 is changed to a low level. Hence, a current according to a potential Vda (equal to a data potential) which is obtained by subtracting the threshold voltage Vth of the driving TFT 910 from the gate potential (Vda+Vth) of the driving TFT 910 flows to the organic EL element 930 from the driving TFT 910, and the organic EL element 930 emits light at a luminance according to the current.
As such, in the pixel circuit 900, the current flowing to the organic EL element 930 from the driving TFT 910 after time t3 is determined by the data potential Vda and thus is not influenced by the threshold voltage Vth of the driving TFT 910. Therefore, according to a display device including the pixel circuits 900, even when there are variations in the threshold voltage Vth of the driving TFT 910, by allowing a current according to the data potential Vda and the threshold voltage Vth to flow through the organic EL element 930, the organic EL element 930 can emit light at a desired luminance.    [Non-Patent Document 1] “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method”, SID'00 Digest, pp. 924-927, Semiconductor Energy Laboratory Co., Ltd.    [Non-Patent Document 2] “Continuous Grain Silicon Technology and Its Applications for Active Matrix Display”, AM-LCD 2000, pp. 25-28, Semiconductor Energy Laboratory Co., Ltd.    [Non-Patent Document 3] “Polymer Light-Emitting Diodes for Use in Flat Panel Display”, AM-LCD' 01, pp. 211-214, Semiconductor Energy Laboratory Co., Ltd.    [Non-Patent Document 4] “A new a-Si:H Thin-Film Transistor Pixel Circuit for Active-Matrix Organic Light-Emitting Diodes”, Electron Device Letters, IEEE, Volume 24, Issue 9, pp. 583-585, Korea Advanced Institute of Science and Technology