THIN FILM ELECTROLUMINESCENT DISPLAY DEVICE

Abstract: A thin film EL display device which comprises a group of parallel scanning electrodes, a group of parallel data electrodes laid so as to extend perpendicular to the group of the scanning electrodes, and an EL layer disposed between the respective groups of the scanning and data electrodes. Each of the electrodes of at least one of the groups of the scanning and data electrodes which apply a writing voltage to the EL layer is connected with a driver circuit of high voltage breakdown characteristics having only a push-pull function or a pull-up and pull-down function. This driver circuit employs thyristors as switching ele­ ments.

Description: The present invention generally relates to an AC driven, capacitive flat matrix display panel, that is, a thin film electroluminescent (EL) display device and, more particularly, to a drive circuit therefor. By way of example, a double insulated (or triple-layered) thin film EL element is constructed in the following manner. As shown in Fig. 6, the EL element shown therein comprises a glass substrate 101 having a group of strip-shaped transparent electrodes 102 made of In₂O₃ and deposited on one surface thereof in parallel relationship with each other, a three-layered structure including a layer 103 of dielectric material such as, for example, Y₂O₃, Si₃N₄ or Al₂O₃, an EL layer 104 made of ZnS doped with an activator such as, for example, Mn and a layer 103′ of dielectric material such as, for example, Y₂O₃, Si₃N₄ or Al₂O₃, which is sequentially formed over the group of the strip-shaped transparent electrodes 102 to a film thickness of 500 to 10,000Å by the use of a thin film technology such as, for example, a vapor-deposition technique or a sputtering technique, and a group of strip-shaped counter-electrodes 105 made of aluminum and deposited over the three-layered structure in parallel relationship with each other so as to extend in a direction perpendicular to the group of the strip-shaped transparent electrodes 102. Since the thin film EL element of the above described construction is such that a sandwich structure wherein the EL layer 104 is sandwiched between the dielectric layers 103 and 103' is disposed between the groups of the electrodes 102 and 105, the thin film EL element in question can be considered an equivalent of a capacitance element. Also, as can be readily understood from the graph of Fig. 7 illustrating the voltage versus luminance characteristic, this thin film EL element of the above described construction is adapted to be driven by the application of a relatively high voltage, for example, about 200V and is featured in that it can be energized by an alternating current field to emit rays of light of high luminance and in that it has a long lifetime. Hitherto, various drive circuits for the thin film EL element of the above described type have been proposed for the purpose of minimizing the consumption of an electric modulating power, the manufacturing cost and the thickness and/or size thereof. An example of such a prior art device is descrIbed in DE-A-3,634,686 in the name of the current applicants. This document describes a thin film electroluminescent display device in which power consumption is reduced by avoiding scanning blank lines of the display. In any one of the proposed prior art drive circuits, it is a general practice to connect scanning electrodes and data electrodes with driver IC circuit having only a push-pull function or a pull-up, pull-down function, and these switching elements are employed in the form of Nch MOS field-effect transistors, Nch transistors, Pch MOS field-effect transistors or Pch transistors because they can contribute to the minimization of the manufacturing cost and the bulkiness and also to the accomplishment of a highly integrated feature. It has, however, been found that, since these switching elements provides an output of low current and a high ON resistance, a phenomenon tends to occur wherein, in the EL display device, the lighting luminance tends to vary depending on the number of picture elements energized to light during the drive of one scanning line. This phenomenon is illustrated in Fig. 4 which illustrates a display screen of the EL display device. In Fig. 4, hatched bars 11 represents non-lighting portions and reference numeral 12 represents a lighting portion. Assuming that the lighting luminances at points (1), (2), (3) and (4) are expressed by B1, B2, B3 and B4, respectively, the luminances at these points have the following relationship. B1 > B2 > B3 > B4 In other words, the increase of the number of picture elements which are energized to light results in reduction of the luminance and, therefore, the display quality tends to be lowered. Where a gradation is desired to be displayed, this phenomenon cannot be neglected because it brings about a considerable adverse effect on the gradation display. By way of example, if the luminance at the point (4) is nine tenths (9/10) of the luminance at the point (1), the level of gradation at the point (1) which is 90% and the level of gradation at the point (4) which is 100% are generally equal to each other and, therefore, a normal gradation display is impossible. Also, when the display device is desired to be able to display a relatively large amount of information, that is, when the load capacitance of one scan line is desired to be increased as a result of the increased number of the data electrodes and the length of time required to accomplish one scan drive is desired to be reduced as a result of the increased number of the scanning electrodes, sufficient writing pulses cannot be applied because of the limitation imposed by the output current capacity and the ON resistance, and accordingly, sufficient lighting luminance cannot be obtained. As hereinabove discussed, there has been a problem associated with the improvement in display quality exhibited by the thin film EL display device. The present invention has been devised with a view to substantially eliminating the above discussed problem and aims to provide an improved thin film EL display device wherein change in luminance resulting from variation of the load capacitance of one scan line is advantageously suppressed, the display device therefore being capable of displaying gradation and an increased amount of information to be displayed more effectively. In accordance with the present invention there is provided a thin film EL dispalay device comprising an EL layer (104) disposed between superposed first and second groups of parallel electrodes, the electrodes in each group being arranged to be substantially perpendicular to those in the other group, and first and second drive circuits of high voltage breakdown characteristics for supplying signal voltages repsectively to said first and second electrode groups to enable writing voltages to be applied to said display device in accordance with data to be displayed, characterised in that the drive circuit for at least one of said electrode groups includes a plurlaity of thyristors, each thyristor being associated with a single electrode of said at least one group and being operable as a switching element to supply a selected signal voltage to that electrode. In the display device of the present invention, since each electrode of one of the groups of the scanning and data electrodes which apply writing voltages to the EL layer is connected with a drive circuit of high voltage break-down characteristic and push-pull function or pull-up and pull-down function and which employs thyristors as switching elements, the thyristors can be considered an equivalent diode. The thyristors are applied with trigger pulses (i.e., driven on) at the time the writing voltage is applied to the EL layer, and, since the current capacity and the ON resistance thereof depend on an external circuit, the current capacity and the ON resistances are sufficiently increased and reduced, respectively, as compared with MOS field-effect transistors. In other words, since the writing voltage can be applied in a relatively short period of time and regardless of the magnitude of the load capacitance, any possible change in luminance resulting from the change in load capacitance of one scan line can be advantageously suppressed and the increase of the capacity per scan line and the reduction in time required to accomplish one scan drive can also be achieved, that is, the thin film EL display device capable of displaying the increased amount of information can be realized. This and other objects and features of the present invention will become clear from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which: Figs. 1(a) and 1(b) and Fig. 2 are diagrams illustrating a drive circuit for a thin film EL display device according to one embodiment of the present invention; Fig. 3 illustrates a timing chart used to explain the operation of the circuit shown in Fig. 2 and waveforms of various voltages applied to picture elements; Fig. 4 is a diagram showing a display screen of the thin film EL display device illustrating change in luminance with change in number of the picture elements energized to light; Fig. 5 is a graph showing the relationship between the output voltage and the current characteristic of switching elements of a driver IC circuit of high breakdown voltage characteristic; Fig. 6 is a schematic perspective view, with a portion cut away, of the thin film EL display device; and Fig. 7 is a graph showing the applied voltage versus luminance characteristic of the thin film EL display device. Referring to Figs. 1(a) and 1(b) and Fig. 2 showing a drive circuit according to one embodiment of the present invention, reference numeral 1 represents a thyristor employed as a pull-up switching element in a push-pull bidirectional driver IC circuit of high voltage breakdown characteristic, reference numeral 2 represents a thyristor employed as a pull-down switching element, and reference numerals 3 and 4 represent respective diodes used to supply an electric current to the associated thyristors 1 and 2 in a reverse direction. Reference numeral 5 represents a drive circuit and a control logic circuit for providing a trigger pulse to the thyristors 1 and 2. Reference numeral 10 represents a thin film EL display device having a lighting threshold voltage Vth (VW <Vth < Vw + Vm) , it being, however, to be noted that a column of electrodes, which serve as data electrodes, and a row of electrodes which serve as scanning electrodes are illustrated in Fig. 2 to show the thin film EL display device 10. Reference numerals 20 and 30 represent scanning-side push-pull bidirectional driver IC circuits of high voltage breakdown characteristic associated respectively with the odd-numbered and even-numbered lines of the row of the scanning electrodes. Each of the driver IC circuits 20 and 30 has a logic circuit 21 or 31 such as, for example, a shift register, which is operable in response to a control signal such as "PDW" to create a condition in which a pull-up element or a pull-down element can be turned ON in correspondence with "Scan data" in the shift register and also to create a condition in which the pull-up element or the pull-down element can be turned ON regardless of "Scan data". Reference numeral 40 represents a data-side push-pull bidirectional driver IC circuit of high voltage breakdown characteristic associated with the column of the data electrodes and including a logic circuit 41 such as, for example, a shift register. Reference numeral 100 represents a switching circuit for switching a pull-down common line potential of the scanning-side driver IC circuits 20 and 30. This switching circuit 100 is comprised of switches SW1, SW2 and SW3 operable in response to respective control signals "NVC", "NGC" and "NM2" to switch over between a writing voltage -Vw of negative polarity, 0V and a modulation voltage 1/2Vm, and a switch SW3′ capable of switching in a direction reverse to that of the switch SW3 in response to a control signal "NM2R". Reference numeral 200 represents a switching circuit for switching a pull-up common line potential of the scanning-side driver IC circuits 20 and 30. This switching circuit 200 is comprised of switches SW4 and SW5 operable in response to respective control signals "PVC" and "PM2" to switch over between a writing voltage Vw+Vm of positive polarity and the modulation voltage 1/2Vm. Reference numeral 300 represents a switching circuit for switching a pull-up common line potential of the data-side driver IC circuit 40. This switching circuit 300 is comprised of a switch SW6 operable in response to a control signal "M1" to switch over between the modulation voltage 1/2Vm and a floating condition, and a switch SW6′ operable in response to a control signal "M1R" to perform a switching operation in a direction reverse to that of the switch SW6. Reference numeral 400 represents a power supply circuit operable in response to a control signal "MDM" to switch a switch SW8 on to charge a modulation voltage 1/4Vm on a capacitor Cm, and also operable in response to a control signal "MUP" to switch the switch SW8 off subsequent to the charging on the capacitor Cm and to switch a switch SW7 on to supply a modulation voltage 1/2Vm subsequent to the supply of the modulation voltage 1/4Vm. This power supply circuit 400 is connected with the switches SW3, SW5 and SW6 adapted to be controlled by the respective control signals "NM2", "PM2" and "M1". On the other hand, when the switch SW3′ or a switch SW6′ is switched on in response to the control signal "NM2R" or the control signal "M1R" and the switch SW8 is switched on in response to the control signal "MDW", this power supply circuit 40 operates to cause a portion of energies accumulated in the EL display device to be stored in the capacitor Cm. Reference numeral 500 represents a data reversal control circuit. The operation of the circuit shown in Figs. 1(a) and 1(b) and Fig. 2 will now be described with reference to the timing chart shown in Fig. 3. For the purpose of the description of the present invention, it is assumed that the scanning electrode Y1 including a picture element A and the scanning electrode Y2 including a picture element B are selected by a line sequence drive. Also, while this drive device is driven by reversing the polarity of the writing voltage applied to the picture element per line, the one-line driving timing during which the pull-down thyristors in the driver IC circuits 20 and 30 connected with the group of the scanning electrodes are triggered on to apply the negative writing pulse to the associated picture elements on the electrode lines is hereinafter referred to as the "N-drive timing", and the one-line driving timing during which the pull-up thyristors in the driver IC circuits 20 and 30 are triggered on to apply the positive writing pulse to the associated picture elements on the electrode lines is hereinafter referred to as the "P-drive timing". Also, the field (picture) in which an N-drive and a P-drive are effected to the odd-numbered scanning-side lines and the even-numbered scanning-side lines is hereinafter referred to as the "NP field", and the converse is referred to as the "PN field". (A) NP Field 1. First Modulation Voltage Charging Period (TN1) During N-Drive All of the scanning electrodes are retained at 0 volt by causing the pull-down thyristors of all scanning-side drivers SDr1 to SDri to be switched on and causing the switch SW2 to be switched on in response to the control signal "NGC". Simultaneously therewith, the switch SW6 is caused to be switched on in response to t he control signal "M1". At this time, data-side drivers DDr1 to DDri cause the pull-up thyristors to be switched on when lighting is to be made according to a data signal "DATA", but causes the pull-down thyristors to be switched on when non-lighting is to be made. Assuming that the lighting takes place when a display data signal is "H" and the non-lighting takes place when the display data signal is "L", it is necessary for an input display data ("DATA″) to be directly inputted to the data-side driver IC circuit 40 and, therefore, a reversing signal "RVC" in the data reversal control signal is left to be "L". (However, it is assumed that, when the data signal is "H", the pull-up thyristors and the pull-down thyristors in the data-side driver IC circuit 40 are switched on and off, respectively, but when the data signal is "L", the pull-up thyristors and the pull-down thyristors in the circuit 40 are switched off and on, respectively. Also, because of the line sequence drive taking place, the display data "DATA" is transferred during the previous line drive and retained by a latch.) Then, the switch SW8 is caused to be turned on in response to the control signal "MDW" to charge the voltage 1/4Vm on the capacitor Cm. Thereafter, by causing the switch SW7 to be switched on in response to the control signal "MUP" after the switch SW8 has been switched off in response to the control signal "MDW", a first modulation voltage of 1/2Vm is stepwisely charged on a data-side only on lighting picture elements and will not be charged on non-lighting picture elements with the consequence that the potential on the data electrodes of the non-lighting picture elements becomes 0V. Upon the completion of the charging, the switches SW6 and SW7 are turned off. 2. Second Modulation Voltage Charging and Writing Period (TN2) During N-Drive The pull-down thyristors only in the drivers connected with the selected scanning electrodes are turned on and the pull-up thyristors in the other scanning-side drivers are turned on. Simultaneously therewith, the modulation voltage 1/4Vm is applied to the pull-up common lines of all the scanning-side driver IC circuits 20 and 30 by causing the switch SW5 to be turned on in response to the control signal "PM2" and, subsequently, the modulation voltage 1/2Vm is applied thereto by causing the switch SW7 to be turned on in response to the control signal "MUP". Also, the negative writing voltage -Vw is applied to the pull-down common lines of all the scanning-side driver IC circuits 20 and 30 by causing the switch SW1 to be turned on in response to the control signal "NVC". On the other hand, the data-side driver IC circuit 40 continues the drive for the first modulation voltage charging period (TN1) during the above described N-drive. Thereby, the modulation voltage 1/2Vm is charged on the lighting picture elements on the data side during the first modulation voltage charging period (TN1) during the N-drive and, therefore, the potential at the data electrodes will be Vm. Also, since at the same time the negative writing voltage -Vw is applied to the selected scanning electrodes, the lighting picture elements are applied with the voltage Vm-(-Vw)=Vw+Vm with the consequence that they are energized to light. Also, since the potential of the data electrodes of the lighting picture element is 0V and the negative writing voltage -Vw is applied to the selected scanning electrodes of the lighting picture elements as hereinabove described, the non-lighting picture elements are applied with the voltage 0V-(-Vm)=Vw , however, the non-lighting picture elements will not be energized because of the voltage being lower than the lighting threshold voltage Vth. 3. Writing Voltage Discharge and Second Modulating Voltage Recovery Period (TN3) During N-Drive When the pull-down thyristors of all the scanning-side drivers SDr1 to SDri are turned on after the switches SW1, SW5 and SW7 have been turned off in response to the respective control signals "NVC", "PM2" and "MUP", the writing voltage is discharged causing the potential at all the scanning electrodes to be 1/2Vm. Then, by causing the switches SW3′ and SW8 to be turned on in response to the respective control signals "NM2R" and "MDW", a portion of the charge accumulated during the second modulation voltage charging period (TN2) with the scanning electrodes being plus is charged on the capacitor Cm, thereby rendering the potential at all the scanning electrodes to be 1/4Vm. On the other hand, the potential of the electrodes connected with the lighting picture elements on the data electrodes becomes 3/4Vm. 4. Second Modulation Voltage Discharge and First Modulating Voltage Recovery Period (TN3) During N-Drive After the switches SW3′ and SW8 have been turned off in response to the respective control signals "NM2R" and "MDW", the switching on of the switch SW2 in response to the control signal "NGC" results in that the potential at the scanning electrodes become 0V. Also, the potential of the electrodes connected with the data-side picture elements becomes 1/2Vm. By causing the switches SW6′ and SW8 to be turned on in response to the respective control signals "M1R" and "MDW", a portion of the charge accumulated during the first modulation voltage charging period (TN1) with the data electrodes being plus is charged on the capacitor Cm, thereby rendering the potential at all the data electrodes to be 1/4Vm. 5. First Modulation Voltage Charging Period (TP1) During P-Drive All of the scanning electrodes are retained at 0 volt by causing the pull-down thyristors of all scanning-side drivers SDr1 to SDri to be switched on and causing the switch SW2 to be switched on in response to the control signal "NGC". Simultaneously therewith, the switch SW6 is caused to be switched on in response to t he control signal "M1". At this time, data-side drivers DDr1 to DDri cause the pull-down thyristors to be switched on when lighting is to be made according to an inverted signal of the data signal "DATA", but causes the pull-up thyristors to be switched on when non-lighting is to be made. Since the inverted signal of the input display data signal "DATA" need be inputted to the data-side driver IC circuit 40, the reversing signal "RVC" in the data reversal control circuit 50 is left to be "H". Then, the switch SW8 is caused to be turned on in response to the control signal "MDW" to charge the voltage 1/4Vm on the capacitor Cm. Thereafter, by causing the switch SW7 to be switched on in response to the control signal "MUP" after the switch SW8 has been switched off in response to the control signal "MDW", the first modulation voltage of 1/2Vm is stepwisely charged on the data-side only on the non-lighting picture elements. At this time, no lighting picture element is charged and the potential on the data electrodes of the lighting picture elements becomes 0V. Upon the completion of the charging, the switches SW6 and SW7 are turned off. 6. Second Modulation Voltage Charging and Writing Period (TP2) During P-Drive The pull-up thyristors only in the drivers connected with the selected scanning electrodes are turned on and the pull-down thyristors in the other scanning-side drivers are turned on. Simultaneously therewith, the positive writing voltage Vw+Vm is applied to the pull-up common lines of all the scanning-side driver IC circuits 20 and 30 by causing the switch SW4 to be turned on in response to the control signal "PVC". Also, the modulation voltage 1/4Vm is applied to the pull-down common lines of all the scanning-side driver IC circuits 20 and 30 by causing the switch SW3 to be turned on in response to the control signal "NM2" and, subsequently, the modulation voltage 1/2Vm is applied thereto by causing the switch SW7 to be turned on in response to the control signal "MUP". On the other hand, the data-side driver IC circuit 40 continues the drive for the first modulation voltage charging period (TP1) during the above described P-drive. Thereby, since the second modulation voltage of 1/2Vm is charged on the lighting picture elements on the data side and since at the same time the positive writing voltage Vw+Vm is applied to the selected scanning electrodes, the lighting picture elements are applied with the voltage (Vw+Vm)-0V=VW+Vm with the consequence that they are energized to light. Also, since the modulation voltage 1/2Vm is charged on the non-lighting picture elements on the data electrodes, the potential at the data electrode becomes Vm. At the same time, since the selected scanning electrodes are applied with the positive writing voltage Vw+Vm as hereinabove described, the non-lighting picture elements are applied with the voltage (Vw+Vm)-Vm=Vw , however, the non-lighting picture elements will not be energized because of the voltage being lower than the lighting threshold voltage Vth. 7. Writing Voltage Discharge and Second Modulating Voltage Recovery Period (TP3) During P-Drive When the pull-down thyristors of all the scanning-side drivers SDr1 to SDri are turned on after the switches SW4, SW3 and SW7 have been turned off in response to the respective control signals "PVC", "NM2" and "MUP", the writing voltage is discharged causing the potential at all the scanning electrodes to be 1/2Vm. Then, by causing the switches SW3′ and SW8 to be turned on in response to the respective control signals "NM2R" and "MDW", a portion of the charge accumulated during the second modulation voltage charging period (TP2) with the scanning electrodes being plus is charged on the capacitor Cm, thereby rendering the potential at all the scanning electrodes to be 1/4Vm. On the other hand, the potential of the electrodes connected with the non-lighting picture elements on the data electrodes becomes 3/4Vm. 8. Second Modulation Voltage Discharge and First Modulating Voltage Recovery Period (TP3) During P-Drive After the switches SW3′ and SW8 have been turned off in response to the respective control signals "NM2R" and "MDW", the switching on of the switch SW2 in response to the control signal "NGC" results in that the potential at the scanning electrodes become 0V. Also, the potential of the electrodes connected with the data-side picture elements becomes 1/2Vm. By causing the switches SW6′ and SW8 to be turned on in response to the respective control signals "M1R" and "MDW", a portion of the charge accumulated during the first modulation voltage charging period (TP1) with the data electrodes being plus is charged on the capacitor Cm, thereby rendering the potential at all the data electrodes to be 1/4Vm. (B) PN Field 1. First Modulation Voltage Charging Period (TP5) During P-Drive The drive similar to that taking place during the first modulating charging period (TP1) under the NP field P-drive takes place. 2. Second Modulation Voltage Charging and Writing Period (TP6) During P-Drive The drive similar to that taking place during the second modulation voltage charging and writing period (TP6) under the P-drive takes place. 3. Writing Voltage Discharge and Second Modulating Voltage Recovery Period (TP7) During P-Drive The drive similar to that taking place during the writing voltage discharge and second modulation voltage recovery period (TP7) under the P-drive takes place. 4. Second Modulation Voltage Discharge and First Modulating Voltage Recovery Period (TP8) During P-Drive The drive similar to that taking place during the second modulation voltage discharge and first modulation voltage recovery period (TP8) under the P-drive takes place. 5. First Modulation Voltage Charging Period (TN5) During N-Drive The drive similar to that taking place during the first modulation voltage charging period (TN1) under the NP field N-drive takes place. 6. Second Modulation Voltage Charging and Writing Period (TN6) During N-Drive The drive similar to that taking place during the second modulation voltage charging and writing period (TN2) under the NP field N-drive takes place. 7. Writing Voltage Discharge and Second Modulating Voltage Recovery Period (TN7) During N-Drive The drive similar to that taking place during the writing voltage discharge and second modulation voltage recovery period (TN7) under the N-drive takes place. 8. Second Modulation Voltage Discharge and First Modulating Voltage Recovery Period (TN8) During N-Drive The drive similar to that taking place during the second modulation voltage discharge and first modulation voltage recovery period (TN4) under the N-drive takes place. As hereinbefore discussed, the drive circuit herein disclosed has the NP and PN field drive timings such t hat, during the NP field, the N-drive and the P-drive are effected to the odd-numbered selection lines on the scanning side and the even-numbered selection lines on the scanning side, respectively, but during the PN field, the drive substantially reverse to that described above is carried out for the purpose that alternating current pulses necessary to effect the lighting can be applied to all of the picture elements in the thin film EL display device. In Fig. 3, examples of waveforms of voltages applied to the picture elements A and B are illustrated. In the meantime, in the conventional drive circuit, as switching elements in driver IC circuits of high voltage breakdown characteristic on the scanning side, Nch MOS field-effect transistors, Nch transistors, Pch MOS field-effect transistors or Pch transistors are generally employed and, therefore, not only are the reduction in manufacturing cost and size limited, but the extent to which it is highly integrated is also limited. Because of these limitations, the output current power tends to be low and the ON resistance tends to be high. The use of the thyristors for these switching elements of high voltage breakdown characteristic brings about such an advantage that the considerable reduction of the manufacturing cost and the size and the highly integrated feature can be advantageously realized (The thyristor is a self-retaining element, that is, an element wherein, once the thyristor is triggered on by the application of a gate signal, that is, when a trigger pulse is applied thereto, the thyristor can retain the ON state even though the gate signal disappears. Because of this feature, as compared with the MOS field-effect transistor and other transistors, a level shifter circuit can be advantageously simplified. Also, a high voltage breakdown characteristic and a high current control can be easily accomplished. In other words, the chip size can be reduced considerably.), and the use of the thyristors can provide such a feature as shown in Fig. 5. Fig. 5 illustrates the relationship between the output voltage and the electric current of the switching element of high voltage breakdown characteristic, wherein the curve 31 is exhibited when the thyristor is employed as the switching element (during the ON state) and the curve 32 is exhibited when a MOS field-effect transistor or a transistor is employed as the switching element. As can be understood from this figure, as compared with the MOS field-effect transistor and the transistor, the thyristor exhibits a high output current power and a sufficiently low ON resistance. Because of this, the phenomenon hitherto experienced wherein the increase of the number of the lighting picture elements results in the reduction in luminance, that is, the increase of the load to such an extent that no writing pulse can longer be applied sufficiently resulting in the difference between the intensity of light when the number of the picture elements lit is large and that when the number of the picture elements lit is small (i.e., resulting in the difference in luminance) can be advantageously eliminated, thereby contributing to the improvement in display quality. The display of gradations can readily be accomplished because of the luminance characteristic such as discussed above can be controlled. Also, since the EL display device is an equivalent to a capacitor, the reduction of the output current power results in the reduction of the length of time required to effect the charging and also the reduction of the length of time required to accomplish one scan drive. In other words, the drive of the EL display device having a multiple of scanning electrodes can be accomplished and the drive is possible even though the load capacitance of one scan line is increased as a result of the increase of the data electrodes. In the foregoing embodiment, both of the scanning electrodes and the data electrodes have been shown and described as connected with the high voltage breakdown driver IC circuits having a push-pull function and employing the thyristors as the switching elements. However, the present invention can be equally applicable to the drive circuit for the EL display device wherein both of the scanning electrodes and the data electrodes are connected with high voltage breakdown driver IC circuits having only a pull-up function or only a pull-down function. Also, in the illustrated thin film EL display device, it suffices that the driver circuit wherein the thyristors are employed as the switching elements is connected only to the electrode to which at least the writing voltage is applied. In the case of the drive circuit wherein the writing voltage is applied from the data electrodes, it is recommended to connect the driver circuit using the thyristors as the switching element to the data electrodes. From the foregoing description of the present invention, since each electrodes of one of the groups of the scanning and data electrodes which applies the writing voltage to the EL layer is connected with the driver circuit of high voltage breakdown characteristic having the push-pull function or the pull-up and pull-down function and employing the thyristors as switching elements, any possible change in luminance dependent on the number of the picture elements lit can be eliminated, the display of gradations can be facilitated, and a large information display capability can be achieved with the increased number of any one of the scanning electrodes and the data electrodes, all without the conventional merits being sacrificed.

Claims: A thin film electroluminescent (EL) display device (10) comprising an EL layer (104) disposed between superposed first and second groups of parallel electrodes (X i ,Y i ;102,105), the electrodes in each group being arranged to be substantially perpendicular to those in the other group, and first and second drive circuits (20,30,40) of high voltage breakdown characteristics for supplying signal voltages repsectively to said first and second electrode groups (X i ,Y i ;102,105) to enable writing voltages to be applied to said display device (10) in accordance with data to be displayed, characterised in that the drive circuit (20,30,40) for at least one of said electrode groups includes a plurality of thyristors (1,2), each thyristor being associated with a single electrode (X i ,Y i ) of said at least one group and being operable as a switching element to supply a selected signal voltage to that electrode. A thin film EL display device as claimed in claim 1 wherein the drive circuit (20,30,40) for said at least one electrode group is operable with a push-pull function. A thin film EL display device as claimed in claim 1 or claim 2 wherein each electrode (X i ,Y i ) of said at least one electrode group is connected to first and second thyristors (1,2) arranged to act with pull-up and pull-down function respectively. A thin film EL display device as claimed in claim 3 wherein said first and second thyristors (1,2) are reciprocally operable to supply said selected signal voltage to the electrode (X i ,Y i ), each thyristor (1,2) supplying one of a respective plurality of signal voltages thereto. A thin film EL display device as claimed in any preceding claim wherein the electrodes of said at least one group are scanning electrodes (Y i ). A thin film EL dispaly device as claimed in any one of claims 1 to 4 wherein the electrodes of said at least one group are data electrodes (X i ).

Inventors: INOHARA, AKIO, KISHISHITA, HIROSHI, OHBA, TOSHIHIRO, SHOJI, KAZUO, UEDA, HISASHI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G09G3
IPC codes: G09G3