Active display device

Abstract: In a display device driven with an active matrix the capacitances associated with the pixels (2) are first discharged or charged as far as beyond the range of transition in the transmission/voltage characteristic before they are accurately adjusted. A capacitive element (10) is used for discharging or charging.

Description: The invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of pixels arranged in rows and columns, with each pixel being defined by a first and a second picture electrode arranged on the facing surfaces of the supporting plates, a system of row and column electrodes for driving the pixels, each pixel being associated with at least one first asymmetrical non-linear switching element being connected in series between the first picture electrode of the pixel and a row electrode, the second picture electrode of a pixel being electrically connected to a column electrode. A display device of this type is suitable for displaying alphanumerical information and video information by means of passive electro-optical display media such as liquid crystals, electrophoretic suspensions and electrochromic materials. A display device of the type described in the opening paragraph is known from NL-A-8701420. In a display device shown in this Application the pixels are given a certain adjustment for each row in that the capacitances associated with these pixels are accurately charged or discharged after they have first been discharged or charged too far (either or not accurately). To this end such a picture display device is provided with means for applying, prior to selection, an auxiliary voltage across the pixels beyond or on the edge of the voltage range to be used for picture display. In one of the embodiments this is effected by means of diodes which are connected to a suitably chosen reference voltage. A drawback of such a display device is that voltage lines must be provided between the pixels in the column direction for the reference voltage. Usually one or two column electrode(s) are alternately provided between the columns of pixels, namely one electrode for the reference voltage, two column electrodes, and so forth. Such a division is not only at the expense of the effective picture surface area but also gives rise to artefacts in the picture. A second drawback is that the picture electrodes, the column electrodes and the switching elements are realised on one and the same supporting plate, while the column electrodes, likewise as the electrodes for the reference voltage, may be implemented as metal lines. The row electrodes are then provided on the other supporting plate and simultaneously constitute the counter electrodes of the picture electrodes. Therefore, these row electrodes are implemented as light-transmissive electrodes of, for example indium tin oxide (having a width which is equal to the height of the picture electrodes). Such indium tin oxide electrodes usually have a high resistance so that accurate charging during one line period is not always possible. Moreover, a so-called delta-colour filter configuration cannot be used without special measures in said display device. It is one of the objects of the present invention to provide a display device of the type described in the opening paragraph, which device has a large effective surface area and in which delta-colour filter configurations are readily applicable. It is a further object of the invention to provide a display device in which an accurate adjustment of the pixels is possible. A display device according to the invention is characterized in that each pixel is further associated with at least one second asymmetrical non-linear switching element connected in series between the end of the first asymmetrical non-linear switching element which is connected to the first picture electrodes and a connection for applying a reference voltage and that at least one capacitive element is connected parallel to the series arrangement of the first and second non-linear switching elements. The invention is based, inter alia, on the recognition that the pixels can be discharged or charged as far as beyond the range to be used for picture display by making use of charge which has been stored in the capacitive element. The capacitive element functions, as it were, as a charge reservoir for charging (positive or negative) charge by means of which the pixel can be charged or discharged as far as beyond the voltage transmission range. This charging or discharging is no longer effected via a reference electrode on the same supporting plate and in the same direction as the column electrodes, but via a reference electrode in the row direction. The electrodes in the row direction (row and reference electrodes) can now be implemented as low-ohmic metal strips, thus precluding a number of said drawbacks (high row resistances, problems in using delta-colour filter configuration). At least a part of a row electrode preferably constitutes a first electrode of the capacitive element. In a first preferred embodiment of a display device according to the invention for pixels associated with a row the connections for applying a reference voltage are interconnected to form a common electrode which is connected to an external connection via at least a third non-linear switching element. The level of the charge in the charge reservoir is maintained via this connection. The third non-linear switching element may be present within or outside the actual display device. The common electrode preferably constitutes a second electrode of the capacitive element. This provides the possibility of implementing the capacitive elements associated with a row of pixels as two substantially superjacent metal lines with a layer of dielectric material being interposed. In this case the drawback of the occurrence of artefacts in the picture is also obviated. In a second preferred embodiment of a display device according to the invention a non-linear resistance element is arranged parallel to the capacitive element. The capacitive element and the non-linear resistance element may be realised as a metal-isolator-metal element. The leakage current through the non-linear resistance element now ensures the supply to the charge reservoir. A first electrode of such a metal-isolator-metal element may form part of a row electrode. In this case it is possible to implement the metal-isolator-metal elements associated with a row of pixels as a row electrode and a substantially subjacent or superjacent row of metal strips with a layer of dielectric material being interposed. For example, tantalum is chosen for the lower metal layer or strip and tantalum oxide is chosen for the layer of dielectric material. The latter may be deposited by means of electro-deposition. On the other hand, for example, chromium or aluminium may be chosen for the metal layer or strip while silicon nitride or oxynitride (provided by way of sputtering or evaporation techniques) is chosen as a dielectric material. For the non-linear switching elements diodes are preferably chosen such as, for example a pn diode, Schottky diode, pin diode, but also other asymmetrical non-linear switching elements are possible such as, for example a transistor having a short-circuited base collector, implemented in monocrystalline, polycrystalline or amorphous silicon, CdSe or another semiconductor material, while the diodes may be implemented both vertically and laterally. For reasons of redundancy, an asymmetrical non-linear switching element may alternatively be built up from a plurality of subelements. To charge or discharge all pixels in a uniform way, it may be advantageous to keep the column voltages equal to zero volt during the reset voltage. Moreover, the reset voltage may then be lower. The invention will now be described in greater detail by way of example with reference to some embodiments and the drawings in which Fig. 1 shows diagrammatically a part of a display device according to the invention, Fig. 2 is a diagrammatic plan view of a part of the display device of Fig. 1, Fig. 3 shows some drive voltages and internal voltages in the display device of Fig. 1, Fig. 4 shows diagrammatically a modification of the display device of Fig. 1, while Fig. 5 is a diagrammatic plan view of a part of the display device of Fig. 4, and Fig. 6 shows some voltages associated with the display device of Fig. 5. Fig. 1 is a diagrammatic representation of a part of a display device 1 according to the invention, for example a liquid crystal display device. The pixels 2 arranged in rows and columns are located at the area of crossings of a system of column electrodes 3 and row electrodes 4. Asymmetrical non-linear switching elements, in this example diodes 5, are arranged between the picture electrodes 2 and the row electrodes 4. Each diode 5 is connected to a picture electrode 6 of a pixel 2. The other picture electrode 7 is connected to a column electrode 3 (see Fig. 1). The display device of Fig. 1 also comprises a second diode 8 arranged in series with the first diode 6, while a capacitive element 10 is arranged parallel to the series arrangement of the two diodes 6, 8 between the row electrode 4 and a node 9 which is common to the diode 8 and the capacitive element 10. In the present example the nodes 9 are interconnected by means of a row electrode 11 which is connected via a diode 12 (or another asymmetrical non-linear switching element) to a terminal 13 for a reference voltage V ref . In this example the row and column electrodes are provided with terminals 14 and 15, respectively. As will be described hereinafter, the display device shown can be driven by means of a similar drive mode as described in the Netherlands Patent Application no. 8701420 laid open to public inspection, referred to in the opening paragraph. Fig. 2 is a diagrammatic plan view of a part of the display device 1 of Fig. 1. A matrix of picture electrodes 6 at the location of the pixels is provided on a first supporting plate 16. The picture electrodes 6 are connected via diodes 5 and 8, shown diagrammatically, to a row electrode 4 and a superjacent electrode 11, respectively. In this example the row electrode 4 is made of tantalum on which a layer of tantalum oxide is deposited by anodic oxidation before the layer 11 of, for example aluminium is deposited thereon. The tantalum-tantalum oxide-aluminium structure constitutes a (divided) capacitance throughout the length of the structure between the lines 4 and 11, which capacitance is the physical realisation of the capacitive elements 10 of Fig. 1. The picture electrodes 7 of, for example indium tin oxide are arranged on the other supporting plate and in this example they coincide with the column electrodes. In Fig. 2 these are shown by means of broken lines 17. After the supporting plates thus formed have been provided, if necessary with a protective coating and/or a layer of orienting material, the display device is completed in a generally known manner by providing spacers, by sealing and filling, whereafter the assembly is provided, if necessary with polarisers, reflectors, etc. The device of Figs. 1, 2 comprises two metal conductors per row of pixels in the row direction. However, the metal conductors are arranged one above the other, thus increasing the effective surface area of the pixels with respect to the device according to NL-A-8701420 in which alternately two metal strips and one metal strip are located between columns of pixels. This also reduces the occurrence of artefacts. Since the row electrodes are now in the form of metal tracks, the pixels have a shorter charge time so that a more accurate adjustment is possible. Moreover, a wider choice of colour filters (for example socalled delta structures) is realised. Other asymmetrical non-linear switching elements may alternatively be chosen for the diodes 5, 8, 12, such as, for example pin diodes, Schottky diodes or a series or parallel arrangement of a plurality of diodes in connection with redundancy. The use of a series arrangement may be notably favourable if the asymmetrical non-linear switching element must be able to withstand a large voltage range. The device shown is very suitable for using a drive method in which V C = Vsat+Vth 2 is chosen for the average voltage across a pixel (with V th being the threshold voltage and V sat being the saturation voltage of the electro-optical effect) so that the absolute value of the voltage for picture display across the pixels 12 is substantially limited to the range between V th and V sat . A satisfactory operation as far as grey scales are concerned is obtained, if dependent on the data voltages V d on the column electrodes 3 the voltage values across the pixels 2 are V c +V dmax =V sat at a maximum and V c -V dmax =V th at a minimum. Elimination of V c yields: |V d | max = 1/2(V sat -V th ), i.e. -1/2(V sat -V th )≦V dmax ≦1/2(V sat -V th ). To charge a row of pixels 2, for example positively, the associated row electrode 4 is given a selection voltage V s =V on1 -1/2(V sat +V th ) in which V on1 is the forward voltage of the diode 5. The voltage across the pixels 2 is therefore V d -V on1 -V s ; it ranges between -1/2(V sat -V th )+1/2(V sat +V th )=V th and 1/2(V sat -V th )+1/2(V sat +V th )=V sat , dependent on V d . In the case of non-selection the requirement must be satisfied that neither diodes 5 nor diodes 8 can conduct, in other words, it must hold for the voltage V A at the node 18 that V A ≦V ns1 (1) and V A ≧V line (2) in which V ns1 is a non-selection voltage and V line is the voltage at line 11, or V Amax ≦V ns1 (1) and V Amin ≧V line = V ns1 -V cli (2) in which V cli is the minimum required voltage across the capacitive element 10 at which it continues to function as a charge reservoir. It follows from (1) that: V ns1 ≧V Amax =1/2(V sat -V th )-V th and it follows from (2) that V ns1 -V cli ≦V Amin =-1/2(V sat -V th )-V sat It follows for V cli that: V cli ≧V ns1 +1/2(V sat -V th )+V sat =2(V sat -V th ) In order to negatively charge the same row of pixels 2 (in a subsequent frame or field period) at a subsequent selection with inverted data voltages, these pixels are first negatively charged too far by means of a reset voltage V reset at the row electrode 11. Subsequently the selected row electrode (in the same line period or in a subsequent period) receives a selection voltage V s2 =-V on1 +1/2(V sat +V th ). The pixels 2 which are negatively charged too far are now charged via the diodes 5 to V d -V on1 -V s2 , i.e. to values between -1/2(V sat -V th )-1/2(V sat +V th )=-V sat and 1/2(V sat -V th )-1/2(V sat +V th )=-V th so that information having an opposite sign is presented across the pixels 2. When negatively charging too far in advance, it must be taken into account that the capacitive element may have lost a part of its charge having a quantity of ΔV Cl . The quantity ΔV Cl is maximum when the pixel 2 (and hence the capacitance Cp) is charged from V sat to -V sat . The capacitance Cl is then discharged by a quantity of ΔV Cl = 2 V sat C p Cl To keep ΔV Cl small, it is preferred to choose the ratio Cl/Cp 〉〉 1, for example 5 to 10. To this end (see Fig. 2) the metal lines 4, 11 can be arranged one over the other with a dielectric as an intermediate layer so that a capacitance is formed which has the value Cl for each width of one pixel (defined by the picture electrode 6 in Fig. 2). For example, the lower line 4 is made of tantalum which is anodised so that a dielectric of tantalum oxide is produced which is free from pin holes and has a high dielectric constant (ε r ≈ 24). With a width of the metal lines of 1/15 of the height of one pixel, it holds for a liquid crystal mixture ZLI 84460 of the firm of Merck (ε r ≈ 6) and thicknesses of the pixel and the tantalum oxide of 4.5 µm and 0.12 µm for Cl/Cp, respectively, that: Cl/Cp≃ 1 15 · 24 6 · 4.5 12 ≃ 10. Further, V sat ≃ 3.5 V so that with (6) ΔV Cl ≃ 0.7 V. As stated hereinbefore, this must be taken into account when charging negatively too far in advance. For the reset voltage used for this purpose it therefore holds in the worst case, namely if the highest voltage (V d = 1/2(V sat -V th )) is present at a column electrode 3: V reset ≧V Amax +V on2 +V Cli +ΔV Cl or V reset ≧1/2(V sat -V th )+V sat +V on2 +2(V sat -V th )+ΔV Cl where V on2 is the voltage across the diode 8 at the end of a reset period. After negatively charging too far and subsequent accurate negative adjustment of the pixels 2 a non-selection voltage V ns2 is applied again to the row electrodes 4. It holds again that V Amax ≦V ns2 (8) while V Amin ≧V line (9) or V ns2 ≧V Amax =1/2(V sat -V th )+V sat (10) (negative selection) and V ns2 -V Cl ≦V Amin =-1/2(V sat -V th )-V th (11) in which V Cl =V Cli +ΔV Cl . Combination of (10) and (11) yields V Cl ≧V ns2 +1/2(V sat -V th )-V th =2(V sat -V th ) (12) At the next selection pulse having a value of V s1 the pixel 2 is again charged positively, and simultaneously the capacitive element 10 (C l ) is charged in a positive sense via a third diode 12. For the reference voltage V ref to be connected to point 13 it then holds that V ref - V s1 + V Cli ≧ V on3 or V ref = V s1 - V Cli + V on3 (13) in which V on3 is the voltage drop across the diode 12 at the end of the selection time t s1 . With V Cli = 2(V sat -V th ) this will be V ref =-1/2(V sat +V th )-V on1 -2(V sat -V th )+V on3 The drive signals on a row electrode 4 for a row of pixels is shown in Fig. 3a, while Fig. 3b shows the associated voltages on the line 11 and Fig. 3c shows the voltage across the capacitive element. In the balanced situation (shown) the reservoir formed by the capacitive element 10 is sufficiently charged positively (to a value of -2(V sat -V th )) so that the loss of charge due to capacitive couplings is compensated again during the reset pulse. When a display device according to Figs. 1, 2 is switched on, the voltage across the capacitive element 10 (C l ) is zero Volt. At each reset pulse for the row 4 (dependent on its use, 25, 30, 50 or 60 times per second) C l is charged slightly more negative in voltage until the diode 12 starts to conduct during a selection pulse and C l charges slightly positively. This results in the situation of Fig. 3. For the cut-off voltage across the diode 12 it holds that it can reach a high value, namely: V cut-off ≦ 1/2(V sat -V th )+V sat +V on2 -V ref It is therefore recommended to use a plurality of diodes in series instead of one diode 12 so that the cut-off voltage for each diode is lower. This also ensures redundancy, which is desirable because a diode 12 must supply the current for an entire row (n pixels) during a reset, hence approximately n times as much as a diode 5. For the same desired current density this diode is also approximately n times as large as a diode 5. The diode 12 may also be common to a plurality of lines 11. Figs. 4 and 5 show modifications of the display device of Figs. 1 and 2. The lines 11 in Fig. 2 are periodically interrupted and constitute metal strips 19 which correspond to the nodes 9 of Fig. 4. Simultaneously, the metal strips 19 constitute the electrodes of a metal-isolator-metal structure comprising an electrode 4 of, for example tantalum, an interposed dielectric of tantalum oxide and the electrode 19. The MIM element implemented in this way is shown in Fig. 4 by the combination of the capacitive element 10 and the non-linear resistor 20. Otherwise, the reference numerals have the same significance as those in Figs. 1, 2. Charging the capacitive element in a positive sense, if it is negatively charged too far due to reset pulses, is now effected via the variable resistor 20 of the MIM. It is dimensioned in such a way that at a voltage value V Cl ≧ 2(V sat -V th ) across the capacitive element 10 (C l ) the leakage through the non-linear resistor 20 is substantially negligible so that it holds for the discharge ΔV Cl2 in the period between two reset pulses (for example 30 msec) that: ΔV Cl2 〈〈 V Cl Also in this case the voltage across Cl becomes slightly more negative at each reset pulse upon switch-on (with a maximum value per reset pulse of ΔV Cl1 = Cp/Cl.2V sat , cf. (6)). This continues until this negative charging is compensated by the leakage current in the non-linear resistor 20 in the period between two reset pulses. A stable state is then reached, at which ΔV Cl1 = ΔV Cl2 Fig. 6a shows the drive voltages on the row electrode 4 in a corresponding manner. The same values can be calculated for these voltages in a manner similar to that described above. Figs. 6b, 6c show, analogously as Figs. 3b, 3c, the voltages at the nodes 9 and those across the capacitive elements 10 C l . Due to the (small) leakage current these voltages are not substantially constant during non-selection, as in the device of Figs. 1, 2. As compared with the device of Figs. 1, 2, the device of Figs. 4, 5 has the advantage that a possible short circuit between the row electrode 4 and a metallisation strip 19 causes only the associated pixel to drop out, whereas in the case of a short circuit between the row electrode 4 and the line 11 in Figs. 1, 2 the entire row of associated pixels 2 drops out. As compared with other display devices, in which a MIM is used as a non-linear switching element, the device has the additional advantage that due to the desired small leakage current the metal-isolator-metal structure has a much thicker dielectric (comparable with the Ta₂O₅ layer in Fig. 2) and a larger surface area. As a result the risk of damage due to static electricity or high drive voltages is considerably smaller. The peak current is also much smaller because the current with which the capacitance Cp associated with the pixel 10 is charged during the reset pulse does not flow through R l but is supplied from C l . This results in a considerable extension of the lifetime. The invention is of course not limited to the examples described hereinbefore, but several variations are possible within the scope of the invention. For example, the diodes 5, 8, 12 can be given a reverse sign while simultaneously changing the values for the drive voltages. The row electrode 4 may alternatively be arranged above instead of below the line 11 and the metallisation strips 15, respectively. The diodes or other non-linear asymmetrical switching elements can be formed to be redundant, for example by using series and/or parallel circuits as described in Netherlands Patent Application NL-A-8800204. It may be advantageous to maintain the column voltages at zero value during the reset pulse so that the reset voltage can be lower, namely V sat +V on2 +2(V sat -V th )+ΔV Cl . All pixels in a row are each time charged to one and the same negative voltage in this case. The duration of the reset pulse is also dependent on the selection time t s , dependent on the use.

Claims: A display device (1) comprising an electro-optical display medium between two supporting plates, a system of pixels (2) arranged in rows and columns, with each pixel being defined by a first (6) and a second (7) picture electrode arranged on the facing surfaces of the supporting plates, a system of row (4) and column (3) electrodes for driving the pixels, each pixel (2) being associated with at least one first asymmetrical non-linear switching element (5) being connected in series between the first picture electrode (6) of the pixel and a row electrode (4), the second picture electrode (7) of a pixel (2) being electrically connected to a column electrode (3), characterized in that each pixel is further associated with at least one second asymmetrical non-linear switching element (8) connected in series between the end of the first asymmetrical non-linear switching element which is connected to the first picture electrode and a connection (9) for applying a reference voltage and that at least one capacitive element (10) is connected parallel to the series arrangement of the first (5) and second (8) non-linear switching elements. A display device as claimed in Claim 1, characterized in that at least a part of a row electrode (4) constitutes a first electrode of the capacitive element (10). A display device as claimed in Claim 1 or 2, characterized in that for pixels associated with a row the connections (9) for applying a reference voltage are interconnected to form a common electrode (11) which is connected to an external connection (13) via at least a third non-linear switching element (12). A display device as claimed in Claim 3, characterized in that the common electrode (11) constitutes a second electrode of the capacitive element (10). A display device as claimed in Claim 4, characterized in that the capacitive elements (10) associated with a row of pixels are formed by means of two substantially superjacent metal lines (4, 11) with a layer of dielectric material being interposed. A display device as claimed in Claim 1, characterized in that a non-linear resistance element (20) is arranged parallel to the capacitive element (10). A display device as claimed in Claim 6, characterized in that the capacitive element and the non-linear resistance element are realised as a metal-isolator-metal element. A display device as claimed in Claim 7, characterized in that a first electrode of the metal-isolator-metal element forms part of a row electrode (4). A display device as claimed in Claim 8, characterized in that the metal-isolator-metal elements associated with a row of pixels are formed by means of a row electrode (4) and a substantially superjacent or subjacent row of metal strips (19) with a layer of dielectric material being interposed. A display device as claimed in any one of the preceding Claims, characterized in that at least one of the non-linear asymmetrical switching elements is formed to be redundant. A display device as claimed in any one of the preceding Claims, characterized in that the electro-optical medium is liquid crystalline. A display device as claimed in any one of the preceding Claims, characterized in that the display device comprises means for maintaining the column voltages equal to zero volt while a reset voltage is being presented.

Inventors: KUIJK KAREL ELBERT, KUIJK, KAREL ELBERT
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: G09G3
IPC codes: G02F1, G09G3, G09F9