GB2217891A - Matrix display device - Google Patents

Abstract

A matrix display device e.g. for TV display, has at each matrix point an electro-optical display element 37 connected between a column conductor 35 and two row conductors 34 in series with a respective one of two bidirectional non-linear resistance elements 33, 36 whereby the voltage at the junction of elements 37, 33, 36 is made dependent simply on the voltages applied to the associated pair of row conductors, assuming the two associated elements 33, 36 to be of similar characteristics. Non-uniformity in display effects across the matrix device arising from dependence of the display element voltage on the characteristics of a single series element 33 which may vary at widely different locations are thus avoided. The elements 33, 36 may alternatively be connected across the columns 35. Each row of display elements may be associated with a separate pair of row conductors and the voltages applied to the display elements may be reversed in every other row. The elements 37 may be of LC, EC or electro-phoretic material. <IMAGE>

Description

DESCRIPTION ACTIVE MATRIX ADDRESSED DISPLAY DEVICE This invention relates to an active matrix addressed display device comprising first and second sets of address conductors carried on first and second supporting plates respectively, the two sets extending in rows and columns, a plurality of picture elements arranged in rows and columns and each comprising a display element having a pair of opposing electrodes on the supporting plates with electro-optical material therebetween which is connected in series with a two terminal bidirectional non-linear resistance element exhibiting a threshold characteristic on the first supporting plate between an associated address conductor of the first set and an associated address conductor of the second set, and means connected to the address conductors for addressing the picture elements.

Display devices of the above kind are known. In these known devices, non-linear elements in the form of dual polarity diode structures such as p-i-n structures, diode rings, MIMs (Metal-Insulator-Metal), and punch-through diodes have been employed. Examples of such display devices are described in US Patent Specification No. 4,223,308, British Patent Specifications Nos. 2050031A and 2091468A, and European Patent Specification No. 184,341A. The non-linear elements act as switches allowing very large numbers of display elements to be driven and providing electrical isolation between individual display elements.The picture elements are addressed by applying a select (scanning) voltage to each of a first set of row address conductors in turn and video data signals to a second set of column address conductors as appropriate in synchronism to set the picture element to a desired display condition until they are selected again by the application of the select voltage.

These display devices are suitable for displaying alphanumeric or video, for example TV, information using liquid crystal material but other electro-optical media such as electrophoretic suspensions and electrochromic materials may be employed instead.

In order to provide an acceptable quality display for TV and the like, it is important that the non-linear elements of the matrix array exhibit substantially similar threshold and voltagelcurrent characteristics in operation so that the same drive voltages applied to any picture elements in the matrix produce substantially identical visual results as regards for example, in the case of a liquid crystal display, picture element transmission levels. Differences in the threshold or turn-on point of the non-linear elements for example will appear directly across the electro-optical material so that different display results are obtained from the picture elements even though the same addressing voltages are applied.Heretofore, it generally has been considered that p-i-n devices are the more preferable from this point of view as they are able to provide sufficiently uniform and consistent operational characteristics for such purposes. Other forms of diode structures, particularly MIMs and punch-through diodes, are regarded as being less suitable in this respect. When considering comparatively large area display devices, localised groups of diode structures normally should have similar characteristics but the operational performance of diode structures at widely different locations in the matrix, for example at opposite sides, can differ to such an extent that noticeable, and unacceptable, display variations can occur even though all the diode structures involved are formed simultaneously in a single fabrication process.

It is an object of the present invention to provide an active matrix addressed display device which can produce a good quality display whilst allowing non-linear elements of various kinds in addition to p-i-n structures to be used.

According to the present invention, an active matrix addressed display device as described in the opening paragraph is characterised in that the display element electrodes on the first supporting plate are each also connected to another address conductor of the first set through a further two terminal bidirectional non-linear resistance element, and in that the picture elements are addressed by applying a voltage across their associated pairs of address conductors of the first set.

By applying a suitable voltage difference, constituting a select voltage, across the pair of address conductors associated with a picture element, a voltage is established at the display element electrode on the first supporting plate which, provided the two associated non-linear elements have substantially similar operational characteristics, is dependent simply on the voltages applied. The non-linear elements have little or no effect on the voltages established at the electrode for applied voltage differences of sufficient magnitude to render the non-linear elements conductive. Upon application of suitable voltages to the pair of address conductors, current flows through the circuit constituted by the address conductors and the two non-linear elements connected in series therebetween.Since the picture element electrode is effectively at the junction between the non-linear elements, the voltage created at this electrode is equal to substantially the average of (or half way between) the voltages applied to the pair of address conductors. Thus, provided the two non-linear elements associated with a picture element have substantially similar operational characteristics, the voltage obtained at a picture element electrode at any position in the matrix is determined merely by the applied voltages regardless of any variations which may occur in operational characteristics of non-linear elements at spaced locations in the matrix.Picture elements in the same row share the same pair of address conductors of the first set, e.g. row conductors so that by applying the voltage difference across the pair of row conductors to select that row of picture elements, the same voltage is obtained at all the display element electrodes of the row. Each row of picture elements is selected in turn by scanning sequentially the appropriate pairs of row conductors with this voltage difference. When each row of picture elements is selected in this manner data signals, constituting video information, are supplied to the appropriate column conductors, in synchronism. Upon removal of the voltage difference, the picture element capacitances (i.e. the display elements) are left charged to a value according to both the applied voltage difference and the magnitude of the respective data signal to provide the desired display effects.The rows of picture elements are addressed in this fashion once in every field period, with the polarity of the data signal voltages applied to the column address conductors of a row of picture elements being reversed periodically, and in certain cases the polarity of the row conductor voltages as well, for example after every complete field. Preferably, for optimum display quality with reduced flicker, the polarity of the addressing voltages applied to the rows of picture elements is reversed after each successive row as well.

The invention offers a further advantage in that the manner in which the picture elements are driven provides substantial immunity from the effects of ambient temperature changes. The operational characteristics of non-linear devices can vary in accordance with temperature. Because of the way voltages are established at the display elements, however, such variations have little or no effect.

The non-linear elements may comprise diode structures in the form of MIMs, punch-through diodes, (operating in the manner of back to back diodes), or diode rings using n-i-n, p-i -p, and pn structures as well as p-i-n structures, although other kinds of diode structures could also be used.

In one embodiment, and considering a series of picture elements extending in the direction of the address conductors of the second set, each picture element in the series is associated with a separate pair of adjacent address conductors of the first set. Thus, where, for example, the first and second sets of conductors extend in rows and columns respectively then adjacent pairs of the row address conductors are associated with only one picture element in a column of picture elements, each picture element in the column being, therefore, connected to a respective and different pair of row conductors. Each pair of row address conductors, however, is common to other picture elements in the same row.

In another embodiment, and likewise considering a series of picture elements extending in the direction of the address conductors of the second set, at least some of the address conductors of the first set are shared by respective adjacent pairs of picture elements in that series. Thus, where for example the first and second sets of conductors extend in rows and columns respectively, adjacent pairs of picture elements in the column direction are connected to a common row conductor. In this arrangement the first and last row conductors are each associated with just one respective picture element in the series. As before, picture elements in the same row share the same row conductors. This arrangement has the advantage that the number of address cpnductors in the first set, for example the row conductors, is reduced compared with the previous embodiment.However because address conductors are shared between rows of picture-elements, driving of the picture elements becomes more demanding.

In a preferred addressing scheme applicable to both these embodiments, each picture element in the series is addressed by the addressing means applying simultaneously to its two associated address conductors of the first set respective voltages of substantially equal magnitude and opposite polarity, with the result that the junction of the two non-linear elements is at approximately zero volts during the selection period.

This, together with the aforementioned preferred data signal polarity inversion after each row of picture elements is addressed, leads to a simplified drive.

Although reference has been made above to first and second sets of address conductors as comprising the row and column conductors respectively, it should be understood that the first and second sets of conductors could alternatively extend in the column and row directions respectively instead.

The two non-linear resistance elements associated with each picture element may be connected to their respective address conductors of the first set through capacitances. Capacitances in series with the non-linear resistance elements provide some fault tolerance in the circuit of the picture element. More particularly, if one non-linear resistance element becomes short circuit, the other non-linear resistance element of the picture element concerned will be protected from failing in consequence so that the possibility of a direct path between the two address conductors is prevented. Such a path would affect the operation of all other picture elements connected to those address conductors.

Embodiments of active matrix addressed liquid crystal display devices in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a simplified block diagram of one embodiment of liquid crystal matrix display device according to the invention suitable for diplaying TV pictures and having a matrix array of individually addressable picture elements using non-linear resistance elements as switches and each including a liquid crystal display element; Figure 2 is a schematic cross-sectional view through a part of the device of Figure 1; Figure 3 illustrates graphically the transmission-voltage characteristic of a typical one of the liquid crystal display elements of the device of Figure 1;; Figure 4 illustrates graphically the voltage (v)/current (i) relationship of the non-linear resistance elements; Figure 5 shows schematically in plan a few of the picture elements of the display device of Figure 1 using MIM structures, back to back diodes, or the like for the non-linear resistance elements,together with their associated address conductors; Figure 6 is a circuit diagram of two typical picture elements of the device of Figures 1 and 5; Figures 7a-d illustrate voltage waveforms applied to row and column conductors of the display device for addressing the picture elements; Figure 8 is similar to Figure 6 and illustrates schematically the circuit configuration of a modified form of the display device in which the picture elements further include capacitive elements;; Figure 9 shows in simpified block diagrammatic form another embodiment of active matrix liquid crystal display device according to the invention similar to that of Figure 1 but using additional addressing conductors; Figure 10 shows schematically a few of the picture elements of the display device of Figure 9 using, MIMs, back to back diodes, or the like as the non-linear resistance elements with their associated address conductors; Figure 11 is similar to Figure 10 but illustrates an alternative form of the picture elements using diode rings as the non-linear resistance elements and further including capacitive elements.

Referring to Figure 1, there is shown schematically and in simplified form a block diagram of an LC matrix display device intended for displaying TV or video, which includes an active matrix addressed liquid crystal display panel 30 consisting of r rows (1 to r) with c horizontal picture elements 32 (1 to c) in each row. In practice, the total number of picture elements (r.c) in the matrix array of rows and columns may be 200,000 or more. The picture elements 32 consist of capacitive liquid crystal display elements 37 with associated two-terminal bidirectional non-linear resistance elements exhibiting a threshold characteristic and acting as switching elements.The picture elements 32 are addressed via sets of row and column address conductors 34 and 35 which, as shown more clearly in Figure 2, are in the form of electrically conductive lines carried on respective opposing faces of two, spaced, glass supporting plates 40 and 41, which also carry the non-linear elements and picture element electrodes. The picture elements are located at the cross-over regions of the two sets of conductors. The display element electrodes on the plate 40 are constituted by portions of the column conductors 35 overlying respective, generally rectangular, display element electrodes carried on the other plate 41, referenced at 42 in Figure 2 Continuous liquid crystal orientation layers 43 and 44 overlying the conductors and electrodes are provided in usual manner.

Polariser and analyser layers 48 and 49 are provided on the outer surfaces of the plates 40 and 41 in accordance with conventional practice.

Referring again to Figure 1, the row conductors 34 serve as scanning electrodes and are controlled by a row driver circuit 45 which applies scanning (selection) signals to the row conductors 34 in sequential fashion as will be described. Data signals are applied to the column conductors 35 from a column conductor driver circuit 47 connected to the output of a video processing circuit 50 in synchronism with the scanning signals, this being achieved by means of a timing circuit 46. In the case of a video or TV display system these data signals comprise video information.

By appropriate selection of the scanning and data signal voltages, the optical transmissivity of the display elements 37 of the picture elements 32 of each row in turn are controlled to produce the required visible display effect. In this respect the display elements 37 have a transmission (T)/voltage (VLC) characteristic as shown in Figure 3 and are only activated to produce a display effect in response to the application of both the scanning and data signals to the picture elements 32 by virtue of the non-linear elements. Below a given threshold (Vth) virtually no transmission occurs whilst above a certain applied voltage (VSat) the display elements show maximum transmission.

At intermediate voltages, various intermediate states of transmission are obtained enabling grey scales to be achieved.

Active matrix addressed liquid crystal display devices employing two terminal non-linear resistance elements as switching elements in series with the display elements are well known. The display device according to the present invention shares many similarities and hence the foregoing description of the principal features of the display device and its operation has been kept brief for simplicity. For further, general, information on the construction and operation of such devices reference is invited to, for example, US Patent Specification 4,223,308 and British Patent Specification 2,147,135, both describing devices using diode structures as non-linear switching elements, and British Patent Specification No. 2,091,468, describing the use of MIMs as non-linear switching elements, details of which are incorporated herein.

The display device according to the present invention, however, uses a drive scheme involving an arrangement of non-linear elements which differs from that of the known display devices. More particularly each of the display element electrodes on one supporting plate is connected to two adjacent address conductors of the set of conductors carried on that plate through respective two terminal, dual polarity, non-linear elements and a picture element is selected by the simultaneous application of predetermined voltages to its two associated address conductors of the first set.

Referring to Figures 1 and 2, a display element electrode (referenced at 42 in Figure 2) of each of the picture elements 32 is in this embodiment connected to two adjacent row conductors 34 through bidirectional non-linear elements 33 and 36 respectively. The elements 33 and 36 comprise MIMs or back to back diode structures such as punch-through diode structures although other suitable forms of two terminal bidirectional non-linear elements exhibiting a threshold characteristic such as diode rings and using pn, p-i-n, p-i-p, or n-i-n diode structures may be used instead.In addition to being bidirectional, the voltage/currentcharacteristic of the non-linear elements 33 and 36, as illustrated in Figure 4, is substantially symmetrical with respect to zero voltage so that the picture elements can be driven with scanning and data voltage signals whose polarities are reversed periodically to avoid a net dc bias across the liquid crystal display elements.

A schematic plan configuration of a typical group of four picture elements is shown in Figure 5. In this figure, three successive row conductors 34 and two successive column conductors 35 are indicated at Xn-1, Xn, Xn+1 and Yn, Yn+1 respectively.

Whilst each column conductor 35 is associated with just one column of picture elements,-each row of picture elements is associated with two row conductors 34, with each of the row conductors, apart from the first and last of the set, being shared by two adjacent rows of picture elements.

Figure 6 illustrates the effective electrical circuits of the two picture elements to the left in Figure 5, that is, the picture elements associated with the column Yn, these two picture elements being denoted Pn and Pn+1.

In order to select a particular row of picture elements, a predetermined voltage difference is applied across the two associated row conductors 34. Referring to Figure 6, the picture element Pn+1 for example is selected by applying simultaneously predetermined voltages, Vx + Vp and Vx - Vp to the row conductors Xn and Xn+1 so that a voltage difference of 2Vp is established across the two address conductors. This difference must exceed the combined turn-on voltages of the two non-linear elements.

The voltages applied to the row conductors individually are insufficient to turn on non-linear elements of adjacent rows not being selected. This voltage causes current flow through the circuit consisting of conductor Xn, the two series connected elements 33 and 36 and the conductor Xn+1. Assuming that the two non-linear elements 33 and 36 have substantially similar electrical characteristics, as is most probable for elements fabricated close together, then the node D, and therefore the display element electrode 42 of picture element Pn+1, is charged to a voltage value substantially equal to the average of the voltages applied to the two row conductors, which in this case is Vx. A data voltage signal, Vd, applied simultaneously to the column conductor Yn, does not affect this voltage.On removing the voltage difference applied to the row conductors Xn and Xn+1 at the end of the select period for this row of picture elements the node D is left charged to a value of approximately Vx - Vd to produce the required display effect, as determined by the value of Vd, from the picture element Pn+1 until that row of picture elements is next addressed in the succeeding field period.

Importantly the elements 33 and 36 are designed to have a small capacitance compared with that of the liquid crystal display element 37, preferably less than one tenth and ideally around one hundredth, and because of this there is hardly any voltage drop at the display element when the voltage difference applied across the row conductors is removed. Of course, other picture elements in the same row are selected at the same time by application of the voltage difference. Each row of picture elements is addressed and loaded in this manner in turn by applying the select voltage difference across successive pairs of row conductors sequentially, and data signals to the column conductors as appropriate build up in one complete field period a display picture.

It is seen therefore that the display elements 37 are charged to a value substantially dependent on simply the average value of the voltages applied to the two addressing row conductors and the value of the simultaneously-applied data signal. It can be assumed that the two non-linear elements associated with each picture element will, by virtue of the fact that they are fabricated close together, have practically the same electrical characteristics. Thus substantially identical responses can be obtained from all picture elements of the display device regardless of variations which can easily occur in the operational electrical characteristics of the non-linear elements for picture elements at more widely spaced locations in the matrix as a result of large area fabrication processes.

Since the voltage across all the liquid crystal display elements over a large display area can therefore be set at a value which is substantially insensitive to long range variations in non-linear element characteristics a high degree of uniformity in the individual display effects of the picture elements over the display area is achievable. In comparison, in conventional types of display devices using one non-linear element connected in series between a respective row conductor and the display element, any variations, even minor, in the operational characteristics of non-linear elements at different locations, or more precisely their threshold points, lead to different voltages being established across the associated display elements in response to the same applied voltages, which, as will be apparent from Figure 3, result in different transmission effects from the display elements.

A suitable addressing scheme for this embodiment will now be described in more detail with reference to Figure 7, which shows examples of voltage/time waveforms applied to particular row and column conductors for addressing picture elements Pn and Pn+1 of Figure 6. Figure 7a shows a typical waveform of the data voltage signal Vd applied by the circuit 47 to the column conductor Yn and Figures 7b to 7d show the select voltage waveforms applied by the circuit 45 to the row conductors Xn-1, Xn and Xn+1 respectively. In this addressing scheme, the polarity of the drive voltages is reversed after each row of picture elements is addressed.

The periods during which picture elements Pn and Pn+1 are addressed, i.e. their select periods, are indicated respectively at tn and tn+1. For PAL system TV display purposes, the row addressing periods, and thus tn and Tn+1, are of approximately 64 microseconds duration.

The voltage signals applied to the row conductors comprise select and hold signal pulse portions, the latter, following known practice, being applied to a row of picture elements during the interval following its selection and prior to that row next being selected in the subsequent field, which for a PAL system TV display will be around 20 milliseconds overall, to maintain the desired display effects from the picture elements of the row.

The data signal Vd (Figure 7a) applied to column conductor Yn, obtained by sampling the appropriate line of the incoming video signal, has maximum and minimum excursions corresponding to 'Vsat and iVth (see Figure 3), and its magnitude during a row select period determines the optical transmission state obtained from the picture element in that row associated with the column conductor Yn. It is seen, therefore, that for the picture elements Pn and Pn+1, the data signal Vd in the example shown has values of approximately -Vth and +Vth respectively.

The waveforms applied to the row conductors Xn-1, Xn and Xn+1 each consist for most of the field period of a regular pulse train alternating between Vh + and Vh- in accordance with our row select periods where Vh+ and Vh comprise the hold signal portion. The value of Vh is chosen to obey the following relationship: Vh+=- Vh = (3Vsat - Vth)/2 which is intended to minimise the voltage appearing across the non-linear elements during'the remainder of the field period.

When it is required to select the row containing the picture element Pn, the voltages applied to the row conductors Xn-1 and Xn are increased in opposite senses, as seen in Figures 7b and 7c, to Vs and Vs+ respectively, where Vs and Vs+ satisfy the relationship: Vs+ = - Vs = Von The voltage applied to the row conductor Xn-1, as will be apparent from Figure 7b, is actually increased from Vh to Vs during the preceding row select period for the purpose of addressing the preceding row (containing the picture element Pn-1) and is maintained at this increased level therefore for two row select periods.Similarly, the voltage applied to row conductor Xn is increased to Vs+ at the beginning of the select period for the row containing picture element Pn and is maintained at this level for a further row select period during which the subsequent picture element Pn+1 is addressed.

The voltage difference across the row conductors Xn-1 and Xn therefore increases during period tn causing the non-linear elements 33 and 36 of the picture element Pn to turn on and producing a voltage at their junction D (Figure 6) substantially equivalent to the average of the voltages applied to the two row conductors, that is, OV. At the end of the select period tn of this row, the voltage applied to row conductor Xn-1 is switched to Vh+, leaving the electrode 42 of picture element Pn charged with a voltage difference across its capacitive display element 37 of approximately Vd.Simultaneously, and corresponding to the begining of the next row select period tn+1, the voltage applied to the next row conductor Xn+1 is increased to Vs which, in conjunction with voltage Vs+ still being applied to row conductor Xn, causes the following row, containing picture element Pn+1, to be selected. With regard to the maximum voltages appearing across the non-linear elements 33 and 36, then immediately following selection of the row containing picture element Pn and during selection of the row containing picture element Pun+1 for example (and bearing in mind that at this stage the voltage of the row conductor Xn is still at Vs+) the condition Von - 2Vth < Voff holds, and for the remainder of the field the condition (Vsat + Vth)/2 < Voff holds.

The picture element Pn+1 and other picture elements in this row, and likewise picture elements in successive rows, are addressed in similar fashion on a row at a time basis, with the polarities of the applied voltages alternating. The row conductor driver circuit 45 is arranged so as also to reverse the polarity of the address voltages applied to a row of picture elements in successive fields.

Referring now to Figure 8, the circuit configuration of a modified form of the above embodiment of display device is shown in which the two non-linear elements of each picture element are each connected to their associated row conductors through a series capacitor, 60. The capacitors 60 makes the circuit more fault tolerant as they serve to protect one non-linear element from failing as a consequence of the other non-linear element becoming short circuit because of a defect, and thereby reduce the possibility of a DC path between the two adjacent row conductors 34 as would occur if both non-linear elements became short circuited. Such a DC path would lead to the entire row of picture elements becoming defective.With the capacitors 60, problems caused by a defective non-linear element are therefore confined to the picture element concerned and even this may still be capable of providing a display response.

A further embodiment of the display device according to the invention is shown in block diagrammatic form in Figure 9. This embodiment is similar in many respects to that shown in Figure 1 and accordingly the same reference numbers are used to designate corresponding components. This further embodiment differs from the previously described embodiment in that each row of picture elements 32 is addressed through a respective and separate pair of row conductors 34, thus almost doubling the number of conductors in the set of row conductors. Figure 10 shows the circuit configuration of a typical group of four picture elements of this display device more. clearly.The upper row of picture elements 32 in this figure are addressed using the row conductors Xn-1, and Xn-11, and the lower row of picture elements addressed using the row conductors Xn and Xn'. As before, each column of picture elements is connected to a single, respective column conductor 35, these being designated Yn and Yn+1 for the picture elements illustrated in the figure.Comparing this configuration with that shown in Figure 8, it can be seen that as the row conductors are not shared between adjacent rows of picture elements the select voltages presented across the two successive row conductors associated with a row of picture elements, for example across row conductors X(n-1) and X(n-1)' for the upper picture elements in Figure 8, to select that row appear only at the picture elements of that row and that picture elements in other rows, for example the lower picture elements in Figure 8, are electrically isolated from these select voltages. Thus the possibility of problems with cross-talk being experienced is avoided and addressing the picture elements is less demanding than with the previous embodiment.

In order to select a row of picture elements, therefore, it is necessary simply to apply predetermined voltages to the two row conductors concerned sufficient to turn on the non-linear elements. Loading of the picture elements is then accomplished in the same manner as before with data voltage signals being applied to the column conductors 35. After selection and loading of a row of picture elements a hold voltage is applied to the two associated row conductors as before to maintain the required display effect from the picture elements for the remainder of the field period until that row is next selected. Because each of the row conductors is dedicated to a single row of picture elements, the nature of the applied voltages, and hence driving of the device, is simplified.Voltages Vp+ and Vp applied to the row conductors Xn-1 and Xn-1' for example causes the picture element electrodes 42 of the row to charge to (Vp++Vp-)/2. Simultaneously, a data voltage signal Vd is applied to the column electrodes Yn and Yn+1. Upon removal of the voltages Vp+ and Vp from the row conductors, the picture elements of the row are left charged to values of approximately (Vp++Vp-)/2-Vd. Voltages Vp+ and Vp are then applied to the row conductors Xn and Xn', of the next row of picture elements to select those elements in similar manner and the voltages on row conductors Xn-1 and Xn-1' returned to zero in which case the maximum voltage which can appear across the non-linear elements of the previously selected row is 2Vsat and therefore the on threshold of the elements must be greater than this value.

Alternatively, by offsetting the row select voltages (i.e. Vp+ is not equal to Vp in magnitude) and by returning the voltages of both row conductors upon removal of the select voltages following selection to a value other than zero then the peak voltage appearing across the non-linear elements can be reduced at the expense of more complicated drive circuitry. The polarity of the applied data signal voltages is reversed after each row and for each row in successive fields as before. In other respects the display device operates in similar manner to that described with regard to the previous embodiment.

The non-linear elements used in this embodiment can again be any suitable two terminal, bidirectional, type as referred to earlier. In Figure 10 these elements are depicted in the manner of back to back diodes and may be in the form of MIMs or punch through diodes for example. Figure 11 illustrates an alternative form of picture elements using diode rings as the non-linear elements instead. Also in the modified circuit shown in Figure 11, capacitors 62 are connected in series between each non-linear element of the picture elements and its associated row conductor in similar manner to, and for the same purpose as, the capacitors 60 in the arrangement of Figure 8.

With regard to all the above-described embodiments, therefore, the value of the voltage obtained across the display elements upon addressing and loading the picture elements, and hence the display effect produced by the individual picture elements, is substantially immune to variations which may occur in the electrical operational characteristics of non-linear elements over a large area of the display region, given that non-Linear elements close together have substantially similar characteristics. As a result, non-linear elements of a kind hitherto considered unsatisfactory for use in an active matrix addressed display device to produce high quality displays because of their insufficient uniformity in operational characteristics over large areas can be employed.

Whilst in the above embodiments liquid crystal material is used as the display medium, other passive electro-optical materials such as electrochromic materials or electrophoretic suspensions may be used instead. Moreover, it will be appreciated that the row and column address conductors could be interchanged.

Claims (11)

CLAIM(S)

1. An active matrix addressed display device comprising first and second sets of address conductors carried on first and second supporting plates respectively, the two sets extending in rows and columns, a plurality of picture elements arranged in rows and columns and each comprising a display element having a pair of opposing electrodes on the supporting plates with electro-optical material therebetween which is connected in series with a two terminal bidirectional non-linear resistance element exhibiting a threshold characteristic on the first supporting plate between an associated address conductor of the first set and an associated address conductor of the second set, and means connected to the address conductors for addressing the picture elements, characterised in that the display element electrodes on the first supporting plate are each also connected to another address conductor of the first set through a further two terminal bidirectional non-linear resistance element, and in that the picture elements are addressed by applying a voltage across their associated pairs of address conductors of the first set.

2. An active matrix addressed display device according to Claim 1, characterised in that each picture element in a series of picture elements in the direction of the address conductors of the second set is associated with a separate pair of adjacent address conductors of the first set.

3. An active matrix addressed display device according to Claim 1, characterised in that address conductors of the first set associated with a series of picture elements extending in the direction of the address conductors of the second set are each connected to a respective adjacent pair of picture elements in the series.

4. An active matrix addressed display device according to Claim 2 or Claim 3, characterised in that the addressing means is arranged to address each picture element in the series by applying simultaneously to its two associated address conductors of the first set respective predetermined voltages of substantially equal magnitudes and opposite polarity and by applying a data signal to its associated address conductor of the second set.

5. An active matrix addressed display device according to any one Claims 2 to 4, characterised in that the polarity of the addressing voltages applied to the second set of address conductors is reversed after each successive picture element in the series is addressed.

6. An active matrix addressed display device according to any one of the preceding claims, characterised in that the addressing means is arranged to apply predetermined hold voltages to the address conductors of the first set associated with each picture element during the interval following addressing that element and prior to it next being addressed so as to minimise voltages appearing across the non-linear elements thereof in said interval.

7. An active matrix address display device according to any one of the preceding claims, characterised in that the first and second sets of address conductors extend in rows and columns respectively.

8. An active matrix addressed display device according to any one of the preceding claims, characterised in that the non-linear resistance elements of each picture element are connected to their associated address conductors of the first set through respective capacitances.

9. An active matrix addressed display device according to any one of the preceding claims, characterised in that the non-linear resistance elements comprise diode structures in the form of MIMs, punch through diodes, or diode rings.

10. An active matrix addressed display device according to any one of the preceding claims, characterised in that the electro-optical material comprises liquid crystal material.

11. An active matrix addressed display device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.