GB2146827A - Display panels - Google Patents

Abstract

The invention relates to a display screen, addressing of which is provided by means of non linear elements formed by two diodes placed in series and in opposition. The invention provides non linear elements 51 formed such that the two diodes forming them have a common doped semi conductor material part, the characteristics of this common part being such that all the non linear elements have, for the same control voltage, the same operating point in the current-voltage characteristic. The desired characteristics are attained by providing the doping level of the common semiconductor material to be such that its conductivity is greater than a particular value. <IMAGE>

Description

SPECIFICATION A display screen with addressing by non linear elements and a process for manufacturing same.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to display screens comprising an electro-optical material layer, for example a liquid crystal layer.

2. Description of the Prior Art As is known, such screens generally comprise a large number of picture elements of square or rectangular shape. These picture elements may be addressed individually. The definition of the screen depends on the number of points capable of receiving data. The control of each point is provided by application of an electric field. For displaying video information, matrix type displays have been proposed. Each picture element is then defined by the intersection of two networks of orthogonal conductors called lines and columns.

The addressing of a picture element by means of control voltages applied to the line and to the column which relate thereto does not need to be maintained if a time multiplexing technique is adopted for refreshing the state of the screen by recursion. This technique is based on a persistence effect which may be physiological or available within the element of the screen. In the case of liquid crystal display devices, a picture element may be likened to a capacitor whose time constant is sufficient to maintain the charge between successive transitory addressing operations.

To apply the control voltage in a short time, a non linear resistor is connected in series with the picture element, i.e. an element of the varistor type which is practically insulating short of a voltage threshold and which becomes more and more conducting beyond this threshold. A convenient way of collectively forming the varistor elements consists in using substrate a block of varistor material which is coextensive with the screen.

Numerous drawbacks are inherent in this process. It introduces not inconsiderable parasite capacities and further, since the materials presenting these properties are generally opaque, they do not allow a screen to be used for transmission. The threshold voltages are not uniform over the whole of the active surface of the screen and are generally high. A distributed structure of these varistors has been proposed in a second embodiment of the prior art, described in French patent application no 81.16 217 filed on the 25th August 1981 in the name of the applicant. A particular arrangement of the control connection is chosen, allowing varistor studs to be formed thereon so as to provide threshold control of the liquid crystal cells.

At the present time, the requirements of the technique in so far as display screens are concerned relates particularly to a better picture definition. In the case of matrix display type screens, devices have been designed comprising a high number of addressing lines or columns. The number thereof may reach 512 or even 1024. This increases correspondingly the switching elements and so the number of varistors in the above mentioned application. For large scale production, it is necessary more especially to obtain good reproducibility and a great stability of such components. It is further more necessary to match, hereagain also with good reproducibility, the electric capacity of a component to that of the associated cell.Now, the materials generally used, such as zinc oxide powder agglomerates containing particles of bismuth oxide and manganese oxide or another similar material, do not entirely satisfy these requirements. The reproducibility and stability of the varistors depend among other things on the grain size and on the techniques for passivating the grain joints used during manufacture.

The parasite capacity of the varistor also tied up with the grain joints is difficult to control.

Other switching elements may be used.

Nevertheless, liquid crystal display screens generally present defects in homogeneity of the contrast depending on the picture elements, due to a dispersion of the characteristics of the switching elements which may be considerable and which is difficult to eliminate over large areas. These defects may also originate, to a lesser extent, in the thickness of the liquid crystal layer and in its fixing layer.

To overcome these disadvantages, the invention provides a display screen whose picture elements are addressed by non linear devices of the voltage-dependent resistance type and formed from semi conductor diodes placed in series and in opposition and whose doping is controlled so as to obtain switching elements all having the same operating point in the current-voltage characteristic.

BRIEF SUMMARY OF THE INVENTION The invention provides a display screen of the type comprising an electro-optical material placed between two plates one at least of which is transparent, said screen comprising non linear elements deposited collectively and associated in series with the picture elements of the screen and allowing control thereof through potentials distributed by electrodes supported by said plates, said control being dependent on the operating point of the non linear elements in its current-voltage characteristic, said non linear elements being formed by two diodes placed in series and in opposition and having a common doped semi conductor layer, wherein the doping of said layer is such that its conductivity is greater than 10-2-l.cm-' so that the non linear ele ments all have the same operating point for a control voltage of a picture element.

The invention also provides a process for manufacturing such a display screen.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and other features and advantages will be clear from the following description and accompanying Figures in which: Figure I shows symbolically a picture element of a display screen; Figures 2 and 3 are electric diagrams; Figure 4 is an explanatory diagram; Figure 5 illustrates a Schottky diode structure; Figure 6 shows a non linear element; Figures 7 and 8 are explanatory diagrams; and Figures 9 and 10 are views of a display screen.

DETAILED DESCRIPTION OF THE PRE FERRED EMBODIMENTS Matrix access display screens are formed by an electro-optical material such as a liquid crystal layer disposed between two sets of crossed electrodes representing the lines and the columns of the display matrix. The intersection of a line and a column defines a picture element of the screen.

Fig. 1 shows an electric diagram equivalent to a picture element of a display screen. The intersection of a column 1 and a line 2 defines the picture element represented by the elementary cell 3 symbolized by a capacitor whose dielectric 5 would be the electro-optical material. The elementary cell 3 is associated with a non linear element 4. Depending on the value of the potential difference applied between column 1 and line 2, the elementary cell 3 may be acted on or not as a function of the threshold voltage of the non linear element.

The non linear element may be formed by the series-opposite association of two diodes D1 and D2. Two possible arrangements exist for these diodes as shown in the electric diagrams of Figs. 2 and 3. Fig. 4 is a diagram whose curve 6 shows the trend of the current I flowing through the non linear element as a function of the potential difference VAB applied to its terminals, i.e. between A and B. It can be seen that curve 6 is very non linear and has a threshold voltage Vs. It is advantageous to use amorphous silicon Schottky diodes which posess very non linear reverse characteristics.

Fig. 5 illustrates an amorphous silicon Schottky diode structure. Typically, this type of diode is obtained by depositing on a substrate 10 a first amorphous silicon layer 11 with n+ type doping, a second non doped amorphous silicon layer 1 2 and a surface layer 1 3 formed by a metal and which forms the Schottky junction with the underlying layer. Usually, the metal is platinum but other metals may be chosen having similar properties such as palladium or gold. So as to provide a good metal-semi conductor contact, the metal layer 1 3 may be treated so as to form a platinum silicide interface.

It is advantageous to form the non linear elements in a co-planar type structure. Fig. 6 illustrates such a configuration. On an insulating substrate 20 which may be common to several non linear devices, an n + doped amorphous silicon layer 21 is first of all deposited then an undoped or very slightly doped amorphous silicon layer 22. Diodes D, and D2 are completed by platinum studs 23 and 24. The n + doped layer 21 provides the electric connection between diodes D, and D2.

Electric connections 25 and 26 connect diodes D, and D2 respectively to contact terminals 27 and 28. Between these terminals, there is thus formed a non linear element formed from two diodes placed in series and in opposition.

Deposition of this amorphous silicon layers may be achieved by numerous processes. For forming non linear devices for display screens a gaseous phase method will be preferred at atmospheric pressure or a CVD method (chemical vapor deposition). It consists in thermal decomposition of silane SiH4 which is the source of silicon. The gas vector used is hydrogen. n type doping is provided by introducing phosphine PH3 diluted in hydrogen.

Doping of the n + layer corresponds generally to a ratio PH3 ~~~~~~ = 10-4toS.10-4, SiH4 which represents a conductivity of the order of 10-6 to 10-5-1.cm-1. The thickness of the deposited layers may vary from 1000 to a few thousand angstroms.

The amorphous silicon samples obtained by CVD deposition at 600'C have a high density of broken bonds. The electronic states corresponding to a broken bond are deep in the prohibited band and give a semi insulating material. By thermal treatment in an atomic hydrogen environment, it is possible to chemically passivate the broken bonds by formation of Si-H bonds. This post hydrogenation is carried out at a temperature of about 400"C in a microwave generated hydrogen plasma.

The different layers may also be deposited by a low pressure gaseous phase method or LPCVD method (low pressure chemical vapor deposition). In this case, deposition is carried out under a pressure of about 500 millitorrs and at a temperature of about 565"C.

The above mentioned methods lend themselves particularly well to depositions on large surfaces. They may therefore be used for forming substrates supporting the non linear elements of a display screen. However, as was mentioned above, it is the dispersion of the current-voltage characteristics of these elements which gives rise to considerable problems, in particular for controlling the contrast.

Fig. 7 is a diagram showing the trend of the current I flowing through the non linear elements as a function of the voltage V applied to the terminals of these elements. The elements considered are of the type shown in Fig. 6. Three curves 30, 31 and 32 have been plotted corresponding to three identical non linear elements formed on the same substrate during the same operation. The axis of the ordinates is graduated in amps according to a logarithmic scale and the axis of the abscissa is graduated in volts. The trend of these curves is substantially linear over the greatest part thereof for a scale of logarithmic amps. These curves are particularly revealing in so far as the dispersion of the currentvoltage characteristics is concerned.In particular, the current difference between curves 30 and 32 for a voltage of 1 2 volts is Al = 7.6x10-8 amps and this difference does not appreciably decrease for voltages higher than 1 2 volts. The corresponding values of the currents are 1, = 9.10-8 amps for curve 32 and 12 = 1.4 x 10amps for curve 30. It can be seen that the current Ii has a value which is 6.4 times higher than the current 12.

The harmful influence of this dispersion on the contrast of the screen may be readily understood.

The invention proposes forming the non linear elements so that they all have the same current-voltage operating point for controlling the liquid crystal layer. This is obtained by limiting the current-voltage characteristic to the same current level for the control voltage applied. This limitation is introduced by controlling the doping of the rear n+ layer common to the two diodes of the non linear element. The i(V) characteristics of the assemblies formed by two diodes placed in series and in opposition are made homogeneous by controlling the value of the series resistance of the n+ doped layer during deposition.This result may be obtained with an n + layer doping which corresponds to PH3 = = 10~3 SiH4 which represents a conductivity of the order of 10-1-1.cm-1. It is thought that doping causing conductivity higher than 10-22-1.cm-1 is satisfactory for homogenizing the characteristics.

Fig. 8 is a diagram showing the trend of the current I flowing through the non linear elements as a function of the voltage V applied to the terminals of these elements. The elements considered are as in the case of Fig. 7, of the type shown in Fig. 6. Three curves 40, 41 and 42 have been plotted corresponding to three identical non linear elements formed on the same substrate during the same operation. The axis of the ordinates is graduated in amps according to a logarithmic scale and the axis of the abscissa is graduated in volts. It can be seen from Fig. 8 that curves 40, 41 and 42 which were separate for low values of V tend to merge when V increases and are practically merged together for voltages higher than 1 2 volts.It can be readily seen from this Figure that, for sufficiently high control voltages, the liquid crystal layer will be uniformly controlled at each point.

It is also possible to homogenize the current-voltage characteristics of the non linear device by controlling the geometrical characteristics of the diodes forming the non linear elements. In particular, the resistance presented by the n + doped layer may be reduced by reducing the thickness of the layer or by increasing the area of the junctions.

Figs. 9 and 10 show partial sectional views of a liquid crystal display screen according to the invention. Fig. 10 is a top view of Fig. 9 through axis XX'. The non linear elements are of the type shown in Fig. 6. The desired amorphous silicon layers were first of all deposited on a plate or substrate 50 by the above described CVD method. The future non linear elements associated with the picture elements were isolated from each other by chemical etching. Then, the metal contacts were deposited by evaporation so as to complete the non linear elements which bear the reference 51. Each non linear element 51 therefore comprises contacts 52 and 53 forming Schottky junctions with the semi conductor layer which supports them.Substrate 50 also supports column connections 54 for conveying the control potentials and electrodes 55 whose areas (for example of the order of 1 mm2) define the picture elements. As can be seen, in particular in Fig. 10, the non linear elements 51 provide the connections between column and electrode connection points 55.

With a vacuum evaporation method, good electric continuity is obtained between connections 54 and contacts 52 on the one hand and between electrodes 55 and contacts 53 on the other. If the screen is intended to be used for reflection, electrodes 55 must be reflecting. They are then made from aluminium preferably. If the screen is to be used for transmission, a mixed tin and indium oxide will be used for example. Connections 54 are sufficiently thin so as not to cause problems.

A second plate 56 is disposed opposite plate 50 at a distance typically of about 1 5 micrometers defined by shims, not shown. It supports line connections 57. They have the same width as electrodes 55. They are made from a transparent conducting material such as mixed tin and indium oxide.

The space between the two plates 50 and 56 is filled with a liquid crystal layer 58 for example of the type comprising a nematic phase.

Thus a display screen is obtained in which each picture element is controlled by a non linear element and which may be used for transmission or reflection. The Schottky diodes forming the non linear elements allow relatively high currents to pass, typically of the order of 100 microamps, i.e. at least ten times greater than the currents transiting through thin layer transistors having a comparable area. Grey tints may be obtained by varying the width of the video voltage pulses which are applied as the case may be to the line or column pulses.

Claims (9)

1. A display screen of the type comprising an electro-optical material placed between two plates one at least of which is transparent, said screen comprising non linear elements deposited collectively and associated in series with the picture elements of the screen and allowing control thereof through potentials distributed by electrodes supported by said plates, said control depending on the operating point of the non linear element in its current-voltage characteristic, said non linear elements being formed by two diodes placed in series and in opposition and having a common doped semi conductor layer, wherein the doping of said layer is such that its conductivity is greater than 10-2-1.cm-1 so that the non linear elements all have the same operating point for the same control voltage of a picture element.

2. The display screen as claimed in claim 1, wherein the diodes forming said non linear element are Schottky diodes.

3. The display screen as claimed in claim 1, wherein said electro-optical material is a liquid crystal.

4. The display screen as claimed in claim 1, wherein the electrodes supported by said plates define a matrix network.

5. A process for manufacturing a display screen as claimed in claim 1, wherein said non linear elements are formed from semi conductor layers deposited on a substrate.

6. The process for manufacturing the display screen as claimed in claim 5, wherein deposition of said semi conductor layers is effected by a gaseous phase method at atmospheric pressure.

7. The process for manufacturing a display screen as claimed in claim 5, wherein deposition of said semi conductor layers is effected by a low pressure gaseous phase method.

8. A display screen substantially as hereinbefore described with reference to Figs. 9 and 10 of the accompanying drawings.

9. A process for manufacturing a display screen as claimed in claim 1 and substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.