EP0589523B1 - Display device - Google Patents
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- Publication number
- EP0589523B1 EP0589523B1 EP19930202700 EP93202700A EP0589523B1 EP 0589523 B1 EP0589523 B1 EP 0589523B1 EP 19930202700 EP19930202700 EP 19930202700 EP 93202700 A EP93202700 A EP 93202700A EP 0589523 B1 EP0589523 B1 EP 0589523B1
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- EP
- European Patent Office
- Prior art keywords
- display device
- pixel
- pattern
- asymmetric
- electrodes
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J31/00—Cathode ray tubes; Electron beam tubes
- H01J31/08—Cathode ray tubes; Electron beam tubes having a screen on or from which an image or pattern is formed, picked up, converted, or stored
- H01J31/10—Image or pattern display tubes, i.e. having electrical input and optical output; Flying-spot tubes for scanning purposes
- H01J31/12—Image or pattern display tubes, i.e. having electrical input and optical output; Flying-spot tubes for scanning purposes with luminescent screen
- H01J31/123—Flat display tubes
- H01J31/125—Flat display tubes provided with control means permitting the electron beam to reach selected parts of the screen, e.g. digital selection
- H01J31/127—Flat display tubes provided with control means permitting the electron beam to reach selected parts of the screen, e.g. digital selection using large area or array sources, i.e. essentially a source for each pixel group
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2300/00—Aspects of the constitution of display devices
- G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J2201/00—Electrodes common to discharge tubes
- H01J2201/30—Cold cathodes
- H01J2201/319—Circuit elements associated with the emitters by direct integration
Definitions
- the invention relates to a display device according to the introductory part of claim 1.
- a display device of this type is usually in the form of a flat display device and is suitable, for example, for displaying video information and alpha-numerical information.
- a display device of the type mentioned in the opening paragraph is described in USP 4,857,161.
- the device shown in this document comprises strip-shaped row electrodes on a substrate on which a plurality of tip-shaped (pointed) field emitters are realised per pixel at the location of crossings with column electrodes.
- the column electrodes which also function as grid electrodes in this case, are separated from the row electrodes by means of a layer of insulating material.
- At the location of the pixels apertures in which the field emitters have been realised on the row electrodes are present in the column electrodes and the subjacent insulating material.
- a face plate provided with phosphors to which electrons released by field emission can be accelerated is present opposite the substrate.
- the display device shown (when displaying moving picture) is driven by selecting a row electrode during a row selection period (which is for example 32 ⁇ sec), for example by presenting a sufficiently low voltage. Simultaneously, data voltages are presented to the column electrodes.
- a row selection period which is for example 32 ⁇ sec
- data voltages are presented to the column electrodes.
- the potential difference between the field emitters connected to the row electrodes and the grid electrodes determines the emission of the associated field emitters and hence the light intensity of a pixel.
- the column electrodes are not too far remote from the field emitter, hence from the row electrodes.
- this increases the capacitance associated with such a field emitter and hence the RC time for a presented addressing signal; as a result, the value of the signal, viewed across the length of a selection electrode may decrease reduced, which also gives rise to a non-uniform emission behaviour.
- an object of the invention to provide a display device of the type mentioned in the opening paragraph in which said problems are solved as much as possible. It is based on the recognition that controlled field emission is possible by charge-controlled driving rather than voltage-controlled driving.
- a display device is characterized according to the characteristic part of claim 1.
- an asymmetric two-pole circuit is understood to mean a circuit having an asymmetrical current-voltage characteristic. It may comprise inter alia, a pn-diode, a Schottky diode, or a pin diode.
- the circuit may also comprise one or more of these diodes which, due to redundance or for other reasons, are arranged in series and/or parallel. Instead of a diode, the use of transistors arranged as diodes is alternatively possible.
- the two-pole circuit is conducting during selection (during the row selection period), so that a capacitance associated with the field emitter is charged to a value determined, inter alia by the voltage across the column electrode (data voltage).
- this charging can be performed within a row selection period.
- the capacitance is discharged within the rest of a frame period (for example 20 msec) via the field emitter due to electron emission. The light intensity of a pixel is now determined by the quantity of charge across the capacitance.
- This capacitance which in the voltage-controlled drive mode results in a delay of the signal and is thus minimized as much as possible by very strict tolerances in the manufacturing process may now have a value which occurs in the conventional method of manufacture (due to, inter alia the conventional wide process tolerances). This value is usually such (or may be adapted in such a way) that the capacitance associated with the field emitter is discharged within a frame period.
- JP-A-4,249,026 emission of a single emitter is rendered uniform by controlling the emission by means of a constant-current element.
- a constant-current element a diode is shown, which is interconnected such that its reverse current or leakage current is utilized during emission. The current is not used for charging an associated capacitance.
- the display device Since the stored charge is decisive of the luminance of a pixel, the display device according to the invention is substantially insensitive to fluctuations in the current-voltage characteristics of the separate field emitters. Since the (electron) current can flow in the field emitters for a longer period of time, lower drive voltages are sufficient, dependent on the capacitance and the quantity of charge. This does not only lead to a reduced power consumption but also reduces the risk of damage due to excessive current passage so that a resistance layer under the field emitters can be dispensed with.
- the power consumption is even further reduced in that the column electrodes are now capacitively loaded to a much lesser extent.
- the capacitance of a single diode is much smaller than that of a complete pixel. Since this capacitance is smaller, it is also possible to provide an extra capacitance which functions as an auxiliary capacitance.
- An embodiment of the display device with pixels arranged in rows and columns and comprising a substrate having a first pattern of strips of conducting material and a layer of insulating material across which a second pattern of strips of conducting material extends, in which the strips of the first and the second pattern constitute a crossbar system and in which, at the location of a pixel, parts of the second pattern and the subjacent insulating material have at least one aperture in which a field emitter is realised is characterized by the characterizing part of claim 6.
- the two-pole circuit may be connected to the field emitters (possibly via a resistance layer), in which the first pattern is formed by column electrodes which are directly or not directly provided on the substrate.
- the first pattern is formed by column electrodes which are directly or not directly provided on the substrate.
- a short-circuit protection may be built in a simple manner by separating the first pattern laterally from the areas where field emission occurs.
- the grid electrodes associated with the second pattern are usually integrated to form strip-shaped row electrodes.
- Fig. 1 shows diagrammatically a part of a known display device 1, based on field emission.
- This device comprises two facing glass substrates 2 and 3.
- the substrate 2 comprises a first pattern of parallel conductors of, for example tungsten or molybdenum which function as row electrodes 4 in this case.
- the entire device is coated with an insulating layer 5 of silicon oxide.
- Column electrodes 6 of, for example molybdenum having a plurality of apertures 7 at the location of the crossings extend across the insulating layer 5 perpendicularly to the row electrodes 4.
- a plurality of field emitters is realised on the row electrodes 4. These field emitters are usually tip-shaped, conical or pointed.
- the pixels 8 are present at the locations of the crossings of the row and column electrodes.
- the substrate 3 has a conducting layer 9 which is provided with a layer 10 having, for example a pattern of phosphors or (in a monochrome display device) a single phosphor layer.
- a conducting layer 9 which is provided with a layer 10 having, for example a pattern of phosphors or (in a monochrome display device) a single phosphor layer.
- Fig. 2 is a simplified representation of an equivalent circuit diagram of the display device of Fig. 1.
- Pixels 8 are present at the location of the crossings of row electrodes 4 and column electrodes 6.
- the pixels 8 are shown by means of triodes 11, a cathode 12 of which is always formed by the field emitters associated with a pixel, while a grid is formed by the part of a column electrode which is provided with apertures 7 at the location of a crossing with a row electrode.
- the anode 9 is common for all triodes 11, which is diagrammatically shown in Fig. 2 by means of a plane 9' in broken lines.
- the row electrodes 4 a , 4 b are selected during successive selection periods while a data signal is presented to the column electrode 6 a , which together with the signal at the row electrodes 4 a , 4 b defines the voltage across the field emitters at the location of the crossings and hence the field emission and consequently the light intensity of the pixels 8 aa , 8 ab .
- the row electrodes receive a voltage of (for example) 0 Volt, so no longer any field emission in the relevant rows occurs.
- the quantity of emitted electrons should be sufficient to cause the pixels 8 to luminesce in the correct way. Since the selection period (32 ⁇ sec) is short with respect to a frame period (20 msec), a high emission should be used so as to realise the light intensity required within the selection period for the relevant frame period.
- the high voltages required not only increase the risk of breakdown (for example, between a field emitter and a grid) but also increase the power required for operation. Another problem is the variation of the current-voltage characteristics of the field emitters, which variation is greatly dependent on the method of manufacture.
- the use of high voltages influences the behaviour of the phosphors, because the phosphor saturation increases with the value of the electron current, notably for red phosphors. Due to this phosphor saturation, a disproportionately large electron current is required for an increasing brightness. This does not only require a higher voltage and more power, but a much larger number of electrons also impinges upon the phosphor, which reduces its lifetime.
- the display device of Fig. 3 shows a plurality of pixels 8 (triodes 11) arranged in the form of a matrix.
- the cathodes 12, i.e. the associated field emitter(s) are now connected in an electrically conducting manner to column electrodes 14 via diodes 13 or other suitable two-pole circuits, while the grids of the triodes 11 are connected in an electrically conducting manner to row electrodes 14.
- the capacitance between a grid and a column electrode 16 is denoted by the reference numeral 15. This capacitance which detrimentally influences the operation of the circuit in the device of Fig. 1 plays an essential role in the device according to the invention, as will be described hereinafter.
- the anode 9' is again shown as common for all triodes 11.
- the display device of Fig. 3 is driven as follows (see Fig. 4).
- t 0 positive selection voltage V s is presented to the row electrode 14 a during a selection period T s (see Fig. 4 a ).
- a data signal V d is presented to the column electrode 16 a (see Fig. 4 d ) which, together with the signal at the row electrode 14 a , defines the voltage across the field emitter(s) and hence the field emission of the pixel 8 aa .
- the row electrode 14 a receives a voltage V ns of (for example) 0 Volt at which no field emission occurs in the relevant row.
- the selection voltage V s is presented to the row electrodes 14 b during a selection period T s (see Fig. 4 b ).
- a data signal V d is presented to the column electrode 16 a (see Fig. 4 d ) which, together with the signal at the row electrode 14 b defines the voltage across the field emitter(s) and hence the field emission of the pixel 8 ab .
- the row electrode 14 b receives a voltage V ns , etc .
- a demultiplex circuit (or shift register) 30 which in this example selects the row electrodes consecutively, while data voltages are presented to the column electrodes 16 via a register 31.
- an incoming signal 32 is sampled via a control circuit 33 and applied to the register 31.
- the control circuit 33 also ensures the mutual synchronization.
- V gc V s -V d -V on , in which V on is the voltage across the diodes 13.
- the series resistance of the diodes 15 is much smaller than that of the field emitters of a pixel which can be considered as a diode (these conduct (emit) only when V gc is positive).
- V gc in which C is the value of the capacitance 15, while the field emitters already emit electrons.
- the voltage across the capacitance 15 is maintained.
- the voltage at the grid of the triode (or row electrode 14 a ) will then be 0 Volt, while the voltage at the cathode of the triode will acquire a negative value of -V gc .
- the capacitance 15 is then discharged during the rest of a frame period because the triode 11 aa continues to conduct (or the associated field emitters continue to emit).
- this emission takes place during a period of slightly less than a frame period (20 msec) whereafter the capacitance 15 is again charged in the manner described hereinbefore (dependent on the signal across the column).
- the selection period T s can be reduced to the period required to charge this capacitance 15. This period is usually shorter than 32 ⁇ sec. As a result, variations in the current-voltage characteristics of field emitters of different pixels are compensated for by contradistinctive variations in discharge time of the associated capacitances. Since the emission takes place for a longer time, lower voltages may be sufficient. This reduces the phosphor saturation and renders a high brightness possible because, spread in time, the same quantity of electrons impinges upon the phosphor. Since the phosphors are now less driven in saturation, higher drive voltages may be used, if necessary, to achieve a higher brightness.
- an extra resistance layer which is usually present to avoid breakdown may be dispensed with.
- Fig. 5 shows diagrammatically a part of a display device according to the invention.
- the display device comprises two facing substrates 2, 3 of, for example glass.
- island-shaped metal regions 17 of, for example niobium or molybdenum which contact semiconductor regions 18 and 19 are present on the first substrate 2.
- the semiconductor region 18 is present on the substrate 2 in this embodiment, but the metal region 17 may also extend completely under this region 18.
- the lower sub-layer 19 a of the semiconductor layer is very weakly doped or substantially intrinsic so that the subjacent metal 17 forms a Schottky diode with this sub-layer.
- the upper sub-layer 19 b is of the n + type and constitutes an ohmic connection with a metal column conductor of, for example molybdenum.
- the column electrodes 16 extend parallel to each other. Column electrodes 16 and semiconductor regions 18, also of the n + type are coated with a layer of insulating material 5 across which parallel row electrodes 14 extend transversely to the direction of the column electrodes
- the row electrodes as well as the subjacent insulating material 5 are provided with apertures which extend as far as the surface of the semiconductor region 18.
- Tip-shaped or conical field emitters are provided on this surface in known manner via the afore-mentioned apertures, which emitters are indicated diagrammatically by means of the reference numeral 20 in one of the pixels of Fig. 6.
- the semiconductor region 18 constitutes a cathode connection for the field emitters with which there are connected in an electrically conducting manner.
- the substrate 3 has a conducting layer 9 of, for example indium-tin oxide which is provided with a layer 10 having, for example a pattern of phosphors.
- Fig. 6 is a diagrammatic cross-section of a modification of the device of Fig. 5.
- the column electrodes 16 are now present on the substrate 2.
- the column electrodes are coated with a thin layer of amorphous silicon 19 in which (Schottky) diodes are realised by forming the lower sub-layers 19 b as highly doped n + layers and the upper sub-layers 19 a as intrinsic layers which are contacted by metal strips 17 of, for example molybdenum.
- the metal strips constitute a Schottky diode together with the subjacent intrinsic amorphous silicon; if necessary, a pn diode may be realised in this case by giving the sub-layer 19 a a p-type doping.
- the other reference numerals refer to the same components as those in Fig. 5.
- Fig. 7 shows a modification of the device of Fig. 6.
- extra insulating layers 21 and metal faces 22 constituting extra capacitances together with the n + type semiconductor regions are present below these regions, as is shown for one pixel in the equivalent circuit diagram of Fig. 8; the extra capacitance is denoted by the reference numeral 23.
- the layer 18 may be formed as a metal layer due to the reduced risk of breakdown at a lower voltage.
- series circuits and/or parallel circuits may be used instead of single diodes 13 or other two-pole circuits for reasons of redundancy.
- the switching unit 13 in Fig. 9 a comprises two series-arranged diodes 25, while the switching unit 13 in Fig. 9 b comprises two parallel-arranged diodes 26.
- a pixel 8 semiconductor region 18 may be divided into sub-pixels (sub-regions), also for reasons of redundancy.
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Description
- The invention relates to a display device according to the introductory part of
claim 1. - A display device of this type is usually in the form of a flat display device and is suitable, for example, for displaying video information and alpha-numerical information.
- A display device of the type mentioned in the opening paragraph is described in USP 4,857,161. The device shown in this document comprises strip-shaped row electrodes on a substrate on which a plurality of tip-shaped (pointed) field emitters are realised per pixel at the location of crossings with column electrodes. The column electrodes, which also function as grid electrodes in this case, are separated from the row electrodes by means of a layer of insulating material. At the location of the pixels apertures in which the field emitters have been realised on the row electrodes are present in the column electrodes and the subjacent insulating material. A face plate provided with phosphors to which electrons released by field emission can be accelerated is present opposite the substrate.
- The display device shown (when displaying moving picture) is driven by selecting a row electrode during a row selection period (which is for example 32 µsec), for example by presenting a sufficiently low voltage. Simultaneously, data voltages are presented to the column electrodes. The potential difference between the field emitters connected to the row electrodes and the grid electrodes determines the emission of the associated field emitters and hence the light intensity of a pixel.
- To obtain a light intensity which is as uniform as possible across the entire surface in such a display device, it is important that the field emitters behave as uniformly as possible, for example with regard to their current-voltage characteristic. This requires a substantially identical shape of all tip-shaped (pointed) field emitters, which imposes very strict requirements from a technological point of view. Since this requirement cannot generally be satisfied in practice, an extra resistor is often arranged in series with the pixel, for example, between the field emitters and the row electrode for the purpose of uniformity. However, this leads to a higher power consumption and higher drive voltages. For these higher drive voltages it may be necessary to realise drive electronics with special circuits, which may additionally increase the cost of these drive electronics, for example because it can no longer be realised with low-cost technologies such as, for example CMOS.
- Moreover, it is necessary for a satisfactory operation that the column electrodes are not too far remote from the field emitter, hence from the row electrodes. However, this increases the capacitance associated with such a field emitter and hence the RC time for a presented addressing signal; as a result, the value of the signal, viewed across the length of a selection electrode may decrease reduced, which also gives rise to a non-uniform emission behaviour.
- It is, inter alia, an object of the invention to provide a display device of the type mentioned in the opening paragraph in which said problems are solved as much as possible. It is based on the recognition that controlled field emission is possible by charge-controlled driving rather than voltage-controlled driving.
- To this end a display device according to the invention is characterized according to the characteristic part of
claim 1. - In this respect an asymmetric two-pole circuit is understood to mean a circuit having an asymmetrical current-voltage characteristic. It may comprise inter alia, a pn-diode, a Schottky diode, or a pin diode. The circuit may also comprise one or more of these diodes which, due to redundance or for other reasons, are arranged in series and/or parallel. Instead of a diode, the use of transistors arranged as diodes is alternatively possible.
- In the device according to the invention the two-pole circuit is conducting during selection (during the row selection period), so that a capacitance associated with the field emitter is charged to a value determined, inter alia by the voltage across the column electrode (data voltage). By using the forward characteristics of the diodes this charging can be performed within a row selection period. After the selection, the capacitance is discharged within the rest of a frame period (for example 20 msec) via the field emitter due to electron emission. The light intensity of a pixel is now determined by the quantity of charge across the capacitance. This capacitance, which in the voltage-controlled drive mode results in a delay of the signal and is thus minimized as much as possible by very strict tolerances in the manufacturing process may now have a value which occurs in the conventional method of manufacture (due to, inter alia the conventional wide process tolerances). This value is usually such (or may be adapted in such a way) that the capacitance associated with the field emitter is discharged within a frame period.
- In JP-A-4,249,026 emission of a single emitter is rendered uniform by controlling the emission by means of a constant-current element. As an example of such a constant-current element a diode is shown, which is interconnected such that its reverse current or leakage current is utilized during emission. The current is not used for charging an associated capacitance.
- Since the stored charge is decisive of the luminance of a pixel, the display device according to the invention is substantially insensitive to fluctuations in the current-voltage characteristics of the separate field emitters. Since the (electron) current can flow in the field emitters for a longer period of time, lower drive voltages are sufficient, dependent on the capacitance and the quantity of charge. This does not only lead to a reduced power consumption but also reduces the risk of damage due to excessive current passage so that a resistance layer under the field emitters can be dispensed with.
- Problems of phosphor saturation are also reduced by using lower drive voltage (and consequently lower currents). Due to saturation effects in the phosphors on the face plate, which phosphors must be driven within a short period of time, it is often necessary in existing devices (notably at a high brightness) to apply high drive voltages to the column electrodes, which increases the power consumption.
- The power consumption is even further reduced in that the column electrodes are now capacitively loaded to a much lesser extent. The capacitance of a single diode is much smaller than that of a complete pixel. Since this capacitance is smaller, it is also possible to provide an extra capacitance which functions as an auxiliary capacitance.
- An embodiment of the display device with pixels arranged in rows and columns and comprising a substrate having a first pattern of strips of conducting material and a layer of insulating material across which a second pattern of strips of conducting material extends, in which the strips of the first and the second pattern constitute a crossbar system and in which, at the location of a pixel, parts of the second pattern and the subjacent insulating material have at least one aperture in which a field emitter is realised is characterized by the characterizing part of
claim 6. - Dependent on the realisation, the two-pole circuit may be connected to the field emitters (possibly via a resistance layer), in which the first pattern is formed by column electrodes which are directly or not directly provided on the substrate. When the column electrodes are provided on the substrate, a short-circuit protection may be built in a simple manner by separating the first pattern laterally from the areas where field emission occurs. The grid electrodes associated with the second pattern are usually integrated to form strip-shaped row electrodes.
- These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings
- Fig. 1 shows diagrammatically a part of a known display device,
- Fig. 2 shows the device of Fig. 1 in an electric circuit diagram,
- Fig. 3 shows a device according to the invention, also in an electric circuit diagram,
- Fig. 4 shows control signals associated with the device of Fig. 3,
- Figs. 5 to 7 show possible embodiments of a part of a display device according to the invention, while
- Fig. 8 shows the electric circuit diagram of a pixel of the display device of Fig. 7 and
- Fig. 9 shows some realisations of an asymmetric two-pole circuit.
- Fig. 1 shows diagrammatically a part of a known
display device 1, based on field emission. This device comprises two facingglass substrates substrate 2 comprises a first pattern of parallel conductors of, for example tungsten or molybdenum which function as row electrodes 4 in this case. With the exception of the areas near the ends 4' of the row electrodes, where they are not insulated for the purpose of connection to external contacts, the entire device is coated with aninsulating layer 5 of silicon oxide.Column electrodes 6 of, for example molybdenum having a plurality ofapertures 7 at the location of the crossings extend across theinsulating layer 5 perpendicularly to the row electrodes 4. In these apertures, which extend across the thickness of the subjacent insulating layer, a plurality of field emitters is realised on the row electrodes 4. These field emitters are usually tip-shaped, conical or pointed. Thepixels 8 are present at the locations of the crossings of the row and column electrodes. - The
substrate 3 has a conductinglayer 9 which is provided with alayer 10 having, for example a pattern of phosphors or (in a monochrome display device) a single phosphor layer. By giving the electrode 9 (anode) a sufficiently high voltage, electrons emitted by the field emitters are accelerated towards the substrate 3 (the face plate) where they cause a part 8' of the phosphor pattern corresponding to apixel 8 to luminesce. The quantity of emitted electrons can be modulated with voltages across grid electrodes integrated tocolumn electrodes 6, via connections 6'. - Fig. 2 is a simplified representation of an equivalent circuit diagram of the display device of Fig. 1.
Pixels 8 are present at the location of the crossings of row electrodes 4 andcolumn electrodes 6. In Fig. 2 thepixels 8 are shown by means oftriodes 11, acathode 12 of which is always formed by the field emitters associated with a pixel, while a grid is formed by the part of a column electrode which is provided withapertures 7 at the location of a crossing with a row electrode. Theanode 9 is common for alltriodes 11, which is diagrammatically shown in Fig. 2 by means of a plane 9' in broken lines. - During operation the row electrodes 4a, 4b are selected during successive selection periods while a data signal is presented to the
column electrode 6a, which together with the signal at the row electrodes 4a, 4b defines the voltage across the field emitters at the location of the crossings and hence the field emission and consequently the light intensity of thepixels - The quantity of emitted electrons should be sufficient to cause the
pixels 8 to luminesce in the correct way. Since the selection period (32 µsec) is short with respect to a frame period (20 msec), a high emission should be used so as to realise the light intensity required within the selection period for the relevant frame period. The high voltages required not only increase the risk of breakdown (for example, between a field emitter and a grid) but also increase the power required for operation. Another problem is the variation of the current-voltage characteristics of the field emitters, which variation is greatly dependent on the method of manufacture. Moreover, the use of high voltages influences the behaviour of the phosphors, because the phosphor saturation increases with the value of the electron current, notably for red phosphors. Due to this phosphor saturation, a disproportionately large electron current is required for an increasing brightness. This does not only require a higher voltage and more power, but a much larger number of electrons also impinges upon the phosphor, which reduces its lifetime. - The display device of Fig. 3 shows a plurality of pixels 8 (triodes 11) arranged in the form of a matrix. The
cathodes 12, i.e. the associated field emitter(s) are now connected in an electrically conducting manner tocolumn electrodes 14 viadiodes 13 or other suitable two-pole circuits, while the grids of thetriodes 11 are connected in an electrically conducting manner to rowelectrodes 14. The capacitance between a grid and acolumn electrode 16 is denoted by thereference numeral 15. This capacitance which detrimentally influences the operation of the circuit in the device of Fig. 1 plays an essential role in the device according to the invention, as will be described hereinafter. The anode 9' is again shown as common for alltriodes 11. - The display device of Fig. 3 is driven as follows (see Fig. 4). At the instant t0 (for example) positive selection voltage Vs is presented to the
row electrode 14a during a selection period Ts (see Fig. 4a). Simultaneously, a data signal Vd is presented to the column electrode 16a (see Fig. 4d) which, together with the signal at therow electrode 14a, defines the voltage across the field emitter(s) and hence the field emission of thepixel 8aa. After the selection period Ts has elapsed, therow electrode 14a receives a voltage Vns of (for example) 0 Volt at which no field emission occurs in the relevant row. At the instant t1 the selection voltage Vs is presented to therow electrodes 14b during a selection period Ts (see Fig. 4b). Simultaneously, a data signal Vd is presented to the column electrode 16a (see Fig. 4d) which, together with the signal at therow electrode 14b defines the voltage across the field emitter(s) and hence the field emission of thepixel 8ab. After the selection period Ts has elapsed, therow electrode 14b receives a voltage Vns, etc. Therow electrodes column electrodes 16 via aregister 31. To this end, anincoming signal 32 is sampled via acontrol circuit 33 and applied to theregister 31. Thecontrol circuit 33 also ensures the mutual synchronization. During a selection period the voltage between the grid and the cathode of a pixel and across the associatedcapacitance 15 is Vgc = Vs-Vd-Von, in which Von is the voltage across thediodes 13. The series resistance of thediodes 15 is much smaller than that of the field emitters of a pixel which can be considered as a diode (these conduct (emit) only when Vgc is positive). Thecapacitances 15 are then charged during a selection period (32 µsec) to a charge of Q = C. Vgc, in which C is the value of thecapacitance 15, while the field emitters already emit electrons. - When the voltage at the
row electrode 14a changes from Vs to Vns at the instant t1 (here 0 Volt), the voltage across thecapacitance 15 is maintained. The voltage at the grid of the triode (or row electrode 14a) will then be 0 Volt, while the voltage at the cathode of the triode will acquire a negative value of -Vgc. Thecapacitance 15 is then discharged during the rest of a frame period because thetriode 11aa continues to conduct (or the associated field emitters continue to emit). For a suitable structure of the field emitters (choice of the material, possibly an extra resistance layer) this emission takes place during a period of slightly less than a frame period (20 msec) whereafter thecapacitance 15 is again charged in the manner described hereinbefore (dependent on the signal across the column). - Since the emission during the selection period is small with respect to that during the rest of the frame period, this emission is now substantially completely defined by the quantity of charge across the capacitance. Since the charge across the
capacitance 15 now substantially defines the emission, the selection period Ts can be reduced to the period required to charge thiscapacitance 15. This period is usually shorter than 32 µsec. As a result, variations in the current-voltage characteristics of field emitters of different pixels are compensated for by contradistinctive variations in discharge time of the associated capacitances. Since the emission takes place for a longer time, lower voltages may be sufficient. This reduces the phosphor saturation and renders a high brightness possible because, spread in time, the same quantity of electrons impinges upon the phosphor. Since the phosphors are now less driven in saturation, higher drive voltages may be used, if necessary, to achieve a higher brightness. - At sufficiently low voltages, an extra resistance layer which is usually present to avoid breakdown may be dispensed with.
- Another advantage is that the column electrodes are now connected to the
diodes 13 instead of constituting grid electrodes themselves. Consequently, the capacitive load of these column electrodes is much smaller. This in turn leads to a smaller load of drive circuits, thus simplifying and realising them at lower cost. Together with the mentioned decrease of the drive voltage, this leads to a considerably smaller energy consumption, and hence to low-cost power supply circuits. - Fig. 5 shows diagrammatically a part of a display device according to the invention. The display device comprises two facing
substrates pixels 8 island-shapedmetal regions 17 of, for example niobium or molybdenum whichcontact semiconductor regions first substrate 2. Thesemiconductor region 18 is present on thesubstrate 2 in this embodiment, but themetal region 17 may also extend completely under thisregion 18. Thelower sub-layer 19a of the semiconductor layer is very weakly doped or substantially intrinsic so that thesubjacent metal 17 forms a Schottky diode with this sub-layer. Theupper sub-layer 19b is of the n+ type and constitutes an ohmic connection with a metal column conductor of, for example molybdenum. Thecolumn electrodes 16 extend parallel to each other.Column electrodes 16 andsemiconductor regions 18, also of the n+ type are coated with a layer of insulatingmaterial 5 across whichparallel row electrodes 14 extend transversely to the direction of thecolumn electrodes 16. - At the location of the pixels 8 (i.e. at locations where the
row electrodes 14 are present abovesemiconductor regions 18 in this embodiment) the row electrodes as well as the subjacent insulating material 5 (for example, silicon oxide) are provided with apertures which extend as far as the surface of thesemiconductor region 18. Tip-shaped or conical field emitters are provided on this surface in known manner via the afore-mentioned apertures, which emitters are indicated diagrammatically by means of thereference numeral 20 in one of the pixels of Fig. 6. Thesemiconductor region 18 constitutes a cathode connection for the field emitters with which there are connected in an electrically conducting manner. Thesubstrate 3 has aconducting layer 9 of, for example indium-tin oxide which is provided with alayer 10 having, for example a pattern of phosphors. - Fig. 6 is a diagrammatic cross-section of a modification of the device of Fig. 5. The
column electrodes 16 are now present on thesubstrate 2. At the location of the pixels the column electrodes are coated with a thin layer ofamorphous silicon 19 in which (Schottky) diodes are realised by forming thelower sub-layers 19b as highly doped n+ layers and theupper sub-layers 19a as intrinsic layers which are contacted bymetal strips 17 of, for example molybdenum. In this embodiment the metal strips constitute a Schottky diode together with the subjacent intrinsic amorphous silicon; if necessary, a pn diode may be realised in this case by giving the sub-layer 19a a p-type doping. The other reference numerals refer to the same components as those in Fig. 5. - Fig. 7 shows a modification of the device of Fig. 6. At the location of the
semiconductor regions 18 extra insulatinglayers 21 and metal faces 22 constituting extra capacitances together with the n+ type semiconductor regions are present below these regions, as is shown for one pixel in the equivalent circuit diagram of Fig. 8; the extra capacitance is denoted by thereference numeral 23. - The invention is of course not limited to the embodiments shown, but several variations are possible within the scope of the invention. For example, the
layer 18 may be formed as a metal layer due to the reduced risk of breakdown at a lower voltage. - As already stated, series circuits and/or parallel circuits may be used instead of
single diodes 13 or other two-pole circuits for reasons of redundancy. For example, the switchingunit 13 in Fig. 9a comprises two series-arrangeddiodes 25, while the switchingunit 13 in Fig. 9b comprises two parallel-arrangeddiodes 26. A pixel 8 (semiconductor region 18) may be divided into sub-pixels (sub-regions), also for reasons of redundancy.
Claims (7)
- A display device (1) comprising a plurality of pixels (8) arranged in rows and columns and comprising row electrodes (4,14) and column electrodes (6,16), each pixel comprising at least one field emitter and a grid electrode, characterized in that each pixel comprises between a column electrode (6,16) and a field emitter an asymmetric two-pole circuit (13) comprising one or more diodes, the asymmetric two-pole circuit being arranged such that a diode is interconnected in its forward direction from the field emitter to the column electrode (6,16).
- A display device as claimed in Claim 1, characterized in that the display device comprises means (30) for selecting row electrodes (4,14) and means (31) for applying voltages to column electrodes (6,16) to charge a capacitance (15) between a grid electrode and a column electrode, the charge of the capacitance determining the emission of the field emitters of a pixel.
- A display device as claimed in Claim 2, characterized in that a pixel (8) is selected each frame period during a selection period while emission takes place during substantially the remainder of the frame period.
- A display device as claimed in Claims 1, 2 or 3, characterized in that the asymmetric two-pole circuit comprises series-arranged diodes (25).
- A display device as claimed in Claims 1, 2 or 3, characterized in that the asymmetric two-pole circuit comprises a parallel arrangement of diodes (26).
- A display device according to one of the claims 1 to 5, comprising a substrate (2) having a first pattern of strips (16) of conducting material as column electrodes and a layer of insulating material (5) across which a second pattern of conducting material (14) as row electrodes extends, in which the strips of the first and the second pattern constitute a crossbar system and in which, at the location of a pixel, parts of the second pattern and the subjacent insulating material have at least one aperture (7) in which a field emitter is realized, wherein each field emitter associated with a pixel is connected in an electrically conducting manner to a first terminal of an asymmetric two-pole circuit comprising diodes, while the second terminal of the asymmetric two-pole circuit is connected in an electrically conducting manner to a strip (16) of the first pattern.
- A display device as claimed in Claim 6, characterized in that the first pattern is present on the first substrate and is laterally separated from conducting regions for field emitters associated with pixels.
Priority Applications (1)
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EP19930202700 EP0589523B1 (en) | 1992-09-25 | 1993-09-17 | Display device |
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
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EP92202948 | 1992-09-25 | ||
EP92202948 | 1992-09-25 | ||
EP19930202700 EP0589523B1 (en) | 1992-09-25 | 1993-09-17 | Display device |
Publications (2)
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EP0589523A1 EP0589523A1 (en) | 1994-03-30 |
EP0589523B1 true EP0589523B1 (en) | 1997-12-17 |
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EP19930202700 Expired - Lifetime EP0589523B1 (en) | 1992-09-25 | 1993-09-17 | Display device |
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Families Citing this family (4)
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JP3278375B2 (en) * | 1996-03-28 | 2002-04-30 | キヤノン株式会社 | Electron beam generator, image display device including the same, and method of driving them |
US5844370A (en) * | 1996-09-04 | 1998-12-01 | Micron Technology, Inc. | Matrix addressable display with electrostatic discharge protection |
US5945968A (en) * | 1997-01-07 | 1999-08-31 | Micron Technology, Inc. | Matrix addressable display having pulsed current control |
FR2809862B1 (en) * | 2000-05-30 | 2003-10-17 | Pixtech Sa | FLAT DISPLAY SCREEN WITH ADDRESSING MEMORY |
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FR2593953B1 (en) * | 1986-01-24 | 1988-04-29 | Commissariat Energie Atomique | METHOD FOR MANUFACTURING A DEVICE FOR VIEWING BY CATHODOLUMINESCENCE EXCITED BY FIELD EMISSION |
FR2641108A1 (en) * | 1988-12-23 | 1990-06-29 | Thomson Csf | Display device having a cathode ray tube screen |
JPH04221990A (en) * | 1990-12-25 | 1992-08-12 | Sony Corp | Image display device |
JP2626276B2 (en) * | 1991-02-06 | 1997-07-02 | 双葉電子工業株式会社 | Electron-emitting device |
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