WO1997005597A1

WO1997005597A1 - Flat panel pixel array incorporating photoconductive switches

Info

Publication number: WO1997005597A1
Application number: PCT/CA1995/000459
Authority: WO
Inventors: T. Gordon Pilkington
Original assignee: Litton Systems Canada Limited
Priority date: 1995-07-31
Filing date: 1995-07-31
Publication date: 1997-02-13

Abstract

A flat panel such as a liquid crystal display (20) includes an array of pixels (24) arranged in rows and columns. A select line (26) interconnects each row of the array while a data line (28) interconnects each column of the array. An array (30) of photoconductive switches (60) is connected between each select line (26) and a voltage bus (50) connected to a potential voltage source (VPP). An array (37) of photoconductive switches (60) is connected between each data line (28) and a voltage bus (56) connected to a potential voltage source VSS. Each photoconductive switch (60) is responsive to incident radiation (34, 39) to connect the select and data lines to the voltage busses and bias the pixels to display data.

Description

FLAT PANEL PIXEL ARRAY INCORPORATING PHOTOCONDUCTIVE SWITCHES

TECHNICAL FIELD

The present invention relates to pixel arrays and in particular to a pixel array incorporating photoconductive semiconductor switches. More particularly, the present invention relates to a flat panel such as a liquid crystal display incorporating a photoconductive bus line switch array.

BACKGROUND ART

Liquid crystal display ("LCD") panels are used in televisions and lap-top computer screens and present video information by modulating light intensity on a pixel-by- pixel basis. Panels of this nature include an array of pixels arranged in rows and columns. Select lines interconnect the individual rows of the array while data lines interconnect the individual columns of the array. Depending on the type of LCD, each pixel may include a TFT switch which uses a semiconductor switch or may be of the passive ferroelectric type. Each pixel has a transparent electrode constituting the output pad of the pixel. The pixels and select and data lines may be fabricated on a common substrate or on separate substrates. A peripheral driver is connected to the select lines on at least one side of the array to select the select lines in succession so that data may be written to a given pixel via the associated data line and onto the transparent electrode. The voltage appearing on the transparent electrode modulates light via the LCD or photoemission effect.

In typical applications, the array of pixels in the flat panel is large and may include as many as one thousand (1000) select and data lines. In the case of ferroelectric LCD panels, since each select and data line must be connected to a driver, a significant number of external electrical connections must be made between the array and the peripheral driver. In cases where redundant drivers are used the number of electrical connections is doubled. It is desired to reduce the number of such connections between the array and peripheral circuits.

It is therefore an object of the present invention to provide a novel flat panel.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a flat panel display comprising: an array of pixels arranged in rows and columns; a plurality of select lines, each interconnecting an individual one of one of the rows or columns of said array for supplying a bias to pixels in said individual one; a plurality of data lines, each interconnecting an individual one of the other of the rows or columns of said array for supplying a bias to pixels in said individual one; and at least one array of photoconductive switches, said at least one array of photoconductive switches being associated with one of said plurality of select or data lines, each photoconductive switch in said at least one array being connected to one of said select or data lines of said plurality, each photoconductive switch being responsive to incident radiation to allow said bias to be supplied to said select or data lines.

Preferably, the flat panel display includes a pair of photoconductive switch arrays, one of the photoconductive switch arrays being connected between each of the select lines and a voltage bus connected to a potential voltage source to supply the bias. The other photoconductive switch array is connected between each of the data lines and a voltage source to supply the bias. It is also preferred that the photoconductive switch array connected to the select lines is configured to supply bias to the select lines to allow the gray shade of the pixels to be selected from a plurality of shades. In one embodiment, this is achieved by configuring each data line as a data bus and sub-dividing each pixel into sub-pixel areas. Each sub-pixel area is connected to the voltage source via a photoconductive switch and a data line of the data bus. The gray shade of a pixel can be selected by directing the incident radiation to one or more of the photoconductive switches connected to the sub-pixel areas of that pixel. In another embodiment, each pixel is connected to three potential voltage sources via a data line and three photoconductive switches. The gray shade of a pixel can be selected by directing the incident radiation to one or more of the photoconductive switches to connect the pixel to one or more of the potential voltage sources.

In one embodiment, each photoconductive switch in the photoconductive switch arrays includes means to accelerate the transition of the photoconductive switch to an off-state upon removal of the incident radiation.

In one embodiment, it is also preferred that the array of pixels and the photoconductive switch arrays include thin film transistors and are formed on a common substrate.

According to another aspect of the present invention there is provided a passive matrix flat panel display comprising: an array of pixels arranged in rows and columns, each of said pixels for displaying a gray shade and including a pair of spaced electrodes; a plurality of select lines, each interconnecting one electrode of an individual one of one of the rows or columns of said array for supplying a bias to said one electrodes in said individual one; a plurality of data lines, each interconnecting the other electrode of an individual one of the other of the rows or columns of said array for supplying a bias to said other electrodes in said individual one; a first array of photoconductive switches, associated with said select lines, each photoconductive switch in said first array being connected to one of said select lines and being responsive to incident radiation to allow said bias to be supplied to said select lines; and a second array of photoconductive switches associated with said data lines, each photoconductive switch in said second array of photoconductive switches being connected to said data lines and being responsive to incident radiation to allow said bias to be supplied to said data lines.

According to yet another aspect of the present invention there is provided an active matrix liquid crystal display comprising: a thin film transistor switch array arranged in rows and columns and formed on a common substrate; a plurality of gate lines formed on said substrate, each interconnecting an individual one of one of the rows or columns of said array; a plurality of source lines formed on said substrate, each interconnecting an individual one of the other of the rows or columns of said array; an array of photoconductive switches formed on said substrate, each being connected to one of said gate lines, each photoconductive switch being responsive to incident radiation to connect said gate line to a potential voltage source and select the pixels connected thereto to allow for data to be written to the selected pixels; and an optical scanning system to direct said incident radiation towards a selected photoconductive switch of said array.

In still yet another aspect of the present invention there is provided a flat panel for radiation imaging comprising: a thin film transistor switch array arranged in rows and columns and formed on a common substrate; a plurality of gate lines formed on said substrate, each interconnecting an individual one of one of the rows or columns of said array; a plurality of source lines formed on said substrate, each interconnecting an individual one of the other of the rows or columns of said array; an array of photoconductive switches formed on said substrate, each being connected to one of said gate lines, each photoconductive switch being responsive to incident radiation to connect said gate line to a potential voltage source and select the pixels connected thereto to allow for data to be read from the selected pixels; and an optical scanning system to direct said incident radiation towards a selected photoconductive switch of said array.

The present invention provides advantages in that since the select and/or data lines are selected by exposing each photoconductive switch to incident radiation, the number of physical electrical connections between the array of pixels and peripheral circuitry is reduced as compared with prior art flat panels.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described more fully with reference to the accompanying drawings in which: Figure 1 is a schematic of a flat panel display;

Figure 2 is a schematic of an optical scanning system forming part of the flat panel display of Figure 1;

Figures 3a and 3b are top plan views of portions of the flat panel display of Figure 1;

Figure 4 is a cross-sectional view a photoconductive switch forming part of the flat panel display of Figure 1; and

Figure 5 is a cross-sectional view an alternative embodiment of a photoconductive switch for a flat panel display.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to Figure l, a flat panel in the form of a ferroelectric liquid crystal display is shown and is generally indicated by reference numeral 20. The liquid crystal display 20 includes an LCD panel 22 constituted by an array of pixels 24 arranged in rows and columns. Each pixel 24 includes a lower electrode 24a and a transparent upper electrode 24b constituting the output pad of the pixel. The pixels 24 in each row of the array are interconnected by a select line 26 while the pixels 24 in each column of the array are interconnected by data busses 28. Specifically, the select lines are connected to the upper electrode 24b of each pixel while the data busses are connected to the lower electrode 24a of each pixel. As is known to those of skill in the art, in order to write data to a pixel, both the upper and lower electrodes of that pixel need to be biased.

A photoconductive switch array 30 is connected to one side of the LCD panel 22 and supplies a bias to the select lines 26 as will be described. The photoconductive switch array 30 includes an array of photoconductive switches 60 (see Figures 3a and 4) , each of which is connected to one of the select lines 26. An optical scanning system 32 is associated with the photoconductive switch array 30 and directs an incident beam of radiation 34 towards the photoconductive switch array in response to input received from a control circuit 36.

A second photoconductive switch array 37 is connected to another side of the LCD panel 22 and supplies a bias to the data busses 28 as will be described. The photoconductive switch array 37 also includes an array of photoconductive switches 60, each of which is connected to one of the data busses 28. An optical scanning system 38 is associated with the photoconductive switch array 37 and directs an incident beam of radiation 39 towards photoconductive switch array 37 in response to input from the control circuit 36.

Turning now to Figure 2, the optical scanning system 32 is better illustrated. As can be seen, the optical scanning system includes a shutter 40 receiving the incident beam of radiation 34. The shutter 40 opens in response to gate pulses received on input line 42 from control circuit 36 to allow the incident beam of radiation to pass. An optical scanner 44 receives the incident beam of radiation 34 when the shutter 40 is open. The optical scanner 44 sweeps the beam of incident radiation across the photoconductive switch array 30 in response to input from control circuit 36 to turn on the photoconductive switches and thereby bias successively the select lines 26. Optical scanner 44 may be of the reflective type having a moving mirror to sweep the incident beam of radiation, or of the refractive type having an optically active crystal to sweep the incident beam of radiation or may be of the electronic type such as a linear LED array similar to those used in photocopiers and facsimile machines.

Although only the optical scanning system 32 is shown, it should be realized that optical scanning system 38 is basically the same as optical scanning system 30. However, in optical scanning system 38 the incident beam of radiation 39 is swept across the photoconductive switch array 37 to select the desired data busses 28 to allow data to be written to pixels 24 that have also been selected via the select lines 26.

Figure 3a best illustrates the photoconductive switch array 30 and as can be seen, the photoconductive switch array is connected between the select lines 26 and a voltage bus 50. The voltage bus 50 leads to a positive potential voltage source V_PP. An accelerate bus 52 is also connected to the photoconductive switch array 30 and leads to a control voltage V_cc. Each photoconductive switch 60 in the array 30 is connected to one of the select lines 26 and to the voltage and accelerate busses 50 and 52 respectively.

Referring now to Figure 3b, the photoconductive switch array 37 is better illustrated. As can be seen, the photoconductive switch array 37 is connected between the data busses 28 and a voltage bus 56 leading to a potential voltage source V^. An accelerate bus 58 is also connected to the photoconductive switch array 37 and leads to the control voltage Vcc. Four photoconductive switches 60 in the array 37 are associated with the pixels 24 of each column and are connected to the data bus 28 extending to that column as well as to the voltage bus 56. One of the pixels 24 is shown divided into four weighted sub-pixel areas 25a to 25d. By selecting one or more of the sub- pixel areas by activating one or more of the photoconductive switches 60 connected to that pixel via the data bus 28, the pixel 24 can be illuminated to different gray shades.

One of the photoconductive switches 60 forming part of both the arrays 30 and 37 is shown in Figure 4. Each photoconductive switch 60 is in the form of a thin film transistor allowing the photoconductive switches of the arrays 30 and 37 to be fabricated on a common substrate 29. The accelerate bus 52 or 58 passes beneath the photoconductive switch 60 and is covered by a dielectric layer 62 formed of Si0₂. A photoconductive semiconductor material film 64 formed of CdSe is deposited on the dielectric layer 62. The photoconductive semiconductor material film 64 contacts both the voltage bus 50 or 56 and the select line 26 or a data line of data bus 28 and establishes an electrical connection between the voltage bus and the select or data line upon exposure to incident radiation.

During operation of the liquid crystal display

20, when it is desired to write data to a particular pixel 24 in the array, the select line 26 connected to that particular pixel must be selected. When a select line 26 is to be selected, the control circuit 36 provides input to the shutter 40 as well as to the optical scanner 44 so that the incident beam of radiation 34 passes through the shutter and is directed by the optical scanner to the photoconductive switch 60 of the photoconductive switch array 30 connected to the appropriate select line 26.

Upon exposure to incident radiation, the photoconductive semiconductor material film 64 establishes an electrical connection between the voltage bus 50 and the select line 26. Since the voltage bus 50 is connected to the positive potential voltage source V_PP, the select line 26 becomes biased to a high potential. The bias on the select line 26 is applied to the lower electrode of each pixel 24 connected to the select line. Once the lower electrodes are biased, data can be written to any of these selected pixels 24. This is achieved by causing the optical scanning system 38 to direct the incident beam of radiation 39 to the appropriate photoconductive switches 60 associated with the selected pixels. When this occurs, the voltage source V^ is connected to one or more of the upper electrodes of the sub-pixel areas of the selected pixels via the lines of the data bus 28. The optical scanning system 38 and the photoconductive switches 60 of array 37 operate in a similar manner as array 30 and optical scanning system 32.

Once a select line 26 and a data bus 28 have been selected and it is desired to deselect the select line and data bus 28, the photoconductive switches 60 connected to that select line and data bus must be isolated from the incident beam of radiation 34 and 39. This can be achieved by closing the shutter 40 or causing the optical scanner 44 to direct the incident beams of radiation 34 and 39 to other photoconductive switches 60. Once the photoconductive switches have been isolated from the incident beam of radiation, the photoconductive switches 60 become conditioned to an off-state to isolate the voltage busses 50 and 56 from the select line 26 and data bus 28. In order to accelerate the transition of the photoconductive switches 60 from the on-state to the off- state, a potential is applied to the accelerate busses 52 and 56. The potential on the accelerate busses speeds up this process by forcing a depletion of current carriers in the semiconductor material of the photoconductive switches 60 rather than relying on natural recombination of current carriers.

During refresh of the LCD panel 22, the optical scanner 44 is conditioned by the control circuit 36 to scan successively each of the photoconductive switches 60 in the arrays 30 and 37 successively so that the select lines and data busses are biased in succession. Thus, data can be re-written to each pixel 24 in the array on a row-by-row basis. If the turn off speed of the photoconductive switches is not of concern, conventional photoconductive switches can be used. Figure 5 shows a conventional photoconductive switch 70. As can be seen, photoconductive switch includes a photoconductive semiconductor material film 72 which contacts both the voltage bus and the select line or data bus and establishes an electrical connection between the voltage bus and the select line or data bus upon exposure of the photoconductive switch to incident radiation. If photoconductive switches 70 of this type are used in the array, no accelerate bus 52 is required.

Referring now to Figure 6, another embodiment of the photoconductive switch array 37 is better illustrated. As can be seen the photoconductive switch array includes three rows 37a, 37b and 37c of photoconductive switches 60. The photoconductive switches in each row of the array are associated with one data line 28. The first row of photoconductive switches is connected between a high potential data bus V^, and the data lines 28. The second row of photoconductive switches is connected between a medium potential data bus V_M*. and the data lines 28. The third row of photoconductive switches is connected between a low potential data bus V_LP and the data lines 28.

When it is desired to write to a pixel 24 that has been selected via a select line connected to it, the optical scanning system 38 is conditioned by control circuit 36 to direct an incident beam of radiation 39 to one or more of the photoconductive switches 60 connecting that pixel to the three voltage buses. The potential voltage buses Vm*, V_M*. and V-j connected to the pixel 24 through the data line, will determine the gray shade of the pixel.

Although the liquid crystal display has been described as including a pair of photoconductive switch arrays, it should be apparent to those of skill in the art that the liquid crystal display need only include one photoconductive switch array associated with either the data or select lines. When the photoconductive switch array is associated with the select lines, either type of photoconductive switch 60 or 70 is suitable since turn-off speed of the conventional photoconductive switches is satisfactory. When the photoconductive switch array is associated with the data lines, it is preferred that the array includes photoconductive switches 60 since fast turn- off speed is important for desired results.

Although the photoconductive switch arrays 30 and

37 have been described for use in a ferroelectric liquid crystal display, the photoconductive switch arrays can also be used in other applications such as in an active matrix

LCD, passive matrix LCD or a plasma display.

In the case of active matrix LCD's which include a TFT switch array, since the photoconductive switches also include TFT's, the photoconductive switches 60 can be formed on the substrate simultaneously with the pixels 24.

The photoconductive switch arrays may also be used in a flat panel for radiation imaging including a TFT switch array. In this application, a photoconductive switch array would be connected to the gate lines of the array to allow the gate lines to be selected by directing an incident beam of radiation to the photoconductive switches in the array. When a gate line is selected, the data stored by the pixels connected to that gate line, which represents the exposure of the pixels to radiation, can be read via the source lines.

Since a light beam can be used to select individual rows of pixels by directing the light beam to the appropriate photoconductive switches, each row of pixels can be selected successively while avoiding the need for electrical connections between each select and data line and a peripheral circuit.

It will also be appreciated that variations and modifications may be made to the present invention without departing from the scope thereof as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A flat panel comprising: an array of pixels arranged in rows and columns; a plurality of select lines, each interconnecting an individual one of one of the rows or columns of said array for supplying a bias to pixels in said individual one; a plurality of data lines, each interconnecting an individual one of the other of the rows or columns of said array for supplying a bias to pixels in said individual one; and at least one array of photoconductive switches, said at least one array of photoconductive switches being associated with one of said plurality of select or data lines, each photoconductive switch in said at least one array being connected to one of said select or data lines of said plurality, each photoconductive switch being responsive to incident radiation to allow said bias to be supplied to said select or data lines.

2. A flat panel as defined in claim 1 further comprising at least two arrays of photoconductive switches, one of said arrays being connected between said select lines and a first potential voltage source and the other of said arrays being connected between said data lines and a second potential voltage source.

3. A flat panel as defined in claim 2 wherein said array of photoconductive switches connected to said select lines is configured to supply bias to the select lines to allow the gray shade of the pixels to be selected from a plurality of shades.

4. A flat panel as defined in claim 3 wherein each of said pixels is subdivided into a plurality of sub-pixel areas, each sub-pixel area being connected to a photoconductive switch via a data line, the gray shade of said pixel being selected by connecting one or more of said sub-pixel areas of a pixel to said second potential voltage source via said photoconductive switches.

5. A flat panel as defined in claim 3 wherein each pixel is connected to a plurality of second potential voltage sources via a data line and a plurality of photoconductive switches, the gray shade of said pixel being selected by connecting the pixel to one or more of said plurality potential voltage sources.

6. A flat panel as defined in claim 1 wherein said array of pixels and said array of photoconductive switches include thin film transistor switches formed on a common substrate.

7. A flat panel as defined in claim 6 wherein said array of photoconductive switches is connected between each of said data lines and a voltage bus connected to said first potential voltage source.

8. A flat panel as defined in claim 2 further comprising an optical scanner to scan sequentially, each photoconductive switch in said arrays with incident radiation.

9. A flat panel as defined in claim 2 wherein each of said photoconductive switches in said arrays includes means to accelerate the transition of said photoconductive switch to an off-state upon removal of said incident radiation.

10. A flat panel as defined in claim 9 wherein said means to accelerate includes a biased conductor covered by a dielectric film disposed beneath photoconductive switch.

11. A passive matrix flat panel display comprising: an array of pixels arranged in rows and columns, each of said pixels for displaying a gray shade and including a pair of spaced electrodes; a plurality of select lines, each interconnecting one electrode of an individual one of one of the rows or columns of said array for supplying a bias to said one electrodes in said individual one; a plurality of data lines, each interconnecting the other electrode of an individual one of the other of the rows or columns of said array for supplying a bias to said other electrodes in said individual one; a first array of photoconductive switches, associated with said select lines, each photoconductive switch in said first array being connected to one of said select lines and being responsive to incident radiation to allow said bias to be supplied to said select lines; and a second array of photoconductive switches associated with said data lines, each photoconductive switch in said second array of photoconductive switches being connected to said data lines and being responsive to incident radiation to allow said bias to be supplied to said data lines.

12. An active matrix liquid crystal display comprising: a thin film transistor switch array arranged in rows and columns and formed on a common substrate; a plurality of gate lines formed on said substrate, each interconnecting an individual one of one of the rows or columns of said array; a plurality of source lines formed on said substrate, each interconnecting an individual one of the other of the rows or columns of said array; an array of photoconductive switches formed on said substrate, each being connected to one of said gate lines, each photoconductive switch being responsive to incident radiation to connect said gate line to a potential voltage source and select the pixels connected thereto to allow for data to be written to the selected pixels; and an optical scanning system to direct said incident radiation towards a selected photoconductive switch of said array.

13. A liquid crystal display as defined in claim 12 wherein said array of photoconductive switches is connected between each of said gate lines and a voltage bus connected to said potential voltage source.

14. A liquid crystal display as defined in claim 13 wherein each of said photoconductive switches includes a thin film transistor.

15. A liquid crystal display as defined in claim 14 wherein each of said photoconductive switches includes means to accelerate the transition of said photoconductive switch to an off-state upon removal of said incident radiation.

16. A liquid crystal display as defined in claim 15 wherein said optical scanning system incudes a shutter and an optical scanner responsive to control means to direct said incident radiation towards the desired photoconductive switch.

17. A flat panel for radiation imaging comprising: a thin film transistor switch array arranged in rows and columns and formed on a common substrate; a plurality of gate lines formed on said substrate, each interconnecting an individual one of one of the rows or columns of said array; a plurality of source lines formed on said substrate, each interconnecting an individual one of the other of the rows or columns of said array; an array of photoconductive switches formed on said substrate, each being connected to one of said gate lines, each photoconductive switch being responsive to incident radiation to connect said gate line to a potential voltage source and select the pixels connected thereto to allow for data to be read from the selected pixels; and an optical scanning system to direct said incident radiation towards a selected photoconductive switch of said array.

18. A flat panel as defined in claim 17 wherein said array of photoconductive switches is connected between each of said gate lines and a voltage bus connected to said potential voltage source.

19. A flat panel as defined in claim 17 wherein each of said photoconductive switches includes a thin film transistor.

20. A flat panel as defined in claim 18 wherein each of s d photoconductive switches includes means to accele. ate the transition of said photoconductive switch to an off-state upon removal of said incident radiation.

21. A flat panel as defined in claim 19 wherein said optical scanning system includes a shutter and an optical scanner responsive to control means to direct said incident radiation towards the desired photoconductive switch.