Flat display

Abstract: The image display comprises a plurality of picture elements (11A₁) arranged in a matrix and each connected to a switching thin film transistor (Tr11) and a capacitor (Cs). The switching thin film transistor is controlled to drive the corresponding picture element. The image display is capable of areal luminace and of displaying images in a satisfactorily high brightness.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an image display, such as a color display, and, more particularly, to a thin image display. Description of the Prior Art A microtips type display such as proposed in Japan Display '86, pp. 512-515 employs micro cold cathodes as electron emitters. This known display has cathodes as electron emitters formed in the shape of a circular cone of 1.0 µm or less in diameter on a substrate by a semiconductor device fabricating process, electrodes formed under the cathodes, and gate electrodes formed on an insulating layer surrounding the cathodes. The cathodes arrays are arranged in an X-Y matrix and are driven individually. When an electric field of 10 6 V/cm or higher is applied across the conical cathode and the corresponding gate electrode, field emission occurs to emit an electron beam from the tip of the cathode. The cathodes arrays are thus driven in an X-Y driving mode to project electron beams selectively on the fluorescent screen of the display to display images. Since X-Y driving is line scanning, the duration of luminance of each picture element (each cathodes array) is very short and hence the image cannot be displayed in a satisfactory brightness. Therefore, the luminous intensity of the picture element must be increased, namely, an increased anode voltage must be applied, to display images in a satisfactory brightness, phosphor for high voltage electron beam must be used and hence only limited phosphor can be used. JP-A-62-010 619 discloses a liquid crystal cell having a MOS capacitor. This capacitor is used for holding an electrical charge necessary for the operation of the liquid crystal cell, i.e. in liquid crystal displays a predetermined electrical voltage has to be maintained in order to prevent the change of the optical transparency of the liqiud crystal. SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing problems in the conventional display and it is therefore an object of the present invention to provide an image display using phosphor for low voltage electron beam and capable of displaying images in a satisfactorily high brightness. The present invention provides an image display having a plurality of picture elements (cathodes arrays) arranged in a matrix and each having micro cold cathodes, switching thin film transistors connected respectively to the picture elements, and capacitors connected respectively to the picture elements. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: Fig. 1 is an enlarged sectional view of an essential portion of an image display embodying the present invention; Fig. 2 is an enlarged partially cutaway perspective view of an essential portion of micro cold cathodes; Fig. 3 is a circuit diagram of an equivalent circuit of the image display embodying the present invention; Fig. 4 is a typical circuit diagram of an equivalent circuit of a picture element; and Fig. 5 is an enlarged sectional view of a front panel in a modification. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, an image display embodying the present invention comprises a front panel 1 provided on its inner surface with fluorescent stripes, and a cathode panel 2 serving as an electron emission source. The front panel 1 comprises of a glass plate 3 disposed opposite to the cathode panel 2, a transparent anode 4 formed of ITO (indium tin oxide) over the inner surface 3a of the glass plate 3, and a fluorescent screen formed by forming fluorescent stripes 5, i.e., red, green, and blue fluorescent stripes and black carbon stripes, in a predetermined pattern on the transparent anode 4. The cathode panel 2 has micro cold cathodes serving as cathodes arrays arranged in a matrix. Each cathodes array is connected to a switching thin film transistor and a capacitor. Referring to Fig. 2, the micro cold cathode comprises cathodes 6, i.e., electron emission sources, a gate electrode 7 for making the cathodes emit electron beams, control lines 8 for giving voltage to the cathodes 6, an insulating layer 9 for insulating the control lines 8 from the gate electrode 7, and a base plate 10. The cathodes 6, the gate electrode 7, the control lines 8 and the insulating layer 9 are formed by a semiconductor device fabricating process on the base plate 10. The cathodes 6 are micro emitters formed of molybdenum, tungsten or lanthanum hexaboride (LaB 6 ) in the shape of a micro circular cone of 1.0 µm or less in diameter. When an electric field is applied to the cathode 6, an electron beam is emitted from the tip of the cathode 6. The insulating layer 9 surrounds the cathodes 6, and the gate electrode 7 formed over the insulating layer 9 has circular holes through which electron beams are emitted from the tips of the cathodes 6 toward the fluorescent stripes 5. A group of several to one thousand of cathodes 6 forms a single cathodes array. A plurality of cathodes array are arranged in a matrix on the glass plate 10. As shown in an equivalent circuit in Fig. 3, a storage capacitor Cs, i.e., a capacitor, is connected in parallel to each of picture elements 11A 1 , 11A 2 , 11B 1 and 11B 2 to suppress flicker noise. The picture elements 11A 1 and 11A 2 in a vertical picture element row are connected through switching thin film transistors Tr 11 and Tr 12 , respectively, to a common signal line 12a, and the picture elements 11B 1 and 11B 2 in another vertical picture element row are connected through switching thin film transistors Tr 21 and Tr 22 , respectively, to a common signal line 12b. The current that flows through the thin semiconductor film of each switching thin film transistors Tr 11 , Tr 12 , Tr 21 and Tr 22 is controlled by applying an electric field vertically to the thin semiconductor film. The switching thin film transistors Tr 11 , Tr 12 , Tr 21 and Tr 22 can be formed on the same plane simultaneously with the micro cold cathode by a semiconductor device fabricating process. The gates of the transistors Tr 11 and Tr 21 connected to the picture elements 11A 1 and 11B 1 in a horizontal picture element row are connected to a common control line 13a, and the gates of the transistors Tr 12 and Tr 22 connected to the picture elements 11A 2 and 11B 2 in another horizontal picture element row are connected to a common control line 13b. The respective gate electrodes 7 of the picture elements 11A 1 , 11A 2 , 11B 1 and 11B 2 are connected to a common bias line 15 as shown in Fig. 4, in which only the picture element 11A 1 is shown typically in an equivalent circuit. When the control line 13a connected to the horizontal row of the picture elements 11A 1 and 11B 1 turns on, the switching thin film transistors Tr 11 and Tr 21 are turned ON to store charge in the storage capacitors Cs of the picture elements 11A 1 and 11B 1 through the signal lines 12a and 12b. Then, the picture elements 11A 1 and 11B 1 emits electron beams owing to the charge stored in the storage capacitors Cs. Consequently, the fluorescent stripe 5 formed on the inner surface 3a of the front panel 1 corresponding to the electron beams remains continuously luminous for, for example, 1/60 sec. It is possible to turn all the control lines ON simultaneously for areal luminance. The duration of luminance in the image display embodying the present invention is longer than that in the conventional image display of an X-Y drive system, and the image display of the present invention is capable of displaying images in a satisfactorily high brightness and in a satisfactorily high resolution. Since the duration of luminance is comparatively long, the accelerating voltage applied to the transparent anode electrode 4 provided on the front panel 1 may be reduced and hence the image display of the present invention may employ phosphor for low voltage electron. Accordingly, degassing from the fluorescent screen is reduced and, consequently, the deterioration of the vacuum and contamination are suppressed. Naturally, a fluorescent screen for an ordinary CRT may be employed when a high voltage is used for driving. The present invention may employ a fluorescent screen for either high-speed electron beams or low-speed electron beams. When a fluorescent screen for high-speed electron beams is employed, fluorescent stripes 17 are formed in a predetermined pattern on a glass plate 16, and a metal film 18, such as an aluminum film, is formed over the fluorescent stripes 17 as shown in Fig. 5. In the image display in this embodiment, a bias is applied to the gate electrode 7 and signal is applied to the cathodes 6 to reduce the load on the element. As is apparent from the foregoing description, the image display in accordance with the present invention is provided with the switching thin film transistor and the capacitor for each picture element, and hence the image display is capable of areal luminance, of displaying images in a satisfactorily high brightness and in a satisfactorily high resolution. The image display of the present invention requires a comparatively low accelerating voltage, which expands the range of selection of the fluorescent screen. Although the invention has been described in its preferred form with a certain degree of particularity, obviously many changes and modifications are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope thereof.

Claims: An image display comprising a plurality of picture elements (11A 1 , 11A 2 , ...) arranged in a matrix and each comprising a plurality of micro cold cathodes (6) as electron emitters; characterized in that each picture element is connected to a switching thin film transistor (Tr 11 , Tr 12 , ...) and a capacitor (C s ) for storing charges to be emitted by the cathodes (6).

Inventors: OHOSHI TOSHIO, WATANABE HIDETOSHI, OHOSHI, TOSHIO, WATANABE, HIDETOSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: H01J31
IPC codes: H01J31, G09G3, G09F9, H01J29