The present invention relates to an electron generating apparatus, an image display apparatus, a driving circuit, and a driving method and, more particularly, to an image display apparatus having a large number of surface-conduction type electron emitters.
Conventionally, two types of devices, namely thermionic and cold cathode devices, are known as electron emitters. Examples of cold cathode devices are field emission type electron emitters (to be referred to as field emitters hereinafter), metal/insulator/metal type electron emitters (to be referred to as MIM-type electron emitters hereinafter), and surface-conduction type electron emitters.
Known examples of the field emitters are described in W. P. Dyke and W. W. Dolan, "Field Emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976). FIG. 38 is a sectional view of a device according to C. A. Spindt et al. Referring to FIG. 38, reference numeral 3010 denotes a substrate, 3011, an emitter wiring layer made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. In this device, a proper voltage is applied between the emitter cone 3012 and the gate electrode 3014 to emit electrons from the distal end portion of the emitter cone 3012.
A known example of the MIM-type electron emitters is described in C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32,646 (1961). FIG. 39 is a sectional view of an MIM-type electron emitter. Referring to FIG. 39, reference numeral 3020 denotes a substrate; 3021, a lower electrode made of a metal; 3022, a thin insulating layer having a thickness of about 100 .ANG.; and 3023, an upper electrode made of a metal and having a thickness of about 80 to 300 .ANG.. In the MIM type, a voltage is applied between the upper electrode 3023 and the lower electrode 3021 to emit electrons from the surface of the upper electrode 3023.
A known example of the surface-conduction type electron emitters is described in, e.g., M. I. Elinson, "Radio Eng. Electron Phys., 10, 1290 (1965) and other examples to be described later.
The surface-conduction type electron emitter utilizes the phenomenon that electron emission takes place in a small-area thin film, formed on a substrate, upon flowing a current parallel to the film surface. The surface-conduction type electron emitter includes electron emitters using an Au thin film (G. Dittmer, "Thin Solid Films", 9,317 (1972)), an In.sub.2 O.sub.3 /SnO.sub.2 thin film (M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975)), a carbon thin film (Hisashi Araki et al., "Vacuum", vol. 26, No. 1, p. 22 (1983), and the like, in addition to an SnO.sub.2 thin film according to Elinson mentioned above.
FIG. 37 is a plan view of the surface-conduction type electron emitter according to M. Hartwell et al. as a typical example of the structures of these surface-conduction type electron emitters. Referring to FIG. 37, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by spattering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 37. An electron-emitting portion 3005 is formed by performing an energization process (referred to as a energization forming process to be described later) with respect to the conductive thin film 3004. Referring to FIG. 37, an interval L is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. For the sake of illustrative convenience, the electron-emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004. However, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface-conduction type electron emitters according to M. Hartwell et al., typically the electron-emitting portion 3005 is formed by performing an energization process called the energization forming process for the conductive thin film 3004 before electron emission. According to the energization forming process, energization is performed by applying a constant DC voltage which increases at a very low rate of, e.g., 1 V/min., across the two ends of the conductive film 3004, so as to partially destroy or deform the conductive film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistor. Note that the destroyed or deformed part of the conductive thin film 3004 has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the energization forming process, electron emission is performed near the fissure.
The above surface-conduction type electron emitters are advantageous because they have a simple structure and can be easily manufactured. For this reason, many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied.
Regarding applications of surface-conduction type electron emitters to, e.g., image forming apparatuses such as an image display apparatus and an image recording apparatus, charged beam sources and the like have been studied.
As an application to image display apparatuses, in particular, as disclosed in the U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open No. 2-257551 filed by the present applicant, an image display apparatus using the combination of a surface-conduction type electron emitter and a phosphor which emits light upon irradiation of an electron beam has been studied. This type of image display apparatus is expected to have more excellent characteristic than other conventional image display apparatuses. For example, in comparison with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require a backlight since it is of a self-emission type and that it has a wide view angle.
The present inventors have experimented on surface-conduction type electron emitters made of various materials, manufactured by various methods, and having various structures as well as the one described above. The present inventors have also studied multi-electron sources each constituted by an array of many surface-conduction type electron emitters, and image display apparatuses using the multi-electron sources.
The present inventors have experimentally manufactured a multi-electron source formed by an electrical wiring method like the one shown in FIG. 40. In this multi-electron source, a large number of surface-conduction type electron emitters are two-dimensionally arrayed and wired in the form of a matrix, as shown in FIG. 40.
Referring to FIG. 40, reference numeral 1002 denotes a surface-conduction type electron emitter which is schematically shown; 1003, a row wiring layer; and 1004, a column wiring layer. In reality, the row and column wiring layers 1003 and 1004 have finite electric resistors. However, FIG. 40 shows these resistors as wiring resistors 4004 and 4005. The above wiring method will be referred to as simple matrix wiring.
For the sake of illustrative convenience, FIG. 40 shows a 6.times.6 matrix. However, the size of a matrix is not limited to this. For example, in a multi-electron source for an image display apparatus, a sufficient number of emitters for a desired image display operation are arrayed and wired.
In the multi-electron source having the surface-conduction type electron emitters wired in the form of a simple matrix, in order to output desired electron beams, proper electrical signals are applied to the row and column wiring layers 1003 and 1004. For example, in order to drive the surface-conduction type electron emitters on an arbitrary row in the matrix, a selection voltage Vs is applied to the row wiring layer 1003 on a selected row, and at the same time, a non-selection voltage Vns is applied to each row wiring layer 1003 on the non-selected rows. A drive voltage Ve is applied to each column wiring layer 1004 in synchronism with the selection voltage Vs. According to this method, ignoring voltage drops across wiring resistors 4004 and 4005, a voltage Ve--Vs is applied to each surface-conduction type electron emitter on the selected row, whereas a voltage Ve--Vns is applied to each surface-conduction type electron emitter on the non-selected rows. If, therefore, the voltages Ve, Vs, and Vns are set to proper voltages, an electron beam having a desired intensity should be output from each surface-conduction type electron emitter on only a selected row. In addition, if different drive voltages Ve are applied to the respective column wiring layers, electron beams having different intensities should be output from the respective emitters on a selected row. Since the response speed of each surface-conduction type electron emitter is high, the length of time that an electron beam is kept output should be changed if the length of time that the drive voltage Ve is kept applied is changed.
Various applications of such a multi-electron source having surface-conduction type electron emitters wired in the form of a simple matrix have therefore been studied. For example, this electron source is expected to be used in an image display apparatus which applies voltage signals in accordance with image information.
In practice, however, when the multi-electron source to which a voltage source is connected is driven by the above voltage application method, voltage drops occur across wiring resistors, resulting in variations in voltages effectively applied to the respective surface-conduction type electron emitters.
The first cause for variations in voltages applied to the respective emitters is that the respective surface-conduction type electron emitters in the simple matrix wiring structure have different wiring lengths (i.e., different wiring resistors).
The second cause is that voltage drops across the wiring resistors 4004 in the respective row wiring layers vary. This is because a current is shunted from the row wiring layer on a selected row to the respective surface-conduction type electron emitters connected thereto so as to cause nonuniform currents to flow in the respective wiring resistors 4004.
The third cause is that the magnitude of a voltage drop across a wiring resistor changes depending on the driving pattern (the image pattern to be displayed in the case of an image display apparatus). This is because a current flowing in a wiring resistor changes depending on the driving pattern.
If the voltages applied to the respective surface-conduction type electron emitters vary due to the above causes, the intensity of an electron beam output from each surface-conduction type electron emitter deviates from a desired value, posing a problem in practical use. For example, when the electron source is applied to an image display apparatus, the luminance of the displayed image becomes nonuniform, or variations in luminance occur depending on the display image pattern.
In addition, variations in voltage tend to increase with an increase in the size of a simple matrix. This tendency is a factor that limits the number of pixels in an image display apparatus.
In the process of studying such techniques in consideration of the above problems, the present inventors have already experimented on a driving method different from the above voltage application method.
In this method, when a multi-electron source having surface-conduction type electron emitters wired in the form of a simple matrix is to be driven, a current source for supplying currents required to output desired electron beams is connected to the column wiring layers, instead of connecting a voltage source for applying the drive voltage Ve to each column wiring layer, so as to drive the multi-electron source. This method was devised in consideration of the strong correlation between the current (to be referred to as an emitter current If hereinafter) flowing in each surface-conduction type electron emitter and the electron beam (to be referred to as an emission current Ie hereinafter) emitted from each emitter. In the method, the magnitude of the emission current Ie is controlled by limiting the magnitude of the emitter current If.
That is, the magnitude of the emitter current If to be supplied to each surface-conduction type electron emitter is determined by referring to the (emitter current If) to (emission current Ie) characteristics of each surface-conduction type electron emitter, and the emitter current If is supplied from the current source connected to each column wiring layer. More specifically, a driving circuit may be constituted by a combination of electric circuits such as a memory storing the (emitter current If) to (emission current Ie) characteristics, an arithmetic unit for determining the emitter current If to be supplied, and a controlled current source. As the controlled current source, a circuit form for temporarily converting the magnitude of the emitter current If to be supplied into a voltage signal, and converting the signal into a current using a voltage/current conversion circuit may be used.
This method is less susceptible to voltage drops across wiring resistors than the above method of driving the multi-electron source using the voltage source connected to each column wiring layer. It was found therefore that this method could reduce variations in the intensity of an electron beam to be output.
However, the following problem is posed in the method of driving the electron source using the current source connected thereto.
This problem will be described with reference to FIGS. 41 and 42.
FIG. 41 is a view for explaining the conventional driving method. FIG. 41 shows a plurality of electron-beam emitters 301 wired in the form of a matrix, and a driving circuit. FIG. 41 shows a case wherein electrons are emitted by driving the electron emitters on the Mth row of the plurality of electron emitters. In the following description, an electron emitter to be driven will be referred to as a selected emitter, and an electron emitter not to be driven will be referred to as a semi-selected emitter.
As shown in FIG. 41, when the emitters on the Mth row are to be driven, a voltage source Vs (for outputting, e.g., a voltage of -7 V) is connected to the row wiring layer on the Mth row, and the remaining row wiring layers are set to the ground level (e.g., 0 V). As is apparent from the polarity of the voltage source Vs, the row wiring layer on the Mth row to be driven is held at a low potential (-7 V) lower than 0 V.
A controlled current source 302 is connected to each column wiring layer, and a drive current is supplied from the controlled current source 302.
FIG. 42 is a circuit diagram showing the detailed arrangement of the controlled current source, which is a voltage/current conversion circuit of a current mirror scheme. Referring to FIG. 42, reference numeral 311 denotes an operational amplifier; 312, a resistor having a resistance of R ohms; 314 and 315, pnp transistors; 313, an npn transistor; and 316, a terminal to connect the current source to each column wiring layer. The following relationship is established between an output current Iout and an input voltage Vin in this circuit:
Iout=Vin/R
That is, the magnitude of the output current Iout can be controlled by changing the magnitude of the input voltage Vin.
The value of an emitter current Ifo which is required to obtain a desired emission current Ie from an electron emitter is determined in advance on the basis of the emission current Ie/emitter current If characteristics of the electron emitter, and the output current Iout from the controlled current source is controlled to be equal to the determined value of the emitter current Ifo.
However, part of the output current Iout from the controlled current source is shunted to a semi-selected emitter. This is because when the controlled current source outputs the current Ifo, the effective voltage at the terminal 316 becomes higher than the ground level.
As shown in FIG. 41, part of the current Iout is shunted to each semi-selected emitter, and the effective drive current Is flowing in a selected emitter becomes considerably lower than the current Iout. As the number of electron emitters wired in the form of a matrix increases, the magnitude of a current is shunted to each semi-selected emitter increases. As a result, the problem becomes more conspicuous. Assume that the current Iout is 1.5 mA, and a current Ihs flowing in each non-selected emitter is 0.001 mA. In this case, in a matrix having 1,000 rows, the sum total of currents Ihs becomes about 1 mA. That is, only Is=0.5 mA can be supplied to each selected emitter (Iout=Is+.SIGMA.Ihs).
For this reason, when this driving method is applied to, e.g., an image display apparatus, in order to ensure the accuracy of luminance of the displayed image, the magnitude of the output current Iout from the controlled current source must be corrected to the sum of the current flowing to each semi-selected emitter and the current Ifo. When a correction circuit for this operation is added to the apparatus, the size and manufacturing cost of the apparatus increase.
In addition, since currents flow to semi-selected emitters which emit no electrons, the power is wasted.
Even if the current controlled sources connected to the column wiring layers in the above driving circuit are replaced with controlled voltage sources, great voltage drops occur across wiring portions when currents flow in the semi-selected emitters. As a result, a drive voltage applied to each selected emitter drops, and the luminance of the displayed image decreases. For this reason, a correction circuit must be added to each controlled voltage source, resulting in increases in the size and manufacturing cost of the apparatus. In addition, the power is wasted in each semi-selected emitter.