This invention relates to a fluorescent display device including an anode which emits light upon impingement of electron beams thereon and a phosphor used for the anode, and more particularly to a fluorescent display device of the type that a phosphor-deposited anode is driven under a driving voltage as low as 1 kV or less (hereinafter also referred to "VFD") and a phosphor used therefor and a fluorescent display device of the type that a field emission type cathode is used for an electron source (hereinafter referred to as "FED") and a phosphor used therefor.
For operation of a VFD or an FED for luminous display while keeping a driving voltage of an anode at a level of 1 kV or less, it is required to use a low-velocity electron excited phosphor. A low-velocity electron excited phosphor conventionally used for this purpose is generally classified into two types depending on a resistance value exhibited by a matrix of the phosphor.
One type is a phosphor including a phosphor matrix of a low resistance value. The phosphors of this type include, for example, a phosphor of a blue-green luminous color represented by a general formula ZnO:Zn, a phosphor of a blue luminous color represented by a general formula ZnGa.sub.2 O.sub.4, which have been put to practical use. Also, the phosphors of this type include, in addition to the phosphors of blue-green and blue luminous colors described above, a phosphor of a red luminous color represented by a formula SnO.sub.2 :Eu and the like. Unfortunately, the phosphors of luminous colors other than blue-green and blue are not only disadvantageously decreased in luminance, but deteriorated in luminous efficiency because luminance is saturated at a driving voltage above a predetermined level. Also, it is deteriorated in life characteristics. This results in the phosphors of luminous colors other than blue-green and blue failing to be put to practical use.
The other type is a phosphor including phosphor matrix of a high resistance value and a conductive material. The conductive material is added to the phosphor matrix, resulting in a phosphor layer then formed of the phosphor being reduced in apparent resistance value. The phosphors of this type include, for example, a phosphor comprising a phosphor matrix such as ZnS, ZnCdS or the like having a luminous center such as Ag, Au, Cu or the like added thereto for formation of a solid solution, as well as In.sub.2 O.sub.3 in an mount of several % mixed with the phosphor matrix so as to act as a conductive material, which has been put to practical use.
More specifically, the phosphors of the latter type include sulfide phosphors including, for example, a phosphor of a blue luminous color such as ZnS:Zn, ZnS:Ag or the like, that of a green luminous color such as ZnS:Cu,Al, (Zn.sub.0.6 Cd.sub.0.4)S:Ag,Cl or the like, and that of a green luminous color such as (Zn.sub.0.22 Cd.sub.0.78)S:Ag,Cl or the like.
Such phosphors each include a phosphor matrix having S contained therein, resulting in being named a sulfide phosphor. The sulfide phosphor tends to be readily decomposed due to impingement of electrons thereon. The decomposition causes a sulfide material to be scattered in a fluorescent display device as widely known in the art. The sulfide material thus scattered is deposited on a filamentary cathode arranged in the fluorescent display device, leading to contamination of the cathode, resulting in the cathode being deteriorated in electron emission capability or characteristics. Also, the sulfide material is deposited on other oxide phosphors as well, leading to contamination of an anode likewise arranged in the fluorescent display device. Further, when the sulfide phosphor includes a ZnCdS phosphor matrix, it would cause a further environmental problem because of containing Cd which is widely known to be a pollutant.
In addition to the above-described sulfide phosphors, there has been also known a phosphor of a red luminous color such as Y.sub.2 O.sub.3 :Eu. The phosphor is disadvantageously increased in insulating properties, so that it is required to add a large amount of In.sub.2 O.sub.3 thereto. Unfortunately, this causes a reactive current flowing through In.sub.2 O.sub.3 to be increased, to thereby reduce efficiency of luminescence of the phosphor in a low-voltage driving region and deteriorate reliability of the phosphor.
Further, a proposal has been made for reducing a particle size of a phosphor to a level as small as 0.1 micron and reduce a thickness of a phosphor layer, to thereby reduce a resistance value of the phosphor. However, the proposal fails to provide a phosphor increased in luminous efficiency while keeping a particle size of the phosphor at a level as low as 1 micron or less.