 or a plastic material. in this case, a vitreous bonding material is preferable for making ohmic layer, while a plastic material is advantageous for nonohmic layer. Numeral 105 indicates impervious semiconductive layer of about micron in thickness which comprises, for example, black paint mixed with powder of nonlinear resistivity such as CdSzCl or powder of linear resistivity such as carbon black and which is formed in a layer. By providing such resistive intermediate layers, dielectric breakdown of EL layer by DC voltage or even by AC voltage is prevented. Moreover, resistivity of the series connected resistive load circuit including the EL layer can be adjusted by varying the resistivity of the intermediate layers, so that the resistivity of said load circuit is set at an appropriate value of the same order as the dark resistivity of the PC layer or lower than that. Therefore, limitation for the resistivity of the EL layer is much relieved. For example, if the EL layer has been produced with extremely low resistivity, the intermediate layer 104 is made so as to have an ohmic resistance asdescribed above, the resistivity being set at an appropriate value higher than that of the EL layer, thereby to attain the matching of the load to the PC layer in the resistances. As described above, the intermediate layers allow easy fabrication of the resistive EL layer and eliminate the effect of limitation of the resistivity to luminescent characteristics, and further facilitate easy matching of the DC resistances between the series connected load circuit including the EL layer and the PC layer, thereby permitting very effective control of the AC photoconductive sensitivity of the PC layer by the DC voltage.
As mentioned previously, by selecting the resistivity of each relevant element so that the transversal resistance of the DC load circuit including the EL layer is similar to or lower than the dark resistance of the PC layer, the DC voltage across the PC layer is made high when the intensity of input energy signal is zero or very low. Thus, gamma value and input-to-output characteristics in low input energy range are improved, and the effective operating range of intensity of input energy signal is remarkably extended.
Further, as the intermediate layers are endowed with resistivity by mixing of powder of resistive material, the resistivity can be freely controlled by varying the amount of the powder over a wide range. This makes very easy the adjustment of the resistance of the series-connected load circuit including the EL layer or the matching of resistances between said load circuit and the PC layer, and accordingly facilitates fabrication of the complete device, and further makes easy improvement of gamma value and the effective operating range of the intensity of input energy.
It will be noted that as the resistive intermediate layers contain powder of ferroelectric material such as BaTi0 the overall dielectric constant of the layers is raised. This raised dielectric constant lowers AC voltage loss in the intermediate layers. This fact is another advantage of the resistive intermediate layers, beside the above-mentioned advantages.
Further, the ferroelectric material such as BaTill has a high specific resistivity. Therefore, when resistive binder material is used for the intermediate layer, the use of the ferroelectric material also makes possible controlling of resistivity of the intermediate layer by varying the amount of the material to be mixed, beside it serves to raise the overall dielectric constant of the layer. On the other hand, in the intermediate layer the resistivity of which is presented by adding of resistive powder, particles of highly resistive ferroelectric material are intermixed with particles of resistive material, as the ferroelectric material is also used. As the result, two dimensional uniformity in the resistivity of the intermediate layer is improved, as the condensation and maldistribution of the resistive powder are thus prevented.
Returning to FIG. 2, numeral 106 indicates the photoconductive layer or PC layer of about 200 to 500 micron in thickness, which is formed of photoconductive powder bound by plastics or a similar binding material, said photoconductive powder being a material which is sensitive not only to the visible light but to a radiation such as X-ray, infrared ray and ultra-violet ray, such as, for example, cadmium sulfide activated with an element of IB group such as Cu or Ag and an element of VII B group such as C1, the latter element of Vll B group being able to be substituted by an element of III B group such as All or Ga. Numeral 107 indicates electrode of vapourdeposited metal, for example, aluminum. This electrode is pervious not only to a radiation such as X-ray, but to the visible light, and can be formed in a gappy pattern such as equispaced parallel lines, lattice or mesh. Numeral 108 represents input energy signal, which is not limited to the visible light, but can be other radiation such as ultra-violet ray, infrared ray or X-ray. Numeral 109 indicates output visible image. Numerals 110 and 111 indicate voltage sources for the solid-state image plate applied across the electrodes 102 and 107, 110 being the AC operating voltage source and 111 being variable DC voltage source. Among the above-described elements 101 to 107, the most important one for realizing the above-mentioned control by DC voltage is the semiconductive EL layer 103.
The resistivity of the EL layer is theoretically required to be appropriately lower than the dark value of resistive component of the PC layer; that is, to be in the semiconductive range of the order of 10 to 10 ohm-cm, where the characteristics is fairly linear. However, this requirement for the resistivity presents several technical problems including difficulties relating to construction and manufacturing method. The conventional techniques for imparting electroconductivity to a solid layer made of a highly resistive material such as plastic or glass, include a process of dispersing resistive material into the plastic or glass body. However, the resistivity obtained by such process is limited to one very near to that of a conductor or to one having directivity, and a solid layer which has a resistivity of 10 to 10 ohm-cm. belonging to the semiconductor range and which maintains ohmic characteristics up to a considerably high electric field, has not been obtained. Though carbon black is comparatively satisfactory as a resistive material, it is not suitable for the material for imparting semiconductivity to the EL layer, as it absorbs the luminescent light from the EL powder. Use of metal powder such as Cu or Sn also presents a difficult problem, since such material is apt to be oxidized or deteriorated at a high temperature expected in the manufacturing process and further, since pulverizin g of such material has a limitation because of its high malleability. Moreover, resistivity of such metallic material is too low to allow easy control of the resistivity of the layer. Especially, if a plastic binding material is used, the ohmic resistivity will be maintained by no means up to a high electric field, partly because of the inferior thermal property of the plastic.
In order to overcome such difficulties, the semiconductive EL layer 103 as shown in FIG. 2 has been introduced by this invention. According to this invention, the resistive material is selected from the semiconductive metal oxides including Sn0,, W0 Sb O and Tit! which are stable at a considerably high temperature in the atmospheric environment and readily available in the form of pulverized product, and which have high reflexibility to the light in the visible spectrum emitted from the EL powder. As the binder of the EL layer, is used a vitreous material which is thermally stable up to a considerably high temperature and into which metal oxide such as Sn0 is preferably fusible in some extent, the ohmic characteristics of the resistivity and thermal stability of the electrical properties being taken into consideration. It is required to select a vitreous binder the softening point of which is lower than that of the support plate 101 and the heat expansion coefficient of which is substantially the same as that of the support plate, so as to ensure satisfactory application of the EL layer to the support plate. Of course, the binder must be light-pervious, as it is used in the EL layer. The control of the resistivity of the semiconductive EL layer according to the above-mentioned construction is made by varying the volumetric ratio of the metal oxide powder in relation to the total volume. In this process, it is important to properly select the relative gradings of the resistive powder, EL powder and binding vitreous powder, in order to ensure sufficient adhesion among the resistive powder, EL powder and support plate 101 and to obtain smooth layer. The following table 1 shows the gradings and volumic percentages of the ingredients of the above-described mixture: that is, Sn0 powder used as the resistive material, ZnS:CuAl powder as the electroluminescent material and the vitreous material as the binder. Table 2 shows an example of the composition of the vitreous binder. Table 3 shows volume expansion coefficients and softening points of the vitreous binder and the light-pervious support plate (a glass plate).
coefiicient point, C.
Vitreous binder X- Support plate (glass) As to the grading of the powder, it is important that the diameter of the particles of vitreous binder is always smaller than those of the other two materials. If this relation is reversed, the mixture will not unite. The percentage of Sn0 powder can be varied in a range of 10 to percent, causing corresponding variation in the resistivity. The heating temperature was set at 640 C. in this embodiment. The resistivity of thus obtained EL layer showed a fairly good linearity in the semiconductor range of 10 to 10 ohm-cm. Moreover, thus obtained EL layer is highly resistive against heat and environmental conditions. The resistivity of the intermediate layers, that is, the semiconductive reflection layer 104 and the semiconductive nonpervious layer 105, is selected so that the total of the resistances of the two intermediate layers and the EL layer is at most not higher than the dark resistance (that is, resistance under no light input) of the PC