Imaging apparatus and operation method of the same

A photoconductive target having a transparent electrode layer and a photoconductive layer on a transparent substrate is disposed opposite to a group of integrated electron beam emitters having gate electrodes. A number of the electron emitters are activated to apply electron beams to the photoconductive target and the activated ones of the electron beam emitters are temporally changed over by an electron emitter selector circuit and a gate selector circuit. Signal charge generated and stored in the photoconductive layer is read. A time-series electric signal corresponding to a spatial distribution of the incident light is generated. A thin imaging apparatus suitable for a larger area is thus provided.

BACKGROUND OF THE INVENTION 
The present invention relates to a thin imaging apparatus having large area 
for reading the distribution of signal charge quantity generated and 
stored in a photoconductive layer by incidence of photons and for 
generating an electric signal corresponding to the spatial distribution of 
the quantity of incident light, and relates to an operation method 
thereof. 
A photoconductive image pickup tube is well known as an imaging apparatus 
which has a photoconductive layer for generating and storing signal 
charges according to the quantity of incident light and which reads out 
the signal charges generated and stored in the photoconductive layer into 
an external circuit in a time series form by using an electron beam 
and-generates an electric signal corresponding to the spatial distribution 
of the quantity of incident light. FIG. 5 is a schematic diagram showing 
the basic structure and operation principle of the photoconductive image 
pickup tube. An electron beam 502 emitted from a cathode electrode 501 is 
accelerated by a mesh electrode 503 to scan a photoconductive layer 504 
under the control of electrostatic and/or electromagnetic deflection and 
focusing means (not illustrated). The electron beam scanning side, i.e., 
scanned surface, of the photoconductive layer 504 has a material and/or 
structure hard of emitting secondary electrons. When the scanning electron 
beam 502 arrives at the scanned surface, the potential of the scanned 
surface gradually falls. If the potential of the scanned surface becomes 
lower than that of the cathode electrode 501, however, the scanning 
electron beam cannot further arrive at the scanned surface. Immediately 
after it has been subjected to electron beam scanning, therefore, the 
potential of the scanned surface balances that of the cathode electrode 
501. Target voltage V.sub.T, which is positive with respect to the cathode 
potential, is applied to a transparent electrode 505. Therefore, an 
electric field so oriented as to be positive on the substrate side and 
negative on the scanned surface side is applied to the photoconductive 
layer 504. If incident light 506 is applied from the outside to the 
photoconductive layer 504 under this state, as many electron-hole pairs as 
determined by the quantity of incident light are generated in the 
photoconductive layer. The above described electric field makes electrons 
run to the substrate side and makes holes run to the scanned surface side. 
The potential of the scanned surface is gradually raised from the cathode 
potential by holes which have arrived at the scanned surface. When the 
scanning electron beam 502 arrives at the scanned surface subsequently, 
the potential of the scanned surface is reset to the cathode potential 
again. At that time, stored signal charge depending upon the quantity of 
incident light at a pertinent location flows through a load resistor 507. 
By means of electron beam scanning, therefore, time-series electric signal 
corresponding to the spatial distribution of the quantity of incident 
light is obtained from an output terminal 508. In FIG. 5, numeral 509 
denotes a transparent substrate, and numeral 510 denotes an electron gun 
tube for vacuum seal. Operation principle of a photoconductive imaging 
tube is disclosed in JP-A-58-194231, for example. 
As described above, a photoconductive imaging tube has a single electron 
emitter. In JP-A-55-25910, however, a plurality of electron emitters 
having negative electron affinities which can be controlled respectively 
independently are disclosed. By using this, a second conventional 
technique in which a target of a vidicon is scanned in a time division 
manner by a plurality of electron beams projected one after another, for 
example, has been disclosed. 
However, the above described photoconductive imaging tube needs magnetic 
and/or electric deflecting and focusing means, such as a coil for 
deflecting and focusing an electron beam emitted from the single electron 
emitter and thereby scanning the photoconductive target, and a 
cylindrically patterned electrode. This results in a problem that the 
distance between the photoconductive target and the electron emitter is 
long and hence a thin imaging apparatus cannot be obtained. 
Furthermore, in an apparatus using the above described second conventional 
technique, the quantity of emitted electrons is controlled by changing the 
potential of the electron emitter itself and it is impossible to make 
electrons arrive at the above described photoconductive target by emitting 
and/or accelerating electrons. As described in detail by referring to FIG. 
5, the potential of the scanned surface in a photoconductive imaging tube 
immediately after electron beam scanning balances the potential of the 
cathode electrode. During a storage interval lasting until that place is 
subjected to electron beam scanning again, the potential is gradually 
raised by the signal charge generated by incident light. Typically, the 
value of this potential rise is approximately several volts. If an imaging 
tube, for example, has a size of 2/3 inch, a signal current of 200 nA, 
storage time of 1/60 sec, and a photoconductive layer made of amorphous Se 
having a thickness of 4 .mu.m, then the potential of the scanned surface 
rises approximately 4 Volt during the storage interval. Since the 
potential rise of the scanned surface of the photoconductive target is 
thus small, it is extremely difficult to sufficiently extract electrons 
emitted from the cathode and make them arrive at the scanned surface. As a 
result of study made by the present inventors, such a configuration that a 
plurality of electron emitters are only disposed opposite to the 
photoconductive target as described above has been found to have the 
following problems. That is to say, it is difficult to make a sufficient 
amount of electron beams incident upon the scanned surface and control the 
quantity of incidence. Furthermore, since the electron beam emitted from 
the electron emitter is not sufficiently accelerated, the configuration is 
poor in property of going straight and beam bending is apt to cause 
resolution degradation and image distortion. 
Furthermore, the present inventors have found that the apparatus using the 
above described second conventional technique has a problem that noise is 
caused by dispersion among the quantities of electrons emitted from 
electron emitters. 
Furthermore, it has been found that the conventional photoconductive 
imaging tube and the apparatus using the above described second 
conventional technique has the following problems. That is to say, if it 
is attempted to obtain a thin imaging apparatus having a shortened 
distance between the photoconductive target and the electron emitter or an 
imaging apparatus, then the electrostatic capacity between the transparent 
electrode and the electron emitter and/or mesh electrode becomes large and 
hence degraded response increases the lag, resulting in one problem. If 
the quantity of incident light is large, then saturation of the output 
signal current due to insufficient quantity of electron beam causes a 
narrow dynamic range, resulting in another problem. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a thin imaging 
apparatus and an operation method thereof; 
A second object of the present invention is to provide an imaging apparatus 
having a higher resolution and less image distortion, and an operation 
method thereof; 
A third object of the present invention is to provide an imaging apparatus 
having reduced noise, and an operation method thereof; and 
A fourth object of the present invention is to provide an imaging apparatus 
having a reduced lag and an increased dynamic range, and an operation 
method thereof. 
The above described first to third objects can be achieved by an imaging 
apparatus including a photoconductive target having at least an electrode 
transmitting incident light from outside and a photoconductive layer 
generating signal charge in response to incidence of the light, a 
plurality of electron beam emitters disposed opposite to the 
photoconductive target, means for temporally changing over electron beam 
emitters emitting electrons among the electron beam emitters, means for 
reading signal charge generated and stored in different places in the 
photoconductive layer, means for generating a time-series electric signal 
corresponding to a spatial distribution of the incident light, and gate 
electrodes for emitting electrons from the electron beam emitters and/or 
accelerating the electrons to make the electrons arrive at the 
photoconductive target. 
Furthermore, the above described fourth object can be achieved in the above 
described imaging apparatus by causing electrons to be emitted from a 
plurality of electron beam emitters at each time point. 
Furthermore, by forming the transparent electrode by using a plurality of 
electrically separated partial electrodes in the imaging apparatus, the 
first and third objects can be achieved more efficiently. 
These and other objects and many of the attendant advantages of the 
invention will be readily appreciated as the same becomes better 
understood by reference to the following detailed description when 
considered in connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The basic configuration and operation principle of an imaging apparatus 
according to the present invention will now be described by referring to 
FIG. 3. Although the present invention can be applied to both 
one-dimensional imaging apparatuses and two-dimensional imaging 
apparatuses, FIG. 3 is a partial sectional view of a two-dimensional 
imaging apparatus and shows the basic configuration of the two-dimensional 
imaging apparatus. In FIG. 3, numeral 301 denotes a transparent substrate, 
302 a transparent electrode, 303 a photoconductive layer, 304 an 
integrated electron emitter, 305 an electron beam, 306 incident light, 307 
gate power supply, 308 target power supply, 309 a load resistor, 310 
capacitance, 311 an amplifier, 312 an output terminal, 320 to 324 gate 
electrodes,-and 331 to 334 switches. A photoconductive target 340 is 
formed by the transparent substrate 301, the transparent electrode 302, 
and the photoconductive layer 303. In the same way as conventional 
photoconductive imaging tubes, the photoconductive layer 303 has such a 
structure as to block injection of holes from the transparent electrode 
302 and injection of electrons from the scanned surface side which is 
scanned by the electron beam 305. In addition, the scanned surface side 
has such a structure that secondary electrons are hardly emitted in 
response to injection of the electron beam 305. 
The electron beams 305 emitted from the electron emitters 304, disposed on 
the opposite side of vacuum space from the photoconductive target, are 
controlled by the gate electrodes 320 to 324. In the case of FIG. 3, the 
gate electrodes are divided into portions 320, 321, . . . 324 respectively 
for controlling a plurality of electron emitters. The gate electrodes 320 
to 324 are connected to switches 330, 331, . . . 334 respectively 
corresponding to them. (The switch 330 is not illustrated.) When these 
switches are in on-states, the potential of corresponding gate electrodes 
is made equal to gate potential V.sub.G which is made higher than the 
potential of the electron emitters by the gate power supply 307. In FIG. 
3, the switch 332 is in the on-state and a plurality of electron beams 305 
emitted from a portion of the electron emitters 304 corresponding to the 
gate electrode 322 arrive at the photoconductive layer 303. In this way, 
the scanned surface of the photoconductive layer 303 is scanned by the 
electron beam 305 emitted from a predetermined electron emitter at each 
time. Since the scanned surface side of the photoconductive layer 303 
hardly emits secondary electrons, the potential of the scanned surface 303 
immediately after it has been subjected to scanning by the electron beam 
305 balances the potential of the electron emitters (depicted to be the 
ground potential in FIG. 3). Since the potential of the transparent 
electrode 302 is made equal to the target potential V.sub.T which is 
higher than the potential of the electron emitters 304 by the potential of 
the target power supply 308, an electric field so oriented as to be 
positive on the substrate side and negative on the scanned surface side is 
applied to inside of the photoconductive layer 303. If in this state the 
incident light 306 from the outside is applied to the photoconductive 
layer 303 through the transparent substrate 301 and the transparent 
electrode 302, as many electron-hole pairs as determined by the quantity 
of incident light are generated in the photoconductive layer. The above 
described electric field makes electrons run to the substrate side and 
makes holes run to the scanned surface side. The potential of the scanned 
surface is gradually raised from the potential of the electron emitters. 
When the scanning electron beam 305 arrives at the scanned surface 
subsequently, the potential of the scanned surface is reset to the 
potential of the electron emitters again. At that time, stored signal 
charge depending upon the quantity of incident light at a pertinent 
location flows through a load resistor 309 via the transparent electrode 
302. From the output terminal 312, a time-series electric signal 
corresponding to the spatial distribution of the quantity of incident 
light is obtained. 
The imaging apparatus according to the present invention has an advantage 
in that the gate electrodes 320 to 324 facilitate control over electron 
beams emitted from the electron emitters 304 to arrive at the scanned 
surface. Furthermore, since the emitted electron beams are incident on the 
scanned surface after they have been accelerated by the gate electrodes, 
the electron beams emitted from respective electron emitters are incident 
on opposite scanned surface efficiently, and resolution degradation and 
image distortion due to beam bending are not caused. 
Furthermore, in the imaging apparatus of the present invention, a plurality 
of electron beams are simultaneously applied to an area subjected to an 
electron beam at a certain time, i.e., to one pixel. Integrated electron 
emitters have a problem that quantities of electrons emitted from electron 
emitters are dispersed. By thus applying electron beams from a plurality 
of electron emitters simultaneously, however, they can be averaged and the 
quantity of electron beams can be made uniform from pixel to pixel. In 
case a photoconductive target basically identical with a photoconductive 
imaging tube is subjected to electron beam scanning to read signals as in 
the imaging apparatus of the present invention, electron beams equivalent 
in quantity to the quantity of charge stored by incident light make a 
landing on the scanned surface as described above and no more electron 
beams arrive at the scanned surface. In principle, therefore, a change in 
the quantity of the scanning electron beam is not reflected in the 
quantity of signal current. More strictly speaking, however, the balance 
value of the potential of the scanned surface after scanning is varied 
with the quantity of the scanning electron beam by the landing 
characteristic of the electron beam, as described in "Imaging 
Engineering", Corona Publishing Co. Ltd., pp. 92-95, for example. The 
present inventors made detailed experiments on this influence. As a 
result, it was found that in the imaging apparatus of the present 
invention dispersion of quantity of electron beam from pixel to pixel 
generated noise in the signal current by the above described effect. 
Therefore, this problem can be solved by simultaneously applying electron 
beams from a plurality of electron emitters to reduce fluctuation of the 
quantity of electron beam from pixel to pixel. 
Furthermore, by making the gate electrodes 320, 321, 323 and 324 having 
switches which are in the off-state as shown in FIG. 3 float electrically 
from the gate electrode 322 having a switch which is in the on-state, the 
electron emitters 304, and the transparent electrode 302, it is possible 
to improve the response characteristic of the imaging apparatus and reduce 
the lag. If capacitance formed by the transparent electrode 302 and the 
gate electrodes is large, there occurs a problem that so-called capacitive 
lag caused by beam resistance of electron beam becomes large. By adopting 
the structure as described above, however, capacitance formed between the 
transparent electrode 302 and the gate electrodes which contributes to 
generation of lag is limited to capacitance formed by the gate electrode 
having a switch which is in the on-state. Therefore, lag can be 
significantly reduced as compared with, for example, the case where all 
gate electrodes are connected and an electron emitter for applying an 
electron beam is selected by switching potentials of the electron 
emitters. This effect is especially effective in case an imaging apparatus 
having a large area has been made and in case the distance between the 
gate electrodes and the scanned surface is short. If the potential of a 
gate electrode which is in the off-state is held to a high value in the 
same way as the on-state, gate electrodes which do not emit electrons 
function as so-called floating gates as if the switches are in the 
on-state. Thus, it is not desirable. Such a problem can be solved by 
bringing the gate electrodes once to, for example, a predetermined low 
potential, lower than the potential of the electron emitters before 
bringing the gate electrodes to the above described electrically floating 
state. 
Operation of the imaging apparatus according to the present invention will 
now be described further by referring to FIG. 4. In FIG. 4, numeral 401 
denotes a transparent substrate, 402 a transparent electrode including a 
plurality of (16 in FIG. 4) stripe-shaped partial electrodes electrically 
separated, 403 an output circuit, 404 a photoconductive layer, 405 a 
scanning circuit having a plurality of electron beam emitters and an 
electron emitter selector circuit, and 406 an electron beam. 
As described above, there is a problem that the response characteristic of 
the apparatus becomes worse and the lag is significant in case the 
electrostatic capacities formed by the gate electrodes, the electron beam 
emitters, and the transparent electrode are large. In the imaging 
apparatus of the present invention, the transparent electrode includes a 
plurality of electrically separated partial electrodes 402, and those 
partial electrodes are connected to the output circuit 403. In the output 
circuit 403, output currents from respective partial electrodes are 
amplified and processed to output a video signal corresponding to the 
spatial distribution of light incident on the entire apparatus. The 
electrostatic capacities relating to the response characteristic of signal 
readout from respective partial electrodes are irrelevant to the area of 
the entire apparatus, and are electrostatic capacities formed by each 
partial electrode, gate electrode, and electron beam emitter. Therefore, 
the lag is significantly reduced as compared with the case where the 
transparent electrode is an electrode having a large area extending over 
the entire apparatus. 
Furthermore, by, for example, simultaneously scanning pixels corresponding 
to two or more adjacent partial electrodes with a plurality of electron 
beams and adding up signals read out by two or more beam scanning 
operations, the effective quantity of beam can be increased and hence the 
dynamic range can be expanded. 
(Embodiment 1) 
FIG. 1 is a diagram showing the configuration of an imaging apparatus 
according to the present invention. For brevity, however, the number of 
electron emitters is omitted. As for the electron emitters 106, common 
electron emitters are formed of every four columns extending in the 
illustrated Y direction. As for the gate electrodes 104, common electrodes 
are formed of every four columns extending in the illustrated X direction. 
The X coordinate of the electron emitting position is determined by an 
electron emitter selector circuit 107, and the Y coordinate is determined 
by a gate selector circuit 108. From 16 electron emitters of the position 
wherein both the electron emitter and gate electrode have been selected, 
electron beams are applied to the scanned surface of a photoconductive 
layer 103. In accordance with predetermined synchronizing signals, a 
control circuit 109 controls the electron emitter selector circuit 107 and 
the gate selector circuit 108 to perform electron beam scanning. In 
addition, the control circuit 109 forms a video signal output from the 
output current of the transparent electrode 102. In FIG. 1, numeral 101 
denotes a transparent substrate and numeral 102 denotes a transparent 
electrode. Numeral 105 denotes an insulation layer for insulating the gate 
electrodes 104 from the electrode emitters 106. 
FIG. 2 is a sectional view of the photoconductive target 204 and the 
electron emitter portion of the above described imaging apparatus. The 
structure and fabrication method of the imaging apparatus according to the 
present invention will now be described in more detail by referring to 
FIG. 2. On the smoothed and cleaned transparent glass substrate 101, the 
transparent electrode 102 having a thickness of 15 nm is formed by 
sputtering. The transparent electrode 102 contains tin and has indium 
oxide as the principal ingredient. As evaporation sources controlled 
separately, CeO.sub.2, Se, As.sub.2 Se.sub.3, and Sb.sub.2 S.sub.3 are 
mounted on a rotational co-evaporation chamber having a turn table. The 
turn table rotates so that samples may pass over respective evaporation 
sources with independently controlled shutters. In this rotational 
co-evaporation chamber, CeO.sub.2 having a thickness-of 10 nm is formed as 
a hole blocking layer 201 for blocking the injection of holes into a 
photoconductive layer 202. As the photoconductive layer 202, an amorphous 
semiconductor layer having a thickness of 4 .mu.m is formed. The amorphous 
semiconductor layer has Se as the principal ingredient and contains one 
atomic percent of As. Furthermore, as both an electron blocking layer for 
blocking injection of electrons into the photoconductive layer and a beam 
landing layer, a porous Sb.sub.2 S.sub.3 layer 203 is formed so as to have 
a thickness of 100 nm by evaporation in Ar gas atmosphere of 0.2 Torr, a 
photoconductive target being thus formed. The electron emitter selector 
circuit 107 is formed on a Si substrate by a process similar to that of a 
conventional integrated circuit. On that electron emitter selector circuit 
107, an electron beam scanning portion is formed. The electron beam 
scanning portion includes field emitters 106 having conical Si electrodes, 
the insulation layer 105 made of SiO.sub.2, and gate electrodes 104 made 
of Nb evaporation layers. The electron beam scanning portion was formed by 
using the photolithography technique or the anisotropic etching. The 
electron beam emitters were disposed at intervals of 4 .mu.m. 
Subsequently, the above described photoconductive target and electron beam 
scanning portion were mounted to a vacuum envelope for holding them so as 
to make them opposite to each other. Then a vacuum seal was formed to 
obtain the imaging apparatus of the present embodiment. At this time, the 
space between the scanned surface and the gate electrode was decided to be 
approximately 100 .mu.m. It is noted that a portion between the porous 
Sb.sub.2 S.sub.3 layer 203 and the gate electrode 104 is formed of a 
vacuum. 
In the imaging apparatus of the present embodiment, operation is conducted 
with the potential of the gate electrodes made 150 V higher than the 
potential of electron emitters at the time of beam emission. Furthermore, 
the potential of the transparent electrode is made 460 V higher than the 
potential of the electron emitters at the time of electron beam emission. 
The potential of the scanned surface after electron beam scanning balances 
the potential of electron emitters. Therefore, an electric field of 
approximately 1.15.times.10.sup.6 V/cm is applied to the photoconductive 
layer having a layer thickness of 4 .mu.m. In amorphous Se, avalanche 
multiplication of charge occurs in an electric field of approximately 
8.times.10.sup.5 V/cm or above. In case of the present embodiment, its 
multiplication factor becomes approximately ten. 
In the imaging apparatus of the present embodiment, the position of 
electron emission can be arbitrarily selected by the electron emitter 
selector circuit and the gate electrode selector circuit. The imaging 
apparatus can be operated with a desired scanning method. At this time, 
gate electrodes and electron emitters which are not selected are 
electrically separated from the selected gate electrode and electron 
emitter. Therefore, the imaging apparatus of the present embodiment has a 
feature that the lag caused by electrostatic capacity formed by the 
transparent electrode 102, the gate electrode 104, and the electron 
emitter 106 is reduced. Amorphous Se used as the photoconductive layer in 
the present embodiment is a material which is high in dark resistance and 
excellent in photoconductivity. Especially, by applying the avalanche 
multiplication operation, the sensitivity can be made significantly high. 
In the present embodiment, As affixed to amorphous Se is a material for 
stabilizing the structure of amorphous Se and improving the heat 
resistance. The electron blocking layer 203 and the hole blocking layer 
201 play an important role in reducing the dark current, improving the 
signal to noise ratio, and reducing the lag. 
In FIG. 1, each pixel is shown to include 16 electron beam emitters for 
brevity. In the present embodiment, however, electron emitters were 
disposed at intervals of 4 .mu.m and the pixel size was decided to be 100 
.mu.m square. Therefore, each pixel includes approximately 600 electron 
emitters. The dispersion in quantity of electron beam emitted from each 
electron emitter is leveled, and the quantity of beam per pixel is made 
uniform. It is necessary that the emitted electron beam does not spread 
out to such a degree that degradation of resolution poses a problem. 
However, it is desirable that the emitted electron beam spread to such a 
degree that electron beam shade (portion whereat no electron beams arrive) 
is not formed in the scanned area. Depending upon the interval of electron 
emitters, the size of pixels, the shape and voltage of gate electrodes, 
and the distance between the photoconductive target and electron emitters, 
an optimum state can be attained. 
In the present embodiment, conical Si was used as the field emitters 106. 
However, a cathode material other than Si such as, for example, Mo, Ta, W, 
or TaC may be used. Its shape may also be a planar electron emitting 
plane. As compared with hot cathodes, cold cathodes such as field emitters 
have an advantage in being easily integrated. Although cold cathodes other 
than field emitters, such as those of tunnel-type, avalanche-type, or 
negative electron affinity-type may be used, use of electron emitters 
having a low electron temperature has especially an advantage of reduced 
lag. 
(Embodiment 2) 
A second embodiment of the present invention will now be described by 
referring to FIG. 6. On a transparent glass substrate 601, a transparent 
electrode 602 containing tin oxide as the principal ingredient and having 
a thickness of 20 nm is formed by using the CVD method. By then using a 
rotational co-evaporation chamber having CeO.sub.2, Se, Te, As.sub.2 
Se.sub.3, and Sb.sub.2 S.sub.3 as evaporation sources, CeO.sub.2 having a 
thickness of 10 nm is formed as a hole blocking layer 603, and an 
amorphous semiconductor layer of a thickness of 6 .mu.m having Se as the 
principle ingredient and containing one atomic percent of As is formed as 
a photoconductive layer 604. At this time, a layer 605 containing 30% Te 
is deposited in a part of the photoconductive layer. Finally, Sb.sub.2 
S.sub.3 is evaporated to have a thickness of 60 nm as an electron blocking 
layer 606. Thereafter, Ar gas of 0.25 Torr is introduced into the 
apparatus, and a beam landing layer 607 having porous Sb.sub.2 S.sub.3 of 
100 nm in thickness is deposited by evaporation to form a photoconductive 
target. The electron beam scanning portion includes planar W cold cathodes 
of field emission-type 610 arranged at intervals of 5 .mu.m, an insulation 
layer 609 made of SiO.sub.2, and gate electrodes 608 made of Mo 
evaporation layers. The above described photoconductive target and 
electron beam scanning portion are mounted to a vacuum envelope for 
holding them so as to make them opposite to each other. Then, a vacuum 
seal is formed to obtain the imaging apparatus of the present embodiment. 
In the present embodiment, the potential of the electron emitters 610 is 
common to pixels. Selection of electron emitters is made by using gate 
electrodes separated for respective pixels. One pixel corresponds to a 
plurality of electron emitters. In FIG. 6, numeral 611 denotes a scanning 
circuit substrate. This has a function of selectively applying voltage of 
200 V with respect to electron emitters to gate electrodes 608. At this 
time, gate electrodes which are not selected are electrically separated 
from the selected gate electrode and electron emitter as described before 
by referring to FIG. 3. Therefore, the electrostatic capacity formed by 
the transparent electrode 602 and the gate electrodes 608 is always kept 
to minimum, resulting in an imaging apparatus having reduced lag. The 
amorphous Se layer 605 containing Te has an advantage in raising 
sensitivity to red light. By combining the imaging apparatus of the 
present embodiment with a suitable color filter, a color imaging apparatus 
excellent in color reproducibility is obtained. 
(Embodiment 3) 
A third embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 7. In the present 
embodiment, an imaging apparatus of the present embodiment has been 
applied to a linear image sensor. FIG. 7 shows a part of sectional view in 
its lengthwise direction. On a transparent glass substrate 701, a 
transparent electrode 702 is deposited by evaporation in oxygen atmosphere 
by using In containing 10% Sn as evaporation sources. Thereafter, 
SiO.sub.2 having a thickness of 15 nm is formed as a hole blocking layer 
703. Subsequently, by high-frequency plasma decomposition of mixture gas 
including SiH.sub.4 and PH.sub.3 diluted with H.sub.2, SiH.sub.4, and 
mixture gas including SiH.sub.4 and B.sub.2 H.sub.6 diluted with H.sub.2, 
and n-type hydrogenated amorphous Si (a-Si:H) layer 704 having a thickness 
of 30 nm, i-type a-Si:H layer 705 having a thickness of 2 .mu.m, and a 
p-type a-Si:H layer 706 having a thickness of 50 nm are deposited one 
after another. Finally as both electron blocking layer and beam landing 
layer 707, porous Sb.sub.2 S.sub.3 is deposited by evaporation in N.sub.2 
gas atmosphere of 0.2 Torr to have a thickness of 150 nm, a 
photoconductive target being thus formed. The electron beam scanning 
portion is formed in the same way as the first embodiment. In the present 
embodiment, however, the gate electrode 708 is a common electrode. 
Selection of the electron emitter which should emit electrons is conducted 
by successively providing an electron emitter 710 with a potential which 
is negative with respect to the gate electrode by using a scanning circuit 
711. The electron emitter is divided to pixels, and each pixel has a 
plurality of cathodes. Each pixel has a shape of 50 nm square arranged at 
intervals of 5 .mu.m. One pixel has approximately 100 electron emitters. 
For brevity, five electron emitters are shown in FIG. 7 to be included in 
one pixel width of the lengthwise direction. 
The a-Si:H used as the photoconductive layer in the present embodiment has 
an advantage over, for example, amorphous Se in being excellent in heat 
resistance and high in sensitivity to red light. Furthermore, in case 
electron beam scanning is performed by changing over the electron emitter 
potential as in the present embodiment, a feature of simple structure is 
obtained because the gate electrode is common. 
(Embodiment 4) 
A scanning method of imaging apparatus according to the present invention 
will now be described by referring to FIG. 8. An imaging apparatus of the 
present embodiment was formed in the same way as the first embodiment. 
FIG. 8 shows the change of emission position of scanning electron beam 
during consecutive time points t.sub.1 to t.sub.5. Numeral 801 denotes a 
photoconductive target. Numeral 802 denotes an electron beam. Numeral 803 
denotes a scanning circuit having a plurality of electron beam emitters 
and an electron emitter selector circuit. The present embodiment is so 
configured that in changing over an electron emitter selector circuit from 
an electron beam emission state at a certain time point to a state at a 
succeeding time point, a part of electron emitters may be selected in 
common in those two states. FIG. 8 shows how scanning is performed while 
emitting electrons simultaneously from eight electron emitters. As 
illustrated, two common electron emitters emit electron beams in two 
temporally adjacent states. Electron beam radiation ranges R.sub.1 to 
R.sub.5 at time points t.sub.1 to t.sub.5 partially overlap with radiation 
ranges at adjacent time points. Such operation can be attained, for 
example, in the second embodiment by performing electron beam scanning 
while simultaneously selecting a plurality of separated gate electrodes 
and by selecting a part of gate electrodes in common at adjacent time 
points. 
According to the present embodiment, boundaries between pixels do not exist 
distinctly. Operation is conducted like an imaging tube for deflecting a 
single electron beam and performing scanning consecutively. Thereby, it is 
possible to prevent occurrence of moire fringes and reduce noise of a 
high-frequency region while maintaining marginal resolution. 
(Embodiment 5) 
A fifth embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 9. In FIG. 9, a 
transparent substrate 901, a photoconductive layer 902, and an electron 
beam emitter and scanning circuit 903 are similar to, for example, those 
of the embodiment 1, and hence description of them will be omitted. In the 
present embodiment, a transparent electrode 904 has a plurality of 
electrically separated stripe-shaped partial electrodes and respective 
partial electrodes are connected to an output circuit 905. Furthermore, in 
the present embodiment, scanning is performed in a direction parallel to 
the stripe-shaped partial electrodes while a plurality of electron beams 
906 are being applied simultaneously to the photoconductive layer 902 on 
respective partial electrodes. Signal currents outputted simultaneously 
from respective partial electrodes are amplified and processed by the 
output circuit 905. A video signal is outputted as a time-series electric 
signal corresponding to the spatial distribution of light incident on the 
entire apparatus. 
In the present embodiment, lag is reduced because the transparent electrode 
has a plurality of electrically separated partial electrodes. In addition, 
it is possible to increase the scanning speed and reduce the bandwidth by 
simultaneously reading output signals from respective partial electrodes 
in parallel. Therefore, the imaging apparatus of the present embodiment is 
suitable for imaging apparatuses each having a large area. For example, a 
contact-type two-dimensional image sensor having an effective imaging area 
equivalent to A4 size can also be obtained. 
(Embodiment 6) 
A sixth embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 10. In the present 
embodiment, a transparent substrate 1001, a photoconductive layer 1002, 
and electrically separated transparent electrodes 1010 to 1025 are similar 
to those of the above described fifth embodiment, and hence description of 
them will be omitted. From an electron beam emitter and scanning circuit 
1003, electron beams 1006 and 1007 are simultaneously applied to the 
scanned surface on two adjacent transparent electrodes. The electron beam 
1006 reads out the stored signal charge into an output circuit 1004 via a 
transparent electrode 1014, a video signal being thus formed. On the other 
hand, the electron beam 1007 further scans the scanned surface which has 
already been scanned by the electron beam 1006. However, a signal current 
read out thereby via a transparent electrode 1013 does not contribute to 
the video signal in the output circuit. 
In case some stored signal charge is left behind due to a change of balance 
potential of the scanned surface caused by an abrupt change of the 
quantity of incident light, for example, or some stored signal charge is 
left behind due to excessive incident light and limit on electron beam 
quantity, the residual charge can be simultaneously erased in the imaging 
apparatus of the present embodiment while normally continuing readout 
scanning. Therefore, the imaging apparatus of the present embodiment has 
an advantage in that lag and so-called comet tail phenomenon are 
restrained. The lag erase electron beam 1007 need not necessarily be 
radiated all the times, but may also be radiated in response to an abrupt 
change of the quantity of incident light and excessive incident light. 
Furthermore, also in case there are no stripe-shaped separate electrodes 
unlike the present embodiment, the lag erase beam can be applied. In that 
case, it is necessary that a current read out at the time of radiation of 
the lag erase beam does not exercise a bad influence upon a signal current 
read out by an ordinary scanning electron beam. 
(Embodiment 7) 
A seventh embodiment different from the above described sixth embodiment 
will now be described by also referring to FIG. 10. Although the imaging 
apparatus of the present embodiment conducts multi-beam scanning in the 
same way as the embodiment 6, operation of the output circuit 1004 is 
different from that of the embodiment 6. That is to say, in embodiment 7, 
a signal current read out from, for example, stripe-shaped transparent 
electrode 1013 by the electron beam 1007 is added to a signal current read 
out when the same position on the transparent electrode 1013 is scanned by 
the electron beam 1006, a video signal being thus formed. 
In case it is necessary to read out a signal current exceeding the limit of 
electron beam quantity unlike the above described sixth embodiment when 
some stored signal charge has been left behind due to excessive incident 
light and limit on electron beam quantity, the imaging apparatus of the 
present embodiment is effective. That is to say, at the time of such high 
illuminance that readout cannot be performed by conducting electron beam 
scanning once, the entire input signal can be read out by conducting 
scanning a plurality of times. Therefore, an imaging apparatus having a 
wide dynamic range is obtained. By the way, two or more electron beams may 
be used for duplicate readout. 
(Embodiment 8) 
An eighth embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 11. In FIG. 11, 
numerals 1101 to 1104 denote imaging apparatuses similar to those of the 
embodiments described above and description of them will be omitted. In 
the present embodiment, four imaging apparatuses 1101 to 1104 are combined 
and held in a vacuum envelope 1105. Signals read out from output circuits 
1106 to 1109 are processed in a signal processing circuit 1110. A video 
signal of light inputted to the above described four imaging apparatuses 
1101 to 1104 as a whole is thus formed. 
In the imaging apparatus of the present embodiment, imaging of a larger 
area is possible as compared with the case where the imaging apparatus 
according to the present invention is used singly. In case an imaging 
apparatus having a large area is formed by combining a plurality of 
imaging apparatuses according to the present invention as in the present 
embodiment, it is desirable to transmit incident light into a junction 
portion to an effective area of picture by using a lens or optical fiber, 
for example, and/or perform pixel interpolation by using image processing, 
as occasion demands so that a lack or abnormality of the video signal may 
not be caused at the junction. 
(Embodiment 9) 
A ninth embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 12. In an imaging 
apparatus similar to the above described first to ninth embodiments, the 
present embodiment has, on the light incidence side of a transparent 
substrate 1201, a fluorescent layer 1202 emitting light in response to 
incidence of radiation. According to the present embodiment, there is 
obtained an imaging apparatus having a sensitivity for such radiation that 
a photoconductive layer 1203 does not have a sufficient sensitivity 
therefor. 
(Embodiment 10) 
A tenth embodiment of an imaging apparatus according to the present 
invention will now be described by referring to FIG. 13. The present 
embodiment is an X-ray imaging apparatus obtained by combining an imaging 
apparatus 1304 according to the present invention with an X-ray image 
intensifier 1303. An X-ray image emitted from an X-ray source 1301 and 
transmitted through a subject 1302 is inputted to the X-ray image 
intensifier 1303. In the X-ray image intensifier, light emitted from an 
input fluorescent screen by inputted X-rays is incident upon a 
photocathode, and emitted electrons are accelerated and focused to make an 
output fluorescent screen luminous. A resultant luminous image is detected 
by an imaging apparatus 1304 according to the present invention. A video 
signal corresponding to an X-ray image transmitted through the subject 
1302 is thus obtained. 
In the X-ray imaging apparatus of the present embodiment, the optical 
system between the X-ray image intensifier and the imaging apparatus can 
be shortened by using an imaging apparatus of the present invention 
increased in area as compared with, for example, conventional imaging 
tubes and solid state imaging apparatuses. Therefore, the X-ray imaging 
apparatus of the present embodiment has advantages in that the apparatus 
can be reduced in size, and the sensitivity drop caused by loss of the 
optical system is reduced. Especially if an imaging apparatus according to 
the present invention having an effective image area nearly equal to or 
larger than that of the output fluorescent screen of the X-ray image 
intensifier as shown in FIG. 13 is used, the above described optical 
system can be removed, being of great advantage. 
(Embodiment 11) 
By referring to FIG. 14, an X-ray digital radiography system which is an 
eleventh embodiment of an imaging apparatus according to the present 
invention will now be described. An X-ray image emitted from an X-ray 
source 1401 and transmitted through a subject 1402 makes a fluorescent 
plate 1403 luminous. The resultant luminous image is detected by an 
imaging apparatus 1404 according to the present invention having a large 
area nearly equivalent to that of the fluorescent plate. The detected 
video signal is converted to a digital signal by an A/D converter 1405, 
and then subjected to processing in an image processor 1406 as occasion 
demands and displayed as an image by a display apparatus 1407. A control 
apparatus 1408 controls the imaging apparatus 1404 and the image processor 
1406. In addition, the control apparatus 1408 preserves the digitized 
video signal in a storage apparatus 1409. The imaging apparatus 1404 was 
decided to have an effective image area of 40 cm square, a thickness of 5 
cm, a pixel size of 100 .mu.m square, and an electron emitter pitch of 5 
.mu.m. Therefore, one pixel has approximately 400 electron emitters, and 
the resolution corresponds to 4000 lines. In the X-ray digital radiography 
system of the present embodiment, an X-ray image intensifier is not needed 
because the imaging apparatus has a large area. Therefore, the X-ray 
digital radiography system of the present embodiment has advantages of 
small-size, high sensitivity, and high resolution. 
As for the embodiments heretofore described, the case where the 
photoconductive target has a sensitivity to visible light has been mainly 
described. However, the imaging apparatus of the present invention is 
suitable also for the case where images of radiation other than visible 
light, such as infrared rays, ultraviolet rays, or X-rays, are directly 
detected by using a photoconductive layer sensitive to them. In that case, 
it is a matter of course that a constituent material depending upon 
radiation to be detected is desired to be selected. For example, Be or BN 
is used as the substrate, and PbO or amorphous Se having a layer thickness 
of at least 10 .mu.m is used as the photoconductive layer to obtain an 
X-ray imaging apparatus. 
As heretofore described in detail, the present invention provides a thin 
imaging apparatus having a large area and having advantages of high 
sensitivity, high resolution, low noise, low lag, and wide dynamic range. 
It is further understood by those skilled in the art that the foregoing 
description covers preferred embodiments of the disclosed apparatus and 
that various changes and modifications may be made in the invention 
without departing from the spirit and scope thereof.