Two-Dimensional Solid-State Image Sensor Device

1. Field of the Invention

The present invention relates generally to a two-dimensional solid-state image sensor device and more particularly to a two-dimensional solid-state image sensor device of the type in which each picture cell consists of a static induction transistor which functions as photodetector and switching element and in which a plurality of such picture cells are two-dimensionally arranged in a mosaic array.

2. Description of the Prior Art

In a conventional solid-state image sensor device, each picture cell comprises a diode for sensing a light image and a switching transistor. That is, a light image is detected by the diode and the sensed signal per se is outputted as a video signal. As a result, the conventional solid-state image sensor device has disadvantages in that the signal output is small and its sensitivity is low. Thus, the degree of integration of the conventional image sensor is limited in terms of sensitivity.

The applicant has disclosed, in Japanese patent application No. 204656/1981, a solid-state image sensor device of the type in which a static induction transistor with a high light sensitivity is used so that a light signal is stored in its gate region and a video signal is derived by controlling the current flowing between the source and the drain in response to the potential at the gate region, whereby a high signal output is obtained.

FIGS. 1 and 2 show sectional views of embodiments of a picture cell used in the above-described solid-state image sensor device. In FIG. 1, reference numeral 1 designates a silicon n.sup.+ substrate; 2, and n.sup.- (or intrinsic) semiconductor region which has a high resistance and serves as a channel; 3, an n.sup.+ drain region which has a high concentration and serves as drain region; 4, a p.sup.+ region which has a high impurity concentration and is formed so as not to interrupt the channel region and serves as a gate; 4-1 and 4-2, p.sup.+ regions in FIG. 2 which have a high impurity concentration and are formed so as not to interrupt the channel region and serve as a control gate and a shielding gate, respectively; and 6, an insulation layer or film such as an SiO.sub.2 or Si.sub.3 N.sub.4 film for providing a capacitor over the gate region 4 (FIG. 1) or over the control gate region 4-1 (FIG. 2). 7, 8 and 10 denote gate, drain and source electrodes, respectively. At least the gate electrode 7 is transparent to an incident light 18. 9 denotes a surface protection layer or film such as SiO.sub.2.

Reference numeral 11 designates a switching transistor; .0..sub.s, a control signal applied thereto; 13, a selection line for applying a read pulse voltage .0..sub.G to the gate electrode 7 from a picture cell selection circuit (not shown); 14, a load resistor; 15, a video voltage supply; 16, a signal readout line; 17, an output terminal; and 18, a light input.

The picture cell as shown in FIG. 2 is different from that as shown in FIG. 1 in that the former has the two gate regions 4-1 and 4-2. Of the two gate regions 4-1 and 4-2, the p.sup.+ region 4-1 is the control gate which controls the current flow between the source and the drain in accordance with the storage of charge induced in response to the light input. The control gate 4-1 together with the insulating layer or film 6 and the electrode 7 constitute a capacitor. Another p.sup.+ region 4-2 is the shielding gate and surrounds the control gate 4-1 and the n.sup.+ drain region 3. The control gate 4-1 and the shielding gate 4-2 produce a potential barrier in the channel.

While FIG. 1 shows the structure of only one picture cell, it should be noted that when many picture cells are formed, the shielding gate 4-2 serves to isolate one picture cell from others by a depletion layer. A potential may be applied to the shielding gate 4-2. Alternatively, the shielding gate 4-2 may be grounded through a resistor.

FIG. 3 shows an equivalent circuit of the picture cell shown in FIG. 1 or 2 and the operation of the picture cell will be explained with reference to FIG. 3.

When the input light 18 is incident to the picture cell, the light excited holes are stored in the gate region 4 or 4-1 so that the light signal is written therein.

Next, when the pulse voltage .0..sub.s is applied to the base (or gate) of the transistor 11, the voltage at the power supply 15 is applied across the source and the drain of the static induction transistor 100 as shown in FIG. 1 and if the pulse voltage .0..sub.G is further applied to the gate region 4 so that the static induction transistor 100 becomes conductive, the drain current corresponding to the light input 18 is derived from the output terminal 17. The output signal derived from the output terminal 17 varies depending upon the intensity of the light input 18. The amplification factor of the static induction transistor 100 is more than 10.sup.3 which is higher at least by one order than a conventional bipolar transistor. Furthermore, the dynamic range of the output signal is large. The capacitor which is connected to the gate is provided in order to block the direct current and to store the light signal.

While the picture cell per se of the solid-state image sensor device of the type described above has various advantageous properties as described above, the inventors have found that a novel readout process is required when a plurality of picture cells of the type described above are two-dimensionally arranged to form a two-dimensional solid-state image sensor device.

The reasons follow in detail. When a television signal is obtained from the two-dimensional solid-state image sensor device, the storage and readout of the video signal must be repeatedly carried out per field or frame unit. It follows, therefore, that the video signal is required to be stored in each cell after the readout of the video signal from one picture cell or from each of the picture cells arranged in one horizontal line (corresponding to one horizontal scanning) is completed and before the readout of the video signal from the same one picture cell or from each of the same picture cells arranged in the same one horizontal line during the next field or frame. Therefore, immediately after the readout from one picture cell or from each of the picture cells arranged in one horizontal line is completed, it is necessary that the same picture cell is refreshed (cleared) and the storage of a new video signal starts.

In a conventional two-dimensional solid-state image sensor device the type in which photodiodes and MOS transistor are combined, carriers corresponding to the intensity of light incident to the photodiode are supplied to the source junction of the MOS transistor, while the MOS transistor is interrupted, thereby rendering the MOS transistor conductive. Simultaneously, a transistor which is connected to the drain of the MOS transistor is rendered conductive so that a charge current flows from the drain to the source. An output signal is obtained depending upon an amount of the charge current. Therefore, the readout process of one picture cell corresponds to the refreshment of that picture cell. Thus, a video signal can be obtained by rendering both the pulses .0..sub.G or .0..sub.s ON precedingly; i.e., the pulse .0..sub.G to be applied to the gate of the MOS transistor or the pulse .0..sub.s to be applied to the gate of a transistor connected between the drain of the MOS transistor and a terminal of a video power supply (serving also as an output terminal).

On the other hand, in the above-described solid-state image sensor device of the type in which the light signal is stored in the gate region of the static induction transistor and the video signal is derived therefrom by controlling the current flowing between the source and the drain in response to the potential of the gate region, it is impossible to store a new video signal unless the video signal stored in the gate region is refreshed immediately after the video signal is readout. As a result, the video signal cannot be obtained.

With this in view, the image sensor device of this type employs the circuit arrangement as shown in FIG. 4 in which a vertical scanning circuit 20 is used to sequentially select read lines 11-1, 11-2, . . . , and 16-L while a horizontal scanning circuit 21 is used to sequentially select lines 13-1, 13-2, . . . , and 13-K. As illustrated in FIG. 5, in response to the pulse .0..sub.s1, the read line 16-1 is selected and during the period of this pulse .0..sub.s1, the pulses .0..sub.G1, .0..sub.G2, . . . , and .0..sub.GK are sequentially selected. The video signal in the picture cells (1-1), (2-1), . . . , and (K-1) which are connected to the read line 16-1 are read out. Thereafter in response to the next pulse .0..sub.s2, the read line 16-2 is selected and during the period of this pulse .0..sub.s2, the pulses .0..sub.G1, .0..sub.G2, . . . , and .0..sub.GK are sequentially selected. When the pulses .0..sub.s3, .0..sub.s4, . . . , and .0..sub.sL are sequentially selected in this manner, the light signals stored in the respective gate regions 4 or 4-1 of the picture cells (1-1), (2-1), . . . , and (K-1) which are connected to the read line 16-1, of which the read out selection is completed, will not be refreshed at all. Therefore, in order to refresh the gate regions 4 or 4-1 of these picture cells (1-1), (2-1), . . . , and (K-1) which are connected to the read line 16-1, it may be proposed to increase the voltage level of the gate pulses .0..sub.G1, .0..sub.G2, . . . , and .0..sub.GK so that the light signals stored in the gate regions 4 or 4-1 are discharged. In this case, however, all the light signals stored in the picture cells connected in common to the selection line 13-1, 13-2, . . . , and 13-K are also discharged, so that it is impossible to attain the two-dimensional image readout.

One of the objects of the present invention is therefore to provide a one-cell-with-one-transistor type two-dimensional solid-state image sensor device in which the above disadvantages are removed in such a way that one static induction transistor with a high degree of light sensitivity is used both for the detection of the light image and for switching

Another object of the present invention is to provide a two-dimensional solid-state image sensor device of the type described above which is also adapted to obtain a video signal.

According to one aspect of the present invention, therefore, there is provided a two-dimensinal solid-state image sensor device which comprises a plurality of picture cells which are two-dimensionalally arranged in column and row directions. Each picture cell comprises a static induction transistor having first and second main electrode regions formed of semiconductor regions with one conductivity type which are disposed on opposite sides of a high resistance semiconductor channel region, and a gate region formed of a semiconductor region with the other conductivity type which is adjacent to the channel region to control a current flowing between the first and second main electrode regions, and a transparent electrode disposed via a capacitance on at least a portion of the gate region, in a manner that light is incident through the transparent electrode to the gate region in which the carrier produced by the light excitation is stored to control the current. Each of a plurality of selection lines connects the gate regions of the picture cells in each column in common via the capacitances. There is provided means for connecting the plurality of selection lines to a readout vertical scanning circuit during each horizontal scanning period so that a read pulse voltage is sequentially applied to the plurality of selection lines from the readout vertical scanning circuit. Each of a plurality of signal readout lines connects the first main electrode regions of the picture cells in each row in common, each picture cell being selected in the column and row directions so that a signal is read out therefrom. There are also provided means for connecting the plurality of signal readout lines to a horizontal scanning circuit during each horizontal scanning period so that a readout pulse is sequentially applied to the plurality of signal readout lines from the horizontal scanning circuit and means for connecting the plurality of selection lines to a refresh vertical scanning circuit during each horizontal scanning period so that a refresh pulse voltage is applied to the selected selection line from the refresh vertical scanning circuit.

Here, an insulation layer is preferably disposed between the gate region and the transparent electrode to form the capacitance.

It is preferable that one of the main electrode region is disposed opposite to one side of the gate region.

It is preferable that the refresh pulse voltage is higher at least five times than the readout pulse voltage.

The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawings.

FIGS. 1 and 2 are cross sectional views showing two embodiments of a picture cell used in a two-dimensional solid-state image sensor device;

FIG. 3 is a circuit diagram showing an equivalent circuit thereof;

FIG. 4 is a circuit diagram showing a two-dimentional solid-state image sensor device in which the picture cells as shown in FIG. 1 or 2 are incorporated;

FIG. 5 is a timing chart used to explain the operations thereof;

FIG. 6 is a circuit diagram showing an embodiment of a two-dimensional solid-state image sensor device in accordance with the present invention;

FIG. 7 is a timing chart used to explain the operations thereof;

FIG. 8 is a circuit diagram showing another embodiment of a two-dimensional solid-state image sensor device in accordance with the present invention;

FIG. 9 is a timing chart used to explain the operations thereof;

FIG. 10 is a partial top view showing an embodiment of a two-dimensional solid-state image sensor device in accordance with the present invention; and

FIG. 11 is a sectional view showing another embodiment of a picture cell used in a two-dimensional image sensor device in accordance with the present invention.

Same reference numerals are used to designate similar or corresponding parts throughout the figures.

FIG. 6 shows a circuit diagram of an embodiment of a two-dimensional image sensor in accordance with the present invention and FIG. 7 shows a timing chart used to explain the operations thereof.

Referring first to FIG. 6, signal readout lines 16-1, 16-2, . . . , and 16-n are connected through switching transistors 11-1, 11-2, . . . , and 11-n to an output terminal 17, respectively. These switching transistors 11-1, 11-2, . . . , and 11-n are selected sequentially in response to pulses .0..sub.s1, .0..sub.s2, . . . , and .0..sub.sn from a horizontal scanning circuit 62, so that the signal readout lines 16-1, 16-2, . . . , and 16-n are sequentially selected. Consequently, a video voltage is applied to the drain regions of the respective picture cells 100 of the selected row. The selection lines 13-1, 13-2, . . . , and 13-m are connected, respectively, through switching transistors 60-1, 60-2, . . . , and 60-m to a readout vertical scanning circuit 63. During a horizontal scanning period, the switching transistors 60-1, 60-2, . . . , and 60-m are rendered to "ON" state, and they are rendered to "OFF" state during a horizontal blanking period.

It follows, therefore, that during each one horizontal line period, one pulse .0..sub.G of voltage V.sub.G supplied from a readout vertical scanning circuit is applied to the selection lines 13-1, 13-2, . . . , and 13-m. The selection lines 13-1, 13-2, . . . , and 13-m are connected through switching transistors 61-1, 61-2, . . . , and 61-m to a refresh vertical scanning circuit 64, respectively. The switching transistors 61-1, 61-2, . . . , and 61-m are rendered to "ON" state during each one horizontal blanking period, and they are rendered to "OFF" state during the horizontal scanning period. As a result, during each horizontal blanking period, one pulse .0..sub.R of voltage V.sub.R is applied to the selection lines 13-1, 13-2, . . . , and 13-m.

The readout vertical scanning circuit 63 and a refresh vertical scanning circuit 64 are synchronized with each other. For instance, as shown in FIG. 7, in response to the pulse .0..sub.G1, the selection line 13-1 is selected. During this period, read pulses .0..sub.s1, .0..sub.s2, . . . , and .0..sub.sn are derived from the horizontal scanning circuit 62. During one horizontal line period immediately after the video signals are read out from the picture cells (1-1), (1-2), . . . , and (1-n), the pulse .0..sub.R1 may be applied to the switching transistor 61-1 so that the gate regions of the picture cells (1-1), (1-2), . . . , (1-n) are refreshed.

It is preferable in this case that the voltage V.sub.R of the refreshing pulse .0..sub.R applied to the selection lines be made sufficiently greater than the voltage V.sub.G of the read pulse .0..sub.G. For instance, it is preferable that V.sub.R .gtoreq.5.times.V.sub.G.

However, it is not necessary that the readout vertical scanning circuit 63 and the refresh scanning circuit 64 be synchronized with each other. For instance, after the application of the pulse .0..sub.G1 as shown in FIG. 7 to the switching transistor 60-1, the refresh pulse .0..sub.R1 may repeatedly be applied during a plurality of horizontal scanning periods H. That is, after the readout from the picture cells connected to, the selection line, for instance, 13-1 is completed, these picture cells are refreshed plural times. In this case, each of the selection lines 13-1, 13-2, . . . , and 13-m is sequentially selected during each one horizontal line period and thereafter the picture cells connected to a plurality of selection lines are simultaneously refreshed.

It should be further noted that according to the present invention it is not necessary that the picture cells connected to a common selection line are refreshed during a horizontal blanking period immediately after the readout from the picture cells is completed. The refreshing can be carried out during any horizontal blanking period.

FIG. 8 shows an embodiment of a solid-state image sensor in accordance with the present invention which has an electronic shutter function. The second embodiment as shown in FIG. 8 is substantially similar in construction to the first embodiment as shown in FIG. 6 except that the refresh pulses .0..sub.R1, .0..sub.R2, . . . , and .0..sub.Rm are directly supplied from the refresh vertical scanning circuit 64 and that, as shown in FIG. 9, the refresh pulse .0..sub.R1, .0..sub.R2 . . . , and .0..sub.Rm are intermittently applied to the selection lines 13-1, 13-2, . . . , and 13-m during a blanking period of each horizontal scanning for a period of time other than a shutter opening period .tau.. That is, the refresh vertical scanning circuit 64 de-energizes the selection lines 13-1, 13-2, . . . , and 13-m sequentially by 1 H (one horizontal scanning period including a blanking period) phase shift in accordance with the timing of the initiation of the shutter opening period .tau..

After the shutter opening period .tau. elapses, the refresh voltage V.sub.R is sequentially applied to the picture cells 100 during each of the horizontal scanning blanking period in such a way that the phase of the voltage is shifted sequentially by 1 H in accordance with the timing of the initiation of the shutter opening period .tau.. During one horizontal scanning period immediately before the shutter is closed, i.e., immediately before the termination of the shutter opening period .tau., the readout vertical scanning circuit 63 and the horizontal scanning circuit 62 generate the read pulses .0..sub.G1, .0..sub.G2, . . . , and .0..sub.Gm and .0..sub.s1, .0..sub.s2, . . . , and .0..sub.sn sequentially, as is the case of the first embodiment as shown in FIG. 6, so that the horizontal lines may be sequentially read out. Unlike the first embodiment shown in FIG. 6, it is preferable that in the second embodiment shown in FIG. 8 the voltage of the refresh pulse is substantially equal to or slightly higher than the diffusion potential between the gate and channel of each picture cell.

In this manner, only during the shutter opening period .tau., the light carrier is stored in each picture cell 100 in response to the intensity of the incident light. At the end of this shutter opening period .tau., the stored signal is read out. Except the time period .tau., the corresponding picture cell 100 always remains in the reset state. The above described operation is substantially similar to that of a mechanical focal plane shutter, except that the function is carried out electronically.

The shutter opening period .tau. can be varied by changing the operation phases in the readout vertical scanning circuit 63 and the refresh vertical scanning circuit 64.

While the timing chart shown in FIG. 9 is used to explain the electronic shutter function which is applied to the recording of a still picture, the second embodiment can be used to record a moving picture, if the electronic shutter is repeatedly opened and closed in synchronism with the vertical scanning of the television signal.

As the picture cells to be used in the solid-state image sensor in accordance with the present invention, the picture cell of the type as shown in FIG. 1 or 2 can be used as it is.

In order to provide a static induction transistor as the picture cell, the impurity concentration in the n.sup.- channel region 2 must be substantially less than 1.times.10.sup.16 cm.sup.-3 and the impurity concentration of the gate, source and drain regions must be substantially higher than 1.times.10.sup.18 cm.sup.-3. In order that the drain current does not flow even when the gate voltage is 0 V, the sizes and impurity concentrations are so selected that the portion between the gates and the portion between the gate and the channel are depleted only at a diffusion potential.

Furthermore, the present invention can use static induction transistors of the type in which the drain current flows even when the gate voltage is 0 V and the drain current is interrupted when a slightly negative voltage is applied to the gate. It is apparent that the fabrication of the picture cells would become easier when the thickness of the gate is increased and the spacing between the adjacent gates are also reduced.

Since the light amplification is needed, care should be so taken that no crystal dislocation or defect results in each step. For instance, in the case of the boron diffusion of the p.sup.+ gate, an atom selected from the group IV is used so as to prevent the lattic distortion. In order to prevent the recombination of the electrons and holes in the n.sup.- channel region, the carrier life in the channel region must be longer. Thus, in the last step, gettering of a heavy metal is carried out so that the carrier life in the channel region is increased.

In order to provide a two-dimensional solid-state image sensor device with a higher degree of density. It is preferable to use the picture cell of separated gate type as shown in FIG. 2 of the picture cells as shown in FIGS. 1 and 2, since better isolation between the adjacent picture cells can be ensured by means of the control gate 4-1 and the shielding gate 4-2. FIG. 10 is a plan view showing the arrangement of the picture cells as shown in FIG. 2 together with their electrodes. The shielding gate regions 4-2 surround the control gate region 4-1 and the drain region (drain electrode) 3. The shielding gate regions 4-2 are formed in common for all picture cells. Reference numeral 16 designates a readout line. The hatched portion of the readout line 16 is made into electrical contact with the drain region 3 while the non-hatched portion is electrically isolated. The readout line 16 has a window which is so formed as to uncover the control gate region 4-1. Reference numeral 13 designates a selection line. The selection line 13 and the control gate region 4-1 form a capacitor at the hatched portion. It is required that the electrode 7 is transparent to light.

FIG. 11 shows a further modification of a picture cell to be used in a solid-state image sensor device in accordance with the present invention. The distance W.sub.2 between the drain region 3 and the shielding gate region 4-2 is considerably smaller than the distance W.sub.1 between the drain region 3 and the control gate region 4-1. That is, W.sub.1 >W.sub.2. The depletion layer from the control gate region 4-1 is further broadened so that the charge produced by the excitation of light can efficiently be stored in the control gate region 4-1.

In the above-described embodiments, the position of the drain region 3 may be replaced with the position of the source region 1 and a voltage may be applied through the load resistor 14 to the n.sup.+ region 1 from the power supply 15. Furthermore, the conductivity type of each region can be reversed. In this case, instead that the positive pulse voltages V.sub.G and V.sub.R are applied to the readout lines 13 in the above embodiments, the negative pulse voltages are applied to the readout lines 13. In like manner, the polarity of the video power supply is reversed. Accordingly, instead of holes, electrons are stored in the gate region.

As described above, according to the present invention, the selection lines are sequentially selected during each horizontal scanning period so that the video signals are readout from the respective picture cells, and during each horizontal blanking period the picture cells connected to the selected selection line are refreshed. Thus, the present invention can provides a two-dimensional solid-state image sensor device is preferably used to obtain a television signal.