Abstract:
There is provided a method of driving a solid-state image sensor, including the steps of transferring signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in at least two pixel lines so that a trailing edge of a first pulse to be applied in a first pixel line corresponds with a leading edge of a second pulse to be applied in a second pixel line, transferring the signal charges from the vertical CCDs to a horizontal CCD, and outputting the signal charges from horizontal CCD to an external circuit. The method makes it possible to prevent an increase in a substrate voltage at which charges are reversely transferred to photodiodes, which increase is caused by simultaneously applying pulses to all signal readers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of driving a solid-state image sensor, and more particularly to a method of reading out signal charges from photoelectric transfer devices to vertical CCDs (charge coupled devices) through signal readers when a pulse is applied to the signal readers, 
     2. Description of the Related Art 
     These days, there has been developed a camera to be used for a personal computer in order to input images to the computer. An image sensor incorporated in a conventional camera employing a television system such as NTSC and PAL has been conventionally designed to have an interlace system in which a frame is displayed with two fields. For instance, such an image sensor has been suggested in Japanese Unexamined Patent Publications Nos. 62-154891 and 9-275206. 
     However, an image sensor with an interlace system is accompanied with a problem of low resolution of images. Hence, an image sensor incorporated in a camera is recently designed to have a non-interlace system in order to enhance resolution of images. In a non-interlace system, signals running through horizontal scanning lines constituting a frame are output in time series. A non-interlace system has an advantage that images can be displayed with ease on a screen such as a personal computer. Hence, an image sensor associated with a non-interlace system, that is, a progressive scan type image sensor has been in demand, and thus, has been researched and developed. 
     FIG. 1 is a plan view of an interline type charge coupled device (CCD) image sensor associated with a progressive scan system. The illustrated CCD image sensor is comprised of an image sensing region  1 , a horizontal CCD  2 , an output section or a charge detector  3 , a plurality of photodiodes  4  arranged in the image sensing region  1  in a two-dimensional matrix, and a plurality of vertical CCDs  5  each located adjacent to each row of photodiodes. 
     Each of the photodiodes  4  converts a light into a signal electric charge, and accumulates the thus converted electric charge therein. Each of the vertical CCDs  5  vertically transfers signal electric charges having been transferred from the photodiodes  4 . An electric charge reader  6  positioned between each of the photodiodes  4  and each of the vertical CCDs  5  reads a signal electric charge out of each of the photodiodes  4  into each of the vertical CCDs  5 . The image sensing region  1  except the photodiodes  4 , the vertical CCDs  5 , and each of the electric charge readers  6  defines an insulating region for insulating a photodiode from another photodiode. The image-sensing region  1  except the photodiodes, the vertical CCDs  5 , and the electric charge readers  6  defines a device isolation region  7 . 
     In operation, a light is converted into an electric charge for a certain period of time in each of the photodiodes  4 , and the thus generated electric charge is accumulated in each of the photodiodes  4 . The electric charges accumulated in the photodiodes  4  are read out into the vertical CCDs  5  through the electric charge readers  6  by applying a certain voltage to the electric charge readers  6 . The electric charges having been read out into the vertical CCDs  5  are transferred towards the horizontal CCD  2  line by line. The electric charges having been transferred to the horizontal CCD  2  are horizontally transferred in the horizontal CCD  2 , and then, detected at the output section  3  as an output voltage. 
     FIG. 2 is an enlarged view of a part of the image sensing region  1  of the image sensor associated with the progressive scan system and capable of reading out signals in every three vertical pixels. A part of the image sensing region  1 , illustrated in FIG. 2, is defined by vertical five pixels x horizontal three pixels. 
     Each of the vertical CCDs  5  includes four vertical transfer electrodes  8   a,    8   b,    8   c,  and  8   d  for each of the photodiodes  4 . At least one of the vertical transfer electrodes  8   a,    8   b,    8   c,  and  8   d  doubles as a read-out electrode for reading out a signal electric charge from the photodiode  4  to the vertical CCD  5 . For instance, the vertical transfer electrode  8   b  doubles as such a read-out electrode in FIG.  2 . 
     In the image sensor illustrated in FIG. 2, transfer of electric charges in the vertical CCD  5  is carried out by four-phase drive pulses. Specifically, four-phase pulses φV 1  to φV 4  are applied to the vertical transfer electrodes  8   a  to  8   d,  respectively, in a four-electrode cycle. 
     Though the four-phase pulses φV 1  to φV 4  are illustrated as applied only to the vertical transfer electrodes  8   a,    8   b,    8   c  and  8   d  in the rightmost vertical CCD  5  for the purpose of simplification, the same phase pulse is applied to the electrodes located in a common line in the vertical CCDs  5 . For instance, the drive pule φV 1  is applied to all the vertical transfer electrodes  8  situated uppermost in the three vertical CCDs  5  illustrated in FIG.  2 . 
     In order to make it possible to read out signals at every three pixel lines, drive pulses φV 3 A, φV 3 B, and φV 3 C can be separately applied to the vertical transfer electrode  8   b  acting as a read-out electrode at every three pixel lines. 
     FIG. 3 is an enlarged plan view of a couple of pixels illustrated in FIG.  2 . As illustrated in FIG. 3, a transfer channel of the vertical CCD  5  vertically extends adjacently to the photodiode  4 . As illustrated in FIG. 4, each of the vertical transfer electrodes  8   a  to  8   d  to which an electric charge transfer pulse is to be applied is composed of three-layered polysilicon. One of the vertical transfer electrodes  8   a  to  8   d,  specifically, the vertical transfer electrode  8   b  doubles as a read-out electrode for transferring signal electric charges to the vertical CCDs  5  from the photodiodes  4 . 
     FIG. 4 is a cross-sectional view taken along the line IV—IV in FIG. 3, and illustrates a structure of the vertical transfer electrode  8 . As illustrated in FIG. 4, the drive pule φV 1  is applied to a vertical transfer electrode  8 - 1  constituted of a first polysilicon layer, the drive pulse φV 2  is applied to a vertical transfer electrode  8 - 2   a  constituted of a second polysilicon layer deposited over the first polysilicon layer, and the drive pulse φV 3  is applied to a vertical transfer electrode  8 - 2   b  constituted of the second polysilicon layer. Since the vertical transfer electrodes  8 - 2   a  and  8 - 2   b  are electrically insulated from each other, it is possible to apply separate pulses to the vertical transfer electrodes  8 - 2   a  and  8 - 2   b,  although they are constituted of the common polysilicon layer. The drive pule φV 3  is applied to a vertical transfer electrode  8 - 3  constituted of a third polysilicon layer deposited over the second polysilicon layer. 
     FIG. 5 is a cross-sectional view taken along the line V—V in FIG.  3 . As illustrated in FIG. 5, an image sensor is comprised of an n-type substrate  9 , a p-type well layer  10  formed in the substrate  9 , including an n-type photodiode layer  11  accomplishing photoelectric transfer and accumulating generated signal electric charges, n-type vertical CCD buried layers  12  for vertically transferring electric charges, and an electric charge reader  6  (not illustrated) for reading out electric charges from the photodiode layer  11  into the vertical CCD buried layers  12 , p-type vertical CCD well layers  13  located just below the vertical CCD buried layers  12 , a heavily doped p-type impurity layer  14  formed above the photodiode layer  11  and between the photodiode layer  11  and the layers  12 ,  13 , an insulating film  15  formed on the p-type well layer  10 , vertical CCD transfer electrodes  8  composed of polysilicon and formed within the insulating film  15  above the vertical CCD buried layers  12 , and a light-impermeable film  16  formed on the insulating film  15 . The light-impermeable film  16  is formed with an opening  17  above the photodiode layer  11 . 
     FIGS. 6A to  6 C illustrate how an electric charge is transferred in progressive scanning operation. For simplification, FIGS. 6A to  6 C illustrate only one vertical CCD  5 , and a part of the horizontal CCD  2  located below the vertical CCD  5 . In FIGS. 6A to  6 C, a solid circle () indicates a packet containing an electric charge therein. 
     With reference to FIG. 6A, photodiodes PD 1  to PD 6  are exposed to a light for a certain period of time to thereby convert a light into signal electric charges, and accumulate the thus generated signal electric charges therein. When a read-out voltage is applied to all the read-out electrodes  6 , the signal electric charges accumulated in the photodiodes PD 1  to PD 6  are read out into the vertical CCD  5 . 
     Then, as illustrated in FIG. 6B, the signal electric charges having been read out into the vertical CCD  5  are vertically transferred towards the horizontal CCD  2  line by line. 
     Then, as illustrated in FIG. 6C, the signal electric charge having been transferred in the vertical CCD  5  reaches the horizontal CCD  2 , and is transferred through the horizontal CCD  2  towards the output section  3 . Finally, the signal electric charge is output through the output section  3  (not illustrated in FIGS. 6A to  6 C). 
     FIG. 7 illustrates waveforms of the vertical drive pulses φV 1  to φV 4  and how the electric charges are transferred in progressive scanning operation. 
     The waveforms of the vertical drive pulses are illustrated over one vertical blanking period and subsequent two horizontal blanking periods. The vertical drive pulses are four-phase pulses. When read-out pulses φTGA, φTGB, and φTGC are applied to the vertical CCDs  5 , the signal electric charges are read out of the photodiodes  4  into the vertical CCDs  5 . 
     In FIG. 7, the read-out pulses φTGA, φTGB, and φTGC are shown as independent pulses, however, it should be noted that the read-out pulses φTGA, φTGB, and φTGC are applied to the vertical transfer electrode  8   b  doubling as a read-out electrode in the form that the read-out pulses φTGA, φTGB, and φTGC are overlapped onto the vertical drive pulses φV 3 A, φV 3 B, and φV 3 C, respectively. 
     How the electric charges are transferred is shown in a lower half of FIG.  7 . The photodiodes  4  and the vertical transfer electrodes  8  are illustrated at the left end. Lowermost rows indicate horizontal transfer electrodes constituting the horizontal CCD  2 . A horizontal drive pulse φH 1  is applied to the horizontal transfer electrodes. 
     In a lower half of FIG. 7, a hollow rectangle indicates a vertical transfer electrode having a packet in which electric charge can be accumulated, but containing no signal electrode charge, and a hatched rectangle indicates a vertical transfer electrode containing a packet in which electric charge is accumulated. Transfer of electric charges with the lapse of time can be understood by virtue of the signal waveforms illustrated in an upper half of FIG. 7, and it is also understood that a signal electric charge is located adjacent to which vertical CCD at a certain timing, by virtue of the photodiodes  4  and the vertical transfer electrodes  8  illustrated at the left. Thus, it is understood how electric charges are transferred. 
     With reference to FIG. 7, when the vertical drive pulses φV 1  to φV 4  are in a middle level, a channel located below the associated vertical transfer electrode  8  is ready to accumulate electric charges therein. When the vertical drive pulses φV 1  to φV 4  are in a low level, a channel located below the associated vertical transfer electrode  8  is not ready to accumulate electric charges therein. 
     When the read-out pulses φTGA, φTGB, and φTGC are in a high level, a signal electric charge is read out from an associated photodiode  4  into the vertical CCD  5 . When the read-out pulses φTGA, φTGB, and φTGC are in a low level, a signal electric charge is not read out. 
     At time t 1 , all the read-out pulses φTGA, φTGB, and φTGC are simultaneously turned into a high level, and signal electric charges accumulated in all the photodiodes  4  are read out into the vertical CCDs  5 . The thus read out signal electric charges are accumulated in both a channel located below the vertical transfer electrode  8  associated with the vertical drive pulse φV 3  which is in a high level, and a channel located below the vertical transfer electrode  8  associated with the vertical drive pulse φV 4  which is in a middle level Thereafter, the signal electric charges are downwardly, vertically transferred in each of the vertical CCDs  5  by a pixel in a horizontal blanking period. 
     FIG. 8A illustrates the photodiodes  4  and the vertical transfer electrodes  8  in an image sensor in which a pulse is applied to vertical transfer electrodes in every three pixel lines, and FIG. 8B illustrates waveforms of a pulse to be applied when electric charges are transferred from the photodiodes  4  to the vertical CCD  5 . Specifically, FIG. 8B illustrates waveforms of the pulses φV 3 A, φV 3 B and φV 3 C to which the pulses φTGA, φTGB and φTGC are overlapped, respectively. 
     The pulses φV 3 A, φV 3 B and φV 3 C are kept at a middle level, for instance, at 0 V, before electric charges accumulated in the photodiodes  4  are read out into the vertical CCDs  5 . At time t 1 , all the pulses φV 3 A, φV 3 B and φV 3 C are raised up to a high level, for instance, 15 V. As a result, channels of the electric charge readers  6  are turned on, and accordingly, signal electric charges accumulated in the photodiodes  4  are read out into the vertical CCDs  5 . 
     In order to ensure that all electric charges are read out into the vertical CCDs  5 , the pulses φV 3 A, φV 3 B and φV 3 C are kept at the high level for about 2 μs. Then, all the pulses φV 3 A, φV 3 B and φV 3 C are fell down to the middle level at time t 2 . As a result, the channels of the electric charge readers  6  are turned off. 
     In the above-mentioned progressive scan type image sensor, high positive voltage pulses of about 15 V are concurrently applied to all the electric charge readers  6 . As a result, there was caused a problem that a ground potential in the p-type well layer  10  was fluctuated due to the application of the pulse voltages, and hence, a substrate voltage at which electric charges were reversely transferred into the photodiodes  4  was raised (hereinafter, such a substrate voltage is referred to as “reverse-transfer substrate voltage”). 
     This problem is explained in detail hereinbelow. 
     FIG. 9 illustrates a relation between potential of electrons and a depth in a substrate at the time when a standard substrate voltage is set. As illustrated in FIG. 5, an impurity profile in a depth-wise direction of the substrate included the heavily doped p-type impurity layer  14 , the n-type photodiode layer  11 , the p-type well layer  10 , and the n-type substrate  9 . In such a structure, the potential is kept at 0V for electrons existing in the heavily doped p-type impurity layer  14 , and the potential makes a valley for electrons existing in the n-type photodiode layer  11 . A ground potential of 0V is applied to the p-type well layer  10 , and a voltage in the range of about 5V to about 8is applied to the substrate  9 . 
     Since the p-type well layer  10  is doped to a less degree than the heavily doped p-type impurity layer  14 , an impurity concentration of the p-type well  10  is influenced by the substrate voltage and other electrodes&#39; voltages. 
     For instance, as illustrated in FIG. 10, if the substrate voltage is raised up to about 10 to 15 V, a potential in the p-type well layer  10  is varied together with the substrate voltage so that a potential in the p-type well layer  10  becomes deeper, resulting in that a capacity of the photodiode is decreased. 
     On the other hand, if the substrate voltage is raised up to about 2V, as illustrated in FIG. 11, a difference in potential between a potential for the photodiode and the substrate voltage is reduced, resulting in that electric charges are reversely transferred from the substrate to the photodiodes. In such a condition, the image sensor cannot operate. For this reason, a substrate voltage is usually set equal to a voltage higher than the reverse-transfer voltage by about 1V. 
     Specifically, since the reverse-transfer voltage is in the range of about 2 to 3V, a minimum substrate voltage can be set at 3 to 4 V. 
     As mentioned above, the reverse-transfer voltage is relatively low in conventional television system such as NTSC and PAL where read-out pulse are not applied to all the transfer electrodes at a time. This is because a timing at which electric charges are to be read out in a line is different from other timings at which electric charges are to be read out in other lines, in a conventional television system usually operated in accordance with the interlace system. 
     However, since pulses are simultaneously applied to all the electrodes in such a progressive scan type image sensor as mentioned above, there was caused a problem that a ground potential in the p-type well layer was fluctuated due to the application of the pulse voltage, and hence, the reverse-transfer voltage was raised. 
     As a result, as illustrated in FIG. 12, since a highly positive voltage, for instance, a voltage of 15V is applied to the read-out electrodes as a pulse for reading out electric charges, a potential for the n-type photodiode layer in the photodiode is fluctuated, and a potential for the photodiodes is fluctuated to be deepened relative to a case where no read-out voltage is applied. 
     Under such a circumstance, even if a substrate voltage is set equal to 5V as usual, for instance, such a substrate voltage is low in view of a relation between a potential for the photodiode and the substrate voltage, As a result, electric charges are readily reversely transferred from the substrate to the photodiodes, as shown with the curve C 2  in FIG.  12 . In such a case, the reverse-transfer voltage is raised up to about 5 to 6V. 
     Hence, the substrate voltage has to be set equal to 6 to 7V, because the substrate voltage is necessary to be set higher than the reverse-transfer voltage by about 1V. This means that an image sensor has to operate with a small amount of electric charges being accumulated in the photodiodes. As a result, it is not possible to accumulate a large amount of electric charges in the photodiodes, and hence, a dynamic range is restricted, and highly qualified image outputs cannot be obtained. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problems, it is an object of the present invention to provide a method of driving an image sensor which method is capable of preventing the reverse-transfer voltage from being increased. 
     There is provided a method of driving a solid-state image sensor, including the steps of (a) transferring signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines so that a pulse to be applied in a first pixel line and a pulse to be applied in a second pixel line are applied at different timings, (b) transferring the signal charges from the vertical CCDs to a horizontal CCD, and (c) outputting the signal charges from the horizontal CCD to an external circuit, 
     There is further provided a method of driving a solid-state image sensor, including the steps of (a) transferring signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines so that a trailing edge of a first pulse to be applied in a first pixel line is in synchronization with a leading edge of a second pulse to be applied in a second pixel line, (b) transferring the signal charges from the vertical CCDs to a horizontal CCD, and (c) outputting the signal charges from horizontal CCD to an external circuit. 
     It is preferable that at least two trailing edges of pulses are in synchronization with leading edges of other pulses, respectively. 
     There is still further provided a method of driving a solid-state image sensor, including the steps of (a) transferring signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines so that a trailing edge of a pulse to be applied in a pixel line is in synchronization with a leading edge of a pulse to be applied in an adjacent pixel line, (b) transferring the signal charges from the vertical CCDs to a horizontal CCD, and (c) outputting the signal charges from horizontal CCD to an external circuit. 
     There is yet further provided a method of driving a solid-state image sensor, including the steps of (a) transferring signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines so that a trailing edge of a pulse applied to the first pixel line is in synchronization with a leading edge of a pulse applied to the last pixel line, (b) transferring the signal charges from the vertical CCDs to a horizontal CCD, and (c) outputting the signal charges from horizontal CCD to an external circuit. 
     It is preferable that the pulses are applied to pixel lines at the same interval. It is also preferable that the pulses are applied to pixel lines at the same period of time. 
     There is still yet further provided a method of reading out signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines s 0  that pulses are applied to adjacent pixel lines at different timings. 
     There is further provided a method of reading out signal charges from photoelectric transfer devices to vertical CCDs constituted of a plurality of pixels, when a pulse is applied to the pixel, the pulse being applied to the pixels in every two or more pixel lines so that a trailing edge of a first pulse to be applied in a first pixel line is in synchronization with a leading edge of a second pulse to be applied in a second pixel line. 
     The advantages obtained by the aforementioned present invention will be described hereinbelow. 
     In accordance with the present invention, a plurality of pulses for reading electric charges from photodiodes to vertical CCDs are synchronized with each other so that they are cancelled with each other, to thereby ensure eliminate fluctuation in a ground potential of a p-type well layer which fluctuation would be caused due to application of high positive voltages to the vertical CCDs, and further prevent an increase in the reverse-transfer voltage. 
     The prevention of an increase in the reverse-transfer voltage in turn make it possible to set a lower substrate voltage, and to enhance a saturated amount of electric charges for a photodiode. This results in that a dynamic range can be widened, and highly qualified images can be obtained. 
     In addition, in accordance with the present invention, a plurality of pulses is cancelled with each other at least once. As a result, it is possible to minimize a gap in time for accumulating electric charges in the photodiodes. Hence, disturbance in images, caused by a difference in time for accumulating electric charges in the photodiodes, can be reduced to such a degree as it is no longer a problem. 
    
    
     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a progressive scan type interlace CCD image sensor. 
     FIG. 2 is a plan view of photodiodes and vertical CCDs in an image sensor in which read-out pulses are applicable in every three pixel lines. 
     FIG. 3 is an enlarged view of a part of FIG.  2 . 
     FIG. 4 is a cross-sectional view taken along the line IV—IV in FIG.  3 . 
     FIG. 5 is a cross-sectional view taken along the line V—V in FIG.  3 . 
     FIGS. 6A to  6 C illustrate electric charges being transferred in progressive scan operation. 
     FIG. 7 illustrates waveforms of vertical drive pulses and transfer of electric charges in progressive scan operation. 
     FIG. 8A is a plan view of photodiodes and a vertical CCD in a conventional image sensor in which read-out pulses are applicable in every three pixel lines. 
     FIG. 8B illustrates waveforms of read-out pulses. 
     FIG. 9 illustrates a relation between a potential and electrons in a depth-wise direction of a substrate, found when a standard substrate voltage is set. 
     FIG. 10 illustrates a relation between a potential and electrons in a depth-wise direction of a substrate, found when a substrate voltage is raised. 
     FIG. 11 illustrates a relation between a potential and electrons in a depth-wise direction of a substrate, found when a substrate voltage is too much lowered, and as a result, electric charges are reversely transferred from a substrate to photodiodes. 
     FIG. 12 illustrates a relation between a potential and electrons in a depth-wise direction of a substrate in a progressive scan type image sensor. 
     FIG. 13A is a plan view of photodiodes and a vertical CCD in an image sensor in which read-out pulses are applicable in every three pixel lines, in accordance with the first embodiment. 
     FIG. 13B illustrates waveforms of read-out pulses applied to pixels illustrated in FIG.  13 A. 
     FIG. 14A is a plan view of photodiodes and a vertical CCD in an image sensor in which read-out pulses are applicable in every four pixel lines, in accordance with the second embodiment. 
     FIG. 14B illustrates waveforms of read-out pulses applied to pixels illustrated in FIG.  14 A. 
     FIG. 15 illustrates waveforms showing a period of time in which electric charges are accumulated in photodiodes. 
     FIG. 16A is a plan view of photodiodes and a vertical CCD in an image sensor in which read-out pulses are applicable in every three pixel lines, in accordance with the third embodiment. 
     FIG. 16B illustrates waveforms of read-out pulses applied to pixels illustrated in FIG.  16 A. 
     FIG. 17A is a plan view of photodiodes and a vertical CCD in an image sensor in which read-out pulses are applicable in every four pixel lines, in accordance with the fourth embodiment. 
     FIG. 17B illustrates waveforms of read-out pulses applied to pixels illustrated in FIG.  17 A. 
     FIG. 18A is a plan view of photodiodes and a vertical CCD in an image sensor in which read-out pulses are applicable in every four pixel lines, in accordance with the fifth embodiment. 
     FIG. 18B illustrates waveforms of read-out pulses applied to pixels illustrated in FIG.  18 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
     FIG. 13A illustrates photodiodes and a vertical CCD in an image sensor used for carrying out a method in accordance with the first embodiment, and FIG. 13B illustrates waveforms of read-out pulses applied to pixels in the image sensor. In the image sensor, different read-out pulses are applied to pixels in every three pixel lines. As illustrated in FIG. 13A, read-out pulses φV 3 A, φV 3 B and φV 3 C are applied to vertical transfer electrodes  8   b  acting also as a read-out electrode. 
     A method of driving an image sensor, in accordance with the first embodiment, is applied to a progressive scan type solid-state image sensor having such a structure as explained with reference to FIGS. 1 to  5 . 
     With reference to FIG. 13B, the pulse φV 3 A is varied from a middle level (for instance, 0V) to a high level (for instance, 15V) at time t 11  in a vertical blanking period. The pulse φV 3 A has a duration of about 2 μs. Hence, the pulse φV 3 A is varied from the high level to the middle level at time t 12  which is later than time t 11  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 12 . 
     In synchronization with time  12  at which the pulse φV 3 A is reduced to the middle level, the pulse φV 3 B is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 A and a leading edge of the pulse φV 3 B are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Then, the pulse φV 3 B is varied from the high level to the middle level at time t 13  which is later than time t 12  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 13 . 
     In synchronization with time  13  at which the pulse φV 3 B is reduced to the middle level, the pulse φV 3 C is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 B and a leading edge of the pulse φV 3 C are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Thus, an increase in the reverse-transfer voltage, caused by application of a high positive voltage to pixels, can be suppressed, resulting in that a substrate voltage can be reduced. The reduced substrate voltage enhances accumulation capacity of a photodiode, and hence, enhances a saturated amount of electric charges for a photodiode. This results in that a dynamic range can be widened, and highly qualified images can be obtained. 
     In the first embodiment, the pulses φV 3 A, φV 3 B and φV 3 C are applied to pixels in this order, but it should be noted that an order in which the pulses φV 3 A, φV 3 B and φV 3 C are applied to pixels is not to be limited to this. The pulses φV 3 A, φV 3 B and φV 3 C may be applied in any order. 
     [Second Embodiment] 
     FIG. 14A illustrates photodiodes and a vertical CCD in an image sensor used for carrying out a method in accordance with the second embodiment, and FIG. 14B illustrates waveforms of read-out pulses applied to pixels in the image sensor. In the image sensor, different read-out pulses are applied to pixels in every four pixel lines. As illustrated in FIG. 14A, read-out pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to vertical transfer electrodes  8   b  acting also as a read-out electrode. 
     With reference to FIG. 14B, the pulse φV 3 A is varied from a middle level (for instance, 0V) to a high level (for instance, 15V) at time t 21  in a vertical blanking period. The pulse φV 3 A has a duration of about 2 μs. Hence, the pulse φV 3 A is varied from the high level to the middle level at time t 22  which is later than time t 21  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 22 . 
     In synchronization with time  22  at which the pulse φV 3 A is reduced to the middle level, the pulse φV 3 B is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 A and a leading edge of the pulse φV 3 B are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Then, the pulse φV 3 B is varied from the high level to the middle level at time t 23  which is later than time t 22  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 23 . 
     In synchronization with time  23  at which the pulse φV 3 B is reduced to the middle level, the pulse φV 3 C is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 B and a leading edge of the pulse φV 3 C are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Similarly, the pulse φV 3 C is varied from the high level to the middle level at time t 24  which is later than time t 23  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 24 , 
     In synchronization with time  24  at which the pulse φV 3 B is reduced to the middle level, the pulse φV 3 D is raised from a middle level to a high level. The pulse φV 3 D is varied from the high level to the middle level at time t 25  which is later than time t 24  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 25 . 
     Thus, an increase in the reverse-transfer voltage, caused by application of a high positive voltage to pixels, can be suppressed, resulting in that a substrate voltage can be reduced. The reduced substrate voltage enhances accumulation capacity of a photodiode, and hence, enhances a saturated amount of electric charges for a photodiode. This results in that a dynamic range can be widened, and highly qualified images can be obtained. 
     In the second embodiment, the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels in this order, but it should be noted that an order in which the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels is not to be limited to this. The pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D may be applied in any order. 
     In addition, the method in accordance with the present embodiment can be carried out in an image sensor where read-out pulses are applied pixels in every two pixel lines or in every five or greater pixel lines. 
     [Third Embodiment] 
     The above-mentioned first and second embodiments may be accompanied with a problem caused by a difference in time for accumulating electric charges in photodiodes. 
     FIG. 15 illustrates signal waveforms showing a difference in a time for accumulating electric charges in photodiodes (hereinafter, “a time for accumulating electric charges in photodiodes” is referred to simply as “accumulation time”). 
     In a solid-state image sensor, there may be carried out an operation in which electric charges being accumulated in photodiodes are swept into a substrate before electric charges having been accumulated in photodiodes are transferred to vertical CCDs to obtain desired sensitivity. This operation is called “a substrate shutter” or “an electron shutter”. 
     In FIG. 15, if the operation “a substrate shutter” is not carried out, the accumulation time is defined as a time between a read-out pulse in a vertical blanking period and a read-out pulse in the next vertical blanking period. That is, the accumulation time is defined as a time between time t 31  and time t 33 . The accumulation time is dependent on a method of driving an image sensor. For instance, the accumulation time is {fraction (1/10)} second or {fraction (1/30)} second. 
     The operation of a substrate shutter is carried out by applying a high positive voltage to a substrate at time  32 , to thereby sweep electric charges accumulated in photodiodes, into a substrate before electric charges accumulated in photodiodes are read out into vertical CCDs. Thus, a time for actually accumulating electric charges in photodiodes is defined as a time between time t 32  and a time at which the next read-out pulse is applied. Specifically, the accumulation time is defined as a period of time p 31 , p 32  or p 33 . 
     In the first and second embodiments, the accumulation time is slightly different among pixel lines. This is shown in FIG. 15 as the fact that the accumulation times p 3 l, p 32  and p 33  are all different from one another. 
     For instance, the accumulation time is usually expected to be in the range of about {fraction (1/10000)} second to about {fraction (1/10)} second. It is now assumed that the accumulation time is set equal to {fraction (1/10)} second in order to take a picture in rather dark environment. In the above-mentioned fist and second embodiments, a difference in the accumulation time between pixel lines is equal to a period of time p 31 , that is, from time t 32  to time t 33 , in a pixel line to which the pulse φV 3 A is applied, and a difference in the accumulation time between pixel lines is equal to a period of time p 33 , that is, from time t 32  to time t 35 , in a pixel line to which the pulse φV 3 C is applied. Hence, a difference in the accumulation time is equal to about 4 μs at greatest. This difference makes merely 0.004% relative to the accumulation time of {fraction (1/10)} second (100 ms), and hence, does not cause any problem. 
     On the other hand, if a picture is to be taken in bright environment, the accumulation time may be set equal to {fraction (1/10000)} second (100 μs). A difference in the accumulation time between pixel lines remains the same, that is, is equal to about 4 μs. Hence, this difference makes 4% relative to the accumulation time of to {fraction (1/10000)} second (100 μs). 
     The difference of 4% is a range which can be recognized in a screen. As a result, there is obtained only images including an output difference among pixel lines. Specifically, output images includes lateral stripes with the result of degradation of quality of images. 
     The above-mentioned example relates to an image sensor where read-out pulses are applied in every three pixel lines. A difference in the accumulation time will become greater in image sensors where read-out pulses are applied in every four or greater pixel lines, resulting in that more remarkable lateral stripes are included in images. 
     The third embodiment solves the above-mentioned problem. 
     FIG. 16A illustrates photodiodes and a vertical CCD in an image sensor used for carrying out a method in accordance with the third embodiment, and FIG. 16B illustrates waveforms of read-out pulses applied to pixels in the image sensor. In the image sensor, different read-out pulses are applied to pixels in every three pixel lines. As illustrated in FIG. 16A, read-out pulses φV 3 A, φV 3 B and φV 3 C are applied to vertical transfer electrodes  8   b  acting also as a read-out electrode. 
     With reference to FIG. 16B, the pulse φV 3 A is varied from a middle level (for instance, 0V) to a high level (for instance, 15V) at time t 41  in a vertical blanking period. The pulse φV 3 A has a duration of about 2 μs. Hence, the pulse φV 3 A is kept at a high level until time t 43  which is later than time t 41  by about 2 μs. 
     The pulse φV 3 B is varied from a middle level to a high level at time t 42  between time t 41  and time t 43 . The pulse φV 3 B has a duration of about 2 μs. Hence, the pulse φV 3 B is kept at a high level until time t 44  which is later than time t 42  by about 2 μs. 
     Then, the pulse φV 3 A is varied from the high level to the middle level at time t 43 , because electric charges are all read out from the associated photodiode at time t 43 . 
     In synchronization with time  43  at which the pulse φV 3 A is reduced to the middle level, the pulse φV 3 C is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 A and a leading edge of the pulse φV 3 C are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Then, the pulse φV 3 B is varied from the high level to the middle level at time t 44  which is later than time t 42  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 44 . The pulse φV 3 C is kept at a high level until t 45  which is later than time t 43  by about 2 μs, and is varied from the high level to the middle level at time t 45 , because electric charges are all read out from the associated photodiode at time t 45 . 
     In accordance with the third embodiment, it is possible to reduce a difference in the accumulation time by half in comparison with the first embodiment. Specifically, if the accumulation time is set equal to {fraction (1/10000)} second, a difference in the accumulation time in the first embodiment is 4%, whereas the same in the third embodiment is 2%, which is smaller than a target difference of 3% with the result that lateral stripes are hardly observed in images to thereby ensure no problem in practical use. 
     In the third embodiment, the pulses φV 3 A, φV 3 B and φV 3 C are applied to pixels in this order, but it should be noted that an order in which the pulses φV 3 A, φV 3 B and φV 3 C are applied to pixels is not to be limited to this. The pulses φV 3 A, φV 3 B and φV 3 C may be applied in any order. 
     [Fourth Embodiment] 
     FIG. 17A illustrates photodiodes and a vertical CCD in an image sensor used for carrying out a method in accordance with the fourth embodiment, and FIG. 17B illustrates waveforms of read-out pulses applied to pixels in the image sensor. In the image sensor, different read-out pulses are applied to pixels in every four pixel lines. As illustrated in FIG. 17A, read-out pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to vertical transfer electrodes  8   b.    
     With reference to FIG. 17B, the pulse φV 3 A is varied from a middle level (for instance, 0V) to a high level (for instance, 15V) at time t 51  in a vertical blanking period. The pulse φV 3 A has a duration of about 2 μs. Hence, the pulse φV 3 A is kept at a high level until time t 53  which is later than time t 51  by about 2 μs. 
     The pulse φV 3 B is varied from a middle level to a high level at time t 52  between time t 51  and time t 53 . The pulse φV 3 B has a duration of about 2 μs. Hence, the pulse φV 3 B is kept at a high level until time t 64  which is later than time t 52  by about 2 μs. 
     Then, the pulse φV 3 A is varied from the high level to the middle level at time t 53 , because electric charges are all read out from the associated photodiode at time t 53 . 
     In synchronization with time t 63  at which the pulse φV 3 A is reduced to the middle level, the pulse φV 3 C is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 A and a leading edge of the pulse φV 3 C are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Then, the pulse φV 3 B is varied from the high level to the middle level at time t 54  which is later than time t 52  by about 2 μs, because electric charges are all read out from the associated photodiode at time t 54 . 
     In synchronization with time t 54  at which the pulse φV 3 B is reduced to the middle level, the pulse φV 3 D is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 B and a leading edge of the pulse φV 3 D are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     The pulse φV 3 C is kept at a high level until t 55  which is later than time t 53  by about 2 μs, and is varied from the high level to the middle level at time t 55 , because electric charges are all read out from the associated photodiode at time t 55 . The pulse φV 3 D is kept at a high level until t 56  which is later than time t 54  by about 2 μs, and is varied from the high level to the middle level at time t 56 , because electric charges are all read out from the associated photodiode at time t 56 . 
     In accordance with the fourth embodiment, it is possible to reduce a difference in the accumulation time by half in comparison with the second embodiment. Specifically, if the accumulation time is set equal to {fraction (1/10000)} second, a difference in the accumulation time in the second embodiment is 6%, whereas the same in the fourth embodiment is 3%, which is smaller than a target difference of 3% with the result that lateral stripes are hardly observed in images to thereby ensure no problem in practical use. 
     In the fourth embodiment, the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels in this order, but it should be noted that an order in which the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels is not to be limited to this. The pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D may be applied in any order. 
     In addition, the method in accordance with the present embodiment can be carried out in an image sensor where read-out pulses are applied pixels in every five or greater pixel lines. 
     [Fifth Embodiment] 
     The above-mentioned third and fourth embodiments make it possible to reduce a difference in the accumulation time among pixel lines in comparison with the first and second embodiments. However, a difference in the accumulation time does still exist, and a problem that a difference in the accumulation time becomes greater in every greater number of pixel lines, remains unsolved. 
     Thus, the inventor tried to minimize a difference in the accumulation time. However, if a difference in the accumulation time were made too small, the reverse-transfer voltage would be raised. To this end, the inventor conducted the experiments many times, and discovered that if a timing at which a pulse rose up was cancelled at least once with a timing at which a pulse was fell down, the reverse-transfer voltage could be lowered. 
     The fifth embodiment explained hereinbelow is based on the above-mentioned discovery. 
     FIG. 18A illustrates photodiodes and a vertical CCD in an image sensor used for carrying out a method in accordance with the fifth embodiment, and FIG.  18 B illustrates waveforms of read-out pulses applied to pixels in the image sensor. In the image sensor, different read-out pulses are applied to pixels in every four pixel lines. As illustrated in FIG. 18A, read-out pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to vertical transfer electrodes  8   b.    
     With reference to FIG. 18B, the pulse φV 3 A is varied from a middle level (for instance, 0V) to a high level (for instance, 15V) at time t 61  in a vertical blanking period. The pulse φV 3 A has a duration of about 2 μs. Hence, the pulse φV 3 A is kept at a high level until time t 63  which is later than time t 61  by about 2 μs. 
     The pulse φV 3 B is varied from a middle level to a high level at time t 62  between time t 61  and time t 64 . The pulse φV 3 B has a duration of about 2 μs. Hence, the pulse φV 3 B is kept at a high level for about 2 μs from t 62 . 
     The pulse φV 3 C is varied from a middle level to a high level at time t 63  between time t 62  and time t 64 . The pulse φV 3 C has a duration of about 2 μs. Hence, the pulse φV 3 C is kept at a high level for about 2 μs from t 63 . 
     The pulse φV 3 A is varied from the high level to the middle level at time t 64 , because electric charges are all read out from the associated photodiode at time t 64 . 
     In synchronization with time t 64  at which the pulse φV 3 A is reduced to the middle level, the pulse φV 3 D is raised from a middle level to a high level. Both a trailing edge of the pulse φV 3 A and a leading edge of the pulse φV 3 D are applied to the image sensing region. Since positive and negative pulse voltages are simultaneously applied to the image-sensing region, influences exerted by those voltages are cancelled with each other. This results in that a ground potential in the p-type well layer is not caused to fluctuate, and an increase in the reverse-transfer voltage can be suppressed. 
     Then, the pulses φV 3 B, φV 3 C and φV 3 D are varied from a high level to a middle level in this order. The pulses φV 3 A to φV 3 D are raised up at the same interval. For instance, an interval between times t 61  and t 62  is equal to an interval between times t 62  and t 68 . In addition, the pulses φV 3 A to φV 3 D has the same duration. 
     In the fifth embodiment, the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels in this order, but it should be noted that an order in which the pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D are applied to pixels is not to be limited to this. The pulses φV 3 A, φV 3 B, φV 3 C and φV 3 D may be applied in any order. 
     In accordance with the fifth embodiment, it is possible to reduce a difference in the accumulation time to 2 μs at greatest. In addition, the same advantages as those of the fifth embodiment can be obtained in an image sensor in which pulses are applied to pixels in every five or greater pixel lines, if all leading edges of pulses are within a term between a leading edge of the first pulse and a trailing edge of the last pulse, and a trailing edge of the first pulse is synchronized with a leading edge of the last pulse. 
     In accordance with the fifth embodiment, a difference in the accumulation time is 2 μs at maximum, which ensures no lateral stripes in images, caused by much difference in the accumulation time among pixel lines. 
     In the above-mentioned embodiments, vertical transfer of electric charges are carried out in four-phase drive, and the vertical transfer electrodes are composed of three-layered polysilicon. However, it should be noted that vertical transfer of electric charges may be carried out in three, five or greater phase drive, and that the vertical transfer electrodes may be composed of four or greater layered polysilicon or other materials. 
     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 
     The entire disclosure of Japanese Patent Application No. 10-188446 filed on Jul. 3, 1998 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.