JP6541513B2 - Imaging device and imaging system - Google Patents

Description

  The present invention relates to an imaging device and an imaging system.

  In recent years, it has been proposed to perform a global electronic shutter in a CMOS image sensor. The imaging devices described in Patent Document 1 and Patent Document 2 have an advantage that the subject image is not distorted even when shooting a fast-moving subject.

Japanese Patent Application Publication No. 2004-111590

  The imaging device described in Patent Document 1 stores, in a photoelectric conversion unit, all the charges generated by photoelectric conversion for obtaining one image or one frame. After that, the charge is transferred from the photoelectric conversion unit to the holding unit at the same time for all the pixels, and photoelectric conversion for obtaining the next image or the next frame is started. Therefore, in order to increase the saturation charge amount of the pixel, it is necessary to secure both the saturation charge amount of the photoelectric conversion portion and the saturation charge amount of the holding portion with substantially the same magnitude. When the saturation charge amount of the photoelectric conversion portion is increased, the area is increased. Therefore, there is a problem that the pixel size is increased.

  As described above, in the prior art, it is difficult to increase the saturation charge amount of the pixel. In view of the above problems, the present invention aims to reduce the size of a pixel while improving the saturation charge amount of the pixel in an imaging device capable of performing a global electronic shutter.

  An embodiment according to one aspect of the present invention includes a photoelectric conversion unit that stores charge generated by incident light, a holding unit that holds the charge, an amplification unit that outputs a signal based on the charge, and the photoelectric unit. A plurality of pixels each having a first transfer switch for transferring the charge from the conversion unit to the holding unit, and a second transfer switch for transferring the charge from the holding unit to the amplification unit; The photoelectric conversion units of the plurality of pixels start to store the charge at a first time, and the first time The first transfer switch of the plurality of pixels is kept off from the second time to the second time, and the first transfer switch of the plurality of pixels is in the first period from the first time to the second time. The photoelectric conversion unit is generated in the first period Load is stored, and at least a part of the second period from the second time to the third time, the holding portion of the plurality of pixels is generated in the first period in the photoelectric conversion portion The charge and the charge generated in the second period in the photoelectric conversion unit are held, and at the third time, the first transfer switch of the plurality of pixels is controlled from on to off, and the photoelectric conversion unit Saturation charge amount A1 of the conversion unit, saturation charge amount A2 of the holding unit defined by the difference between the depletion voltage of the photoelectric conversion unit and the depletion voltage of the holding unit, the first period P1, the second period Period P2 satisfies the predetermined relation.

  An embodiment according to another aspect of the present invention is a photoelectric conversion unit that stores charge generated by incident light, a holding unit that holds the charge, an amplification unit that outputs a signal based on the charge, and the photoelectric unit. A plurality of pixels each having a first transfer switch for transferring the charge from the conversion unit to the holding unit, and a second transfer switch for transferring the charge from the holding unit to the amplification unit; The photoelectric conversion units of the plurality of pixels start to store the charge at a first time, and the first time The first transfer switch of the plurality of pixels is kept off from the second time to the second time, and the first transfer switch of the plurality of pixels is in the first period from the first time to the second time. The photoelectric conversion unit generates electricity during the first period. Are stored, and at least a part of the second period from the second time to the third time, the charges generated in the first period in the photoelectric conversion portion by the holding portions of the plurality of pixels. And the charge generated in the second period in the photoelectric conversion unit is held, and at the third time, the first transfer switches of the plurality of pixels are controlled from on to off, and the photoelectric conversion is performed. Saturation charge amount A1 of the portion, charge amount A2 held by the holding portion within a range in which the output of the amplification portion changes linearly with respect to incident light amount, the first period P1, and the second period P2 A predetermined relationship is satisfied.

  According to the present invention, the size of the pixel can be reduced while improving the saturation charge amount.

It is a figure showing the equivalent circuit of an imaging device. It is a figure which represents the cross-section of an imaging device typically. It is a figure which shows the drive pulse of an imaging device. It is a figure which shows the drive pulse of an imaging device. It is a figure which shows operation | movement of an imaging device typically. It is a figure which represents the cross-section of an imaging device typically. It is a figure showing the equivalent circuit of an imaging device. It is a figure which represents the cross-section of an imaging device typically. It is a figure which shows the drive pulse of an imaging device. It is a figure which represents the cross-section of an imaging device typically. It is a figure which represents the cross-section of an imaging device typically. It is a figure which shows the drive pulse of an imaging device. It is a figure which shows operation | movement of an imaging device typically. It is a figure which shows the potential structure of an imaging device. It is a figure explaining the relationship between the incident light quantity and output of an imaging device. It is a block diagram showing composition of an imaging system.

  One embodiment according to the present invention is an imaging device provided with a plurality of pixels and an output line to which signals from the plurality of pixels are output. Each of the plurality of pixels includes a photoelectric conversion unit, a holding unit that holds a charge, and an amplification unit that outputs a signal based on the charge. Furthermore, a first transfer switch for transferring charge from the photoelectric conversion unit to the holding unit, and a second transfer switch for transferring charge from the holding unit to the amplification unit are arranged. With such a configuration, it is possible to perform an imaging operation in which photoelectric conversion periods coincide with one another among a plurality of pixels, so-called global electronic shutter. The electronic shutter is to electrically control the accumulation of charge generated by incident light.

  In some embodiments according to the present invention, photoelectric conversion units of a plurality of pixels simultaneously start accumulating charge at a first time. The first transfer switch is kept off in the plurality of pixels from the first time to the second time. The charge generated in this period is accumulated in the photoelectric conversion unit. The period from the first time to the second time is the first period.

  The holding units of the plurality of pixels hold a charge in a second period from the second time to the third time. In at least a part of the second period, the holding unit holds the charge generated in the first period and the charge generated in the second period. At a third time, the first transfer switches of the plurality of pixels are simultaneously controlled from on to off.

  In some embodiments according to the present invention, the ratio of the saturated charge amount of the photoelectric conversion unit to the saturated charge amount of the holding unit is the length of the first period with respect to the total length of the first period and the second period. It is almost equal to the ratio of

  By performing driving in accordance with the saturation charge amount of the photoelectric conversion portion and the saturation charge amount of the holding portion, the respective sizes can be reduced. Therefore, with such a configuration, it is possible to perform global electronic shutter while achieving both a small pixel size and a high saturation charge amount.

  The saturated charge amount of the holding portion is defined by the difference between the depletion voltage of the photoelectric conversion portion and the depletion voltage of the holding portion. The depletion voltage of the photoelectric conversion unit is a voltage of a portion having the lowest potential with respect to the signal charge in the region when the entire region where the signal charge of the photoelectric conversion unit is accumulated is depleted. The depletion voltage of the holding portion is the voltage of the portion with the lowest potential for the signal charge in the region when the entire region where the signal charge of the holding portion is held is depleted. According to such a configuration, it is possible to maintain good linearity of the output with respect to the amount of incident light.

  In another embodiment, the ratio of the saturation charge amount of the photoelectric conversion unit to the charge amount held by the holding unit is within a range in which the output of the amplification unit changes linearly with the amount of incident light during the first period. It is approximately equal to the ratio of the length of the first period to the total length of the second period. According to such a configuration, it is possible to maintain good linearity of the output with respect to the amount of incident light.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Of course, the embodiments according to the present invention are not limited to the embodiments described below. For example, an example in which a part of the configuration of any one of the following embodiments is added to another embodiment or is replaced with a configuration of a part of another embodiment is also an embodiment of the present invention. In the following embodiments, the first conductivity type is N-type, and the second conductivity type is P-type. However, the first conductivity type may be P-type, and the second conductivity type may be N-type.

  Example 1 will be described. FIG. 1 shows an equivalent circuit of a pixel of an imaging device. Although four pixels 20 are shown in FIG. 1, the imaging device has more pixels.

  Each pixel 20 includes a photoelectric conversion unit 1, a holding unit 2, an amplification unit 10, a first transfer switch 4, and a second transfer switch 5. Furthermore, the pixel 20 includes a reset transistor 9 and a selection transistor 7.

  The photoelectric conversion unit 1 accumulates the charge generated by the incident light. The first transfer switch 4 transfers the charge of the photoelectric conversion unit 1 to the holding unit 2. The holding unit 2 holds the charge generated by the incident light at a place different from the photoelectric conversion unit 1. The second transfer switch 5 transfers the charge of the holding unit 2 to the input node 3 of the amplification unit 10. The reset transistor 9 resets the voltage of the input node 3 of the amplification unit 10. The selection transistor 7 selects a pixel 20 that outputs a signal to the output line 8. The amplification unit 10 outputs a signal based on the charge generated by the incident light to the output line 8. The amplification unit 10 is, for example, a source follower. The first transfer switch 4 and the second transfer switch 5 are each a MOS transistor.

  A control line Tx1 is connected to the first transfer switch 4. A control line Tx2 is connected to the second transfer switch 5. In this embodiment, a plurality of pixels are arranged in a matrix. A common control line is connected to the pixels included in one row. Therefore, for example, the pixel in the n-th row is described as a control line Tx1 (n).

  With such a configuration, the photoelectric conversion unit 1 can accumulate the charge generated while the holding unit 2 holds the charge. Therefore, an imaging operation in which photoelectric conversion periods coincide with one another among a plurality of pixels, that is, so-called global electronic shutter can be performed.

  FIG. 2 schematically shows the cross-sectional structure of the imaging device. A cross section of one pixel is shown in FIG. Parts having the same functions as those in FIG. 1 are denoted by the same reference numerals. Although FIG. 2 shows a front side illumination type imaging device, it may be a backside illumination type.

  The photoelectric conversion unit 1 has a buried photodiode structure. The photoelectric conversion unit 1 includes an N-type semiconductor region 11 and a P-type semiconductor region 12. The N-type semiconductor region 11 and the P-type semiconductor region 12 constitute a PN junction. The P-type semiconductor region 12 can suppress interface noise.

  The P-type semiconductor region 14 is a well. Under the N-type semiconductor region 11, an N-type semiconductor region 13 is disposed. The impurity concentration of the N-type semiconductor region 13 is lower than the impurity concentration of the N-type semiconductor region 11. As a result, charges generated at deep positions are collected in the N-type semiconductor region. Here, the N-type semiconductor region 13 may be P-type. Below the N-type semiconductor region 13, a P-type semiconductor region 17 serving as a potential barrier to charge is disposed.

  The holding unit 2 includes an N-type semiconductor region 201. The charge serving as a signal is held in the N-type semiconductor region 201. The impurity concentration of the N-type semiconductor region 201 is higher than the impurity concentration of the N-type semiconductor region 11.

  The gate electrode 40 constitutes the gate of the first transfer switch 4. Further, the gate electrode 50 constitutes the gate of the second transfer switch 5. A part of the gate electrode 40 is disposed on the N-type semiconductor region 201 via the gate insulating film. By applying a negative voltage to the gate electrode 40, holes can be induced on the surface of the N-type semiconductor region 201. Thereby, the noise generated at the interface can be suppressed.

  The holding unit 2 is shielded by the light shielding unit 203. The light shielding portion 203 is formed of a metal such as tungsten or aluminum which hardly transmits light to visible light. The color filter 100 and the micro lens 101 are disposed on the opening of the light shielding unit 203.

  The photoelectric conversion unit 1 and the holding unit 2 are disposed on a semiconductor substrate. In this embodiment, the area of the orthographic projection of the photoelectric conversion unit 1 on a plane parallel to the surface of the semiconductor substrate is smaller than the area of the orthographic projection of the holding unit 2 on the plane. According to such a configuration, it is possible to obtain an effect that the saturation charge amount of the pixel can be increased while reducing the noise.

  In order to improve the saturation charge amount of the pixel, it is preferable that the holding unit 2 have a large saturation charge amount. The saturation charge amount of the holding unit 2 can be increased by increasing the impurity concentration of the N-type semiconductor region 201 of the holding unit 2 or increasing the area of the N-type semiconductor region 201 in plan view. However, when the impurity concentration of the N-type semiconductor region 201 is high, leakage current and the like are likely to be large, and noise may be large. Therefore, the saturation charge amount can be increased while suppressing the impurity concentration of the N-type semiconductor region 201 by increasing the area of the N-type semiconductor region 201 in plan view.

  As described above, by increasing the area of the holding unit 2 in plan view, that is, the area of the orthogonal projection of the holding unit 2, it is possible to increase the saturation charge amount of the pixel while reducing noise. Then, the area of the photoelectric conversion unit 1 in plan view tends to be relatively small, and it becomes difficult to increase the saturation charge amount of the photoelectric conversion unit 1. Therefore, even if the saturation charge amount of the photoelectric conversion unit 1 is small, the effect of being able to maintain the saturation charge amount of the pixel becomes more remarkable.

  A driving method of the imaging device of the embodiment will be described. FIG. 3 schematically shows driving pulses used in the present embodiment. FIG. 3 shows drive pulses supplied to the control line Tx1 of the first transfer switch 4 and the control line Tx2 of the second transfer switch 5 in the pixels in the nth to n + 2th rows. When the drive pulse is high, the corresponding transistor or switch is turned on. When the drive pulse is low, the corresponding transistor or switch is turned off. These drive pulses are supplied by a control unit disposed in the imaging device. In the control unit, logic circuits such as shift registers and address decoders are used.

  First, exposure of the previous frame is performed before time T1. Exposure means that charge generated by photoelectric conversion is accumulated or held as a signal. The charge generated before time T1 is held in the holding unit 2. The end of the exposure of the previous frame is to control the first transfer switch 4 of the charge from the photoelectric conversion unit 1 to the holding unit 2 from on to all pixels simultaneously (time T1 in FIG. 1).

  Further, at time T1, all charges of the photoelectric conversion unit 1 are transferred to the holding unit. That is, the photoelectric conversion unit 1 is in the initial state. Therefore, at time T1, the photoelectric conversion units 1 of the pixels in the three rows simultaneously start accumulating charge. As described above, in the present embodiment, when the first transfer switch 4 is turned off, accumulation of charges by the photoelectric conversion unit 1 is started.

  The first transfer switch 4 is maintained off from time T1 to time T2 when the first period elapses. In this embodiment, the first transfer switches 4 of all the pixels are kept off. However, the first transfer switch 4 may be kept off from time T1 to time T2 in at least one pixel.

  The time when the first period has elapsed from time T1 is time T2. That is, the period from time T1 to time T2 is the first period. In the first period, charges generated in the first period are accumulated in the photoelectric conversion unit 1. On the other hand, in the first period, the holding unit 2 holds the charge generated in the previous frame.

  Then, during the first period, the charge of the holding unit 2 is sequentially read out to the input node 3 of the amplification unit 10. Specifically, by turning on the second transfer switch 5 in the nth row, the charge of the holding unit 2 of the pixel in the nth row is transferred to the input node 3. The voltage of the input node 3 changes in accordance with the capacitance of the input node 3 and the amount of transferred charge. The amplifier 10 outputs a signal based on the voltage of the input node to the output line 8. Next, the same operation is performed on the (n + 1) th row of pixels. This operation is performed in each of the pixels in the first row to the pixels in the last row. After the readout in the last pixel, the first transfer switch 4 and the second transfer switch 5 of all the pixels are turned off.

  At time T2, the first transfer switch 4 is turned on. Thereby, the charge of the photoelectric conversion unit 1 is transferred to the holding unit 2. That is, after time T2, the charge generated in the first period is held by the holding unit 2. In this embodiment, the first transfer switches 4 of all the pixels simultaneously transition from off to on. However, as long as the first transfer switches 4 of the plurality of pixels are turned on by time T2, the transition timings may be different from each other. For example, the first transfer switch 4 may be turned on sequentially from the pixel on which the above-described read operation is completed.

  After that, the holding unit holds both the charge generated in the first period and the charge generated in the second period from time T2 to time T3 when the second period elapses. In this embodiment, the first transfer switch 4 is kept on in the second period. Therefore, the charge generated in the second period is immediately transferred to the holding unit 2. Note that the period for transferring the charge from the photoelectric conversion unit 1 to the holding unit 2 can be set freely. The first transfer switch 4 may be off during part of the second period.

  At time T3, the first transfer switches 4 of all the row pixels are simultaneously controlled from on to off. Thus, the exposure period of one frame ends. Thus, the exposure periods coincide with one another among all the pixels. That is, exposure starts at time T1 and ends at time T3 in all the pixels. At time T3, exposure of the next frame is started, and thereafter, the operation from time T1 to time T3 is repeated.

  Next, the operation of reading out a signal from one pixel will be briefly described. FIG. 4 schematically shows drive pulses used in the imaging device. FIG. 4 shows the drive pulse SEL supplied to the selection transistor 7, the drive pulse RES supplied to the reset transistor 9, and the drive pulse Tx2 supplied to the second transfer switch 5. When the drive pulse is high, the corresponding transistor or switch is turned on. When the drive pulse is low, the corresponding transistor or switch is turned off.

  According to the drive pulse shown in FIG. 4, pixel selection, reset, noise signal readout (N reading), charge transfer, and light signal readout (S reading) are performed. The output signal may be AD converted outside the imaging device. AD conversion may be performed inside the imaging device.

  FIG. 5 schematically shows the operation of the imaging device. FIG. 5 shows imaging operations from the nth frame to the n + 1th frame. The operation for the nth frame is shown by a solid line, and the operation for the n + 1th frame is shown by a dotted line.

  FIG. 5 shows an exposure period in each frame, a period in which the photoelectric conversion unit 1 stores electric charge, and a period in which the holding unit 2 holds electric charge. Further, FIG. 5 shows that the reading operation of a plurality of pixels is performed in the first period. The read operation in FIG. 5 is an operation including the transfer of charge by the second transfer switch 5 and the output of the signal by the amplification unit 10 described with reference to FIGS. 3 and 4.

  As shown in FIG. 5, the next exposure can be started as soon as the exposure of one frame is completed. Therefore, the image quality can be improved because the period in which information is lost can be almost eliminated.

  Further, as shown in FIG. 5, the read operation is performed on each of the plurality of pixels in the first period in which the photoelectric conversion unit 1 stores the charge. For this reason, even if the saturation charge amount of the photoelectric conversion unit 1 is small, the saturation charge amount of the pixel can be increased. The saturation charge amount of the pixel is the maximum value of the charge amount that can be treated as a signal among the charges generated in one exposure. The saturation charge amount of the photoelectric conversion unit 1 is the maximum value of the charge amount that can be accumulated by the photoelectric conversion unit 1.

  One exposure period is the sum of the first period and the second period. Here, the charge of the previous frame held in the holding unit 2 is read out in the first period. Therefore, when the first period ends, the holding unit 2 can hold the charge. Therefore, the photoelectric conversion unit 1 only needs to be able to store the charge generated in at least the first period. Usually, the amount of charge generated in the first period is smaller than the amount of charge generated in one exposure period, so the amount of saturation charge of the photoelectric conversion portion 1 can be reduced.

  As shown in FIG. 5, in the present embodiment, the second period in which the holding unit 2 holds the charge is longer than the first period. Therefore, the saturation charge amount of the photoelectric conversion unit 1 can be further reduced. However, the first period may be equal to the second period, or the first period may be longer than the second period.

  FIG. 5 shows an example in which the read operation is performed sequentially from the first row. However, the order of performing the read operation is not limited to this example. During the first period, reading may be performed at least once for each of the pixels constituting one frame. In addition, in at least some pixels, the period from when the holding unit 2 starts holding charges in a certain frame until the holding unit 2 starts holding charges in the next frame is equal to the exposure period. .

It is preferable that the ratio of the first period to the total of the first period and the second period be substantially equal to the ratio of the saturated charge amount of the photoelectric conversion unit 1 to the saturated charge amount of the holding unit 2. More specifically, the saturation charge amount of the photoelectric conversion unit 1 is A ₁ , the saturation charge amount of the holding unit 2 is A ₂ , the first period is P ₁ , and the second period is P ₂ . Then, A ₁ , A ₂ , P ₁ and P ₂ satisfy the relationship of the following equation (1). Here, the first period _{P 1} and the sum of the second period _{P 2} is that of one exposure period _P 1 + _{P 2.}

More preferably, the ratio of the first period to the sum of the first period and the second period is equal to the ratio of the saturation charge amount of the photoelectric conversion unit 1 to the saturation charge amount of the holding unit 2. That is, A ₁ , A ₂ , P ₁ and P ₂ satisfy the relationship of the following formula (2).

  In this embodiment, the ratio of one exposure period to the first period is four. That is, the first period is 1⁄4 of one exposure period. For example, in the case of shooting a moving image at 60 frames per second, the exposure period is 1/60 seconds, and the first period is 1/240 seconds.

  Therefore, the ratio of the saturation charge amount of the photoelectric conversion unit 1 to the saturation charge amount of the holding unit 2 is preferably close to 1⁄4. This is because while the holding unit 2 holds all of the charges generated in one exposure period, the photoelectric conversion unit 1 may hold a 1/4 amount of charge. Specifically, when the saturation charge amount of the holding unit 2 is 40000 electrons, the saturation charge amount of the photoelectric conversion unit 1 is preferably in the range of 5000 electrons or more and 25000 electrons or less. Preferably, the saturation charge amount of the photoelectric conversion unit 1 is 10000 electrons.

  Thus, the sizes of the photoelectric conversion unit 1 and the holding unit 2 can be optimized by setting the ratio of the saturated charge amount represented by the above-mentioned formula (1).

When the holding unit 2 is configured to hold the charge overflowing from the photoelectric conversion unit 1, A ₁ , A ₂ , P ₁ , and P ₂ may satisfy the following formula (3).

  Specifically, when the saturation charge amount of the holding unit 2 is 40000 electrons, the saturation charge amount of the photoelectric conversion unit 1 is in the range of 5000 electrons or more and less than 10000 electrons. With such a configuration, the charge overflowing from the photoelectric conversion unit 1 can be held by the holding unit 2, so that the mixing of charges can be reduced.

  On the other hand, when A1, A2, P1 and P2 satisfy the following equation (4), the saturation charge amount of the photoelectric conversion unit 1 can have a margin, so that the charge overflow can be reduced.

  FIG. 14 shows the potential structure of the pixel 20 of the imaging device. Parts having the same functions as those in FIG. 1 are denoted by the same reference numerals. The potential is the potential for the signal charge. In the present embodiment, the signal charge is an electron. Therefore, the lower the voltage in FIG. Among the photoelectric conversion unit 1, the holding unit 2, and the input node 3, the potential of the input node 3 is deepest or low. Then, the potential of the holding unit 2 is shallower or higher than the potential of the input node 3. The potential of the photoelectric conversion unit 1 is shallower or higher than the potential of the holding unit 2. With such a potential structure, complete transfer from the photoelectric conversion unit 1 to the holding unit 2 becomes possible. That is, when transferring the signal charge from the photoelectric conversion unit 1 to the holding unit 2, all the charges can be transferred to the holding unit 2 without leaving the charge in the photoelectric conversion unit 1. Therefore, noise associated with charge transfer can be reduced. Similarly, complete transfer from the holding unit 2 to the input node 3 is also possible.

  The saturation charge amount A1 of the photoelectric conversion unit 1 is the maximum charge amount that can be accumulated in the photoelectric conversion unit 1. In FIG. 14, the charge Q <b> 1 schematically indicates the maximum charge amount that can be accumulated in the photoelectric conversion unit 1.

  In FIG. 14, the charge Q2 schematically shows the saturation charge amount A2 of the holding unit 2. The saturation charge amount A2 of the holding unit 2 is defined by the difference ΔVdep between the depletion voltage Vdep1 of the photoelectric conversion unit 1 and the depletion voltage Vdep2 of the holding unit 2. In FIG. 14, arrows schematically indicate the difference ΔVdep. The sum of the charge Q2 and the charge Q3 corresponds to the maximum charge amount that can be held by the holding unit 2. The maximum charge amount that can be held by the holding unit 2 is the lower one of the potential when the first transfer switch 4 is off and the potential when the second transfer switch 5 is off, and the depletion voltage Vdep 2 of the holding unit 2. Defined by the difference in

  FIG. 15A shows the relationship between the amount of light incident on the photoelectric conversion unit 1 and the output signal. The horizontal axis indicates the amount of incident light. The vertical axis shows the level of the output signal. When the amount of incident light to the photoelectric conversion unit 1 increases, linearity is maintained between the amount of incident light and the level of the output signal when the amount of incident light is equal to or less than the amount of light L2 shown in FIG. However, if the amount of incident light exceeds the amount of light L2, the linearity may be reduced. For example, the amount of change in the level of the output signal relative to the amount of change in the amount of incident light decreases.

  15B to 15D show the potential structure of the pixel 20 after the charge generated in the photoelectric conversion unit 1 is transferred to the holding unit 2. FIG. 15 (b) schematically shows the charge generated by the incidence of the light of the light amount L1 shown in FIG. 15 (a). FIG. 15 (c) schematically shows the charges generated by the incidence of the light of the light amount L2 shown in FIG. 15 (a). FIG. 15 (d) schematically shows the charge generated by the incidence of the light of the light amount L3 shown in FIG. 15 (a). The amount of charge generated by the incidence of the light of the light amount L1 is smaller than the charge Q2 shown in FIG. The amount of charge generated by the incidence of the light of the light amount L2 is equal to the charge Q2. The amount of charge generated by the incidence of the light of the light amount L3 is larger than the charge Q2.

  As shown in FIGS. 16B and 16C, when the amount of incident light is equal to or less than the amount of light L2, no charge remains in the photoelectric conversion unit 1 after charge transfer. On the other hand, as shown in FIG. 16D, when the amount of incident light is larger than the amount of light L2, charges not transferred to the holding unit 2 may remain in the photoelectric conversion unit 1. This is because there is no difference between the potential of the photoelectric conversion unit 1 and the potential of the holding unit 2 for the charge generated beyond the charge Q2. The charge transferred to the input node 3 of the amplification unit as the signal charge is the charge held by the holding unit 2. Therefore, the charge remaining in the photoelectric conversion unit 1 does not contribute to the output signal. As a result, the level of the output signal decreases, and as shown in FIG. 15A, the slope of the level of the output signal with respect to the amount of incident light decreases.

  Such a decrease in linearity can be suppressed by setting the charge amount to be handled to the charge amount generated at the incident light amount up to the light amount L2, that is, the charge Q2 or less in FIG. Therefore, in the present embodiment, the saturation charge amount A2 of the holding unit 2 is defined by the difference ΔVdep between the depletion voltage Vdep1 of the photoelectric conversion unit 1 and the depletion voltage Vdep2 of the holding unit 2.

  In another viewpoint, the saturation charge amount A2 of the holding unit 2 may be defined as the charge amount held by the holding unit in a range in which the output of the amplification unit changes linearly with the amount of incident light. Such a configuration can suppress the decrease in the linearity of the output signal.

  Note that the imaging device of the present embodiment may have an operation mode in which a rolling shutter is performed. In the rolling shutter operation mode, charge accumulation by the photoelectric conversion units 1 of a plurality of pixels is sequentially started. After that, the first transfer switches 4 of the plurality of pixels are sequentially controlled to be turned on. In addition, it may have an operation mode for performing another type of global electronic shutter. Another type of global electronic shutter is an operation in which the period during which the photoelectric conversion unit 1 stores charges is equal to the exposure period.

  As described above, according to the imaging device of this embodiment, it is possible to perform the global electronic shutter while improving the saturation charge amount.

  Another embodiment will be described. In the present embodiment, the structure of the holder is different from that of the first embodiment. Therefore, only differences from the first embodiment will be described, and the description of the same parts as the first embodiment will be omitted.

  The equivalent circuit of this embodiment is the same as that of the first embodiment. That is, FIG. 1 shows an equivalent circuit of a pixel of the imaging device of this embodiment. The description of FIG.

  The driving method of this embodiment is the same as that of the first embodiment. That is, FIGS. 3 and 4 schematically show driving pulses used in the present embodiment. FIG. 5 schematically shows the operation of the image pickup apparatus of this embodiment. The description of FIGS. 3 to 5 is the same as that of the first embodiment, and thus is omitted.

  FIG. 6 schematically shows the cross-sectional structure of the imaging device. The cross section of one pixel is shown in FIG. Parts having the same functions as those in FIGS. 1 to 5 are denoted by the same reference numerals.

  The holding unit 2 includes an N-type semiconductor region 201 and a P-type semiconductor region 202. The P-type semiconductor region 202 is disposed on the N-type semiconductor region 201. The P-type semiconductor region 202 can suppress interface noise.

  Also, the gate electrode 40 of the first transfer switch 4 does not extend over the N-type semiconductor region 201. As a result, the layout restriction is reduced, and the degree of freedom in design can be increased.

  As described above, according to this embodiment, noise can be reduced in addition to the effects of the first embodiment.

  Another embodiment will be described. The present embodiment differs from the first and second embodiments in that the pixel has an ejection switch. Therefore, only differences from the first embodiment and the second embodiment will be described, and the description of the same portions as the first embodiment or the second embodiment will be omitted.

  FIG. 7 shows an equivalent circuit of a pixel of the imaging device. The same parts as in FIG. 1 are denoted by the same reference numerals. In addition, in order to simplify the drawing, reference numerals of the control line Tx1 and the control line Tx2 are omitted. The control line Tx1 and the control line Tx2 have the same configuration as in the first embodiment.

  Each pixel has a discharge switch 18. The discharge switch 18 discharges the charge of the photoelectric conversion unit 1 to a power supply node such as an overflow drain. A control line OFG is connected to the discharge switch 18. The discharge switch 18 is, for example, a MOS transistor.

  In the first embodiment, charge accumulation by the photoelectric conversion unit 1 is started by controlling the second transfer switch 5 from on to off. In this embodiment, as shown in FIG. 9, it is also possible to control the discharge switch 18 to control the start of exposure. Specifically, charge control by the photoelectric conversion unit 1 is started by controlling the discharge switch 18 from on to off. Thereby, it is possible to freely set the exposure time.

  FIG. 8 schematically shows the cross-sectional structure of the imaging device. Parts having the same functions as those in FIGS. 1 and 2 are denoted by the same reference numerals. FIG. 8 shows an example in which the holding unit 2 includes the P-type semiconductor region 202 as in the second embodiment. As in FIG. 1, the holding portion 2 may not include the P-type semiconductor region 202.

  The discharge switch 18 has an overflow control electrode 16 and an overflow drain 15. The charge of the photoelectric conversion unit 1 is discharged to the overflow drain 15 in accordance with the voltage supplied to the overflow control electrode 16. The overflow drain 15 is supplied with a predetermined voltage. The overflow control electrode 16 and the overflow drain 15 are shielded by the light shielding portion 203.

  A driving method of the imaging device of the embodiment will be described. FIG. 9 schematically shows drive pulses used in the present embodiment. FIG. 9 shows drive pulses supplied to the control line Tx1, the control line Tx2, and the control line OFG of the pixels in the nth to n + 2th rows. The drive pulses supplied to the control line Tx1 and the control line Tx2 are the same as in the first embodiment.

  When the drive pulse is high, the corresponding transistor or switch is turned on. When the drive pulse is low, the corresponding transistor or switch is turned off. These drive pulses are supplied by a control unit disposed in the imaging device. In the control unit, logic circuits such as shift registers and address decoders are used.

  9 (a) and 9 (b) change the timing at which the discharge switch 18 operates. In FIG. 9A, the discharge switch 18 is controlled from on to off at time T4. While the discharge switch 18 is on, the generated charge is discharged. Therefore, according to the drive of FIG. 9A, the exposure period is from time T4 to time T3. In FIG. 9B, the discharge switch 18 is controlled from on to off at time T4 at time T5. Therefore, according to the drive of FIG. 9B, the exposure period is from time T5 to time T3.

  According to this embodiment, the driving method can be changed according to the brightness of the subject. For example, normally, the drive pulse of FIG. 3 is used, the drive pulse of FIG. 9 (a) is used when bright, and the drive pulse of FIG. 9 (b) is used when brighter.

  In FIG. 9A, accumulation of charges by the photoelectric conversion unit 1 is started at time T4. Then, the discharge switch 18 is maintained off from time T4 to time T3. Also, the read operation is performed based on the drive pulse shown in FIG.

  As described above, according to the present embodiment, in addition to the effects of the first embodiment, the exposure period can be set freely.

  Another embodiment will be described. The present embodiment is different from Embodiments 1 to 3 in that a waveguide for guiding light to the photoelectric conversion portion is provided. Therefore, only the points different from the first to third embodiments will be described, and the description of the same parts as any of the first to third embodiments will be omitted.

  The equivalent circuit of this embodiment is the same as that of the first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7 show equivalent circuits of pixels of the imaging device of the present embodiment. The descriptions of FIGS. 1 and 7 are the same as in the first and third embodiments, respectively, and are thus omitted.

  The driving method of this embodiment is the same as that of the first embodiment or the third embodiment. That is, when there is no discharge switch, the drive pulses shown in FIGS. 3 and 4 are used. If the pixel has an eject switch, the drive pulses shown in FIGS. 9 and 4 are used. FIG. 5 schematically shows the operation of the image pickup apparatus of this embodiment. Descriptions of FIGS. 3 to 5 and FIG. 9 are the same as in the first embodiment and the third embodiment, respectively, and thus are omitted.

  FIG. 10 schematically shows the cross-sectional structure of the imaging device. The same parts as in FIG. 1, FIG. 2, FIG. 6, FIG. 7, or FIG. FIG. 10 shows an example in which the holding unit 2 includes the P-type semiconductor region 202 as in the second embodiment and the pixel includes the discharge switch 18 as in the third embodiment. However, the P-type semiconductor region 202 and the discharge switch 18 may be omitted.

  In the present embodiment, the waveguide 301 is disposed corresponding to the photoelectric conversion unit 1. The waveguide 301 guides the incident light to the photoelectric conversion unit 1. Thereby, the sensitivity can be improved. In particular, the reduction in sensitivity to obliquely incident light can be reduced.

  For the waveguide 301, a known structure is used. In the present embodiment, the waveguide 301 is made of a material having a refractive index higher than that of the surrounding insulating film. For example, an interlayer insulating film formed of a silicon oxide film is used as the surrounding insulating film, and a silicon nitride film is used for the waveguide 301. Alternatively, a reflective layer is provided around the waveguide 301. The waveguide 301 may be disposed corresponding to the photoelectric conversion units 1 of all the pixels, or may be disposed only in the photoelectric conversion units 1 of some of the pixels.

  An intralayer lens 302 may be disposed between the color filter 100 and the waveguide 301. The in-layer lens 302 condenses the light passing through the color filter 100 in the waveguide 301. The in-layer lens 302 can improve the sensitivity. In particular, the reduction in sensitivity to obliquely incident light can be reduced.

  As described above, according to this embodiment, in addition to the effects of the first embodiment, the sensitivity can be improved. In particular, when the area of the photoelectric conversion unit 1 in planar view is reduced to increase the area of the holding unit 2 in planar view, the effect of improving the sensitivity is remarkable.

  Another embodiment will be described. The present embodiment differs from the first to fourth embodiments in the structure of the holder. Therefore, only differences from the first to fourth embodiments will be described, and the description of the same parts as any of the first to fourth embodiments will be omitted.

  The equivalent circuit of this embodiment is the same as that of the first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7 show equivalent circuits of pixels of the imaging device of the present embodiment. The descriptions of FIGS. 1 and 7 are the same as in the first and third embodiments, respectively, and are thus omitted.

  The driving method of this embodiment is the same as that of the first embodiment or the third embodiment. That is, when there is no discharge switch, the drive pulses shown in FIGS. 3 and 4 are used. When the discharge switch is provided, the drive pulses shown in FIGS. 9 and 4 are used. FIG. 5 schematically shows the operation of the image pickup apparatus of this embodiment. Descriptions of FIGS. 3 to 5 and FIG. 9 are the same as in the first embodiment and the third embodiment, respectively, and thus are omitted.

  FIG. 11 schematically shows the cross-sectional structure of the imaging device. The same parts as those in FIG. 1, FIG. 2, FIG. 6, FIG. 7, FIG. 8, or FIG. FIG. 11 shows an example in which the holding unit 2 includes the P-type semiconductor region 202 as in the second embodiment and the pixel includes the discharge switch 18 as in the third embodiment. However, the P-type semiconductor region 202 and the discharge switch 18 may be omitted. Further, FIG. 11 shows an example in which the waveguide 301 and the intralayer lens 302 are disposed. However, the waveguide 301 and the intralayer lens 302 may be omitted.

  In this embodiment, the P-type semiconductor region 303 and the P-type semiconductor region 304 are disposed under the N-type semiconductor region 201 which is included in the holding unit 2 and holds the charge. The P-type semiconductor region 304 is disposed below the P-type semiconductor region 303. The impurity concentration of the P-type semiconductor region 303 is higher than the impurity concentration of the P-type semiconductor region 304. Such a configuration can prevent charges in the deep portion of the substrate from intruding into the N-type semiconductor region 201. As a result, noise can be reduced.

  Further, in the present embodiment, the P-type semiconductor region 304 extends until reaching the P-type semiconductor region 17. With such a configuration, it is possible to reduce color mixture of charges between pixels.

  As described above, according to this embodiment, noise can be reduced in addition to the effects of the first embodiment.

  Another embodiment will be described. The present embodiment differs from the first to fifth embodiments in the driving method. Therefore, only differences from the first to fifth embodiments will be described, and description of the same parts as any of the first to fifth embodiments will be omitted.

  The equivalent circuit of this embodiment is the same as that of the first embodiment or the third embodiment. That is, FIG. 1 and FIG. 7 show equivalent circuits of pixels of the imaging device of the present embodiment. Descriptions of FIG. 1 and FIG. 7 are the same as in the first embodiment and the third embodiment, respectively, and thus are omitted.

  The cross-sectional structure of the pixel of this embodiment is the same as that of the first to fifth embodiments. That is, FIG. 2, FIG. 6, FIG. 8, FIG. 10, and FIG. 11 schematically show the cross-sectional structure of the pixel of this embodiment.

  A driving method of the imaging device of the embodiment will be described. FIG. 12 schematically shows drive pulses used in the present embodiment. FIG. 12 shows drive pulses supplied to the control line Tx1, the control line Tx2, and the control line OFG of the pixels in the nth to n + 2th rows. The drive pulses supplied to the control line Tx1, the control line Tx2, and the control line OFG are the same as in the first embodiment or the third embodiment. If the pixel does not have the discharge switch 18, the drive pulse supplied to the control line OFG is unnecessary.

  When the drive pulse is high, the corresponding transistor or switch is turned on. When the drive pulse is low, the corresponding transistor or switch is turned off. These drive pulses are supplied by a control unit disposed in the imaging device. In the control unit, logic circuits such as shift registers and address decoders are used.

  In the present embodiment, the first transfer switch 4 is turned off in part of the second period. Specifically, at time T6, the first transfer switch 4 is controlled from on to off. Thereafter, at time T7, the first transfer switch 4 is controlled from off to on. With such a configuration, the period during which the first transfer switch 4 is on can be shortened. As a result, noise generated in the first transfer switch 4 can be reduced.

  In the present embodiment, at time T8, the first transfer switch 4 is controlled from off to on again. As described above, the control from the off to the on of the first transfer switch 4 is performed multiple times in the second period. Such a configuration can further reduce noise.

  Further, the number of times of control from off to on is preferably equal to or larger than the ratio of the saturation charge amount of the holding unit 2 to the saturation charge amount of the photoelectric conversion unit 1. In the present embodiment, the ratio of the saturation charge amount of the holding unit 2 to the saturation charge amount of the photoelectric conversion unit 1 is four. Therefore, control from the off to on of the first transfer switch 4 is performed four times in the second period.

  As described above, according to this embodiment, noise can be reduced in addition to the effects of the first embodiment.

  Another embodiment will be described. In the present embodiment, the driving method is different from that of the first embodiment. Therefore, only differences from the first embodiment will be described, and the description of the same parts as the first embodiment will be omitted.

  The equivalent circuit of this embodiment is the same as that of the first embodiment. That is, FIG. 1 shows an equivalent circuit of a pixel of the imaging device of this embodiment. The description of FIG.

  The sectional structure of the pixel of this embodiment is the same as that of the first embodiment. That is, FIG. 2 schematically shows the cross-sectional structure of the pixel of this embodiment. The description of FIG.

  FIG. 13 schematically shows the operation of the imaging device of this embodiment. FIG. 13 shows imaging operations from the nth frame to the n + 1th frame. The operation for the nth frame is shown by a solid line, and the operation for the n + 1th frame is shown by a dotted line. FIG. 13 shows an exposure period in each frame, a period in which the photoelectric conversion unit 1 stores electric charge, and a period in which the holding unit 2 holds electric charge. Further, FIG. 13 shows the read operation. The read operation in FIG. 13 is an operation including the transfer of charge by the second transfer switch 5 and the output of a signal by the amplification unit 10 described with reference to FIGS. 3 and 4. In FIG. 13, the first period is equal to the second period.

  As shown in FIG. 13, the read operation of the plurality of pixels is performed after the corresponding exposure period ends, in other words, after the first period and the second period have elapsed. Charge is not accumulated during the read operation. In this embodiment, the exposure period is the sum of the first period and the second period.

For such an operation, the ratio of the saturation charge amount A ₁ of the photoelectric conversion unit 1 to the saturated charge amount A ₂ of the holder 2, the first to the sum of the first period P ₁ and the second period P ₂ it is preferably substantially equal to the ratio of the period P ₁ of the. That is, it is preferable that A1, A2, P1, and P2 satisfy Formula (5).

  For example, in the present embodiment, a period in which the photoelectric conversion unit 1 stores charges, that is, a first period is half of one exposure period. Then, the saturation charge amount of the holding unit 2 may be about twice the saturation charge amount of the photoelectric conversion unit 1. This is because the charge overflows in the photoelectric conversion unit 1 even if the holding unit 2 has a saturation charge amount larger than that. Therefore, the size of the photoelectric conversion part 1 and the holding part 2 can be optimized by setting it as ratio of the amount of saturation charges represented by the above-mentioned Formula (4).

  An embodiment of an imaging system according to the present invention will be described. Examples of imaging systems include digital still cameras, digital camcorders, copiers, fax machines, mobile phones, in-vehicle cameras, observation satellites, and the like. A camera module including an optical system such as a lens and an imaging device is also included in the imaging system. FIG. 16 shows a block diagram of a digital still camera as an example of an imaging system.

  In FIG. 16, reference numeral 1001 denotes a barrier for protecting the lens, 1002 denotes a lens for forming an optical image of an object on an imaging device 1004, and 1003 denotes a stop for varying the amount of light passing through the lens 1002. An image pickup apparatus 1004 described in each of the above-described embodiments converts an optical image formed by the lens 1002 as image data. Here, it is assumed that an AD conversion unit is formed on the semiconductor substrate of the imaging device 1004. A signal processing unit 1007 compresses various corrections and data in the imaging data output from the imaging device 1004. Referring to FIG. 16, a timing generation unit 1008 outputs various timing signals to the image pickup apparatus 1004 and the signal processing unit 1007, and a general control unit 1009 controls the entire digital still camera. Reference numeral 1010 denotes a frame memory unit for temporarily storing image data, 1011 an interface unit for recording or reading out on a recording medium, and 1012 a detachable semiconductor memory or the like for recording or reading out imaging data It is a recording medium. An interface unit 1013 communicates with an external computer or the like. Here, timing signals and the like may be input from the outside of the imaging system, and the imaging system may include at least the imaging device 1004 and a signal processing unit 1007 that processes the imaging signal output from the imaging device 1004.

  In the present embodiment, the configuration in which the imaging device 1004 and the AD conversion unit are formed on the same semiconductor substrate has been described. However, the imaging device 1004 and the AD conversion unit may be provided on different semiconductor substrates. In addition, the imaging device 1004 and the signal processing unit 1007 may be formed on the same semiconductor substrate.

REFERENCE SIGNS LIST 1 photoelectric conversion unit 2 holding unit 4 first transfer switch 5 second transfer switch 8 output line 10 amplification unit

Claims (19)

A photoelectric conversion unit for accumulating charge generated by incident light, a holding unit for holding the charge, an amplification unit for outputting a signal based on the charge, and transferring the charge from the photoelectric conversion unit to the holding unit A plurality of pixels each having a first transfer switch, and a second transfer switch for transferring the charge from the holding unit to the amplification unit;
An output line through which a signal from the amplification unit of the plurality of pixels is output;
At a first time, the photoelectric conversion units of the plurality of pixels start to store the charge;
The first transfer switch of the plurality of pixels is kept off from the first time to the second time, and in a first period from the first time to the second time, The photoelectric conversion units of the plurality of pixels accumulate charges generated in the first period;
The charge generated in the first period in the photoelectric conversion portion by the holding portion of the plurality of pixels in at least a part of the second period from the second time to the third time, and Holding the charge generated in the second period in the photoelectric conversion unit,
At the third time, the first transfer switches of the plurality of pixels are controlled from on to off,
A saturated charge amount A1 of the photoelectric conversion portion, a saturated charge amount A2 of the holding portion defined by a difference between a depletion voltage of the photoelectric conversion portion and a depletion voltage of the holding portion, the first period P1, The second period P2 satisfies the following relationship:

An imaging device characterized by A photoelectric conversion unit for accumulating charge generated by incident light, a holding unit for holding the charge, an amplification unit for outputting a signal based on the charge, and transferring the charge from the photoelectric conversion unit to the holding unit A plurality of pixels each having a first transfer switch, and a second transfer switch for transferring the charge from the holding unit to the amplification unit;
An output line through which a signal from the amplification unit of the plurality of pixels is output;
At a first time, the photoelectric conversion units of the plurality of pixels start to store the charge;
The first transfer switch of the plurality of pixels is kept off from the first time to the second time, and in a first period from the first time to the second time, The photoelectric conversion units of the plurality of pixels accumulate charges generated in the first period;
The charge generated in the first period in the photoelectric conversion portion by the holding portion of the plurality of pixels in at least a part of the second period from the second time to the third time, and Holding the charge generated in the second period in the photoelectric conversion unit,
At the third time, the first transfer switches of the plurality of pixels are controlled from on to off,
The saturated charge amount A1 of the photoelectric conversion unit, the charge amount A2 held by the holding unit in a range where the output of the amplification unit changes linearly with the amount of incident light, the first period P1, the second period P2 satisfies the following relationship,

An imaging device characterized by Each of the plurality of pixels has a discharge switch that discharges the charge of the photoelectric conversion unit,
The discharge switch of the at least one pixel is kept off from the first time to the second time,
The imaging device according to claim 1 or 2, wherein Start the charge accumulation by controlling the discharge switch from on to off,
The imaging device according to claim 3, characterized in that: Starting the charge accumulation by controlling the first transfer switch from on to off,
The imaging device according to any one of claims 1 to 3, characterized in that: The first transfer switch of the plurality of pixels is controlled to be turned on by the second time at the latest,
The first transfer switch is controlled to be off in a part of the second period,
The imaging device according to any one of claims 1 to 5, characterized in that: In the second period, the first transfer switch is controlled from off to on a plurality of times,
The imaging device according to claim 6, The plurality of times is larger than the ratio of the saturation charge amount of the holding unit to the saturation charge amount of the photoelectric conversion unit,
The imaging device according to claim 7, characterized in that: The first switches of the plurality of pixels are kept on from the second time to the third time,
The imaging device according to any one of claims 1 to 5, characterized in that: The holding unit includes a first semiconductor region of a first conductivity type that holds the charge.
The imaging device according to any one of claims 1 to 9, characterized in that: The holding portion includes a second semiconductor region of a second conductivity type disposed on the first semiconductor region.
The imaging device according to claim 10, characterized in that: The holding portion includes a third semiconductor region of a second conductivity type disposed below the first semiconductor region, and a fourth conductivity type of a second conductivity type disposed below the third semiconductor region. Including semiconductor regions,
The impurity concentration of the third semiconductor region is higher than the impurity concentration of the fourth semiconductor region,
The imaging device according to claim 10 or 11, characterized in that: And a waveguide disposed corresponding to the photoelectric conversion unit of each of the plurality of pixels.
The imaging device according to any one of claims 1 to 12, characterized in that: In the first period, the second transfer switches of the plurality of pixels are sequentially turned on, and the amplification units of the plurality of pixels sequentially output the signal to the output line.
The imaging device according to any one of claims 1 to 13, characterized in that: Controlling the first switch of the plurality of pixels from off to on at the second time;
The imaging device according to any one of claims 1 to 14, characterized in that: In the first period, the first transfer switch is controlled from off to on sequentially from the pixel at which the output of the signal by the amplification unit ends.
The imaging device according to claim 14, characterized in that: The saturated charge amount A1 of the photoelectric conversion unit, the saturated charge amount A2 of the holding unit, the first period P1, and the second period P2 satisfy the following relationship:

The imaging device according to claim 1, The saturation charge amount A1, the charge amount A2, the first period P1, and the second period P2 satisfy the following relationship:

The imaging device according to claim 2, characterized in that: An imaging apparatus according to any one of claims 1 to 18.
A signal processing device that processes a signal from the imaging device;
An imaging system characterized by