JPH08307772A - Image sensor - Google Patents

Abstract

PURPOSE: To improve sensitivity by providing a stored time changing means making the time for storing incident light electric charge in capacitance for storage and the time for detecting background light different on appearance. CONSTITUTION: At first, a light emitting part is turned off and only background light is received. Because background light current components are amplified to plural times by a current amplifier 1 at this time, the background light electric charge component of an opposite phase which is stored in capacitance CA for storage becomes plural times as much as the optical electric charge during this period. The magnification is defined as an A. Next, the light emitting part is turned on for the period 8, both of background light and signal light are received and the positive phase storage of the incident light electric charge corresponding to incident light including bald light and signal light is performed for the capacitance CA for storage. If the period 8 of this positive storage is made A times as much as the period 7 of the opposite phase storage, the background light electric charge components stored in the positive phase is cancelled by the background light electric charge component stored in the opposite phase. As a result, only the signal electric charge is stored in the capacitance CA for storage. In this case, a current amplifier 1 is used as a stored time changing means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor capable of performing time / space calculus of a signal at each pixel.
Especially, the sensitivity is improved.

[0002]

2. Description of the Related Art As a conventional image sensor, for example, there is one disclosed by the applicant of the present invention as shown in FIG. 8 (Japanese Patent Application No. 6-258287). Explaining the configuration, each pixel as a photoelectric conversion unit for photoelectrically converting incident light including background light corresponding to noise and signal light in which light emitted from a light emitting unit (light or the like) is reflected by an object to a photoelectric current. PIN
A photodiode PDA is provided, and its output side is connected to the positive electrode and the negative electrode of the storage capacitance CA for temporarily storing charges via the switches SA1 and SA3, respectively. The positive electrode of the storage capacitance CA is grounded via the switch SA4, and the negative electrode is grounded via the switch SA2. Further, the positive electrode of the storage capacitance CA is connected to the positive electrode of the storage capacitance CI for additionally storing the signal charge component via the switch SA5.
The negative electrode of the holding capacitance CI is grounded. Reference numeral TA denotes a source follower type MOS transistor serving as an output unit, a positive electrode of the holding capacitance CI is connected to its gate, and a negative electrode of the storage capacitance CA is a switch SA6.
Connected to its source via. The relatively large gate capacitance of the MOS transistor TA holds the holding capacitance C.
It can be used as I. R is load resistance, SR
Is a switch for resetting the holding capacitance CI, and M
It is used to set the gate voltage of the OS transistor TA to the initial value Vb. Vd is a power supply. MOS
Source potential (= output potential) Vout of the transistor TA
Is equal to the threshold voltage Vt of the potential Vc-TA at the positive electrode of the holding capacitance CI, the MOS transistor TA
Operates as a unity gain buffer.

The operation of the conventional example configured as described above will be described with reference to FIG. First, during the period (1), the light emitting unit is turned off and only the background light is received. At this time, switch SA
1 and SA2 are turned off and SA3 and SA4 are turned on to perform the reverse phase accumulation of the background photocharge component corresponding to the background light in the accumulation capacitance CA. Next, during the period (2), the light emitting section is turned on,
It receives both background light and signal light. At this time, switch S
A3 and SA4 are turned off and the switches SA1 and SA2 are turned on to perform positive-phase accumulation of incident photoelectric charges corresponding to incident light including background light and signal light in the storage capacitance CA. As a result, only the signal charge component obtained by subtracting the background photocharge component from the incident photocharge is stored in the storage capacitance CA per cycle. By repeating the accumulation of the positive and negative phases, it is possible to integrate and amplify only the signal charge component while removing the background photocharge component that becomes noise, and a large output can be obtained. To read out the signal, switch SA at regular intervals
5, SA6 is turned on, and the signal charge component is transferred from the storage capacitance CA to the holding capacitance CI. That is, the switches SA5 and SA6 form a transfer unit that transfers the signal charge component stored in the storage capacitance CA to the holding capacitance CI. Further, in this case, since the background photocharge components are removed by accumulating the positive and negative phases in a short cycle, the background light does not change during one cycle of the positive and negative phase accumulation. That is, the shorter the accumulation cycle of the positive / negative phases, the stronger the fast change of background light. However, the background light does not change rapidly for long-distance shooting, and the cycle need not be so short.

[0004]

However, in such a conventional image sensor, half of the frame period is used for the antiphase accumulation of the background photocharge component corresponding to the background light which becomes noise. Therefore, the time that can be used for accumulating the signal charge component is at most half of the frame period, and the sensitivity becomes lower than the case where the entire period of the frame period can be used for accumulating the signal charge component. In addition, the photodiode PDA of the light receiving portion has parasitic capacitance as shown by the dotted line, and a part of the signal charge accumulated in the storage capacitance CA when switching between positive and negative phases is parasitic. A signal loss occurs due to flow to the capacity. In the conventional image sensor, the switching between the positive and negative phases is frequent, so that the loss of the signal charge is large and the sensitivity becomes lower than that in the case where the number of switching is small. Therefore, the conventional image sensor still needs improvement in these points.

The present invention has been made in view of the above,
An object of the present invention is to provide an image sensor capable of improving sensitivity.

[0006]

In order to solve the above-mentioned problems, the invention according to claim 1 includes, in each pixel, a photoelectric conversion portion for photoelectrically converting incident light into photocurrent, and signal light and background light. An accumulating capacitance for temporarily accumulating the incident photocharge corresponding to the photocurrent due to the incident light and a signal charge component obtained by subtracting the background photocharge component corresponding to the background light from the incident photocharge are accumulated and accumulated. A holding capacitance and a transfer unit that subtracts the background photocharge component from the incident photocharge stored in the storage capacitance and transfers the signal charge component to the holding capacitance are provided. In this image sensor, it is a gist to provide a storage time changing unit that apparently makes the time for storing the incident photocharges in the storage capacitance different from the time for detecting the background light.

According to a second aspect of the present invention, in the image sensor according to the first aspect, the storage time changing means is
The background photocharge component obtained by detecting the background light for a predetermined time is held for a plurality of times of the predetermined time, and the incident photocharge is stored in the storage capacitance a plurality of times at the predetermined time. The background photocharge component held is subtracted from the incident photocharge and transferred to the holding capacitance every time the predetermined time elapses.

According to a third aspect of the present invention, in the image sensor according to the first aspect, the storage time changing means is
Means for charging and holding the background light charge component obtained by detecting the background light for a predetermined time to a voltage multiple times the voltage corresponding to the background light charge component, or background light obtained by detecting the background light for a predetermined time The incident light charge is stored in the storage capacitance for a time period multiple of the predetermined time, and the incident light stored in the storage capacitance is formed by any one of means for amplifying a charge component by a plurality of times. It is configured to subtract either the background photocharge component corresponding to the multiple times voltage charged and held from the electric charge or the background photocharge component amplified to the multiple times and transferred to the holding capacitance. The point is to become.

According to a fourth aspect of the invention, in the image sensor according to the first aspect, the accumulation time changing means is
Means for accumulating the background light charge component obtained by detecting the background light for a plurality of times of the predetermined time or the background light charge obtained by detecting the background light for a plurality of times of the predetermined time The incident photocharge is stored in the storage capacitance for the predetermined time, and the incident photocharge is stored in the storage capacitance. The gist is that the background light component accumulated for a predetermined time or divided into a plurality of times is subtracted and transferred to the holding capacitance.

[0010]

According to the present invention, if the background photocharge component, which is accumulated in the opposite phase corresponding to the background light, is amplified by the accumulating time changing means by a plurality of times, the incident photocharge is accumulated in the storage capacitance. By increasing the positive phase accumulation time to multiple times, the background photocharge component accumulated during the positive phase is canceled out by the background photocharge component accumulated during the opposite phase, and only the signal charge component is retained. It is added and stored in the capacitance. Therefore, the time for accumulating the incident photocharges in the accumulating capacitance can be made longer than the time for detecting the background light, and the sensitivity can be improved. Further, the number of times of switching between the positive and negative phases is reduced, the loss of the signal charge at the time of switching is reduced, and the sensitivity can be further improved. Also, for example, when the signal light intensity is sufficiently strong and a large output can be obtained in a short positive-phase accumulation time, conversely, if the background photocharge component is accumulated for a longer time, the influence of background light noise is reduced and the accuracy is improved. It is possible to detect a good image signal.

In the second aspect of the present invention, the background photocharge component obtained by the antiphase accumulation for a predetermined time is held for a plurality of times of the predetermined time, and the incident photocharge is stored for a plurality of times at a predetermined time. The time to accumulate the incident photocharge in the positive phase by accumulating in the capacitance and subtracting the above-mentioned held background photocharge component from the incident photocharge at every elapse of the predetermined time and transferring to the holding capacitance. Can be made multiple times the background light detection time, and the sensitivity can be improved.

In the invention according to claim 3, the background photocharge component obtained by the reverse phase accumulation is charged and held at a voltage which is a multiple of the voltage corresponding to the background photocharge component, or a plurality of background photocharge components are held. Amplify the incident photocharges in the positive phase in the storage capacitance for the above multiple times and multiply the background photocharge components corresponding to the voltage held by the incident photocharges or the above multiple times. By subtracting the amplified background photocharge component and transferring it to the holding capacitance, the time to accumulate the incident photocharge in the positive phase can be extended to multiple times the background light detection time. It is possible to reduce the number of switching times and improve the sensitivity.

In the invention of claim 4, for example,
If the intensity of the signal light is sufficiently strong and a sufficiently large output can be obtained with a short positive-phase accumulation time, conversely to the above, by increasing the time for accumulating the background photocharge components in the opposite phase, the background light noise The influence is reduced and the image signal can be detected with high accuracy.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. 1 to 3 are views showing a first embodiment of the present invention. It should be noted that in FIG. 1 and the drawings showing the second and subsequent embodiments, the same or equivalent elements as those in FIG. 8 are denoted by the same reference numerals as those described above, and the duplicated description will be omitted. .

First, the structure of the image sensor will be described.
In this embodiment, the current amplifier 1 for amplifying the background photocurrent component corresponding to the background light by a predetermined multiple is connected as a storage time changing means between the switch SA3 and the negative electrode of the storage capacitance CA. ing. FIG. 2 shows a specific configuration example of the current amplifier 1, for example, a MOS transistor 1a.
Then, a current mirror circuit is used which amplifies the current I to a predetermined multiple times AI by driving the (A-1) MOS transistors 1b to 1n.

Next, the operation of the image sensor configured as described above will be described with reference to FIG. First, during the period (1), the light emitting unit is turned off to receive only the background light. At this time, since the background photocurrent component has been amplified by the current amplifier 1 multiple times, the background photocharge component of the opposite phase accumulated in the storage capacitance CA is multiple times the photocharge during this period. The magnification is A. Next, during the period (8), the light emitting portion is turned on to receive both the background light and the signal light and store the storage capacitance C.
Positive-phase accumulation of incident photocharges corresponding to incident light including background light and signal light is performed on A. If the positive-phase accumulation period (8) is multiplied by A times the negative-phase accumulation period (7), the background photocharge components accumulated in the positive phase are offset by the background photocharge components accumulated in the reverse phase. To be done. As a result, only the signal charge component is stored in the storage capacitance CA. In this way, this embodiment operates in the same manner as the conventional example in that the background photocharge component is removed by repeating the accumulation of the positive and negative phases and only the signal charge component is accumulated in the accumulation capacitance CA. . Further, in such an operation of accumulating the signal charge component, the present embodiment can accumulate the signal light for a longer time than the conventional example. Therefore, the sensitivity is improved. Further, since the number of times of switching between the positive and negative phases is reduced, the loss of the signal charge at the time of switching is reduced and the sensitivity is further improved.

FIG. 4 shows a second embodiment of the present invention. First, the configuration will be described. In this embodiment, the photodiode P
The output side of DA is connected to the respective positive electrodes of the positive-phase storage capacitance CB1 and the negative-phase storage capacitance CB2 via switches SB1 and SB2. Both storage capacitance CB
The negative electrodes of 1 and CB2 are grounded. The positive electrode of the positive phase storage capacitance CB1 is connected to the inverting input terminal (−) of the operational amplifier 2 via the switch SB3, and the positive electrode of the negative phase storage capacitance CB2 is connected to the operational amplifier 2 via the switch SB4. It is connected to the non-inverting input terminal (+). The output terminal of the operational amplifier 2 is connected to the negative electrode of the holding electrostatic capacitance CB3, and this connection point becomes the output terminal OUT as it is. The positive electrode of the holding capacitance CB3 is connected to the inverting input terminal (−) of the operational amplifier 2. Switch SB5
SB6 and SB7 are capacitances CB1, CB2 and C, respectively.
It is a reset switch for B3. Further, between the positive electrode of the capacitance CB2 for storing negative phase and the switch SB4,
As the storage time changing means, the reverse phase storage capacitance CB2
A voltage amplifier 3 for amplifying the background photoelectric charge component accumulated in the above is multiplied by a predetermined multiple A is connected.

Next, the operation will be described. First, the light emitting unit is turned off to receive only the background light. At this time, the switch SB1 is turned off and the switch SB2 is turned on so that the reverse phase accumulation capacitance C
In B2, the background light charge component corresponding to the background light is stored in the reverse phase. Then, the light emitting unit is turned on to receive both the background light and the signal light. At this time, the switch SB2 is turned off and the switch S
B1 is turned on to perform positive-phase accumulation of incident photoelectric charges corresponding to incident light including background light and signal light in the positive-phase accumulation capacitance CB1. After that, when the switches SB3 and SB4 are turned on, the signal charge component obtained by subtracting the background photocharge component from the incident photocharge is transferred to the holding capacitance CB3 by the action of the operational amplifier. That is, in the image sensor having this configuration, the background photocharge component is subtracted from the incident photocharge accumulated in the positive phase accumulation capacitance CB1 by the switches SB3, SB4 and the operational amplifier 2 to hold the signal charge component in the capacitance CB3. The transfer means for transferring to is configured. In the subtraction of the background photocharge component from the incident photocharge, the background photocharge component stored in the anti-phase storage capacitance CB2 is amplified A times by the voltage amplifier 3 in the present embodiment. By setting the time for accumulating the incident photocharges in the positive-phase accumulation capacitance CB1 in the positive phase to be a multiple of the antiphase accumulation time A, the background photocharge components accumulated in the positive phase are accumulated in the opposite phase. Cancels out the background photocharged components. As a result, only the signal charge component is added and accumulated in the holding capacitance CB3. Therefore, also in this embodiment, the same operation as that shown in FIG. 4, that is, the signal charge accumulation time can be lengthened, the number of times of switching between positive and negative phases can be reduced, and the sensitivity can be improved.

FIG. 5 shows a third embodiment of the present invention. In the present embodiment, in the image sensor having substantially the same configuration as the image sensor of the second embodiment, the capacitance value of the reverse phase storage capacitance CB4 is the capacitance value of the positive phase storage capacitance CB1. It is set to be 1 / A times smaller than. The storage time changing means is composed of the reverse-phase storage capacitance CB4 whose capacitance value is set small. Due to such a configuration, in the same storage time, the negative phase storage capacitance CB4 is charged with a voltage A times as high as the positive phase storage capacitance CB1. Therefore, also in this embodiment, as in the case of the second embodiment, the time for accumulating the incident photocharges in the positive-phase accumulation capacitance CB1 in the positive phase is lengthened to a multiple A of the reverse-phase accumulation time, and the The number of phase switching can be reduced and the sensitivity can be improved.

FIG. 6 shows a fourth embodiment of the present invention. In the present embodiment, an image sensor having the same configuration as the image sensor of the second embodiment is provided with a reset control circuit 4 for controlling a reset switch SB6 of the anti-phase storage capacitance CB2 as storage time changing means. There is. The background charge component obtained by detecting the background light for a predetermined time by the reset control circuit 4 can be held in the anti-phase storage capacitance CB2 for a plurality of times of the predetermined time. . FIG. 7 shows the operation timing chart. During the period (9), the light emitting unit is turned off to receive only the background light, and the background light charge component is stored in the reverse phase storage capacitance CB.
2 accumulates in reverse phase. Next, during the period (10), the light emitting unit is turned on to receive both the background light and the signal light, and the incident photocharges corresponding to the incident light including the background light and the signal light are stored in the positive-phase storage capacitance CB1. Accumulate in normal phase. When the background photocharge component for the positive phase accumulation is accumulated by the same amount as the background photocharge component during the reverse phase accumulation, the switches SB3 and SB4 are turned on to transfer only the signal charge component to the holding capacitance CB3. Continued period (1
In 1), the light emitting unit is left lit as it is, and the incident photocharge is stored in the positive phase storage capacitance CB1 in the positive phase again. At this time, unlike the prior art, the reset switch SB6 is kept in the cutoff state and the background photoelectric charge component stored in the anti-phase storage capacitance CB2 is not reset. At the end of the period (11), the switches SB3 and SB4 are turned on again, subtraction is performed using the background photocharge component accumulated in the antiphase capacitance CB2, and only the signal charge component is stored in the holding capacitance CB3. Transfer accumulation. Repeat this A times. With such an operation, the time for accumulating the incident photocharges including the signal charge component in the positive-phase accumulating capacitance CB1 in the positive phase can be made multiple times the background light detection time, and the sensitivity can be improved. .

In addition to this, the above-mentioned Japanese Patent Application No. 6-258287.
The present invention can be applied to all the embodiments disclosed in the publication.

Contrary to what has been described above, it is also conceivable to make the reverse phase accumulation time of the background photocharge component longer than the positive phase accumulation time of the signal charge component. For example, when the intensity of the signal light is sufficiently strong and a sufficiently large output can be obtained in a short positive phase accumulation time, if the background photocharge component is accumulated for a long time, the influence of the noise of the background light is reduced and an accurate image signal is obtained. It becomes possible. In this case, for example, FIGS.
The amplification factor A or the multiplication factor A shown in (1) may be set to be smaller than 1, or an amplifier or a timing control circuit may be provided on the positive phase accumulation side contrary to the cases shown in FIGS. 1, 4, 5, and 6.

[0023]

As described above, according to the first aspect of the invention, for example, when the time for accumulating the incident photocharge in the accumulating capacitance is set longer than the time for detecting the background light, the sensitivity is increased. Can be improved. Further, the number of times of switching between the positive and negative phases is reduced, the loss of the signal charge at the time of switching is reduced, and the sensitivity can be further improved. on the other hand,
For example, if the signal light intensity is sufficiently strong and a large output is obtained in a short positive-phase accumulation time, and if the time for detecting the background light is set longer than the above, the effect of the background light noise decreases. An accurate image signal can be obtained.

According to the second aspect of the present invention, it is possible to increase the time for accumulating the incident photocharges in the storage capacitance in the positive phase to be a multiple of the background light detection time and improve the sensitivity.

According to the third aspect of the present invention, it is possible to lengthen the time for accumulating the incident photocharges in the accumulating capacitance in the positive phase to be a plurality of times longer than the background light detection time, and switching between the positive and negative phases. The number of times can be reduced and the sensitivity can be improved.

According to the fourth aspect of the invention, for example, when the intensity of the signal light is sufficiently strong and a sufficiently large output can be obtained in a short positive phase accumulation time, the influence of the noise of the background light is reduced and the accuracy is high. An image signal can be obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an image sensor according to the present invention.

FIG. 2 is a circuit diagram showing an internal configuration of the current amplifier shown in FIG.

FIG. 3 is a diagram for explaining the operation of the first embodiment.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a third embodiment of the present invention.

FIG. 6 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 7 is a diagram for explaining the operation of the fourth embodiment.

FIG. 8 is a circuit diagram showing a conventional image sensor.

FIG. 9 is a diagram for explaining the operation of the conventional example.

[Explanation of symbols]

1 current amplifier (accumulation time changing means) 2 operational amplifier 3 voltage amplifier (accumulation time changing means) 4 reset control circuit (accumulation means changing means) CA storage capacitance CB1 positive phase storage capacitance CB2 reverse phase storage static Capacitance CB3, CI Holding capacitance CB4 Capacitance for reverse phase accumulation with small capacitance value (accumulation time changing means) PDA Photodiode (photoelectric conversion section) SA5, SA6 Switch serving as switch SB3, SB4 Operational amplifier A switch that acts as a transfer means with

Claims (4)

[Claims] 1. A photoelectric conversion unit for photoelectrically converting incident light into a photocurrent in each pixel, and a storage unit for temporarily accumulating an incident photocharge corresponding to the photocurrent by the incident light including signal light and background light. A capacitance, a holding capacitance for additionally storing a signal charge component obtained by subtracting a background photocharge component corresponding to the background light from the incident photocharge, and an incident photocharge stored in the storage capacitance. And a transfer means for transferring the signal charge component to the holding capacitance by subtracting the background photocharge component from the image sensor, and a time for storing the incident photocharge in the storage capacitance. And an accumulation time changing means for apparently differentiating the time for detecting the background light and the time for detecting the background light. 2. The accumulation time changing means comprises means for holding a background photoelectric charge component obtained by detecting the background light for a predetermined time, for a time that is a multiple of the predetermined time,
The incident photocharge is accumulated in the storage capacitance a plurality of times at the predetermined time, and the held background photocharge component is subtracted from the incident photocharge at the elapse of the predetermined time to hold the holding static charge. The image sensor according to claim 1, wherein the image sensor is configured to transfer to an electric capacity. 3. The means for changing the accumulation time, wherein the background light charge component obtained by detecting the background light for a predetermined time is set to a voltage that is a multiple of the voltage corresponding to the background light charge component, and held. The background photocharge component obtained by detecting the background light for a predetermined time is amplified by a plurality of times, and the incident photocharge is stored in the storage capacitance for a plurality of times of the predetermined time. Along with the subtraction of either the background photocharge component corresponding to the multiple voltage that has been charged and held or the background photocharge component that has been amplified multiple times from the incident photocharge stored in the storage capacitance The image sensor according to claim 1, wherein the image sensor is configured to be transferred to a use capacitance. 4. The storage time changing means stores the background light charge component obtained by detecting the background light for a plurality of times of a predetermined time or a plurality of the background light for a predetermined time. The input photocharge is stored in the storage capacitance for the predetermined time and stored in the storage capacitance for a predetermined time. It is configured to subtract the background light component accumulated for the predetermined time or divided by a plurality of times from the incident photocharge accumulated in the capacitance and transfer it to the holding capacitance. The image sensor according to claim 1, wherein: