JP2001203941A - Solid-state image pickup element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid-state image pickup element which has a wide light receiving sensitivity range, a high contrast detecting function, and high environment adapting capability, is highly integrated, and actualizes low power consumption. SOLUTION: The solid-state image pickup element is constituted by connecting pixel circuits equipped with photodetectors, respectively, and each pixel circuit computes the mean value of the quantity of incident light detected by itself and the quantity of incident light detected by a pixel circuit adjacent to this pixel circuit and varies its photodetection sensitivity range according to the computed mean value to detect the quantity of incident light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor imaging device which realizes a wide light receiving sensitivity range comparable to the retina of a living body and a high contrast detection function. In particular, the present invention relates to an imaging device having high environmental adaptability. The present invention relates to a solid-state imaging device that can be used for an outdoor monitoring camera, a vehicle-mounted camera, and the like used in various environments.

[0002]

2. Description of the Related Art In the human visual system, each photoreceptor cell
It has the function of outputting the difference between the amount of light received by itself and the average value of the amount of light received by the surrounding photoreceptors as a signal for the amount of light received. With this function, each photoreceptor cell shifts its own light receiving sensitivity range according to the average of the light intensity incident on its periphery. That is, when subjects having extremely different luminances are within the same field of view, the photoreceptor cells that see a bright subject shift their light receiving sensitivity range to the bright side, and the photoreceptors that see the dark subject shift to the dark side. With such a function of shifting the light receiving sensitivity range, the width of the light receiving sensitivity range of each photoreceptor cell is constant, but the human visual system as a whole realizes a light receiving sensitivity range wider than the light receiving sensitivity of individual photoreceptors. (FIG. 7).

[0003] Further, since the difference from the output of an adjacent photoreceptor cell is output, the human visual system reacts strongly to a portion where luminance changes. For this reason, the human visual system automatically extracts the outline of the subject and gives a high contrast to the viewed object for recognition. On the other hand, with solid-state imaging devices such as CCD and CMOS imagers, when shooting an object with extremely different brightness within the same field of view, the range of the brightness distribution of that object can be sufficiently recognized by human vision. Even within the image, there is often seen an image in which a bright subject causes halation or a dark subject becomes black.

A solid-state imaging device such as a CCD or a CMOS imager has a structure in which a plurality of functional units (pixels) for converting light incident on the device into an electric signal corresponding to light intensity and exposure time are arranged. . Generally, each pixel has the same light receiving sensitivity range, and the light receiving sensitivity range is fixed.
The light receiving sensitivity range of the element is also equal to the light receiving sensitivity range of each pixel.
Therefore, when photographing a target having a luminance spread exceeding the light receiving sensitivity range of each pixel, the above-described phenomena such as halation of a bright subject and darkening of a dark subject occur. Further, in an image captured by a solid-state imaging device, the outline of an object is less clear than when a person views the same object.

For this reason, it is often necessary to perform an outline emphasis process to clarify the contrast before displaying the image on a cathode ray tube or the like. The conventional solid-state imaging device is significantly inferior to the human visual system in the light receiving sensitivity range and the contrast detection function. A substitute for human vision, such as a surveillance camera, or recording an image and then viewing it
Considering that such a usage is a general usage of the solid-state imaging device, it is clear that the performance of the conventional solid-state imaging device is insufficient in terms of the light receiving sensitivity range and the contrast detection function.

[0006] From such a background, in order to realize a wide light receiving sensitivity range, research has been made to widen the light receiving sensitivity range of each pixel. As an example, there is a method of expanding the dynamic range by superimposing images taken with different exposure times (Reference: Hirohito Kobuchi, "Wide Dynamic Range Imaging Technology", Proceedings of the 7th Image Input Technology Symposium) , pp.85-92). In this method, when photographing an object having an extremely different luminance in the same field of view, an image photographed with a long exposure time for photographing a dark object and a short exposure time for photographing a bright object are taken. By superimposing the photographed image on the image, both bright and dark subjects are represented on the image without halation or blackout (first conventional example).

[0007] In addition, studies have been made to expand the light receiving sensitivity width of each pixel itself by using nonlinear input / output characteristics in a sub-threshold region of a transistor (Reference: SGChamberlain, and JPY Lee, "A Novel Wide").
Dynamic Range Silicon Photodetector and Linear Ima
ging Array ", IEEE Trans on ED, vol.ED-32, No.2, p
p.175-182, Feb., 1984 ... second conventional example).

Further, a solid-state image pickup device, such as a human retina, in which each pixel has a function of shifting the light receiving sensitivity range in accordance with the output of a peripheral pixel has already been prototyped by C. Mead (third). Conventional example). The pixel circuit of this element is shown in FIG.
(References: C. Mead, "Adaptive Retina", Analog
VLSI Implementations of Neural Systems, 1989, pp.2
39-246). In this element, peripheral pixel outputs are averaged via a resistor network, and the difference between the average of the peripheral pixel outputs and its own output is calculated using a differential amplifier. In this element, like the visual cells of the human visual system, each pixel shifts the light receiving sensitivity range to realize a wide light receiving sensitivity range as the whole element. According to this method,
Since the light receiving sensitivity range of each pixel is not widened, the problem that the contrast of the entire screen is reduced in the display system due to the widening of the light receiving sensitivity range as an element does not occur.

[0009]

However, the first conventional example of expanding the dynamic range of each pixel and the second conventional example of expanding the light-receiving sensitivity range involve the contrast of the entire screen when displaying a captured image. There is a problem that becomes weak. Therefore, a method has been proposed in which an image is divided into several regions and a large number of display system gradations are given to the luminance range concentrated in each region (references: Atsushi Morimura, Takeo Azuma, Kenya Uomori) , "Wide Dynamic Range Image Synthesis Technology", Journal of the Institute of Image Information and Television Engineers vol.51, No.2, pp228-23
2), this method has a problem that a complicated image post-processing is required. Further, the third conventional example has a practical problem that a circuit scale is large and power consumption is large because an operation amplifier is required for each pixel.

Therefore, the present invention has a wide light receiving sensitivity range comparable to the retina of a living body, a high contrast detection function, and a high environmental adaptability that can be used in various environments, and realizes high integration and low power consumption. It is an object of the present invention to provide a solid-state imaging device capable of performing the following.

[0011]

In order to achieve the above object, a solid-state imaging device according to the present invention is a solid-state imaging device in which a plurality of pixel circuits each having a photodetector are connected. Calculates the average value of the incident light amount detected by the pixel circuit and the incident light amount detected by the pixel circuit adjacent to the pixel circuit, and changes the light receiving sensitivity range based on the calculated average value. And detecting the amount of incident light. This makes it possible to set the optimum light receiving sensitivity range according to the amount of incident light and detect the amount of incident light. Further, with such a configuration, a mechanism for shifting the light receiving sensitivity range in accordance with the intensity of incident light on each pixel and peripheral pixels can be configured with a simple circuit, and the power consumption can be reduced.

Further, in the solid-state imaging device according to the present invention, the circuit for calculating the average potential includes a capacitance element having a capacitance sufficiently smaller than the capacitance of the photodetector, and one end of the capacitance element. A simple configuration including a first switch connected between one end of the photodetector and a second switch connected between one end of the capacitance element and one end of a capacitance element of an adjacent pixel circuit. can do.

Still further, in the solid-state imaging device according to the present invention, the pixel circuit compares an average potential with a reference value by setting an initial setting switch for setting a potential at one end of the photodetector to an initial potential. When the average potential is equal to or less than the reference value, a comparator that controls the initial setting switch so as to set the potential at one end of the photodetector to an initial potential can be easily set by a simple circuit. can do.

Further, in the solid-state imaging device according to the present invention, it is preferable that the reference value is set to の of the initial potential, so that the effective accumulation time can be set in a preferable range.

In the solid-state imaging device according to the present invention, in order to simplify the control circuit, the average potential is calculated, and the interval at which the potential at one end of the photodetector is set to the initial potential is set to one of the maximum accumulation time. It is preferable to decrease by a power of / 2.

Further, in the solid-state imaging device according to the present invention, in order to simplify the configuration, it is preferable that the comparator includes an inverter composed of a MOS transistor.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. The solid-state imaging device (semiconductor imaging device) according to the first embodiment of the present invention includes a plurality of pixel circuits 50 described in detail below. FIG. 1 is a circuit diagram conceptually showing a configuration of a pixel circuit of the solid-state imaging device according to the present embodiment. 1 includes a photodetector PD1, an amplification transistor Tr1, reset switches SW1 and SW2, a reading transistor Tr2, an electronic shutter switch SW3, a capacitance element C1, and a comparator Comp1. Here, the photodetector PD1 is a photodetector composed of a photodiode, and generates a photocurrent according to the amount of received light. The switch S
Wn1 to SWn4 are switches connected between adjacent pixel circuits.

More specifically, in the pixel circuit 50 of FIG.
The photodetector PD1 has one end grounded to the ground GND, and the other end node 1 connected to the reset switch SW.
1, connected to a bias terminal for supplying a power supply voltage Vdd via SW2 so as to be reverse biased. The node 1 is connected to the gate of the transistor for amplification Tr1, and the transistor for readout Tr2 is connected in series to the transistor for amplification Tr1. The node 1 connected as described above is initialized to a reset potential (initial potential) Vsat when the reset switch SW1 is turned on. Node 1 is connected to an electronic shutter switch SW
3 is connected to the node 2 to which one end of the capacitance element C1 is connected. Note that the capacitance element C1 is connected between the node 2 and the ground GND. Further, the node 2 is connected to one input of the comparator 1, and the output node 3 of the comparator Comp1 is connected to the reset switch SW2.
Are connected as control signals. As shown in FIG. 2, the solid-state imaging device according to the first embodiment includes a plurality of pixel circuits 50 each configured as described above, which are arranged in a matrix in a matrix corresponding to each of the switches SWni (i = 1, 2,. It is configured by being connected by 3,4). SWni (i = 1, 2, 3, 4) is connected between the nodes 2 of the adjacent pixel circuits.

FIG. 3 is a circuit diagram of a pixel circuit which is an example in which the pixel circuit 50 shown in FIG. 1 is actually constituted by an integrated circuit. In the pixel circuit of FIG. 3, each of the switches SW1, SW
2 and SW3 are NMOS transistors Tr3 and
Tr4 and Tr5. The switches SWn1 to SWn4 connecting the respective pixel circuits are respectively
It comprises NMOS transistors Tr9 to Tr12. Further, in each pixel circuit, an output signal line 21 connected to one end of the reading transistor Tr2, a reset control signal line 22 for controlling the transistor Tr3,
Shutter control signal line 2 for controlling transistor Tr5
3, a read control signal line 24 for controlling the read transistor Tr2, and an average value calculation control signal line 25 for controlling the transistors Tr9 to Tr12.

Next, the operation of the pixel circuit according to the first embodiment will be described. In the pixel circuit 50 of FIG. 1, when light enters the reverse-biased photodetector PD1, a photocurrent corresponding to the amount of incident light flows into the photodetector PD1, and the potential V1 of the node 1 changes from Vsat. The rate at which the potential V1 of the node 1 changes from Vsat is proportional to the amount of incident light. By detecting the rate of change, the amount of incident light can be detected.

In the pixel circuit 50 of the first embodiment, when the potential V1 of the node 1 drops to the ground GND (0) potential, the gate voltage of the amplifying transistor Tr1 becomes 0.
Therefore, the image signal is saturated on the output signal line 21 (fixed to the maximum value). Therefore, when the amount of incident light is strong, the decay time T1 during which the potential of the potential V1 of the node 1 decreases from Vsat to the ground GND (0) potential becomes shorter than the accumulation time T. Becomes saturated. In order to avoid this saturated state, in the image circuit 50 of the first embodiment, the saturated pixel circuit is repeatedly reset as described below, and the saturated state is avoided at the time when a predetermined accumulation time T elapses. The image signal is output immediately after the predetermined accumulation time T has elapsed.

More specifically, the period for repeatedly resetting the pixel circuit 50 is sequentially reduced so that a saturation state can be avoided when a predetermined accumulation time T elapses. That is, as shown in the timing chart of FIG. 4, first, in the initial state (t = 0) of the frame, the node 1 of the photodetector PD1 is switched to the switch S.
Turning on W1 resets it to the initial value Vsat.
In FIG. 4, the accumulation time T is set to 32 milliseconds. Then, after a lapse of T / 2 milliseconds (16 milliseconds), the switch SW3 is turned on by the shutter control signal (VSH) to make the potential of the node 1 of the photodetector PD1 the same as that of the node 2. Here, the photodetector PD1
Is set to be sufficiently larger than the capacitance element C1 (Cpd >> C1).

Next, after the switch SW3 is turned off, the switches SWn1 to SWn4 are switched to the average value calculation signal (VAV).
To turn on. Thereby, the switches SWn1 to SWn
Charges flow in and out of adjacent pixel circuits through n4,
As a result, the potential V2 of the node 2 becomes a value averaged between the peripheral pixels. That is, in the solid-state imaging device of the first embodiment, the average value of the amount of light incident on each pixel circuit and the amount of light incident on the adjacent pixel circuit is determined by the capacitance element C1, the switch SW3, and the switches SWn1 to SWn4. An arithmetic circuit for calculating a corresponding potential is configured. The potential V2 of the photodetector PD1 averaged between adjacent pixels thus obtained is equal to the saturation voltage Vs
If it is smaller than 1/2 of at (Vsat / 2), the comparator Comp1 outputs an "H" level signal and turns on the switch SW2. As a result, the node 1 of the photodetector PD1 is reset to Vsat again.

When the average value calculation signal (VAV) is turned off, the output node 3 of the comparator Comp1 goes low again, the switch SW3 turns off, and the potential V1 of the node 1 of the photodetector PD1 changes according to the amount of incident light. Change (decrease). Hereinafter, the repetition period (reset period) is defined as T / 4 (8 milliseconds), T / 8 (4 milliseconds),. . . . , T / 2 ^n,. . And sequentially decrease
When the potential V2 of the photodetector PD1 averaged between adjacent pixels is equal to or more than 1/2 (Vsat / 2) of the saturation voltage Vsat, the potential of the node 1 of the photodetector PD1 at the time when the accumulation time T elapses is calculated. The signal is amplified by the amplifying transistor Tr1 and output as an output signal.

For example, as shown in FIG.
When the cycle is decreased by two, the potential of the photodetector PD1 is reset after the lapse of T / 2 time and after the lapse of the next T / 4 time in the accumulation time T, and is averaged at the lapse of the next T / 8 time. V2 is the saturation voltage Vsat
Ｖ (Vsat / 2) or more, the node 1 is not reset thereafter, and the node 1 at the time when T / 8 hours have passed since that time (when the storage time T has passed since the first reset) Is amplified and output, and an output signal inverted by an external inverting amplifier can be obtained.

In this way, of the accumulation time T, T
Since the reset is performed after the lapse of / 2 hours and after the lapse of the next T / 4 time, the period from t = 0 to 3T / 4 in the accumulation time T is excluded. As a result, only the remaining time (T-3T / 4 = T / 4) in the accumulation time T has a substantial meaning as the accumulation time, and the effective accumulation time substantially having the meaning as the accumulation time. In Te, the potential amount at which the potential V1 of the node 1 changes from Vsat can be detected. That is, in the example shown in FIG. 4, at a certain effective accumulation time Te, the incident light amount corresponding to the decay time T1 satisfying 2Te ≧ T1> Te is detected. It should be noted that the averaged potential V2 of the photodetector PD1 after the elapse of T / 2 time is 1/1 of the saturation voltage Vsat.
With a small incident light amount of 2 (Vsat / 2) or more, the potential V1 of the node 1 at the time when the accumulation time T elapses is output without being reset once within the accumulation time T, and the effective accumulation The time Te matches the accumulation time T.

In the pixel circuit 50 of the first embodiment, the repetition
Return cycle (reset cycle) is T / 2^k(K = 1,
2,. . . n)
For example, by setting n = 10, the accumulation time
Illumination range 2 that can be covered by T^TenDouble Dynamic Ren
(60 dB dynamics)
Cleanse can be secured. In the first embodiment, the configuration of the control circuit is simplified.
In order to make the repetition cycle (reset cycle) T / 4 (8 mm
Second), T / 8 (4 milliseconds),. . . . , T / 2 ⁿ,. .
However, the present invention is not limited to this and other rules
Even if it is made to decrease sequentially with a repetition cycle according to the rule
Good.

In the pixel circuit according to the first embodiment,
It is desirable that the control signal is commonly applied to a plurality of pixel circuits so that each pixel operates uniformly between pixels to be averaged.

FIG. 5 schematically shows a change in the potential V1 of the node 1 of the photodetector PD1 with respect to the incident light amount D. For example, when the incident light amount corresponding to the average value of the potential of the node 1 and the potential of the peripheral pixels is in the range from a to b, the potential V1 of the node 1 with respect to the illuminance changes on the solid line P. When the upper limit of the average value changes from b to b ', the change of the node 1 moves on the solid line P'. As the average value changes according to the illuminance, the solid line of the potential change moves in order.

As described above, in the pixel circuit according to the first embodiment, the pixel circuit 50 is provided corresponding to each pixel, and based on the average value of the amount of incident light between the pixel and the pixels around the pixel. When the incident light amount is relatively large, the effective accumulation time is shortened, and when the incident light amount is relatively small, the effective accumulation time is lengthened to detect the incident light amount. Thus, the solid-state imaging device according to the first embodiment including the pixel circuit 50 shifts the light receiving sensitivity range in each pixel according to the intensity of the incident light amount, and detects the incident light amount in an appropriate light receiving sensitivity range. Therefore, a wide light receiving sensitivity range can be secured as a whole. In the pixel circuit according to the first embodiment, a large output amplitude (Vsat to 0) can be assigned to each of a relatively narrow illuminance range, and detection with high contrast can be easily performed.

In each pixel circuit 50 of the first embodiment, the potential corresponding to the average value of the amount of incident light between adjacent pixels is calculated by the capacitance element C1, the switch SW3, and the switches SWn1 to SWn4. The configuration of the arithmetic circuit can be simplified. Thus, a circuit for shifting the light receiving sensitivity range can be simplified.

In the pixel circuit 50 according to the first embodiment,
Although it has been assumed that the potential V1 of the node 1 changes linearly with respect to the amount of incident light, the present invention may be changed in a non-linear manner.

Embodiment 2 FIG. FIG. 6 is a diagram illustrating a pixel circuit included in the solid-state imaging device according to the second embodiment. In the second embodiment, the comparator C according to the first embodiment is used.
The configuration is similar to that of the first embodiment, except that omp1 is configured using an inverter including two transistors, a PMOS transistor Tr6 and an NMOS transistor Tr7. In the second embodiment, the reference voltage is Vdd / 2. The output node 3 of the inverter is pulled down to GND by the NMOS transistor Tr8.

The pixel circuit of FIG. 6 according to the second embodiment configured as described above operates and has the same effects as the pixel circuit 50 of the first embodiment, and further has the following advantages. That is, in the pixel circuit of Embodiment 2, the number of transistors in the pixel circuit can be significantly reduced and the sensitivity of the solid-state imaging device can be improved by substituting the comparator with an inverter.

[0035]

As is apparent from the above description, in the solid-state imaging device according to the present invention, each pixel circuit is detected by the amount of incident light detected by the pixel circuit and the pixel circuit adjacent to the pixel circuit. The average potential corresponding to the average value of the incident light amount is calculated, and the effective storage time is changed based on the average potential to change the light-receiving sensitivity range to detect the incident light amount and to respond to the incident light amount. Since the incident light amount can be detected by setting the optimum light receiving sensitivity range, the light receiving sensitivity has a wide light receiving sensitivity range comparable to the retina of a living body, a high contrast detection function, and a high environmental adaptability that can be used in various environments. Further, the solid-state imaging device according to the present invention can realize a mechanism for shifting the light receiving sensitivity range in accordance with the intensity of incident light to each pixel and peripheral pixels with a simple circuit configuration, so that high integration and low integration can be achieved. It can be operated with power consumption.

Further, in the solid-state imaging device according to the present invention, the circuit for calculating the average potential includes a capacitance element having a capacitance sufficiently smaller than the capacitance of the photodetector, and one end of the capacitance element. And a second switch connected between one end of the capacitance element and one end of a capacitance element of an adjacent pixel circuit. As a result, a simple circuit configuration can be achieved, so that higher integration can be achieved.

Still further, in the solid state imaging device according to the present invention, the pixel circuit compares the average potential with a reference value by setting an initial setting switch for setting the potential at one end of the photodetector to an initial potential. An initial setting circuit can be easily configured by using a comparator that controls the initial setting switch so that the initial setting is performed when the average potential is equal to or less than the reference value.

Further, in the solid-state imaging device according to the present invention, by setting the reference value to 好 ま し い of the initial potential, the effective accumulation time can be set in a preferable range. The light receiving sensitivity range can be set in a suitable range, and a high contrast ratio can be detected more effectively.

In the solid-state imaging device according to the present invention, the interval for calculating the average value and setting the potential at one end of the photodetector to an initial value is reduced by a power of 1/2 of the maximum accumulation time. The control circuit can be simplified.

Further, in the solid-state image pickup device according to the present invention, by configuring the comparator with an inverter composed of a MOS transistor, higher integration and lower power consumption can be achieved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of each pixel circuit forming a solid-state imaging device according to a first embodiment of the present invention;

FIG. 2 is a circuit diagram illustrating an overall configuration of the solid-state imaging device according to the first embodiment;

FIG. 3 is a circuit diagram illustrating a specific configuration of each pixel circuit forming the solid-state imaging device according to the first embodiment;

FIG. 4 is a timing chart illustrating an operation of the pixel circuit according to the first embodiment;

FIG. 5 is a graph showing the potential V1 of the node 1 with respect to the amount of incident light in the pixel circuit according to the first embodiment.

FIG. 6 is a circuit diagram showing a specific configuration of a pixel circuit forming a solid-state imaging device according to a second embodiment of the present invention;

FIG. 7 is a diagram illustrating a state of local sensitivity automatic adjustment in a human retina.

FIG. 8 is a circuit diagram of a conventional pixel having a function of shifting a light receiving sensitivity range.

[Explanation of symbols]

1, 2, 3 nodes, PD1 photodetector, SW
1, SW2, SW3, SWni switch, Tr1, T
r2, Tr3, Tr4, Tr5, Tr6, Tr7, Tr
8, Tr9, Tr10, Tr11, Tr12 MOS transistor, Comp1, Comp10 comparator, C1 capacitance element, 50 pixel circuit.

Claims (6)

[Claims] 1. A solid-state imaging device in which a plurality of pixel circuits each having a photodetector are connected, wherein each of the pixel circuits detects an incident light amount detected by the pixel circuit and a pixel circuit adjacent to the pixel circuit. A solid-state imaging device that calculates an average potential corresponding to an average value of the incident light amount and changes an effective accumulation time based on the average potential to change a light receiving sensitivity range and detect an incident light amount. element. 2. The circuit for calculating the average potential is connected between a capacitance element having a capacitance sufficiently smaller than a capacitance of the photodetector and one end of the capacitance element and one end of the photodetector. The solid-state imaging device according to claim 1, further comprising a first switch, wherein a second switch is connected between one end of the capacitance element and one end of a capacitance element of an adjacent pixel circuit. 3. The pixel circuit according to claim 1, wherein an initial setting switch for setting a potential at one end of the photodetector to an initial potential, and comparing the average potential with a reference value, the average potential being equal to or less than the reference value. 3. The solid-state imaging device according to claim 1, further comprising a comparator that controls the initial setting switch to set a potential at one end of the photodetector to the initial potential. 4. The solid-state imaging device according to claim 3, wherein said reference value is set to の of said initial potential. 5. An interval for calculating the average potential and setting the potential at one end of the photodetector to the initial potential,
5. The solid-state imaging device according to claim 3, wherein the power is sequentially reduced by a power of 1/2 of the maximum accumulation time. 6. The solid-state imaging device according to claim 3, wherein said comparator includes an inverter formed of a MOS transistor.