KR101209506B1 - CMOS image sensor and operating method thereof - Google Patents

Abstract

An image sensor and its operating method are disclosed. The image sensor may include an active pixel sensor array region including a plurality of unit pixels; And a digital block, wherein each unit pixel outputs pixel signals to the digital block in the order of data potential and reference potential according to a driving signal input from the digital block. According to the present invention, the chip area can be reduced by eliminating the clamp circuit used for the prevention of the black sun phenomenon, and the productivity can be increased due to the reduction of the chip area.

Description

Image sensor and its operation method

The present invention relates to an image sensor and a method of operating the same.

An image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor is classified into a charge coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor. The CCD image sensor has a complicated driving method and a relatively high power consumption. Therefore, a CMOS image sensor is widely used.

CMOS image sensor uses CMOS technology that uses a control circuit, a signal processing circuit, and the like as peripheral circuits to form MOS transistors corresponding to the number of unit pixels on a semiconductor substrate, thereby forming each unit pixel by MOS transistors. The device adopts a switching method for sequentially detecting the output of the.

CMOS image sensor has advantages such as low power consumption, simple manufacturing process according to a few photo process step because of the CMOS manufacturing technology. In addition, since the CMOS image sensor can integrate various circuits on a single chip, it is easy to miniaturize the product.

The image sensor generates fixed pattern noise due to an offset voltage on the manufacturing process. To correct this fixed pattern noise, the image sensor reads the reset voltage signal at each unit pixel of the pixel array, reads the data voltage signal, and then outputs the difference. Correlated Double Sampling (CDS) method is used.

The operation principle of the CDS method of obtaining pixel output from unit pixels of an image sensor is as follows.

The unit pixel is reset by turning on the transfer transistor Tx and the reset transistor Rx. At this time, the photodiode PD starts to deplete, and carrier charging occurs. In the floating diffusion region FD, charge is accumulated in proportion to the supply voltage VDDP.

The transfer transistor Tx, the reset transistor Rx, and the select transistor Sx are turned off and the photocharges are collected in the photodiode PD. The unit pixel is turned on by turning off the transfer transistor Tx and the reset transistor Rx and turning on the select transistor Sx. The first output voltage V1 of the drive transistor Dx is measured, and this value represents a DC level shift of the floating node FN.

The transfer transistor Tx is turned on so that all photocharges of the photodiode PD are transferred to the floating diffusion region FD. When the second output voltage V2 of the drive transistor Dx is measured, the difference between the first and second output signals V1-V2 is the result of photocharge transfer, which is a pure pixel output with noise removed. It becomes a (voltage) signal.

The first output voltage and the second output are measured by measuring the reset voltage (that is, the first output voltage), which is the voltage due to noise, and the data voltage (that is, the second output voltage), which is a voltage obtained by adding noise components and image information (data). By calculating the difference in voltage, accurate image information can be obtained.

As described above, the photodiode PD is reset using the transfer transistor Tx and the reset transistor Rx before performing the operation of reading out the photocharges accumulated in the photodiode using the CDS method.

However, the first output voltage, which is a voltage due to noise output from the unit pixel, may cause distortion of the output image of the image sensor because its voltage level is out of an appropriate range by strong light from an external environment such as sunlight. . That is, the distortion of the output image causes a black sun phenomenon, which is a phenomenon in which the output image is black.

In order to prevent the black sun phenomenon, when the first output voltage, which is a voltage due to noise, becomes below a predetermined reference voltage level, the external direct reset to maintain the level of the first output voltage supplied to the CDS circuit within a certain range. Clamp circuits have been developed that provide voltage.

However, the conventional CMOS image sensor adopting the clamp circuit has a problem in that the chip area is increased because the clamp circuit occupies a predetermined space and the productivity is reduced due to the increase in the chip area.

The above-described background technology is technical information that the inventor holds for the derivation of the present invention or acquired in the process of deriving the present invention, and can not necessarily be a known technology disclosed to the general public prior to the filing of the present invention.

An object of the present invention is to provide an image sensor and a method of operating the same, which can prevent a black sun phenomenon only by improving pixel timing.

The present invention is to provide an image sensor and a method of operating the same that can reduce the chip area by removing the clamp circuit used to prevent the black sun phenomenon, and can increase the productivity due to the reduction of the chip area.

Other objects of the present invention will be readily understood through the following description.

According to an aspect of the present invention, an image sensor comprising: an active pixel sensor array region including a plurality of unit pixels; And a digital block, wherein each unit pixel outputs a pixel signal to the digital block in order of a data potential Vsig and a reference potential Vref according to a driving signal input from the digital block. Is provided.

The digital block may calculate a difference value between the value of the data potential and the value of the reference potential as an output of a pure pixel signal.

Calculation of the pure pixel signal may be performed by a correlated double sampler.

The digital block turns on a transfer transistor to transfer charges generated in the photodiode PD to a floating diffusion region so that the data potential is output, and then the photodiode and the floating diffusion region A driving signal may be input to each unit pixel to reset the driving signal to output the reference potential.

The image sensor may be a complementary metal oxide silicon (CMOS) image sensor.

According to another aspect of the present invention, there is provided a driving method of an image sensor including an active pixel sensor array region including a plurality of unit pixels and a digital block, the digital block outputting a driving signal to each unit pixel; And reading, by the digital block, a pixel signal input from each unit pixel in the order of a data potential and a reference potential, and calculating a difference value between the data potential and the reference potential as a pure pixel signal. A method is provided.

Calculation of the pure pixel signal may be performed by a correlated double sampler.

The driving signal turns on the transfer transistor of the unit pixel so that the charge generated in the photodiode is transferred to the floating diffusion region so that the data potential is output, and then the photodiode and the floating diffusion region are reset. The reference potential may be a signal for outputting.

The image sensor may be a complementary metal oxide silicon (CMOS) image sensor.

Other aspects, features, and advantages will become apparent from the following drawings, claims, and detailed description of the invention.

According to the exemplary embodiment of the present invention, the black sun phenomenon can be prevented only by improving the pixel timing.

In addition, it is possible to reduce the chip area by removing the clamp circuit used for the prevention of the black sun phenomenon, there is an effect that can increase the productivity by reducing the chip area.

1 is a block diagram of a general image sensor.
2 shows a planar layout of a typical image sensor.
3 is a circuit diagram of a unit pixel of a general image sensor for preventing a black sun phenomenon.
4 is a timing diagram of a typical image sensor.
5 is a circuit diagram of a unit pixel of an image sensor for preventing a black sun phenomenon according to an embodiment of the present invention.
6 is a flowchart illustrating a method of operating an image sensor according to an exemplary embodiment of the present invention.
7 is a timing diagram of an image sensor according to an embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may be present in between. Should be. On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, the terms "comprises" or "having" and the like refer to the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

In the following description of the present invention with reference to the accompanying drawings, the same components are denoted by the same reference numerals regardless of the reference numerals, and redundant explanations thereof will be omitted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

1 is a block diagram of a general image sensor, FIG. 2 is a diagram illustrating a planar layout of a general image sensor, FIG. 3 is a circuit diagram of a unit pixel of a general image sensor for preventing a black sun phenomenon, and FIG. Timing diagram of the image sensor.

Referring to FIG. 1, the image sensor includes an active pixel sensor (APS) array region 10 in which pixels including a light receiving element are two-dimensionally arranged, and a digital block 20 for controlling the APS array region 10. do.

The APS array region 10 includes a plurality of unit pixels arranged in a two-dimensional matrix, and converts an optical signal into an electrical signal.

2 and 3 show a planar layout of the image sensor and a circuit diagram of the unit pixels.

As shown in FIGS. 2 and 3, the unit pixel 100 of the image sensor includes one photo diode (PD) 111, a transfer transistor (TGT) 112, and floating diffusion. Diffusion (FD) region 113, a reset transistor (RST) 114, a drive transistor (DX) 115, and a select transistor (RSEL) 116 are included.

The photodiode 111 is formed in a wide portion of the active region and receives light to generate photocharges. The transfer transistor 112 transfers the photocharge collected from the photodiode 111 to the floating diffusion region 113.

The reset transistor 114 sets or resets the potential of the floating diffusion region 113. The driving transistor 115 having a gate is connected to the selection transistor 116 having a gate. Dopant doped source / drain regions are provided around the gates.

When sunlight or strong light is incident in the unit pixel 100, the photo potential 111 passes over the limit of the capacity that can be accepted by the incident light to the adjacent pixel or the floating diffusion region 113. Rather, the black sun phenomenon occurs in which the output image is black.

4 is a timing diagram of a general image sensor. 4A is a timing diagram in a normal state, and FIG. 4B is a timing diagram when sunlight or strong light is incident.

As shown in (a) of FIG. 4, when light having a normal operating intensity is incident, a pixel out of the reference potential Vref and the data potential Vsig is generated to generate a pixel out of the digital block 20. A correlated double sampler (CDS) 60 is input, and the correlated double sampler 60 obtains a final output by using a difference value between the reference potential and the data potential (ie, Vref-Vsig). Since the reference potential is not a fixed value, the reference potential may be output as a different value for each unit pixel.

Here, the reference potential Vref refers to a voltage value after the floating diffusion region 113 is reset, and the data potential Vsig indicates that the electrons generated when the photodiode 111 receives light are transferred to the transistor 112. ) Is on to mean the voltage value of the floating diffusion region 113 after being transferred to the floating diffusion region 113.

However, as shown in (b) of FIG. 4, when a strong light is incident on a specific unit pixel, the reference potential value is lowered so that the difference with the data potential value is small or becomes the same value and is represented in black.

That is, when strong light enters, the potential of the floating diffusion region 113 should be close to the supply voltage VDDP after the transfer transistor 112 is turned off, but there are too many electrons in the photodiode 111. Generated and overflowed to the floating diffusion region 113, whereby the reference potential of the floating diffusion region 113 is lowered, and the data potential is equal to or smaller than the reference potential. maintain. Therefore, the difference between the reference potential and the data potential becomes zero (zero) or a value close to zero, which is an object that emits strong light such as the sun, but is output in black.

Therefore, as shown in FIG. 3, the conventional image sensor is operated when the reference potential Vref is lowered below a specific potential level to prevent the bright area from darkening. The added configuration is adopted.

Referring back to FIG. 1, the APS array region 10 may be driven by receiving driving signals such as a pixel selection signal, a reset signal, and a charge transfer signal from the row driver 50 of the digital block 20. The converted electrical signal is provided to the correlated double sampler 60 of the digital block 20 via a vertical signal line.

The digital block 20 may include a timing generator 30, a row decoder 40, a row driver 50, a correlated double sampler (CDS) 60, and an analog to digital converter ( An analog to digital converter (ADC) 70, a latch 80, and a column decoder 90 may be included.

The timing generator 30 provides a timing signal and a control signal to the row decoder 40 and the column decoder 90.

The row driver 50 provides a plurality of driving signals to the APS array region 10 for driving the plurality of unit pixels according to the result decoded by the row decoder 40. In general, when unit pixels are arranged in a matrix form, a driving signal is provided for each row.

The correlated double sampler 60 receives, holds, and samples electrical signals formed in the APS array region 10 through vertical signal lines. In other words, the signal level due to the specific noise level and the formed electrical signal is sampled twice, and the difference level corresponding to the difference between the noise level and the signal level is output.

The analog-to-digital converter 70 converts an analog signal corresponding to the difference level into a digital signal and outputs the digital signal.

The latch unit 80 latches the digital signal, and the latched signal is sequentially output from the column decoder 90 to a subsequent image signal processing unit (not shown) according to the decoding result.

FIG. 5 is a circuit diagram of a unit pixel of an image sensor for preventing a black sun phenomenon according to an embodiment of the present invention. FIG. 6 is a flowchart illustrating a method of operating an image sensor according to an embodiment of the present invention. Is a timing diagram of an image sensor according to an embodiment of the present invention.

As shown in FIG. 5, a unit pixel of an image sensor according to an embodiment of the present invention may include a photo diode (PD), a transfer transistor (TGT), a floating diffusion (FD) region, The reset transistor includes a reset transistor (RST), a drive transistor (DX), and a select transistor (RSEL).

That is, as described above with reference to FIG. 1, the image sensor according to the embodiment of the present invention includes an active pixel sensor (APS) array region 10 and a digital block 20 for controlling the APS array region 10. It is configured to have a function to prevent the black solar phenomenon, but has a structural feature that is not provided with a clamp circuit for managing the reference potential to prevent the black solar phenomenon as shown in FIG.

6 and 7, a method in which the image sensor according to the present embodiment without a clamp circuit is driven to prevent a black sun phenomenon will be described.

Referring to FIG. 6, in operation 610, the digital block 20 outputs a driving signal to each unit pixel. The driving signal may be output from, for example, the row driver 50, and may include one or more of a pixel selection signal, a reset signal, a charge transfer signal, and the like. In addition, the driving signal may include a control signal for generating the pixel output in the order of the data potential Vsig and the reference potential Vref as shown in FIG. 7.

The digital block 20 receives the pixel output from each unit pixel in step 620, proceeds to step 630, reads the potential value in the order of the data potential value Vsig and the reference potential value Vref, Produces a pure pixel output that is the difference (ie, Vsig-Vref). The input of the pixel output and the calculation of the pure pixel output can be performed by, for example, a correlated double sampler (CDS) 60 of the digital block 20.

7 illustrates a timing diagram of pixel outputs input to the digital block 20. FIG. 7A is a timing diagram in a normal state in which light is incident from a suitable light source, and (b) is a timing diagram in the case where sunlight or strong light is incident.

As shown, when the selection transistor is switched to the high state, the reset transistor RST is switched from the high to the low state, and the transfer transistor PTG is turned on, the data potential is turned on. The value Vsig is output so that the digital block 20 reads the data potential value Vsig.

Even if strong light is incident on the unit pixel, electrons from the photodiode overflow into the floating diffusion region after the reset transistor is turned off and before the transfer transistor is turned on, so that the data potential value already falls to a low voltage. When the transfer transistor is turned on, electrons are transferred from the photodiode to the floating diffusion region so that the data potential value is lower or maintains a similar potential value, and the digital block reads the potential value as the data potential value.

After that, when the transfer transistor and the reset transistor are turned on again and the photodiode PD and the floating diffusion FD region are reset, the reference potential value Vref is output, and the digital block 20 generates the reference potential value Vref. Read out. As described above, after resetting the photodiode PD and the floating diffusion region, the reference potential value is read out so that strong light is incident and electrons overflow from the photodiode to the floating diffusion region so that the reference potential value is abnormally lowered. The phenomenon is eliminated.

Thereafter, the digital block 20 calculates a pure pixel output which is a difference value between the read data potential value and the reference potential value.

As described above, the digital block 20 reads the pixel outputs in the order of the data potential value and the reference potential value, and resets the photodiode and the floating diffusion region before the reference potential value is read out, even though no clamp circuit is provided. Accurate pure pixel output can be calculated not only when proper light is introduced but also when strong light such as sunlight is introduced.

Naturally, the above-described method of operating the image sensor may be performed by an automated procedure according to a time series sequence by a program built in the image sensor. Codes and code segments constituting the program can be easily inferred by a computer programmer in the art. The program is also stored in a computer readable media that can be read by a digital processing device, and read and executed by the digital processing device to implement the method. The information storage medium includes a magnetic recording medium, an optical recording medium and a carrier wave medium.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the following claims And changes may be made without departing from the spirit and scope of the invention.

10: active pixel sensor array area 20: digital block
30: timing generator 40: low decoder
50: low driver 60: correlated double sampler
70: analog to digital converter 80: latch portion
90: column decoder 100: unit pixel
111: photodiode 112: transfer transistor
113: floating diffusion region 114: reset transistor
115: driving transistor 116: selection transistor

Claims (10)

In the image sensor,
An active pixel sensor array region including a plurality of unit pixels; And
Including digital blocks,
Each unit pixel outputs a pixel signal to the digital block in the order of data potential (Vsig) and reference potential (Vref) according to a driving signal input from the digital block.
The method of claim 1,
And the digital block calculates a difference value between the value of the data potential and the value of the reference potential as an output of a pure pixel signal.
The method of claim 2,
The calculation of the pure pixel signal is performed by a correlated double sampler.
The method of claim 1,
The digital block turns on a transfer transistor to transfer charges generated in the photodiode PD to a floating diffusion region so that the data potential is output, and then the photodiode and the floating diffusion region And a driving signal inputted to each unit pixel to reset the reference potential.
The method of claim 1,
The image sensor is a complementary metal oxide silicon (CMOS) image sensor.
A driving method of an image sensor including a digital block and an active pixel sensor array region including a plurality of unit pixels,
Outputting a driving signal to each unit pixel by the digital block; And
Reading a pixel signal input from each unit pixel in the order of a data potential and a reference potential, and calculating a difference value between the data potential and the reference potential as a pure pixel signal. .
The method according to claim 6,
The method of driving the image sensor, characterized in that the calculation of the pure pixel signal is performed by a correlated double sampler.
The method according to claim 6,
The driving signal turns on the transfer transistor of the unit pixel so that the charge generated in the photodiode is transferred to the floating diffusion region so that the data potential is output, and then the photodiode and the floating diffusion region are reset. And a signal for outputting the reference potential.
The method according to claim 6,
And the image sensor is a complementary metal oxide silicon (CMOS) image sensor.
A program of instructions that can be executed by a digital processing apparatus is tangibly embodied in order to perform the method of driving an image sensor according to any one of claims 6 to 9, and a program that can be read by the digital processing apparatus is recorded. Record carrier.

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