KR20090015286A - Image sensor and operating method thereof - Google Patents

Abstract

An image sensor and a driving method thereof for appropriately controlling the potential level of a reset device are provided not to be limited within subjects mentioned and to make the other subjects, which are not mentioned, specifically understood as the person skilled in the art from the following material. An image sensor comprises a photoelectric conversion device, a charge coupled device, a charge transfer device, a pixel array(10), and a matrix driving unit(20). The charge transfer device transmits an electric charge accumulated in the photoelectric conversion device to the charge coupled device. In the pixel array, a unit pixel including a reset device for resetting the charge coupled device is arranged in a matrix type. The matrix driving unit provides a boosting reset signal to the reset device which is boosted in order to have a level higher than a power supply voltage.

Description

Image sensor and driving method thereof

The present invention relates to an image sensor, and more particularly, to an image sensor and a driving method thereof capable of appropriately adjusting the potential level of the reset element.

An image sensor is an element that converts an optical image into an electrical signal. Recently, with the development of the computer industry and the communication industry, the demand for improved image sensors in various fields such as digital cameras, camcorders, personal communication systems (PCS), gaming devices, security cameras, medical micro cameras, robots, etc. is increasing. have.

The image sensor includes a charge coupled device (CCD) and a CMOS image sensor. In particular, the CMOS image sensor has an advantage of a simple driving method and a low manufacturing cost because the CMOS process technology can be used interchangeably. The CMOS image sensor may be implemented in various structures, but a structure mainly used is a structure using four transistors and a photodiode (hereinafter, referred to as a “4Tr structure”). 4Tr structure is fabricated using general CMOS fabrication process.

The driving of an image sensor using a 4Tr structure is as follows. The photodiode absorbs light energy to accumulate charge corresponding to the amount of light, and the charge transfer element transfers the charge accumulated in the photodiode to the charge detection element. The amplifying element acts as a source follower buffer amplifier in combination with a constant current source to output a voltage that changes in response to the potential of the charge detection element as a vertical signal line.

In particular, the reset element serves to reset the signal of the charge detection element. Typically, the low level of the potential of the reset element is made higher than the low level of the potential of the charge transfer element. In addition, the high level of the potential of the reset element should be made higher than the high level of the potential of the charge transfer element. Conventionally, the potential level of a reset element was adjusted by injecting N-type ions into the reset gate.

The problem to be solved by the present invention is to provide an image sensor capable of appropriately adjusting the potential level of the reset element.

Another technical problem to be solved by the present invention is to provide a method of driving an image sensor image sensor capable of appropriately adjusting the potential level of the reset element.

Problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

An image sensor according to an embodiment of the present invention for solving the above problems is a photoelectric conversion element, a charge detection element, a charge transfer element for transferring the charge accumulated in the photoelectric conversion element to the charge detection element and the charge detection element reset The pixel array unit includes a pixel array unit including a reset element to form a matrix, and a row driver configured to provide a reset signal boosted to the reset element to have a level higher than a power supply voltage.

According to another aspect of the present invention, there is provided a method of driving an image sensor, including: a photoelectric conversion device, a charge detection device, a charge transfer device coupled between the photoelectric conversion device and the charge detection device, and the charge detection device; Providing a unit pixel including a reset device coupled with the device, and providing a reset signal boosted to have a level higher than a power supply voltage to the reset device to reset the charge detection device and charge the charge transfer device. Providing a transfer signal to transfer the charge accumulated in the photoelectric conversion element to the charge detection element.

Other specific details of the invention are included in the detailed description and drawings.

According to the image sensor according to the present invention, the boosted reset signal can be used to appropriately adjust the potential level of the reset element.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms. It is provided to fully convey the scope of the invention to those skilled in the art, and the present invention is defined only by the scope of the claims. Thus, in some embodiments, well known process steps, well known device structures and well known techniques are not described in detail in order to avoid obscuring the present invention. Like reference numerals refer to like elements throughout.

Unless otherwise defined, all terms used in the present specification (including technical and scientific terms) may be used in a sense that can be commonly understood by those skilled in the art. In addition, the terms defined in the commonly used dictionaries are not ideally or excessively interpreted unless they are specifically defined clearly.

An image sensor according to embodiments of the present invention includes a charge coupled device (CCD) and a CMOS image sensor. Here, the CCD has less noise and better image quality than the image sensor, but requires a high voltage and a high process cost. CMOS image sensors are simple to drive and can be implemented in a variety of scanning methods. In addition, since the signal processing circuit can be integrated on a single chip, the product can be miniaturized, and the CMOS process technology can be used interchangeably to reduce the manufacturing cost. Its low power consumption makes it easy to apply to products with limited battery capacity. Therefore, hereinafter, a CMOS image sensor will be described as an image sensor of the present invention. However, the technical idea of the present invention can be applied to the CCD as it is.

An image sensor according to embodiments of the present invention can be well understood by referring to FIGS. 1 to 6.

1 is a block diagram illustrating an image sensor according to an exemplary embodiment.

Referring to FIG. 1, an image sensor 1 according to an exemplary embodiment may include a pixel array unit 10, a row driver 20, a correlated double sampler (CDS) 70, and an analog-digital camera. And a converter (Analog to Digital Converter, ADC) 80.

The pixel array unit 10 includes a plurality of unit pixels arranged in two dimensions. A plurality of unit pixels serve to convert an optical image into an electrical signal. The pixel array unit 10 is driven by receiving a plurality of driving signals such as a selection signal ROW, a charge transfer signal TG, and a boosted reset signal BRST from the row driver 20. In addition, the converted electrical signal is provided to the correlated double sampling unit 70 through the vertical signal line 12.

The row driver 20 receives a timing signal and a control signal from a timing generator (not shown), and supplies a plurality of driving signals for driving a read operation of the plurality of unit pixels, and the like. 10) to provide. In general, when the unit pixels are arranged in a matrix form, a driving signal is provided for each row.

The row driver 20 includes a driving signal providing unit 30 and a boosting unit 40.

The driving signal providing unit 30 provides the selection signal ROW and the charge transfer signal TG to the pixel array unit 10 in a row unit, and provides the reset signal RST to the boosting unit 50.

The booster 40 boosts the reset signal RST to provide a boosted reset signal BRST having a level higher than the power supply voltage Vdd.

The select signal ROW is a signal for controlling the select element in the pixel array unit 10, and is provided to the select element in the i th row through, for example, the i th select signal line 14.

The charge transfer signal TG is a signal for controlling the charge transfer element in the pixel array 10, and is provided to the charge transfer element in the i th row through, for example, the i th charge transfer signal line 18.

The reset signal RST provided to the boosting unit 40 is boosted by the boosting unit 40 to be a boosted reset signal BRST. The boosted reset signal BRST is a signal for controlling the reset element in the pixel array unit 10 and is provided to the reset element in the i-th row through, for example, the i-th reset signal line 18.

The correlated double sampling unit 70 receives, holds, and samples an electrical signal formed in the pixel array unit 10 through the vertical signal line 12. That is, a specific reference voltage level (hereinafter referred to as "noise level") and a voltage level (hereinafter referred to as "signal level") by the formed electrical signal are sampled twice, corresponding to the difference between the noise level and the signal level. Output the difference level. It serves to suppress a fixed noise level due to dispersion of characteristics of the unit pixel and the vertical signal line 12. An amplifier receives a difference level from the correlated double sampling unit 70 and outputs an analog signal having a proper gain through a programmable gain.

The analog-to-digital converter 80 receives an analog signal from an amplifier (not shown) and outputs a digital signal for offset correction. The digital signal is latched by a latch unit (not shown), and the data selection element (not shown) provides the latched signal to the multiplexer MUX (not shown). The multiplexer (not shown) arranges all of the provided signals in series and provides the serialized signal to an image signal processor (not shown).

2 is a circuit diagram of a unit pixel of an image sensor according to an exemplary embodiment. 3 is a schematic plan view of a unit pixel of an image sensor according to an exemplary embodiment.

2 and 3, the unit pixel 100 of the image sensor 1 according to the exemplary embodiment may include the photoelectric conversion element 110, the charge detection element 120, and the charge transfer element 130. And a reset device 140, an amplifying device 150, and a selection device 160.

The photoelectric conversion element 110 absorbs light energy reflected from an object and accumulates charges generated in correspondence with the amount of light. The photoelectric conversion element 110 may be a photo diode, a photo transistor, a photo gate, a pinned photo diode (PPD), and a combination thereof.

As the charge detection device 120, a floating diffusion region (FD) is mainly used, and the charge accumulated in the photoelectric conversion device 110 is received. Since the charge detection element 120 has a parasitic capacitance, the charge is accumulated cumulatively. The charge detecting device 120 is electrically connected to the gate of the amplifying device 150 to control the amplifying device 150.

The charge transfer element 130 transfers charges from the photoelectric conversion element 110 to the charge detection element 120. The charge transfer element 130 generally consists of one transistor and is controlled by the charge transfer signal TG.

The reset device 140 periodically resets the signal of the charge detection device 120. The source of the reset device 140 is connected to the charge detection device 120 and the drain is connected to Vdd. It is also driven in response to the boosted reset signal BRST.

Here, the low level of the potential of the reset element 140 is higher than the low level of the potential of the charge transfer element 130. In addition, the high level of the potential of the reset element 140 must be higher than the high level of the potential of the charge transfer element 130.

The image sensor 1 according to an embodiment of the present invention provides a boosted reset signal BRST in order to appropriately adjust the potential level of the reset element 140. This will be described later with reference to FIGS. 4A, 4B, and 5.

The amplifier 150 serves as a source follower buffer amplifier in combination with a constant current source (not shown) located outside the unit pixel 100, and responds to a potential of the charge detection device 120. The changing voltage is output to the vertical signal line 12. The source is connected to the drain of the select element 160, and the drain is connected to the power supply voltage Vdd.

The selection element 160 selects the unit pixel 100 to be read in units of rows. Driven in response to the select signal ROW, the source is coupled to a vertical signal line 12.

Meanwhile, the driving signal lines 18, 16, and 14 of the charge transfer element 130, the reset element 140, and the selection element 160 are driven in the row direction (horizontal direction) so that the unit pixels included in the same row are simultaneously driven. Is extended.

4A, 4B, and 5, the boosted reset signal BRST is provided to properly adjust the potential level of the reset element 140. 4A is a diagram illustrating a reset signal and a boosted reset signal in an image sensor according to an exemplary embodiment of the present invention. 4B is a diagram illustrating a boosted reset signal in an image sensor according to another exemplary embodiment of the present invention. 5 is a diagram illustrating potential levels of a charge transfer signal and a boosted reset signal in an image sensor according to an exemplary embodiment of the present invention.

When the driving signal providing unit provides the reset signal RST to the boosting unit, the boosting unit boosts the reset signal RST to provide a boosted reset signal BRST having a level higher than the power supply voltage Vdd. The boosted reset signal BRST is a signal common to the unit pixels positioned in a specific row of the pixel array unit. One boosting unit provides a boosted reset signal BRST to all rows of the pixel array unit.

Referring to FIG. 4A, in the exemplary embodiment of the present invention, the low level of the reset signal RST is 0 and the high level is the power supply voltage Vdd, but the boosted reset signal BRST has a voltage α due to the boosting unit. The low level becomes α, and the high level becomes power supply voltage Vdd + α. As a result, the potential of the reset element 140 may be higher than the potential of the charge transfer element 130.

Referring to FIG. 5, the low level RGL and the high level RGH of the potential of the reset element 140 have the low level TGL and the high level TGH of the charge transfer element 130, respectively. It can be made as high as α. In addition, the high level RGH of the potential of the reset element 140 may have a level higher than the power supply voltage Vdd, that is, the power supply voltage Vdd + α.

In the image sensor according to another exemplary embodiment of the present disclosure, the boosting unit may provide the boosted reset signal BRST through two or more boosts, for example. That is, the reset signal BRST boosted to have a level higher than the power supply voltage Vdd may have a plurality of different levels (for example, two or more levels). As an example, FIG. 4B shows the boosted reset signal BRST raised in two steps.

In this manner, the image sensor 1 according to another exemplary embodiment of the present invention may reduce stress that may be generated by sudden application of a voltage higher than the power supply voltage Vdd having a large difference in voltage level from the power supply voltage Vdd. .

6 is a conceptual diagram and potential diagram of an image sensor according to an embodiment of the present invention. 6 shows a change in potential according to the boosted reset signal BRST and the charge transfer signal TG. The dislocation diagram is a direction in which the dislocation increases.

In general, all the unit pixels located in the pixel array unit (see 10 of FIG. 1) commonly integrate charges. In addition, the boosted reset signal BRST and the selection signal ROW are signals common to the unit pixels positioned in a specific row of the pixel array unit.

The pixel array section is composed of N rows, each of which is ROW (1),... … , ROW (i), ROW (i + 1),... … , ROW (N) is read sequentially. For convenience of explanation, the description will focus on ROW (i).

6, operation of the image sensor 1 using the photoelectric conversion element 110 as a pinned photodiode will be described.

The section 0 <t <t1 until time t1 is in an unselected state. That is, the selection signal ROW and the charge transfer signal TG are low, and only the boosted reset signal BRST is high. However, the charge transfer device 130 may use a depletion type transistor or a low threshold voltage Vth to prevent an overflow phenomenon in the photoelectric conversion device 110 that may occur when excessive light energy is irradiated. Enhancement type transistors may be used. Therefore, even when the charge transfer device 130 is inactive, a predetermined channel is formed so that a predetermined amount or more of the charges are discharged to the charge detection device 120 through the charge transfer device 130.

At this time, since the boosted reset signal BRST is used to reset the reset element 140, the potential level of the reset element can be appropriately adjusted. In addition, the high level RGH of the potential of the reset element 140 may be higher than the power supply voltage Vdd. Therefore, the charge of the charge detection element 120 can be sufficiently discharged to the outside.

In one embodiment of the present invention, the reset gate 145 may or may not be controlled by a separate ion implantation. In other words, in another embodiment of the present invention, the potential level of the reset device 140 may be appropriately adjusted by using a boosted reset signal BRST while performing separate ion implantation into the reset gate 145.

When the selection signal ROW becomes high at time t1, the selection element 160 is activated. That is, the charges stored in the charge detection device 120 are prepared to be read through the vertical signal line (see 12 in FIG. 1) connected to the selected unit pixel 100.

The reset signal BRST boosted at time t2 becomes low. When the boosted reset signal BRST goes low, a different offset level, that is, a noise level, is read through the vertical signal line 12 for each pixel. Although not shown in the figure, the noise level on the vertical signal line 12 is held in the correlated double sampling section (see 70 in FIG. 1) by the sample hold pulse SHP.

When the charge transfer signal TG becomes high at time t3, the charge transfer element 130 is activated. That is, the charge accumulated in the photoelectric conversion element 110 is transferred to the charge detection element 120. At this time, since the charge detection device 120 has parasitic capacitance, charges are accumulated cumulatively, and thus the potential of the charge detection device 120 is changed.

At time t4, the charge transfer signal TG goes low. When the charge transfer signal TG (i) goes low, the potential of the changed charge detection element 120, that is, the signal level, is read through the vertical signal line 12. Although not shown in the figure, the signal level on the vertical signal line 12 is retained in the correlated double sampling section 70 by the sample hold pulse SHD.

That is, the noise level and the signal level are sequentially sampled in one unit pixel (100 of FIG. 2). Of course, the signal level may be sampled first, and then the noise level may be sampled.

This operation first prevents a fixed noise level from occurring even if the same path is used, since the output of the noise level and signal level is controlled using a predetermined switch. In addition, since the output is sequentially, the difference between the noise level and the signal level can be obtained by the correlated double sampling unit 70 which is a differential circuit without using a separate memory, thereby simplifying the system.

Thereafter, the image signal processor (not shown) undergoes a plurality of processes until the screen is displayed. For example, the correlated double sampling unit 70 outputs the difference level between the noise level and the signal level. Therefore, the fixed noise level due to the characteristic dispersion of the unit pixel 100 and the vertical signal line 12 is suppressed. In addition, the analog-to-digital converter (80 in FIG. 1) receives the analog signal output from the correlated double sampling unit 70 and outputs it as a digital signal.

Although not shown in the drawing, after the selection signal ROW (i) goes low, the selection signal ROW (i + 1) of the next row becomes high. The subsequent operation is the same as the i th row.

Although an all pixel independent reading mode in which signals of all the unit pixels 100 are read independently has been described, the present invention is not limited thereto. Of course, a frame reading mode is also possible in which signals of odd lines are read in the first field and signals of even lines are read in the second field. In addition, a field reading mode is also possible in which signals from two adjacent lines are read at the same time to add a voltage and change a combination of two lines added for each field.

The unit pixel 100 of the image sensor 1 according to an embodiment of the present invention uses a negative charge as a carrier and uses an NMOS transistor, but is not limited thereto. That is, a positive charge can be used as a carrier and a PMOS transistor can be used, and the polarity of the voltage can be changed accordingly.

The image sensor 1 according to an embodiment of the present invention may be modular, including a signal processing chip and / or a lens system and embedded in a predetermined electric device.

Although embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may implement the present invention in other specific forms without changing the technical spirit or essential features thereof. You will understand that. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

1 is a block diagram illustrating an image sensor according to an exemplary embodiment.

2 is a circuit diagram of a unit pixel of an image sensor according to an exemplary embodiment.

3 is a schematic plan view of a unit pixel of an image sensor according to an exemplary embodiment.

4A is a diagram illustrating a reset signal and a boosted reset signal in an image sensor according to an exemplary embodiment of the present invention.

4B is a diagram illustrating a boosted reset signal in an image sensor according to another exemplary embodiment of the present invention.

5 is a diagram illustrating potential levels of a charge transfer signal and a boosted reset signal in an image sensor according to an exemplary embodiment of the present invention.

6 is a conceptual diagram and potential diagram of an image sensor according to an embodiment of the present invention.

(Explanation of symbols for the main parts of the drawing)

1: image sensor 10: pixel array

20: row driver 30: drive signal providing unit

40: boosting section 70: correlated double sampling section

80: analog-digital converter 110: photoelectric conversion element

120: charge detection element 130: charge transfer element

140: reset device 145: reset gate

147: reset drain 150: amplifying element

160: selection element

Claims (7)

A pixel array in which unit pixels including a photoelectric conversion element, a charge detection element, a charge transfer element for transferring charges accumulated in the photoelectric conversion element to a charge detection element, and a reset element for resetting the charge detection element are arranged in a matrix form part; And And a row driver configured to provide a reset signal boosted to the reset device to have a level higher than a power supply voltage. The method of claim 1, The row driving unit includes a driving signal providing unit for providing a reset signal and a boosting unit for boosting and providing the reset signal to a level higher than a power supply voltage. The method of claim 2, The boosting unit increases the potential of the reset element higher than the potential of the charge transfer element. The method of claim 2, And the boosting unit boosts and supplies the power voltage at least once. The method of claim 1, The unit pixel further includes a selection element for selecting the unit pixel. The method of claim 1, And the unit pixel further comprises an amplifying element for outputting a signal corresponding to a potential of the charge detecting element as a vertical signal line. Providing a unit pixel including a photoelectric conversion element, a charge detection element, a charge transfer element coupled between the photoelectric conversion element and the charge detection element, and a reset element coupled with the charge detection element, Providing a reset signal boosted to have a level higher than a power supply voltage to the reset device to reset the charge detection device, And providing a charge transfer signal to the charge transfer element, thereby transferring charge accumulated in the photoelectric conversion element to the charge detection element.

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