JP2016019232A - Imaging device and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging element capable of effectively suppressing noise due to 1/f noise of a current source transistor while suppressing an increase in chip area.SOLUTION: An imaging element has a plurality of pixels each having a photoelectric conversion part and an amplification transistor outputting a signal amplified based upon electric charges transferred to a floating diffusion part, and also has color filters of a plurality of color filters, and a current source transistor connected to a pixel through a vertical output line provided for each column and constituting a source follower circuit with the amplification transistor of the pixel, gate area of a first current source transistor connected to a first color pixel corresponding to at least one of a plurality of colors of the color filters of the plurality of colors through a vertical output line being made larger than gate area of a second current source transistor connected to a second color pixels corresponding to at least the one of the colors of the color filters of the plurality of colors through the vertical output line.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an imaging element and an imaging apparatus.

  CMOS type solid-state image sensors are widely used in imaging devices such as digital still cameras and digital video cameras. For example, Patent Document 1 discloses a configuration including a pixel portion including a photodiode, a transfer transistor, a floating diffusion portion, a reset transistor, an amplification transistor, and a selection transistor, a vertical output line, and a current source transistor. An amplification transistor and a current source transistor in the pixel portion are connected via a vertical output line to form a source follower, and the potential on the vertical output line is read out to the outside.

  A general solid-state image pickup device that picks up a color image has a structure in which a color filter and a microlens are mounted on a monochrome image pickup device having the same structure in all pixels, and color information is separated by changing the color of the color filter. For example, the color filter includes a primary color filter composed of three colors of red (R), green (G), and blue (B).

JP 2003-51989

  If a trap level exists in the gate insulating film of the current source transistor, so-called 1 / f noise in which the noise power spectrum is proportional to the reciprocal of the frequency f may occur. In the imaging device described in Patent Document 1, when 1 / f noise occurs in the current source transistor, fluctuation occurs in the current flowing through the vertical output line, and as a result, noise is generated in the output signal of the amplification transistor in the pixel portion. It will occur. When noise due to the 1 / f noise of the current source transistor is generated in a low-sensitivity pixel such as an R (red) pixel, a large white balance gain is multiplied during white balance adjustment. The noise will be increased.

  Incidentally, it is known that 1 / f noise decreases as the gate area of a transistor increases. However, if the gate areas of the current source transistors in all the columns are increased in order to reduce the 1 / f noise, the chip area is greatly increased. Accordingly, an object of the present invention is to provide an imaging device capable of effectively suppressing the occurrence of noise due to 1 / f noise of a current source transistor while suppressing an increase in chip area.

  The imaging device according to the present invention includes a photoelectric conversion unit, a floating diffusion unit to which charges of the photoelectric conversion unit are transferred, and an amplification transistor that outputs an amplified signal based on the charges transferred to the floating diffusion unit. A plurality of pixels having a plurality of color filters and arranged in a matrix, a vertical output line provided for each column, to which the plurality of pixels are connected, and the pixels via the vertical output line A first color pixel corresponding to at least one color of the plurality of color filters and the vertical output line, the amplification transistor included in the pixel and a current source transistor constituting a source follower circuit. A gate area of the first current source transistor connected via the at least one of the color filters of the plurality of colors One of being larger than the gate area of the second current source transistor connected via the vertical output line and the second color pixel corresponding to the color.

  According to the present invention, it is possible to effectively suppress the occurrence of noise due to the 1 / f noise of the current source transistor while suppressing an increase in chip area, and an image with good image quality can be obtained.

It is a figure which shows the structural example of the imaging device in embodiment of this invention. It is a figure which shows the structural example of the image pick-up element in 1st Embodiment. It is a figure which shows the example of the spectral sensitivity characteristic of each color pixel of an image sensor. It is a figure which shows the structural example of the unit pixel in 1st Embodiment. It is a figure which shows the detailed structure of the pixel area and current source circuit of the image pick-up element in 1st Embodiment. FIG. 2 is a diagram illustrating a detailed configuration of an image sensor readout circuit, a horizontal scanning circuit, and an output amplifier according to the first embodiment. It is a figure which shows the example of the layout of the current source circuit in 1st Embodiment. It is a figure which shows the example of the gate width and gate length of the current source transistor in 1st Embodiment. It is a figure which shows the detailed structure of the pixel area and current source circuit of an image pick-up element in 2nd Embodiment. It is a figure which shows the example of the gate width and gate length of the current source transistor in 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
A first embodiment of the present invention will be described.
FIG. 1 is a block diagram illustrating a configuration example of an imaging apparatus according to the present embodiment. The taking lens 1110 is subjected to zoom control, focus control, aperture control, and the like by a lens driving circuit 1109, and forms an optical image of a subject on the image sensor 1101. The image sensor 1101 photoelectrically converts an optical image of a subject formed by the photographing lens 1110 and takes it as an image signal. The timing generation circuit 1102 outputs a drive timing signal to the image sensor 1101.

  The signal processing circuit 1103 performs analog-digital conversion processing, various correction processing, and the like on the image signal output from the image sensor 1101. The overall control / arithmetic circuit 1104 controls various calculations and the entire imaging apparatus. The overall control / arithmetic circuit 1104 has a white balance adjustment unit (not shown), and adjusts the white balance by multiplying the pixel signal of each color pixel of the captured image by a white balance gain. The memory circuit 1105 has a memory for temporarily storing image data. A display circuit 1106 displays various information and captured images. A recording circuit 1107 is a detachable recording circuit such as a semiconductor memory for recording and reading image data. The operation circuit 1108 electrically receives an operation member of the imaging apparatus.

  Next, the configuration of the image sensor 1101 in the present embodiment will be described. FIG. 2 is a diagram illustrating a configuration example of the image sensor 1101 in the present embodiment. The image sensor 1101 includes a pixel region 1, a vertical scanning circuit 2, a current source circuit 3, a readout circuit 4, a horizontal scanning circuit 5, and an output amplifier 6.

  In the pixel area 1, a plurality of unit pixels 100 are arranged in a matrix (in the row direction and the column direction). In order to simplify the explanation, FIG. 2 shows an array of 16 pixels of 4 rows and 4 columns, but in practice, a large number of pixels are arranged. Each unit pixel 100 is provided with a color filter of a plurality of colors (a plurality of types of colors). Here, a red color filter is provided, a pixel that captures red light is R, a green color filter is provided, a pixel that captures green light is G, a blue color filter is provided, and a pixel that captures blue light is B It describes as. Each pixel having three color filters of R (red), G (green), and B (blue) is arranged according to a Bayer array.

  The vertical scanning circuit 2 selects the pixels in the pixel area 1 in units of one row, and sends a drive signal to the pixels in the selected row. The current source circuit 3 is provided in each column and is paired with the unit pixel 100 to constitute a source follower circuit. The readout circuit 4 is provided in each column, amplifies the output signal of the source follower circuit composed of the unit pixel 100 and the current source circuit 3, and samples and holds the output signal. The horizontal scanning circuit 5 sends out a signal for sequentially outputting the signal sampled and held by the reading circuit 4 to the output amplifier 6 for each column. The output amplifier 6 outputs the signal output from the readout circuit 4 to the outside by the operation of the horizontal scanning circuit 5.

  An example of spectral sensitivity characteristics of each color pixel having three color filters of R, G, and B is shown in FIG. In addition, in FIG. 3, the spectral characteristic of an infrared cut filter is also included. For example, when an achromatic color illuminated with a light source having a color temperature of 5000K is photographed, the signal levels of R, G, and B are about 0.5: 1: 0.6. In this case, the white balance gain multiplied by the pixel signal of each color in the white balance adjustment unit of the overall control / arithmetic circuit 1104 is about twice for the R pixel and about 1.6 times for the B pixel.

  FIG. 4 is a circuit diagram illustrating a configuration example of the unit pixel 100 in the present embodiment. The unit pixel 100 includes a photodiode 101, a transfer switch 102, a floating diffusion unit 103, a reset switch 104, an amplification transistor 105, and a selection switch 106.

  The photodiode 101 functions as a photoelectric conversion unit that receives light incident on the pixel 100 and generates a signal charge corresponding to the amount of light received. The transfer switch 102 is driven by the transfer pulse signal PTX and transfers the signal charge generated by the photodiode 101 to the floating diffusion unit 103. The floating diffusion unit 103 functions as a charge-voltage conversion unit that temporarily holds the charge transferred from the photodiode 101 and converts the held charge into a voltage signal.

  The amplification transistor 105 is a source follower MOS transistor, amplifies a voltage signal based on the electric charge held in the floating diffusion unit 103, and outputs it as a pixel signal. The reset switch 104 is driven by a reset pulse signal PRES, and resets the potential of the floating diffusion unit 103 to the potential VDD. The selection switch 106 is driven by the vertical selection pulse signal PSEL, and outputs the pixel signal amplified by the amplification transistor 105 to the vertical output line 107.

  FIG. 5 is a diagram illustrating a detailed configuration of the pixel region 1 and the current source circuit 3 of the image sensor according to the present embodiment. The current source circuit 3 includes current source transistors 301A and 301B. A current source transistor 301A is connected to the vertical output line 107A to which the R pixel and the G pixel are connected, and a current source transistor 301B is connected to the vertical output line 107B to which the B pixel and the G pixel are connected.

  The current source transistors 301A and 301B function as constant current sources. The current source transistors 301A and 301B are connected to the pixel 100 via the vertical output lines 107A and 107B, and constitute a source follower circuit with the amplification transistor 105 included in the pixel. The amount of current flowing through the current source transistors 301A and 301B is controlled to be the same. The magnitude of the current flowing through the current source transistor 301A is controlled by the bias potential applied to the bias signal line 302A, and the magnitude of the current flowing through the current source transistor 301B is controlled by the bias potential applied to the bias signal line 302B. Is done. Further, the potential Vsf on the vertical output lines 107 </ b> A and 107 </ b> B of each column is input to the reading circuit 4.

  FIG. 6 is a diagram illustrating a detailed configuration of the readout circuit 4, the horizontal scanning circuit 5, and the output amplifier 6 of the image sensor according to the present embodiment. The read circuit 4 has a column read circuit 400 for each column, but since the configuration of the column read circuit 400 of each column is common, only the first column is shown in detail. 401 is an operational amplifier for amplifying the potential Vsf on the vertical output line, 402 is a clamp capacitor (C0), 403 is a feedback capacitor (Cf), and 404 short-circuits both ends of the feedback capacitor (Cf) 403. It is a switch for. Switch 404 is controlled by signal PC0R.

  Reference numeral 405 denotes a capacitor (CTS) that holds the output signal voltage, and reference numeral 406 denotes a capacitor (CTN) that holds the reset signal voltage. Reference numerals 407 and 408 denote switches for controlling writing to the capacitor (CTS) 405 and the capacitor (CTN) 406, respectively. The switch 407 is controlled by the signal PTS, and the switch 408 is controlled by the signal PTN. Reference numerals 409 and 410 denote switches for receiving the signal PH from the horizontal scanning circuit 5 and outputting the signals written in the capacitor (CTS) 405 and the capacitor (CTN) 406 to the common output lines 411 and 412, respectively.

  A signal written to the capacitor (CTS) 405 is input to the output amplifier 6 via the common output line 411, and a signal written to the capacitor (CTN) 406 is input to the output amplifier 6 via the common output line 412. Is done. The output amplifier 6 outputs a differential voltage between a signal from the capacitor (CTS) 405 input via the common output line 411 and a signal from the capacitor (CTN) 406 input via the common output line 412. Reference numerals 413 and 414 denote switches for resetting the common output lines 411 and 412 to the reset voltage VCHR. The reset switches 413 and 414 are controlled by a signal PCHR.

  Since the driving method of the imaging apparatus having the circuit configuration as described above is known, the description thereof is omitted.

  FIG. 7 is a plan view showing an example of the layout of the current source circuit 3 in the present embodiment. A current source transistor 301A in which the R pixel and the G pixel are connected via the vertical output line 107A, and a current source transistor 301B in which the B pixel and the G pixel are connected via the vertical output line 107B are arranged. Drain source regions 41 and 42 of the current source transistor 301A and drain source regions 44 and 45 of the current source transistor 301B are formed on the semiconductor substrate, respectively. A gate electrode 43 of the current source transistor 301A and a gate electrode 46 of the current source transistor 301B are formed on the semiconductor substrate via a gate insulating film.

  In the present embodiment, the gate length L1 of the current source transistor 301A is formed larger than the gate length L2 of the current source transistor 301B. Note that the gate width W1 of the current source transistor 301A and the gate width W2 of the current source transistor 301B are the same.

  Here, assuming that the gate width of the MOS transistor is W, the gate length is L, and the gate insulating film capacitance per unit area is Cox, the 1 / f noise generated in the MOS transistor is a square root of (W × L × Cox). It is known to be inversely proportional. Therefore, by setting the gate area (gate width W × gate length L) of the current source transistors 301A and 301B to (W1 × L1)> (W2 × L2), 1 / f noise of the current source transistor 301A is reduced to the current source transistor. Compared with 301B, it can be suppressed. For example, when the gate width W and the gate length L of the current source transistors 301A and 301B are set to the values shown in FIG. 8, the 1 / f noise generated in the current source transistor 301A is 1 / f generated in the current source transistor 301B. The noise can be reduced to about 50%.

  Therefore, in white balance adjustment, even if a large white balance gain is multiplied to the pixel signal of the R pixel, it is possible to suppress an increase in noise of the pixel signal of the R pixel. On the other hand, a pixel column including B pixels generally has a white balance gain multiplied in white balance adjustment smaller than that of R pixels. Therefore, even if noise due to the 1 / f noise of the current source transistor occurs, the increase in noise due to the white balance gain is small compared to the R pixel. Further, in this embodiment, when the gate areas of the current source transistors in all the columns are increased by increasing only the gate area of the current source transistors to which the vertical output lines corresponding to the columns including the R pixels are connected. In comparison, an increase in chip area can be suppressed. Therefore, it is possible to obtain an image with good image quality in which noise is effectively reduced while suppressing an increase in chip area.

  In this embodiment, the gate length of the current source transistor 301A is set to be larger than the gate length of the current source transistor 301B. However, the gate length is increased or both the gate length and the gate width are increased. There may be. Further, instead of the primary color filter composed of three colors of R, G, and B, a complementary color filter composed of four colors obtained by adding G (green) to C (cyan), M (magenta), and Y (yellow) is used. The present invention can also be applied to the existing configuration. In the case of the configuration using the complementary color filter composed of four colors, the gate area of the current source transistor to which the vertical output line corresponding to the column including the low-sensitivity M pixel is connected is set to the gate of the current source transistor in the other column It may be made larger than the area.

  According to the first embodiment, the gate area of a current source transistor in a column that includes a pixel that has a low sensitivity, such as an R pixel, and that has a large gain when white balance adjustment is performed, is greater than the gate area of a current source transistor in another column. Also make it bigger. As a result, the 1 / f noise of the current source transistors in the column including the low-sensitivity pixels can be reduced, and the noise caused by the 1 / f noise of the current source transistors can be effectively suppressed while suppressing an increase in the chip area. Occurrence can be suppressed and an image with good image quality can be obtained.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. Hereinafter, only differences from the above-described first embodiment will be described. In the first embodiment, the gate area of the current source transistor in the column from which the pixel signal of the R pixel and the G pixel is read out is larger than the gate area of the current source transistor in the column from which the pixel signal of the G pixel and B pixel is read out. Was configured to do. In the second embodiment, the pixel signal of each color pixel is read from a different vertical output line. Since the pixel signals of the R pixel, G pixel, and B pixel are read from different vertical output lines, the gate area of the current source transistor can be changed for each of the R pixel, G pixel, and B pixel.

  FIG. 9 is a diagram illustrating a detailed configuration of the pixel region 1 and the current source circuit 3 of the imaging element according to the second embodiment. 9, components having the same functions as those shown in FIG. 5 are given the same reference numerals, and redundant descriptions are omitted. In the pixel region 1 of the image sensor in the second embodiment, two vertical output lines are provided for each column. An R pixel is connected to the vertical output line 503A, a B pixel is connected to the vertical output line 503B, and a G pixel is connected to the vertical output line 503C.

  The current source circuit 3 includes current source transistors 501A, 501B, and 501C. The current source transistor 501A is connected to the vertical output line 503A to which the R pixel is connected and the pixel signal of the R pixel is read. A current source transistor 501B is connected to a vertical output line 503B to which a B pixel is connected and a pixel signal of the B pixel is read out. A current source transistor 501C is connected to a vertical output line 503C to which a G pixel is connected and a pixel signal of the G pixel is read out.

  The current source transistors 501A, 501B, and 501C function as constant current sources. The current source transistors 501A, 501B, and 501C are connected to the pixel 100 via the vertical output lines 503A, 503B, and 503C, and form a source follower circuit with the amplification transistor 105 included in the pixel. The amount of current flowing through the current source transistors 501A, 501B, and 501C is controlled to be the same. The magnitude of the current flowing through the current source transistors 501A, 501B, and 501C is controlled by the bias potential applied to the bias signal lines 502A, 502B, and 502C, respectively.

  Here, the gate width of the current source transistor 501A is W1, the gate length is L1, the gate width of the current source transistor 501B is W2, the gate length is L2, the gate width of the current source transistor 501C is W3, and the gate length is L3. In the present embodiment, the gate areas of the current source transistors 501A, 501B, and 501C satisfy (W1 × L1) = (W2 × L2)> (W3 × L3). Therefore, 1 / f noise generated in the current source transistor can be reduced so that current source transistor 501A = current source transistor 501B <current source transistor 501C. For example, when the gate widths and gate lengths of the current source transistors 501A, 501B, and 501C are set to the values shown in FIG. 10, 1 / f noise generated in the current source transistors 501A and 501B is 1 generated in the current source transistor 501C. / F Noise can be reduced to about 50%.

  Therefore, in white balance adjustment, even when a large white balance gain is multiplied to the pixel signals of the R pixel and the B pixel, the noise of the pixel signals of the R pixel and the B pixel increases compared to the pixel signal of the G pixel. That can be suppressed. In this embodiment, the gate areas of the current source transistors 501A and 501B are the same, but may be different sizes. However, the current source transistors can be arranged efficiently when the gate areas of the current source transistors 501A and 501B are made equal as in the present embodiment.

  According to the second embodiment, the gate areas of the current source transistors of the R pixel column and the B pixel column multiplied by a large white balance gain are made larger than the gate areas of the current source transistors of the G pixel column, respectively. Thereby, noise caused by 1 / f noise of the current source transistors of the R pixel column and the B pixel column can be reduced. As a result, even when white balance adjustment is performed, an increase in noise of the pixel signals of the R pixel and the B pixel can be suppressed, and an image with good image quality can be obtained. Further, an increase in the chip area can be suppressed as compared with the case where the gate areas of the current source transistors in all the columns are increased.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

1101: Imaging device 1102: Timing generation circuit 1103: Signal processing circuit 1104: Overall control / arithmetic circuit 100: Unit pixel 101: Photodiode 103: Floating diffusion section 105: Amplifying transistor 107 (107A, 107B): Vertical output line 301A, 301B: Current source transistor

Claims (7)

Each of the color filters includes a photoelectric conversion unit, a floating diffusion unit to which charges of the photoelectric conversion unit are transferred, and an amplification transistor that outputs an amplified signal based on the charges transferred to the floating diffusion unit. A plurality of pixels arranged in a matrix having
A vertical output line provided for each column to which the plurality of pixels are connected;
A current source transistor that is connected to the pixel via the vertical output line and that forms the source follower circuit and the amplification transistor included in the pixel;
The gate area of the first current source transistor connected to the first color pixel corresponding to at least one color of the plurality of color filters via the vertical output line is at least the other color filter of the plurality of color filters. An image pickup device having a larger area than a gate area of a second current source transistor connected to a second color pixel corresponding to one color via the vertical output line.   The imaging device according to claim 1, wherein a gate width of the first current source transistor is larger than a gate width of the second current source transistor.   The imaging device according to claim 1, wherein a gate length of the first current source transistor is larger than a gate length of the second current source transistor.   The color filter of the plurality of colors is a color filter composed of red, green, and blue, and the first color pixel includes a pixel corresponding to a red color filter. The imaging device according to item 1.   The color filter of the plurality of colors is a color filter composed of cyan, magenta, yellow, and green, and the first color pixel includes a pixel corresponding to a magenta color filter. The imaging device according to any one of the above. The first color pixel further includes a pixel corresponding to a blue color filter;
The image sensor according to claim 4, wherein the second color pixel is a pixel corresponding to a green color filter. The image sensor according to any one of claims 1 to 6,
An image pickup apparatus comprising: a processing circuit that performs white balance adjustment related to a pixel signal output from the image pickup element.

Images