Patent Publication Number: US-2021175269-A1

Title: Image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims benefit of priority to Korean Patent Application No. 10-2019-0160516 filed on Dec. 5, 2019 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to an image sensor. 
     An image sensor is a semiconductor-based sensor for receiving light and generating an electrical signal. For example, an image sensor converts light into an electrical signal that conveys information used to form an image. An image sensor may include a pixel array including a plurality of pixels, a logic circuit for driving the pixel array and generating an image, and the like. The plurality of pixels may include a photodiode for generating electric charge by reacting to external light, a pixel circuit for converting the electric charge generated by the photodiode into an electrical signal, and the like. An image sensor has traditionally been employed in cameras for capturing still and video images, but is now widely applied to smartphones, tablet personal computers (PCs), laptop computers, televisions, vehicles, and the like. Recently, a variety of methods for improving noise characteristics, a dynamic range, and the like, of an image sensor have been developed. 
     SUMMARY 
     Example embodiments provide an image sensor, capable of limiting and/or preventing crosstalk from occurring when providing an autofocusing function. 
     According to an example embodiment, an image sensor includes: a first photodiode group including photodiodes and a second photodiode group including photodiodes, a first transfer transistor group including at least one transfer transistor connected to the first photodiode group; a second transfer transistor group including at least one transfer transistor connected to the second photodiode group; a floating diffusion region of a substrate, the floating diffusion region configured to store electric charges generated in the first photodiode group as the at least one transfer transistor of the first transfer transistor group is turned on; and a power supply node connected to the second photodiode group, the power supply node configured to have a power supply voltage applied thereto for applying the power supply voltage to the second photodiode group. The power supply voltage allows electric charges generated in the second photodiode group to migrate to the power supply node. The at least one transfer transistor of the second transfer transistor group is configured to have a barrier voltage applied thereto. The barrier voltage forms a potential barrier between the second photodiode group and the floating diffusion region. 
     According to an example embodiment, an image sensor includes a first pixel area and a second pixel area sharing a single microlens, a device isolation layer electrically isolating the first pixel area and the second pixel area from each other, at least one transfer transistor connected to the first pixel area, and at least one drain terminal connected to the second pixel area. The at least one drain terminal is configured to have a power supply voltage applied thereto. 
     According to an example embodiment, an image sensor includes a normal pixel group configured to generate an image and an autofocusing pixel group configured to provide an autofocusing function. The autofocusing pixel group and the normal pixel group have substantially a same sensitivity as each other. The autofocusing pixel group includes unit pixels sharing a single microlens and each including a single photodiode, a device isolation layer electrically isolating the unit pixels from each other, and a drain terminal configured to have a constant voltage input thereto. Photodiodes in the autofocusing pixel group include a first photodiode group and a second photodiode group. The drain terminal is connected to the second photodiode group. The first photodiode group is configured to generate electric charges that migrate from the first photodiode group to a floating diffusion region of a substrate. The second photodiode is configured to generate electric charges that are discharged to the drain terminal. 
     According to an example embodiment, an image sensor includes a pixel array including a normal pixel group and an autofocusing pixel group and a controller configured to generate an image based on a signal generated from the normal pixel group and to provide an autofocusing function based on a signal generated from the autofocusing pixel group. The autofocusing pixel group includes unit pixels sharing a single microlens and each including either one of an autofocusing photodiode and a drain photodiode, a device isolation layer electrically isolating the unit pixels from each other, and a drain terminal connected to the drain photodiode. The autofocusing photodiode is configured to generate electric charges that migrate to a floating diffusion region of a substrate, and electric charges generated from the drain photodiode are discharged to the drain terminal. The pixel array has either one of a red-green-green-blue (RGGB) pattern and a red-green-blue-white (RGBW) pattern. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects, features, and effects of the present disclosure will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings. 
         FIG. 1  is a block diagram of an image sensor according to an example embodiment. 
         FIGS. 2 and 3  are schematic diagrams of an image sensor according to an example embodiment, respectively. 
         FIG. 4  illustrates an operation of an image sensor according to an example embodiment. 
         FIG. 5  is a schematic circuit diagram illustrating a pixel included in an image sensor according to an example embodiment. 
         FIG. 6  illustrates an operation of an image sensor according to an example embodiment. 
         FIGS. 7A and 7B  illustrate a pixel array according to an example embodiment. 
         FIG. 8  is a schematic circuit diagram illustrating a pixel group according to an example embodiment. 
         FIG. 9A  is a plan view of a normal pixel group according to an example embodiment. 
         FIGS. 9B and 9C  are plan views of autofocusing pixel groups according to an example embodiment, respectively. 
         FIGS. 10A to 10F  illustrate an operation of an autofocusing pixel group according to an example embodiment. 
         FIG. 11A  is a plan view of an autofocusing pixel group according to an example embodiment,  FIG. 11B  is a cross-sectional view taken along line II-IT in  FIG. 11A , and  FIG. 11C  is a cross-sectional view taken along line III-III′ in  FIG. 11A . 
         FIG. 12A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 12B  is a cross-sectional view taken along line IV-IV′ in  FIG. 12A . 
         FIG. 13A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 13B  is a cross-sectional view taken along line V-V′ in  FIG. 13A . 
         FIG. 14A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 14B  is a cross-sectional view taken along line VI-VI′ in  FIG. 14A . 
         FIG. 15  is a plan view of an autofocusing pixel group according to an example embodiment. 
         FIG. 16  illustrates a pixel array according to an example embodiment. 
         FIG. 17  is a plan view of autofocusing pixel groups according to an example embodiment. 
         FIG. 18  is a partial cross-sectional view of pixels according to an example embodiment. 
         FIGS. 19A and 19B  illustrate sensitivity of an image sensor according to an example embodiment. 
         FIG. 20  is a schematic block diagram of an electronic device including an image sensor according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     When the terms “about” or “substantially” are used in this specification in connection with a numerical value, it is intended that the associated numerical value include a tolerance of s-sectional view taken along line III-III′ in  FIG. 11 . Action based on a range includes all values therebetween such as increments of 0.1%. 
     Hereinafter, example embodiments will be described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of an image sensor according to an example embodiment. 
     Referring to  FIG. 1 , am image sensor  100  according to an example embodiment may include a pixel array  110 , a row driver  120 , a read-out circuit  130 , a timing controller  140 , and a signal processing circuit  150 . 
     The image sensor  100  may convert light, received from an external device, into an electrical signal and may generate image data. The pixel array  110  included in the image sensor  100  may include a plurality of pixels PX, and the plurality of pixels PX may include a photoelectric device configured to receive light and to generate electric charge. The photoelectric device may be, for example, a photodiode PD. The plurality of pixels PX may be connected to a plurality of row lines, extending in a first direction, and a plurality of column lines extending in a second direction. In an example embodiment, each of the plurality of pixels PX may include two or more photoelectric devices to generate pixel signals corresponding to light of various colors or to provide an autofocusing function. 
     Each of the plurality of pixels PX may include a pixel circuit configured to generate a pixel signal from electric charges generated by photodiodes. As an example, the pixel circuit may include a transfer transistor, a driving transistor, a select transistor, a reset transistor, a floating diffusion region, and the like. A pixel circuit may output a reset voltage and a pixel voltage. A pixel voltage may correspond to electric charges generated by photodiodes included in each of the plurality of pixels and stored in the floating diffusion region. In an example embodiment, two or more adjacent pixels PX may constitute a single pixel group, and two or more pixels PX included in a pixel group may share at least a portion of the transfer transistor, the driving transistor, the select transistor, and the reset transistor. 
     The row driver  120  may input a driving signal to a plurality of row lines to drive a pixel array  110 . For example, the driving signal may include a transfer control signal TS configured to control a transfer transistor of a pixel circuit, a reset control signal RS configured to control a reset transistor of the pixel circuit, and a select control signal SEL configured to control a select transistor of the pixel circuit, or the like. As an example, the row driver  120  may sequentially drive a plurality of row drivers. 
     The read-out circuit  22  may include a ramp signal generator  131 , a sampling circuit  132 , an analog-to-digital converter (ADC)  133 , and a buffer  134 . The sampling circuit may include a plurality of samplers connected to pixels PX through a plurality of column lines. In example embodiments, the sampler may be a correlated double sampler (CDS). The sampler may detect a rest voltage and a pixel voltage from the pixels PX connected to a selected row line, driven by the row driver  120 , among a plurality of row lines. The samplers may output a difference between the reset voltage and the pixel voltage as an analog signal. 
     The analog-to-digital converter  133  may receive the ramp voltage V RMP  from the ramp signal generator  131  and may receive an analog signal from the sampling circuit  132 . The analog-to-digital converter  133  may compare the analog signal with the ramp voltage V RMP , and may convert a comparison result into a digital signal and output the digital signal as the image data LSB. The image data LSB may have a value increased as a difference between the reset voltage and the pixel voltage is increased. Therefore, the image data LSB may have a value increased as the intensity of light, received by the photodiode, is increased. 
     The buffer  134  may temporarily store the image data LSB received from the analog-to-digital converter  133 . 
     The row driver  120  and the read-out circuit  130  may be controlled by the timing controller  140 . The timing controller  140  may control operation timings of the row driver  120  and the read-out circuit  130 . The row driver  120  may control an operation of the pixel array  110  in units of rows under the control of the timing controller  140 . 
     The signal processing circuit  150  may generate an image using the image data LSB transmitted from the buffer  134 . The signal processing circuit  150  may process the image data LSB and may output the processed image data as an image. 
     In some embodiments, the row driver  120 , timing controller  140 , analog-to-digital converter (ADC)  133 , ramp signal generator  131 , and/or other features of the of the image sensor  100  may be implemented with processing circuitry processing circuitry such as hardware including logic circuits; a hardware/software combination such as a processor executing software; or a combination thereof. For example, the processing circuitry more specifically may include, but is not limited to, a central processing unit (CPU), an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, application-specific integrated circuit (ASIC), an analog element (e.g., circuit), an application processor, etc. The buffer  134  may include a volatile memory chip such as dynamic random access memory (RAM) (DRAM) or static RAM (SRAM), or may include a non-volatile memory chip such as phase-change RAM (PRAM), magnetoresistive RAM (MRAM), ferroelectric RAM (FeRAM), resistive RAM (RRAM), a high bandwidth memory (HBM), or a combination thereof. 
     An autofocusing function may be provided by shielding a portion of a photodiode with a metal, or the like, and detecting only light, incident on an unshielded portion of the photodiode, in a plurality of pixels PX. However, in a method of detecting a phase difference using a metal or the like, light may be reflected by the metal, or the like, to cause crosstalk between adjacent pixels. 
     An image sensor according to an example embodiment may adjust a doping concentration between a substrate and at least one photodiode, among photodiodes constituting a pixel group, to form a drain terminal connected to the at least one photodiode. Electric charges, generated by the at least one photodiode, may not migrate to a floating diffusion region and may be discharged to the drain terminal. Thus, crosstalk may be limited and/or prevented from occurring when the pixel group provides the autofocusing function. 
       FIGS. 2 and 3  are schematic diagrams of an image sensor according to an example embodiment, respectively. 
     Referring to  FIG. 2 , an imaging device  10  according to an example embodiment may include a first layer  11  and a second layer  12  provided below the first layer  11 . The first layer  11  and the second layer  12  may be stacked in a direction perpendicular to each other. 
     The first layer  11  may have a sensing area SA, a peripheral circuit PC, and a first pad area PAL In the sensing area SA, a plurality of pixels PX may be provided. The peripheral circuit PC may be provided with elements for driving the plurality of pixels PX. The first pad area PA 1  may be provided around the sensing area SA and the peripheral circuit PC. 
     The first pad area PA 1  includes a plurality of upper pads PAD, and the plurality of upper pads PAD may be connected to pads and logic circuit LC provided in a second pad area PA 2  of the second layer  12  through vias or the like. 
     Each of the plurality of pixels PX may include a photodiode receiving light to generate electric charges, a pixel circuit processing the electric charges generated by the photodiode, and the like. The pixel circuit may include a plurality of transistors for outputting a voltage corresponding to the electric charges generated by the photodiode. 
     The first layer  11  may include a plurality of elements for providing a peripheral circuit PC. A plurality of elements, included in the peripheral circuit PC, may provide circuits for driving a pixel circuit provided in the first layer  11 , for example, a row driver, a column driver, a CDS, an ADC, a timing controller, and the like. The plurality of elements, included in the peripheral circuit PC, may be connected to the pixel circuit through the first and second pad areas PA 1  and PA 2 . The peripheral circuit PC may obtain a reset voltage and a pixel voltage from the plurality of pixels PX to generate a pixel signal. 
     The second layer  12  may include a logic circuit LC and a second pad area PA 2 . The logic circuit LC may include a power supply circuit, an input/output interface, an image signal processor (ISP), and the like. The second pad area PA 2  may be provided around the logic circuit LC. 
     Referring to  FIG. 3 , an imaging device apparatus  20  according to an example embodiment may include a first layer  21  and a second layer  22 . The first layer  21  may include a sensing area SA, in which a plurality of pixels PX are provided, and a first pad area PA 1  provided around the sensing area SA. The first pad area PA 1  may include a plurality of upper pads PAD. Through vias, or the like, the plurality of upper pads PAD may be connected to pads, the peripheral circuit PC, and the logic circuit LC, provided in the second pad area PA 2  of the second layer  22 . 
       FIG. 4  illustrates an operation of an image sensor according to an example embodiment. 
     Referring to  FIG. 4 , a pixel array PA of an image sensor according to an example embodiment may include a plurality of pixels PX. The plurality of pixels PX may be connected to a plurality of row lines ROW 1  to ROWm (ROW) and a plurality of column lines COL 1  to COLn (COL). The image sensor may drive the plurality of pixels PX in units of a plurality of row lines ROW. As an example, time required to drive a selected driving line among the row lines ROW and to read a reset voltage and a pixel voltage from pixels PX connected to the selected driving line may be defined as one horizontal period. The image sensor may operate in a rolling shutter manner in which the plurality of row lines ROW are sequentially driven. 
     A frame period FT of the image sensor may be defined as times required to read a reset voltage and a pixel voltage from all pixels included in the pixel array PA. As an example, the frame period FT may be greater than or equal to a product of the number of row lines ROW and the horizontal period. The shorter the frame period FT of the image sensor, the greater the number of image frames the image sensor may generate during the same time. 
       FIG. 5  is a schematic circuit diagram illustrating a pixel included in an image sensor according to an example embodiment. 
     Referring to  FIG. 5 , a pixel included in an image sensor may conclude a photodiode PD configured to generate electric charges in response to light, and a pixel circuit configured to process the electric charges generated by the photodiode PD to output an electrical signal, and the like. As an example, a pixel circuit may include a reset transistor RX, a driving transistor DX, a select transistor SX, a transfer transistor TX, and the like. 
     The reset transistor RX may be turned on and off by a reset control signal RS. When the reset transistor RX is turned on, a voltage of the floating diffusion region FD may be reset to a power supply voltage VPIX. When the voltage of the floating diffusion region FD is reset, the select transistor SX may be turned on by a select control signal SEL to output the reset voltage to a column line COL. 
     In an example embodiment, the photodiode PD may generate electrons or holes as a main charge carrier by reacting to light. The transfer transistor TX may be turned on and off by the transfer control signal TS. When the transfer transistor TX is turned on after a reset voltage is output to the column line COL, electric charges generated by exposing the photodiode PD to light may migrate to the floating diffusion region FD. The driving transistor DX may operate as a source-follower amplifier for amplifying the voltage of the floating diffusion region FD. When the select transistor SX is turned on by the select control signal SEL, a pixel voltage corresponding to the electric charges, generated by the photodiode PD, may be output to the column line COL. 
     Each of the reset voltage and the pixel voltage may be detected by a sampling circuit connected to the column line COL. The sampling circuit may include a plurality of samplers having a first input terminal for receiving a reset voltage and a second input terminal for receiving a pixel voltage. The sampler may compare the reset voltage, input to the first input terminal, with the pixel voltage input to the second input terminal. An analog-to-digital converter ADC may be connected to an output terminal of the sampler, and the analog-to-digital converter may output image data corresponding to a result of the comparison between the reset voltage and the pixel voltage. The signal processing circuit may generate an image using the image data. 
       FIG. 6  illustrates an operation of an image sensor according to an example embodiment. 
     Referring to  FIG. 6 , an image sensor  200  according to an example embodiment may include a pixel array  210 , a row driver  220 , and a read-out circuit  230 . The pixel array  210  may include a plurality of row lines ROW 1  to ROWm (ROW), extending in a direction, and a plurality of column lines COL 1  to COLn (COL), intersecting with the plurality of row lines ROW. The row lines ROW and the column lines COL may be connected to the pixels PX 11  to PXmn, and each of the pixels PX 11 -PXmn may include a photodiode and a pixel circuit. 
     The row driver  220  may input a transfer control signal, a reset control signal, a select control signal, and the like, to each pixel circuit through row lines ROW 1  to ROWm (ROW). The read-out circuit  230  may detect the pixel voltage and the reset voltage through the column line COL from the pixels PX connected to the row line ROW selected by the row driver  220 . The read-out circuit  230  may include a sampling circuit  231  configured to output a difference between the pixel voltage and the reset voltage, an analog-to-digital converter  232  configured to convert the outputs of the sampling circuit  231  into digital data DOUT, and the like. 
     The pixel array  210  according to an example embodiment may include a plurality of unit pixels PX 11  to PXmn. Each of the plurality of unit pixels PX 11  to PXmn may include at least one photodiode. A device isolation layer may be formed between the photodiodes. For example, the device isolation layer may have a back deep trench isolation (BDTI) structure. The device isolation layer may electrically isolate adjacent unit pixels from each other. 
     Among the plurality of unit pixels PX 11  to PXmn, two or more adjacent unit pixels (for example, PX 11 , PX 12 , PX 21 , and PX 22 ) may constitute a single pixel group. Unit pixels, included in a single pixel group, may each include color filters having the same color. The unit pixels, included in the single pixel group, may share at least one floating diffusion region. 
     The pixel array  210  may include normal pixel groups and autofocusing pixel groups. A controller, included in the image sensor, may generate an image based on a signal generated from the normal pixel groups and may provide an autofocusing function based on a signal generated from the autofocusing pixel groups. The number of normal pixel groups may be greater than the number of autofocusing pixel groups. The autofocusing pixel groups may include a pixel group, generating first image data (for example, left image data), and a pixel group generating second image data (for example, right image data). 
     In an image sensor according to an example embodiment, a doping concentration between at least one of the photodiodes, included in each of the autofocusing pixel groups, and a substrate may be adjusted to form a drain terminal. Electric charges, generated in the at least one photodiode, may be discharged to the drain terminal to be limited and/or prevented from migrating to the floating diffusion region. Thus, crosstalk can be limited and/or prevented from occurring when the autofocusing pixel groups provide an autofocusing function. 
       FIGS. 7A and 7B  illustrate a pixel array according to an example embodiment. 
     Referring to  FIG. 7A , a pixel array  300  may have a red-green-green-blue (RGGB) pattern. R 1  to R 4  may each refer to a red pixel, G 1  to G 4  may each refer to a green pixel, and B 1  to B 4  may each refer to a blue pixel. 
     The first to fourth unit pixels G 1  to G 4  adjacent to each other may constitute a first pixel group  310 . When the first pixel group  310  is a normal pixel group, each of the first to fourth unit pixels G 1  to G 4  may include a green color filter and a microlens ML 1 . Each of the first to fourth unit pixels G 1  to G 4  may include at least one photodiode. The first to fourth unit pixels G 1  to G 4  may share at least one floating diffusion region. 
     The first to fourth unit pixels B 1  to B 4  adjacent to each other may constitute a second pixel group  320 . When the second pixel group  320  is a normal pixel group, each of the first to fourth unit pixels B 1  to B 4  may include a blue color filter and a microlens ML 1 . Each of the first to fourth unit pixels B 1  to B 4  may include at least one photodiode. The first to fourth unit pixels B 1  to B 4  may share at least one floating diffusion region. 
     The first to fourth unit pixels G 1  to G 4  adjacent to each other may constitute a third pixel group  330 . When the third pixel group  330  is an autofocusing pixel group, each of the first to fourth unit pixels G 1  to G 4  may include a green color filter. Each of the first to fourth unit pixels G 1  to G 4  may include at least one photodiode. The first to fourth unit pixels G 1  to G 4  may share at least one floating diffusion region. Each of the first to fourth unit pixels of the normal pixel group includes the microlens ML 1 , whereas the first to fourth unit pixels G 1  to G 4  of the autofocusing pixel group  330  may share a single microlens ML 2 . 
     In  FIG. 7A , each of the first to fourth unit pixels G 1  to G 4  included in the third pixel group  330  is illustrated as including a green color filter. However, each of the first to fourth unit pixels G 1  to G 4 , included in the third pixel group  330 , may include one of a red color filter and a blue color filter. In addition, a portion of the first to fourth unit pixels G 1  to G 4 , included in the third pixel group  330 , may include different color filters to each other. For example, each of a portion of the first to fourth unit pixels G 1  to G 4 , included in the third pixel group  330 , may include a green color filter, and each of the other unit pixels may include a red color filter. 
     The third pixel group  330  may generate one image data among first image data (for example, left image data) or second image data (for example, right image data). 
     The microlens ML 1  of the normal pixel group and the microlens ML 2  of the autofocusing pixel group may have different characteristics. For example, a protruding height of the microlens ML 1  and a protruding height of the microlens ML 2  are the same, but a radius of curvature of the microlens ML 1  and a radius of curvature of the microlens ML 2  may be different from each other. According to example embodiments, the protruding height of the microlens ML 1  and the protruding height of the microlens ML 2  may be different from each other, and the radius of curvature of the microlens ML 1  and the radius of curvature of the microlens ML 2  may also be different from each other. 
     In an image sensor according to an example embodiment, when four adjacent pixels constitute an autofocusing pixel group, the autofocusing pixel group may include two autofocusing photodiodes and two drain photodiodes. The image sensor may generate first image data or second image data using electric charges generated from the two autofocusing photodiodes. Electric charges, generated from the two drain photodiodes, may be discharged to a drain terminal connected to the drain photodiodes. 
     Unlike the pixel array  300  of  FIG. 7A , the pixel array  400  of  FIG. 7B  may have a red-green-blue-white (RGBW) pattern. R 1  to R 4  may each refer to a red pixel, G 1  to G 4  may each refer to a green pixel, B 1  to B 4  may each refer to a blue pixel, and W 1  to W 4  may each refer to a white pixel. 
     First to fourth unit pixels W 1  to W 4  included in a third pixel group  330 , an autofocusing pixel group, may each include a white color filter. However, an example embodiment is not limited thereto, and each of the first to fourth unit pixels W 1  to W 4 , included in the third pixel group  330 , may include one of a green color filter, a red color filter, and a blue color filter. 
     In addition, a portion of the first to fourth unit pixels W 1  to W 4 , included in the third pixel group  330 , may include different color filters to each other. For example, each of a portion of the first to fourth unit pixels W 1 -W 4 , included in the third pixel group  330 , may include a green color filter, and each of the other unit pixels may include a white color filter. 
       FIG. 8  is a schematic circuit diagram illustrating a pixel group according to an example embodiment. 
     Referring to  FIG. 8 , a pixel group according to an example embodiment may include first to fourth photodiodes PD 1  to PD 4 , first to fourth transfer transistors TX 1  to TX 4 , a reset transistor RX, a driving transistor DX, and a select transistor SX. The first transfer transistor TX 1  may be connected to the first photodiode PD 1 , and the second transfer transistor TX 2  may be connected to the second photodiode PD 2 . The third transfer transistor TX 3  may be connected to the third photodiode PD 3 , and the fourth transfer transistor TX 4  may be connected to the fourth photodiode PD 4 . A first unit pixel including the first photodiode PD 1 , a second unit pixel including the second photodiode PD 2 , a third unit pixel including the third photodiode PD 3 , and a fourth unit pixel including a fourth photodiode PD 4  may be shared by a single floating diffusion region FD. 
     A first transfer control signal TS 1  may be a first turn-on signal for turning on the first transfer transistor TX 1 , and the first transfer transistor TX 1  may be turned on in response to the first transfer control signal TS 1 . A second transfer control signal TS 2  may be a second turn-on signal for turning on the second transfer transistor TX 2 , and the second transfer transistor TX 2  is turned on in response to the second transfer control signal TS 2 . In contrast, a third transfer control signal TS 3  may be a first turn-off signal for turning off the third transfer transistor TX 3 , and the third transfer transistor TX 3  may be turned off in response to the third transfer control signal TS 3 . A fourth transfer control signal TS 4  may be a second turn-off signal for turning off the fourth transfer transistor TX 4 , and the fourth transfer transistor TX 4  may be turned off in response to the fourth transfer control signal TS 4 . For example, each of the first turn-off signal and the second turn-off signal may be a ground voltage. 
     In  FIG. 8 , four photodiodes and four transfer gate electrodes are illustrated and described as being separated from each other. However, at least two photodiodes may be connected to each other to constitute a single photodiode, and at least two transfer gate electrodes may be connected to each other to constitute a single transfer gate electrode. 
     When the pixel group illustrated in  FIG. 8  is an autofocusing pixel group, the pixel group may include two autofocusing photodiodes PD 1  and PD 2  and two drain photodiodes PD 3  and PD 4 . The image sensor may generate first image data or second image data using electric charges generated from the two autofocusing photodiodes PD 1  and PD 2 . The image sensor may discharge electric charges, generated from the drain photodiodes PD 3  and PD 4 , to drain terminals connected to the drain photodiodes. 
     Specifically, the image sensor may simultaneously turn on the first and second transfer transistors TX 1  and TX 2 . When the first and second transfer transistors TX 1  and TX 2  are simultaneously turned on, electric charges output from the first and second photodiodes PD 1  and PD 2  may be summed in the floating diffusion region FD. 
     When the select transistor SX is then turned on in response to the selection control signal SEL, a read-out circuit may detect a pixel voltage corresponding to the amount of the electric charges, summed in the floating diffusion region FD, through a column line COL. 
     The image sensor may turn off the third and fourth transfer transistors TX 3  and TX 4 . The image sensor may apply a power supply voltage V PIX  to the third photodiode PD 3  between the third photodiode PD 3  and the third transfer transistor TX 3 . In addition, the image sensor may apply the power supply voltage V PIX  to the fourth photodiode PD 4  between the fourth photodiode PD 4  and the fourth transfer transistor TX 4 . In this application, although the power supply voltage V PIX  is illustrated and described as being applied to the third photodiode PD 3  and the fourth photodiode PD 4 , a constant voltage or a positive (+) voltage may applied to the third photodiode PD 3  and the fourth photodiode PD 4 . As the power supply voltage V PIX  is applied to the third photodiode PD 3  and the fourth photodiode PD 4 , electric charges output from each of the third and fourth photodiodes PD 3  and PD 4  may be limited and/or prevented from migrating to the floating diffusion region FD. 
       FIG. 9A  is a plan view of a normal pixel group according to an example embodiment. The pixel group of  FIG. 9A  may be one of the pixel groups  310  to  350  of  FIG. 7A . 
     Referring to  FIG. 9A , a pixel group PG may include first to fourth photodiodes PD 1  to PD 4 , first to fourth transfer gate electrodes TG 1  to TG 4 , a select transistor SX, a driving transistor DX, a reset transistor RX, and a dummy transistor DMX. The first to fourth photodiodes PD 1  to PD 4  may share a floating diffusion region FD. 
     The first transfer gate electrode TG 1  may be a gate electrode of the first transfer transistor TG 1  connected to the first photodiode PD 1 , and the second transfer gate electrode TG 2  may be a gate electrode of the second transfer transistor TG 2  connected to the second photodiode PD 2 . The third transfer gate electrode TG 3  may be a gate electrode of the third transfer transistor TG 3  connected to the third photodiode PD 3 , and the fourth transfer gate electrode TG 4  may be a gate electrode of the second transfer transistor TG 4  connected to the four photodiode PD 4 . 
     The pixel group PG may share the select transistor SX, the driving transistor DX, the reset transistor RX, and the dummy transistor DMX. 
     The dummy transistor DMX may be used to separate a node of the floating diffusion region FD and a charge output node of an adjacent pixel group from each other. According to example embodiments, a device isolation layer such as shallow trench isolation (STI) may be used instead of the dummy transistor DMX. According to an example embodiment, the node of the floating diffusion region FD and the charge output node of the adjacent pixel group may be isolated from each other using a PN junction rather than the dummy transistor DMX. For example, the node of the floating diffusion region FD may be doped with n-type, and the active region of the charge output node of the adjacent pixel group may also be doped with n-type. Therefore, p-type impurities may be introduced between the node of the floating diffusion region FD and the charge output node of the adjacent pixel group to isolate the node of the floating diffusion region FD and the charge output node of the adjacent pixel group. 
     In  FIG. 9A , four photodiodes and four transfer gate electrodes are illustrated and described as being separated from each other. However, at least two photodiodes may be connected to each other to constitute a single photodiode, and at least two transfer gate electrodes may be connected to each other to constitute a single transfer gate electrode. When at least two transfer gate electrodes are connected to each other to constitute a single transfer gate electrode, the number of contacts connected to the transfer gate electrode may be decreased. 
     According to a first embodiment, a pixel group PG may have a first structure in which the first transfer gate electrode TG 1  and the second transfer gate electrode TG 2  are connected to each other to constitute a single transfer gate electrode, and the third transfer gate electrode and the fourth transfer gate electrode TG 4  are connected to each other to constitute a single transfer gate electrode. Alternatively, the pixel group PG may have a second structure in which the first photodiode PD 1  and the second photodiode PD 2  are connected to each other to constitute a single photodiode, and the third photodiode PD 3  and the fourth photodiode PD 4  are connected to each other to constitute a single photodiode. Alternatively, the pixel group PG may have both the first structure and the second structure. 
     According to a second embodiment, the pixel group PG may have a third structure in which the first transfer gate electrode TG 1  and the third transfer gate electrode TG 3  are connected to each other to constitute a single transfer gate electrode, and the second transfer gate electrode TG 2  and the fourth transfer gate electrode TG 4  are connected to each other to constitute a single transfer gate electrode. Alternatively, the pixel group PG connects the first photodiode PD 1  and the third photodiode PD 3  to constitute a single photodiode and connects the second photodiode PD 2  and the fourth photodiode PD 4 . The fourth structure may be connected to constitute a single photodiode. Alternatively, the pixel group PG may have both the third structure and the fourth structure. 
     According to the first embodiment, when the pixel group PG is an autofocusing pixel group, the pixel group PG may obtain autofocusing information from left and right photodiodes. According to the second example embodiment, when the pixel group PG is an autofocusing pixel group, the pixel group PG may obtain autofocusing information from upper and lower photodiodes. 
     According to a third embodiment, when the pixel group PG is a normal pixel group, the pixel group PG may have a fifth structure. in the fifth structure, a first transfer gate electrode TG 1 , a second transfer gate electrode TG 2 , a third transfer gate electrode TG 3 , and the fourth transfer gate electrode TG 4  are connected to each other to constitute a single transfer gate electrode, and a first photodiode PD 1 , a second photodiode PD 2 , a third photodiode PD 3 , and the fourth photodiode PD 4  are connected to each other to constitute a single photodiode. 
     According to the third embodiment, the image sensor may detect only a pixel voltage corresponding to the amount of summed electric charges output from the first to fourth photodiodes PD 1  to PD 4 . 
       FIGS. 9B and 9C  are plan views of autofocusing pixel groups according to an example embodiment, respectively. The autofocusing pixel groups of  FIGS. 9B and 9C  will be described while focusing on differences from the normal pixel group of  FIG. 9A . An autofocusing pixel group according to an example embodiment may include a first pixel area and a second pixel area. A first photodiode PD 1  included in a first unit pixel and a second photodiode PD 2  included in a second unit pixel may constitute a first photodiode group PG 1  in the first pixel area. A third photodiode PD 3  included in a third unit pixel and a fourth photodiode PD 4  included in a fourth unit pixel may constitute a second photodiode group PG 2  in the second pixel area. 
     Specifically, referring to  FIG. 9B , an autofocusing pixel group  330   a  may generate first image data (for example, left image data). In this case, the autofocusing pixel group  330   a  may obtain first image data using electric charges output from the first photodiode PD 1  and the second photodiode PD 2  included in the first photodiode group PG 1 . The first photodiode PD 1  and the second photodiode PD 2  may be an autofocusing photodiode group. 
     Electric charges, output from the third photodiode PD 3  and the fourth photodiode PD 4  included in the second photodiode group PG 2 , may be discharged to at least one drain terminal. The at least one drain terminal may be connected to the third photodiode PD 3  and the fourth photodiode PD 4 . The third photodiode PD 3  and the fourth photodiode PD 4  may be a drain photodiode group. 
     Referring to  FIG. 9C , an autofocusing pixel group  330   b  may generate second image data (for example, right image data). In this case, the autofocusing pixel group  330   b  may obtain second image data using electric charges output from the third photodiode PD 3  and the fourth photodiode PD 4  included in the second photodiode group PG 2 . The third photodiode PD 3  and the fourth photodiode PD 4  may be an autofocusing photodiode group. 
     Electric charges, output from the first photodiode PD 1  and the second photodiode PD 2  included in the first photodiode group PG 1 , may be discharged to at least one drain terminal. The at least one drain terminal may be connected to the first photodiode PD 1  and the second photodiode PD 2 . The first photodiode PD 1  and the second photodiode PD 2  may be a drain photodiode group. 
     The image sensor may perform an autofocusing function using the first image data and the second image data generated from the autofocusing pixel groups  330   a  and  330   b.    
     Since a structure and an operation of the autofocusing pixel group for generating the first image data are similar to those of the autofocusing pixel group for generating the second image data, examples of an autofocusing pixel for generating the second image data will now be described. 
       FIGS. 10A to 10F  illustrate an operation of an autofocusing pixel group according to an example embodiment. 
       FIG. 10A  is a plan view of an autofocusing pixel group according to an example embodiment,  FIG. 10B  is a cross-sectional view taken along line I-I′ in  FIG. 10A ,  FIG. 10C  illustrates a potential level of an image sensor in a photonic integration mode according to an example embodiment, and  FIGS. 10D to 10F  are cross-sectional views illustrating portions of an autofocusing pixel group according to an example embodiment. 
     Referring to  FIG. 10A , an autofocusing pixel group  330 - 1  may include a first photodiode PD 1  and a second photodiode PD 2  formed in a first pixel area, a third photodiode PD 3  and a fourth photodiode PD 4  formed in a second pixel area, a first drain terminal DN 1  connected to the third photodiode PD 3 , and a second drain terminal DN 2  connected to the fourth photodiode PD 4 . The first to fourth photodiodes PD 1  to PD 4  may be doped with n-type, and the first drain terminal DN 1  and the second drain terminal DN 2  may be doped with n-type. 
     Referring to  FIG. 10B , an autofocusing pixel group  330 - 1  may include a second photodiode PD 2 , a second color filter CF 2  disposed on the second photodiode PD 2 , a fourth photodiode PD 4 , a fourth color filter CF 4  disposed on the fourth photodiode PD 4 , a device isolation layer  331 , a pixel circuit  333 , a floating diffusion region FD, and a second drain terminal DN 2 . 
     The device isolation layer  331  may electrically isolate the first pixel area and the second pixel area. For example, the device isolation layer  331  may electrically isolate the second photodiode PD 2  and the fourth photodiode PD 4  disposed adjacent to each other. The floating diffusion region FD may store electric charges generated by each of the second photodiode PD 2  and the fourth photodiode PD 4 . The pixel circuit  333  may include a second transfer gate electrode TG 2 , a fourth transfer gate electrode TG 4 , and metal interconnections. 
     A transfer control signal VTG may be input to the second transfer gate electrode TG 2  of the second transfer transistor connected to the second photodiode PD 2  in the first pixel area. Thus, the second transfer transistor may be turned on. As the second transfer transistor is turned on, electric charges generated in the second photodiode PD 2  may be discharged to the floating diffusion region FD. 
     A barrier voltage V PB  may be applied to the fourth transfer gate electrode TG 4  of the fourth transfer transistor connected to the fourth photodiode PD 4  in the second pixel area. The barrier voltage may refer to a voltage for forming a sufficient potential barrier between the fourth photodiode PD 4  and the floating diffusion region FD. A power supply voltage V PIX  may be input to the second drain terminal DN 2  of the second pixel area from a power supply node. As the power supply voltage VPIX is input to the second drain terminal DN 2 , electric charges generated by the fourth photodiode PD 4  may be discharged to the second drain terminal DN 2 . For example, the electric charges generated in the fourth photodiode PD 4  by the power supply voltage V PIX  may not migrate to the floating diffusion region FD due to the potential barrier, and may migrate to the power supply node. 
     Referring to  FIG. 10C , a potential level is a negative potential in an upward direction and a positive potential in a downward direction. The potential level may be decreased in the upward direction and may be increased in the downward direction. 
     Referring to both  FIGS. 10B and 10C , when a barrier voltage is applied to the fourth transfer gate electrode TG 4  to form a sufficient potential barrier between the fourth photodiode PD 4  and the floating diffusion region FD, a channel region below the fourth transfer gate electrode TG 4  may have a first potential level V 1 . Each of the second photodiode PD 2  and the fourth photodiode PD 4  may have a second potential level V 2 . When the second transfer transistor is turned on, the channel region below the second transfer gate electrode TG 2  may have a third potential level V 3 . Each of the floating diffusion region FD and the second drain terminal DN 2  may have a fourth potential level V 4 . A potential level of the floating diffusion region FD and a potential level of the second drain terminal DN 2  may be different from each other. 
     The first potential level V 1  may be lower than the second potential level V 2 , the second potential level V 2  may be lower than the third potential level V 3 , and the third potential level V 3  may be lower than the fourth potential level V 4 . 
     Electric charges (−), generated in the second photodiode PD 2  due to a difference between a potential level of the second photodiode PD 2  and a potential level of the channel region below the second transfer gate electrode TG 2 , may migrate to the floating diffusion region FD through a first path P 1 . 
     However, a potential barrier PB may be provided between the fourth photodiode PD 4  and the floating diffusion region FD due to the difference between a potential level of the fourth photodiode PD 4  and a potential level of the channel region below the fourth transfer gate electrode TG 4 . Therefore, electric charges (−) generated in the fourth photodiode PD 4  may not migrate to the floating diffusion region FD and may be discharged to the second drain terminal DN 2  through a second passage P 2 . 
     According to an example embodiment, the autofocusing pixel group  330  of  FIG. 10A  may not include the third transfer gate electrode TG 3  and the fourth transfer gate electrode TG 4 . When a potential barrier between the third photodiode PD 3  and the floating diffusion region FD and a potential barrier between the fourth photodiode PD 4  and the floating diffusion region FD are formed to be sufficiently high, electric charges generated in the third photodiode PD 3  and the fourth photodiode PD 4  may not migrate to the floating diffusion region FD and may be discharged to the drain terminals DN 1  and DN 2  even the case in which a third transfer gate electrode and a third transfer gate electrode are not provided. 
     Referring to  FIG. 10D , unlike the autofocusing pixel groups  330 - 1  of  FIGS. 10A to 10C , an autofocusing pixel group  330   c  of  FIG. 10D  has a second pixel region in which at least one of a third unit pixel and a fourth unit pixel may not have a drain terminal. For example, the fourth unit pixel may not have a drain terminal. Instead, a metal may be further provided between a fourth color filter CF 4  of the fourth unit pixel and a microlens ML 2 . Light, incident on the fourth photodiode PD 4 , may be blocked by the metal. Thus, the autofocusing pixel group may generate image data for performing an autofocusing function even when the fourth unit pixel does not have a drain terminal. 
     Unlike the autofocusing pixel group  330   c  of  FIG. 10D , an autofocusing pixel group  330   d  of  FIG. 10E  has a second pixel area which may not include at least one of a third photodiode and a fourth photodiode. For example, the second pixel area may not include a fourth photodiode PD 4 . 
     Unlike an autofocusing pixel group  330   d  of  FIG. 10E , an autofocusing pixel group  330   e  of  FIG. 10F  is an autofocusing pixel group in which a metal, or the like, may not be provided between a microlens ML 2  and a fourth color filter CF 4  of a fourth unit pixel. Instead, the fourth unit pixel of the second pixel area may further have a drain terminal DN 2 . A power supply voltage may be applied to the drain terminal DN 2 . A device isolation layer  331  may physically completely isolate the second unit pixel and the fourth unit pixel from each other. 
     The device isolation layer  331  may refer to a DTI region extending (or formed) from a first surface (or a lower portion) of a semiconductor substrate SL in a vertical direction. In addition, the device isolation layer  331  may refer to a DTI formed in a process of manufacturing a backside illuminated (BSI) image sensor. The device isolation layer  331  may prevent crosstalk between one pixel and another pixel. 
     When the semiconductor substrate SL of the fourth unit pixel is formed of silicon, electric charges may be generated in the semiconductor substrate SL of the fourth unit pixel by light. Since the device isolation layer  331  may physically completely isolate one pixel from another pixel, the electric charges generated in the semiconductor substrate SL of the fourth unit pixel by light may not migrate to an adjacent second unit pixel and may be discharged to the second drain terminal DN 2 . 
     As described with reference to  FIGS. 10A to 10D , the number of photodiodes included in the second pixel area may be the same as the number of photodiodes included in the first pixel area. According to an example embodiment, as described with reference to  FIGS. 10E and 10F , the number of photodiodes included in the second pixel area may be smaller than the number of photodiodes included in the first pixel area. According to an example embodiment, the second pixel area may not include a photodiode. 
     In  FIGS. 10A to 10D , a single pixel group has been illustrated and described as including two autofocusing photodiodes and two drain photodiodes. However, a single pixel group may include at least one autofocusing photodiode and at least one drain photodiode. 
       FIG. 11A  is a plan view of an autofocusing pixel group according to an example embodiment,  FIG. 11B  is a cross-sectional view taken along line II-IT in  FIG. 11A , and  FIG. 11C  is a cross-sectional view taken along line III-III′ in  FIG. 11A . 
     Unlike the autofocusing pixel group  330 - 1  of  FIG. 11A , an autofocusing pixel group  330 - 2  of  FIG. 11A  is an autofocusing pixel group in which a third transfer gate electrode TG 3  and a fourth transfer gate electrode TG 4  may be connected to each other to constitute a single transfer gate electrode TG 3 ′. In addition, a third photodiode PD 3  and a fourth photodiode PD 4  may be connected to each other to constitute a single photodiode PD 3 ′. A single drain terminal DN may be connected to the photodiode PD 3 ′. 
     The photodiode PD 3 ′ may have a shape different from a shape of each of the first photodiode PD 1  and the second photodiode PD 2 . In addition, the photodiode PD 3 ′ may have an area different from an area of each of the first photodiode PD 1  and the second photodiode PD 2 . For example, the photodiode PD 3 ′ may have an area larger than an area of each of the first photodiode PD 1  and the second photodiode PD 2 . In this case, a light receiving area of the photo gate electrode TG 3 ′ may be larger than a light receiving area of each of the first photodiode PD 1  and the second photodiode PD 2 . 
     Referring to  FIG. 11B , the third photodiode PD 3  and the fourth photodiode PD 4  may be connected to each other to constitute a single photodiode PD 3 ′, and a single drain terminal DN may be connected to the photodiode PD 3 ′. A power supply voltage V PIX  may be input to the drain terminal DN. As the power supply voltage V PIX  is input to the drain terminal DN, electric charges generated by the photodiode PD 3 ′ may be discharged to the drain terminal DN. 
     Referring to  FIG. 11C , the transfer gate electrode TG 3 ′ may be disposed between a floating diffusion region FD and the drain terminal DN. The floating diffusion region FD may be doped with n − -type, while the drain terminal DN may be doped with n + -type. A barrier voltage V PB  may be input to the transfer gate electrode TG 3 ′ of the transfer transistor connected to the photodiode PD 3 ′. The power supply voltage V PIX  may be input to the drain terminal DN. Therefore, electric charges generated in the photodiode PD 3 ′ may not migrate to the floating diffusion region FD and may be discharged to the drain terminal DN. 
     In the case in which the transfer gate electrode TG 3 ′ is not present between the floating diffusion region FD and the drain terminal DN, when a sufficient potential barrier is not formed between the floating diffusion region FD and the photodiode PD 3 ′, a portion of electric charges generated in the photodiode PD 3 ′ may migrate to the floating diffusion region FD. 
     According to an example embodiment, the third transfer gate electrode TG 3  and the fourth transfer gate electrode TG 4  may connected to each other to constitute a single transfer gate electrode TG 3 ′, and the barrier voltage V PB  may be input to the transfer gate electrode TG 3 ′. As the barrier voltage V PB  is input to the transfer gate electrode TG 3 ′, a high potential barrier may be formed between the floating diffusion region FD and the photodiode PD 3 ′. Accordingly, it may be difficult for the electric charges, generated in the photodiode PD 3 ′, to migrate to the floating diffusion region FD. As a result, an effect of discharging the electric charges, generated in the photodiode PD 3 ′, to the drain terminal DN may be significantly improved. 
       FIG. 12A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 12B  is a cross-sectional view taken along line IV-IV′ in  FIG. 12A . 
     Unlike the autofocusing pixel group  330 - 2  of  FIG. 11A , an autofocusing pixel group  330 - 3  of  FIG. 12A  may further include a ground terminal GN. The ground terminal GN may supply a ground voltage to unit pixels included in a pixel array. 
     The ground terminal GN may be disposed in each normal pixel group. As a p + -type ground terminal GN approaches n-type photodiodes, dark current may be increased. As the dark current is increased, a signal detected even when light is not irradiated (a dark level) may be increased. As the dark level is increased, the ground terminal GN may act as a factor causing noise in an image sensor. 
     In an image sensor according to an example embodiment, a ground terminal GN may be disposed in respective autofocusing pixel groups smaller in number than normal pixel groups. Accordingly, the number of factors, causing noise in the image sensor, may be decreased. Even when the ground terminal GN is disposed in each of the autofocusing pixel groups, a sufficient ground voltage may be supplied to unit pixels included in the pixel array. 
     Referring to  FIG. 12B , a barrier voltage V PB  may be input to a transfer gate electrode TG 3 ′ of a transfer transistor connected to a photodiode PD 3 ′, a power supply voltage V PIX  may be input to a drain terminal DN, and a ground voltage GND may be input to the ground terminal GN. As the ground voltage GND is input to each of the ground terminals GN included in the autofocusing pixel groups, the ground voltage GND may be supplied to unit pixels included in the pixel array. 
       FIG. 13A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 13B  is a cross-sectional view taken along line V-V′ in  FIG. 13A . 
     Referring to  FIG. 13A , unlike the autofocusing pixel group  330 - 3  of  FIG. 12A , an autofocusing pixel group  330 - 4  of  FIG. 13A  is an autofocusing pixel group in which a ground terminal GN is disposed in a region including a drain photodiode PD 3 ″ formed therein, rather than in a region including autofocusing photodiodes PD 1  and PD 2  formed therein. As an n + -type drain terminal DN and a p + -type ground terminal GN approach, an electric field may be increased between a drain terminal DN and the ground terminal GN. As the electric field is increased, an image defect (for example, a white spot) may occur. 
     In the image sensor according to the example embodiment, a transmission gate electrode TG 3 ″ may be formed to extend from a photodiode PD 3 ″. For example, the transfer gate electrode TG 3 ″ may have a Y shape. The transfer gate electrode TG 3 ″ may extend from the photodiode PD 3 ″ to be disposed between the drain terminal DN and the ground terminal GN. The transfer gate electrode TG 3 ″, disposed between the drain terminal DN and the ground terminal GN, may prevent an electric field from increasing between the drain terminal DN and the ground terminal GN. 
     In addition, when a third transfer gate electrode TG 3  and a fourth transfer gate electrode TG 4  are separately disposed, a contact may be disposed on each of the third transfer gate electrode TG 3  and the fourth transfer gate electrode TG 4 . The contact may be connected to a metal layer. However, when the third transfer gate electrode TG 3  and the fourth transfer gate electrode TG 4  are connected to each other to constitute a single transfer gate electrode TG 3 ″ and the transfer gate electrode TG 3 ″ extends from the photodiode PD 3 ″, a single contact may be disposed on the transfer gate electrode TG 3 ″. The contact may be connected to the metal layer. 
     Referring to  FIG. 13B , the photodiode PD 3 ″ of the autofocusing pixel group  330 - 4  of  FIG. 13B  may include a first region S 1  and a second region S 2 . The first region S 1  may be disposed on the ground terminal GN, and the second region S 2  may be disposed on the drain terminal DN. 
     The first region S 1  may have a thickness T 1  different from a thickness T 2  of the second region S 2 . For example, the first region S 1  may have a thickness T 1  less than a thickness T 2  of the second region S 2 . In this case, an upper surface UP 1  of the first region S 1  may have the same depth as an upper surface UP 2  of the second region S 2 , and a lower surface LP 1  of the first region S 1  may have the same depth as a lower surface LP 1  of the second region S 2 , and the second region S 2  may have a height LP 1  different from a height LP 2  of the lower surface S 2 . 
     A barrier voltage V PB  may be input to a transfer gate electrode TG 3 ′ of the transfer transistor connected to the photodiode PD 3 ′, a power supply voltage V PIX  may be input to the drain terminal DN, and a ground voltage GND may be input to the ground terminal GN. 
       FIG. 14A  is a plan view of an autofocusing pixel group according to an example embodiment, and  FIG. 14B  is a cross-sectional view taken along line VI-VI′ in  FIG. 14A . 
     Referring to  FIG. 14A , an autofocusing pixel group  330 - 5  may have a first drain terminal DN 1 , connected to a third photodiode PD 3 , and a second drain terminal DN 2  connected to a fourth photodiode PD 4 . The ground terminal GN may be disposed between the first drain terminal DN 1  and the second drain terminal DN 2 . 
     Unlike the autofocusing pixel group of  FIG. 13A , an autofocusing pixel group of  FIG. 14A  may have the same layout of the drain photodiodes PD 3  and PD 4  and transfer gate electrodes TG 3  and TG 4  as a normal pixel group. Therefore, it may be advantageous in terms of process management. 
     Referring to  FIG. 14B , a first power supply voltage V PIX   1  may be input to a first drain terminal DN 1  connected to a third photodiode PD 3 , and a second power supply voltage V PIX   2  may be input to a second drain terminal DN 2  connected to the fourth photodiode PD 4 . A ground voltage GND may be input to a ground terminal GN. 
       FIG. 15  is a plan view of an autofocusing pixel group according to an example embodiment. Unlike the autofocusing pixel group  330 - 5  of  FIG. 14A , a autofocusing pixel group  330 - 6  of  FIG. 15  may not include the third transfer gate electrode and the fourth transfer gate electrode. This is because, when both a potential barrier between a third photodiode PD 3  and a floating diffusion region FD and a potential barrier between a fourth photodiode PD 4  and the floating diffusion region FD are formed to be sufficiently high, electric charges generated in the third photodiode PD 3  and the fourth photodiode PD 4  do not migrate to the floating diffusion region FD and may be discharged to terminals DN 1  and DN 2 . 
       FIG. 16  illustrates a pixel array according to an example embodiment, and  FIG. 17  is a plan view of autofocusing pixel groups according to an example embodiment. 
     Referring to  FIG. 16 , a third pixel group  330 ′ and a fifth pixel group  350 ′, adjacent to each other in the pixel array  500 , may each be an autofocusing pixel group. For example, the third pixel group  330 ′ may generate first image data (for example, left image data), and the fifth pixel group  350 ′ may generate second image data (for example, right image data). 
     Referring to both  FIGS. 16 and 17 , a third photodiode PD 3  of the third pixel group  330 ′ and a first photodiode PD 1  of the fifth pixel group  350 ′ may connected to each other to constitute a photodiode PD 3 ′. Therefore, a single first drain terminal DN 1  may be connected to the photodiode PD 3 ′. 
     In addition, a fourth photodiode PD 4  of the third pixel group  330 ′ and a second photodiode PD 2  of the fifth pixel group  350 ′ may be connected to each other to constitute a single photodiode PD 4 ′. Therefore, a single second drain terminal DN 2  may be connected to the photodiode PD 4 ′. 
     The ground terminal GN may be disposed between the first drain terminal DN 1  and the second drain terminal DN 2 . 
       FIG. 18  is a partial cross-sectional view of pixels according to an example embodiment. 
     Referring to  FIG. 18 , a third pixel group  330  of a pixel array  600  may be an autofocusing pixel group. A second unit pixel of the third pixel group  330  may include a second photodiode PD 2  and a color filter CF 2 , and a fourth unit pixel of the third pixel group  330  may include a fourth photodiode PD 4  and a color filter CF 4 . The second unit pixel and the fourth unit pixel of the third pixel group  330  may share a single microlens ML 2 . The second photodiode PD 2  of the third pixel group  330  may be an autofocusing photodiode, and the fourth photodiode PD 4  of the third pixel group  330  may be a drain photodiode. 
     Each of the fourth pixel group  330  and a fifth pixel group  350  may be a normal pixel group. The fourth unit pixel of the fourth pixel group  330  may include a fourth photodiode PD 4 , a fourth color filter CF 4 , and a microlens ML 1 . The second unit pixel of the fifth pixel group  350  may include a second photodiode PD 2 , a second color filter CF 2 , and a microlens ML 1 . 
     A BDTI structure may be formed between unit pixels to vertically extend from a first surface LP of a semiconductor substrate SL. A second drain terminal DN 2  may be formed between the fourth photodiode PD 4  of the third pixel group  330  and a second surface UP of a semiconductor substrate SL by increasing an n-type doping concentration. 
     A transfer control signal V T G may be input to a second transfer gate electrode TG 2  of the second transfer transistor connected to the second photodiode PD 2  of the third pixel group  330 . Thus, the second transfer transistor can be turned on. When the second transfer transistor remains in a turned-on state, a potential level of a channel region below the second transfer gate electrode TG 2  is higher than a potential level of the second photodiode PD 2 , and thus, electric charges generated in the second photodiode PD 2  may migrate to a floating diffusion region FD. 
     A barrier voltage V PB  may be input to a fourth transfer gate electrode TG 4  of the fourth transfer transistor connected to the fourth photodiode PD 4  of the third pixel group  330 . A power supply voltage V PIX  may be input to the second drain terminal DN 2 . Therefore, a potential barrier may be provided between the fourth photodiode PD 3  and the floating diffusion region FD due to a difference between a potential level of the fourth photodiode PD 4  and a potential level the channel region below the fourth transfer gate electrode TG 4 . Electric charges, generated in the fourth photodiode PD 4 , may not migrate to the floating diffusion region FD due to the potential barrier and may be discharged to the second drain terminal DN 2 . According to an example embodiment, the third pixel group  330  may not include the fourth transfer gate electrode TG 4 . 
     An image sensor may generate first image data or second image data using the electric charges generated in the second photodiode PD 2  of the third pixel group  330 . The image sensor may perform an autofocusing function using the first image data and the second image data. 
     The image sensor may generate image data using the electric charges generated in the fourth photodiode PD 4  of the fourth pixel group  330  and the second photodiode PD 2  of the fifth pixel group  350 . The image sensor may generate an image using the image data. 
       FIGS. 19A and 19B  illustrate sensitivity of an image sensor according to an example embodiment. 
     Referring to  FIGS. 19A and 19B , an X-axis may represent illuminance (LUX), and a Y-axis may represent an output of an analog-to-digital converter, for example, a digital signal. There may be an allowable range in an analog input values which may be converted by an analog-to-digital converter. When an input which may be out of the allowable range is applied, the input may have a saturated value SAT within an electrically allowable range. For example, the analog-to-digital converter may represent resolutions of up to 10 bits. In this case, the analog-to-digital converter may output 0 to 1023 LSB, and the saturated value SAT may be 1023 LSB. 
     In  FIGS. 19A and 19B , a graph “a” may denote a first signal S 1  generated from a normal pixel group, and a graph “b” may denote a second signal S 2  generated from an autofocusing pixel group. The first signal S 1  may correspond to the amount of summed electric charges output from four photodiodes included in the normal pixel group. The second signal S 1  may correspond to the amount of summed electric charges output from two autofocusing photodiodes included in the autofocusing pixel group. 
     As illustrated in  FIG. 19A , sensitivity of the autofocusing pixel group may be less than sensitivity of the normal pixel group. Therefore, when the sensitivity of the autofocusing pixel group increases by the sensitivity of the normal pixel group as shown in the graph “c” of  FIG. 19B , the sensitivity of the normal pixel group and the sensitivity of the autofocusing pixel group may be substantially the same. 
     For example, conversion gains CG, applied when electric charges generated in photodiodes included in the autofocusing pixel group into a voltage, may be adjusted. For example, a conversion gain of electric charges, generated in the photodiode included in the autofocusing pixel group, may be greater than a conversion gain of electric charge generated in the photodiode included in the normal pixel group. 
     As a first embodiment, a distance between a metal layer connected to a floating diffusion region and a metal layer around the floating diffusion region in the autofocusing pixel group may be adjusted. 
     For example, the shorter a distance between a first metal layer connected to the floating diffusion region and a metal layer around the first metal layer, the higher capacitance between the metal layers. Thus, a conversion gain CG of a photodiode may be decreased. However, the longer the distance between the metal layer connected to the floating diffusion region and the metal layer around the metal layer, the lower the capacitance between the metal layers. Thus, the conversion gain CG of the photodiode may be increased. 
     For example, the distance between the first metal layer connected to the floating diffusion region of the normal pixel group and the metal layer around the metal layer may be shorter than the distance between the second metal layer connected to the floating diffusion region of the autofocusing pixel group and the metal layer around the second metal layer. Thus, the sensitivity of the normal pixel group and the sensitivity of the autofocusing pixel group may be substantially the same. 
     As a second embodiment, at least one of a width and a length of the metal layer connected to the floating diffusion region may be adjusted. For example, the width of the second metal layer connected to the floating diffusion region of the autofocusing pixel group may be smaller than a width of the first metal layer connected to the floating diffusion region of the normal pixel group. The less the width of the metal layer connected to the floating diffusion region, the more the conversion gain CG of the photodiode may be increased. In addition, the length of the second metal layer connected to the floating diffusion region of the autofocusing pixel group may be less than the length of the first metal layer connected to the floating diffusion region of the normal pixel group. The less the length of the metal layer connected to the floating diffusion region is shorter, the more the conversion gain CG of the photodiode may be increased. 
     As a third embodiment, when a color filter of each of the unit pixels included in the autofocusing pixel group is a white color filter, sensitivity of the photodiode may be increased. 
     As a fourth embodiment, a concentration of impurities, implanted into the floating diffusion region, may be adjusted. For example, a concentration of impurities, implanted into the floating diffusion region of the autofocusing pixel group, may be lower than a concentration of impurities implanted into the floating diffusion region of the normal pixel group. The lower the concentration of the floating diffusion region, the more the conversion gain CG of the photodiode may be increased. 
     In this application, a single pixel group has been illustrated and described as including two autofocusing photodiodes and two drain photodiodes. However, a single pixel group may include at least one autofocusing photodiode and at least one drain photodiode. 
       FIG. 20  is a schematic block diagram of an electronic device including an image sensor according to an example embodiment. 
     A computer device  1000  according to an example embodiment illustrated in  FIG. 20  may include an image sensor  1010 , an input/output device  1020 , a memory  1030 , a processor  1040 , a port  1050 , and the like. In addition, the computer device  1000  may further include a wired/wireless communications device, a power supply, and the like. Among the components illustrated in  FIG. 20 , the port  1050  may be an element for communicating with a video card, a sound card, a memory card, an USB card, or the like. The computer device  1000  may be a smartphone, a tablet personal computer (PC), a smart wearable device, or the like, in addition to a general desktop computer or a laptop computer. 
     The processor  1040  may perform specific operations, commands, tasks, or the like. In some embodiments, the processor  1040  may be implemented with processing circuitry such as hardware including logic circuits; a hardware/software combination such as a processor executing software; or a combination thereof. For example, the processing circuitry more specifically may include, but is not limited to, a central processing unit (CPU), an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, application-specific integrated circuit (ASIC), an analog element (e.g., circuit), an application processor, etc. The processor  1040  may be a central processing unit (CPU), a microprocessor unit (MCU), a system on chip (SoC), or the like, and the processor  1040  may communicate with the image sensor  1010 , the display  1020 , and the memory  1030 , as well as other devices connected to the port  1050 , via a bus  1060 . 
     The memory  1030  may be a storage medium for storing data used for operation of the electronic device  1000 , multimedia data, and the like. The memory  1030  may include a volatile memory such as a random-access memory (RAM), or a non-volatile memory such as a flash memory. The memory  1030  may also include a solid-state drive (SSD), a hard disk drive (HDD), or an optical disk drive (ODD) as a storage device. An input and output device may also be included in the electronic device  1000  or be connected with the electronic device  1000  and may include an input device such as a keyboard, a mouse, a touch screen, and the like, and an output device such as a display, an audio output unit, and the like, provided to a user. 
     The image sensor  1010  may be mounted on a package substrate and may be connected to the processor  1040  by the bus  1060  or by another communication means. The image sensor  1010  may be employed in the electronic device  1000  as in the example embodiments described with reference to  FIGS. 1 to 19 . 
     As described above, a drain terminal, connected to at least one photodiode among photodiodes constituting a pixel group, may be formed by adjusting a doping concentration between the at least one photodiode and a substrate. Electric charges, generated in the at least one photodiode, may not migrate to a floating diffusion region and may be discharged to the drain terminal. As a result, crosstalk may be limited and/or prevented from occurring when the pixel group provides an autofocusing function. 
     While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of inventive concepts as defined by the appended claims.