Patent Publication Number: US-11658193-B2

Title: Image sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/840,663 filed on Apr. 6, 2020, which is a continuation of U.S. patent application Ser. No. 16/218,704 filed on Dec. 13, 2018, no U.S. Pat. No. 10,714,517 issued on Jul. 14, 2020, which claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2018-0008297 filed on Jan. 23, 2018, Korean Patent Application No. 10-20184-060446 filed on May 28, 2018, and Korean Patent Application No. 10-2018-0110823 filed on Sep. 17, 2018 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     The present inventive concept relates to an image sensor. 
     DESCRIPTION OF RELATED ART 
     An image sensor is a semiconductor-based sensor for receiving light to produce an electrical signal. For example, the image sensor converts light into an electrical signal that conveys information used to make an image. The image sensor may include a pixel array having a plurality of pixels, a circuit for driving the pixel array and generating an image, and the like. The plurality of pixels may include a photodiode for generating an electric charge in response to external light, and a pixel circuit for converting the electric charge generated by the photodiode into an electrical signal. The image sensor was traditionally employed in cameras for capturing still and video images, but is now widely applied to smartphones, tablet personal computers (PC)s, laptop computers, televisions, automobiles, and the like. In recent years, studies have been carried out to increase a dynamic range of the image sensor, and to accurately detect light from a light source in which a flicker phenomenon occurs. 
     SUMMARY 
     According to an exemplary embodiment of the present inventive concept, there is provided an image sensor including: a first photodiode, a first circuit including an overflow transistor and a first transfer transistor connected to the first photodiode, a switch element connected to the first transfer transistor and a capacitor disposed between the first transfer transistor and the switch element, wherein the capacitor is a physical capacitor, a second photodiode; and a second circuit including a second transfer transistor connected to the second photodiode, a reset transistor connected to an output of the first circuit and a driving transistor connected to the second transfer transistor and the output of the first circuit. 
     According to an exemplary embodiment of the present in concept, there is provided an image sensor including: a first photodiode; a first circuit including an overflow transistor and a first transfer transistor connected to the first photodiode, a switch element connected to the first transfer transistor and a metal-insulator-metal (MIM) capacitor disposed between the first transfer transistor and the switch element a second photodiode; and a second circuit including a second transfer transistor connected to the second photodiode, a reset transistor connected to an output of the first circuit and a driving transistor connected to the second transfer transistor and the output of the first circuit. 
     According to an exemplary embodiment of the present inventive concept, there is provided an image sensor including: a first photodiode; a first circuit including an overflow transistor and a first transfer transistor connected to the first photodiode, a switch element connected to the first transfer transistor and a capacitor disposed between the first transfer transistor and the switch element; a second photodiode; and a second circuit including a second transfer transistor connected to thre second photodiode a reset transistor connected to an output of the first circuit and a driving transistor connected to the output of the first circuit and a floating diffusion node, wherein a conversion gain is different when the driving transistor is turned on and off. 
     According to an exemplary embodiment of the present inventive concept, there is provided an image sensor including: a pixel array including a plurality of pixels, a first pixel including a first photodiode, a first pixel circuit, a second photodiode, and a second pixel circuit, wherein the first pixel circuit includes an overflow transistor and a first transfer transistor connected to the first photodiode, a switch element connected to the first transfer transistor and a capacitor connected between the first transfer transistor and the switch element, wherein the second pixel circuit includes a second transfer transistor connected to the second photodiode, a driving transistor connected to a floating diffusion node, and a reset transistor connected to a pixel voltage, wherein the image sensor further comprises a controller configured to turn the driving transistor on and off to sense the second photodiode a plurality of times. 
     According to an exemplary embodiment of the present inventive concept, there is provided a method of operating an image sensor including a first photodiode; a second photodiode; a first circuit and a second circuit, the first circuit including an overflow transistor and a first transfer transistor connected to the first photodiode, a switch element connected to the first transfer transistor and a capacitor disposed between the first transfer transistor and the switch element, the second circuit including a second transfer transistor connected to the second photodiode, a reset transistor connected to an output of the first circuit and a driving transistor connected to the second transfer transistor, the method including: sensing the second photodiode for a first period of time when the driving transistor is off; and sensing the second photodiode for a second period of time when the driving, transistor is off, wherein the first period of time is longer than the second period of time, wherein the first period of time overlaps a period of time in which the first photodiode is sensed and read, and the second period of time does not overlap the period of time in which the first photodiode is sensed and read. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other features of the present inventive concept will be more clearly understood by describing in detail exemplary embodiments thereof in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a block diagram illustrating an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  2  and  3    are views illustrating an image processing device including an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  4  and  5    are plan views illustrating a pixel array included in an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIG.  6    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIG.  7    is a view illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  8  and  9    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  10 ,  11  and  12    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIG.  13    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  14 ,  15 ,  16  and  17    are views illustrating, an operation of an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIG.  18    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  19 ,  20 A and  20 B  are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept; 
         FIGS.  21 ,  22 ,  23 ,  24 ,  26  and  27    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept; and 
         FIG.  28    is a block diagram illustrating an electronic device including an image sensor according to an exemplary embodiment of the present inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present inventive concept will be described with reference to the accompanying drawings. Like reference numerals may refer to like elements in the drawings. 
       FIG.  1    is a block diagram illustrating an image processing device including an image sensor according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG.  1   , an image processing device  1  may include an image sensor  10 , and an image processor  20 . The image sensor  10  may include a pixel array  11 , a row driver  12 , a column driver  13 , a read-out circuit  14 , a timing controller  15 , and the like. 
     The image sensor  10  may operate according to a control command received from the image processor  20 , and may convert light transmitted from an object  30  into an electrical signal and output the electrical signal to the image processor  20 . The pixel array  11  included in the image sensor  10  may include a plurality of pixels PXs. The plurality of pixels PXs may include photodiodes for receiving light to generate an electric charge. In an exemplary embodiment of the present inventive concept, each of the plurality of pixels PXs may include two or more photodiodes. 
     Each of the plurality of pixels PXs may include a pixel circuit for generating an electrical signal from an electric charge generated by the photodiode. In an exemplary embodiment of the present inventive concept, the pixel circuit may include a transfer transistor, a driving transistor, a selection transistor, a reset transistor, or the like. When one pixel PX has two or more photodiodes, each pixel PX ma include a pixel circuit for processing an electric charge generated in each of the two or more photodiodes. For example, when one pixel PX has two or more photodiodes, the pixel circuit may include two or more of at least the transfer transistor, the driving transistor, the selection transistor, and the reset transistor. 
     In an exemplary embodiment of the present inventive concept, one pixel PX may include a first photodiode and a second photodiode. In addition, in an exemplary embodiment of the present inventive concept, one pixel PX may include a first pixel circuit or processing an electric charge generated in the first photodiode, and a second pixel circuit for processing an electric charge generated in the second photodiode. The first pixel circuit and the second pixel circuit may comprise a plurality of semiconductor elements, respectively. The first pixel circuit may generate a first pixel signal from an electric charge generated in the first photodiode and output the first pixel signal to a first column line. The second circuit may generate a second pixel signal from an electric charge generated in the second photodiode, and output the second pixel signal to a second column line. Each of the first and second pixel signals may include a reset voltage and a pixel voltage. 
     The row driver  12  may drive the pixel array  11  on a row basis. For example, the row driver  12  may generate a transfer control signal for controlling the transfer transistor of each pixel PX, a reset control signal for controlling the reset transistor of each pixel PX, a selection control signal for controlling the selection transistor of each pixel PX, or the like. 
     The column driver  13  may include a correlated double sampler (CDS), an analog-to-digital converter (ADC), or the like. The correlated double sampler may perform correlated double sampling by receiving a pixel signal through column lines connected to pixels PXs included in a row selected by a row selection signal supplied by the row driver  12 . The analog-to-digital converter may convert an output of the correlated double sampler into a digital signal, and deliver the digital signal to the read-out circuit  14 . 
     The read-out circuit  14  may include a latch or buffer circuit for temporarily storing a digital signal, an amplifying circuit, and the like. The read-out circuit  14  may process the digital signal received from the column driver  13  to generate image data. Operational timings of the row driver  12 , the column driver  13  and the read-out circuit  14  may be determined by the timing controller  15 . The timing controller  15  may control the row driver  12 , the column driver  13  and the read-out circuit  14  based on a control command from the image processor  20 . The image processor  20  may process image data output from the read-out circuit  14 , and output the processed image data to a display device or the like, or store the processed image data in a storage device such as a memory. Alternatively, when the image processing device  1  is mounted on an autonomous vehicle, the image processor  20  may process image data, and may transfer the processed image data to a main controller for controlling the autonomous vehicle. 
       FIG.  2    is a view illustrating an image processing device including an image sensor according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG.  2   , an image processing device  2  may include a pixel array region  40 , a logic circuit region  50  below the pixel array region  40 , a memory region  60  below the logic circuit region  50 , and the like. The pixel array region  40 , the logic circuit region  50  and the memory region  60  may be stacked on each other. In an exemplary embodiment of the present inventive concept, the pixel array region  40  may be stacked on the logic circuit region  50  at a wafer level, and the memory region  60  may be attached to a lower portion of the logic circuit region  50  at a chip level. 
     The pixel array region  40  may include a sensing area SA in which a plurality of pixels PXs 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 PADs. The plurality of upper pads PADs may be connected to a second pad area PA 2  of the logic circuit area  50  through a via, and to a logic circuit LC. 
     Each of the plurality of pixels PXs may include a photodiode for receiving light to generate an electric charge, a pixel circuit for converting the electric charge generated by the photodiode into an electrical signal, and the like. The photodiode may include an organic photodiode or a semiconductor photodiode. In an exemplary embodiment of the present inventive concept, a plurality of semiconductor photodiodes may be included in each of the plurality of pixels PXs. The pixel circuit may include a plurality of transistors for converting the electric charge generated by the photodiode into an electrical signal. 
     The logic circuit region  50  may include a plurality of circuit elements formed in the logic circuit LC. The plurality of circuit elements included in the logic circuit LC may be circuits for driving a pixel circuit provided in the pixel array region  40 , such as a row diver, a column driver, and a timing controller. The plurality of circuit elements included in the logic circuit LC may be connected to the pixel circuit through the first and second pad areas PA 1  and PA 2 . 
     The memory region  60  provided on the lower portion of the logic circuit region  50  may include a memory chip MC, a dummy chip DC, and a protection layer EN for sealing the memory chip MC and the dummy chip DC. The memory chip MC may be a dynamic random access memory (DRAM) or a static random access memory (SRAM). The dummy chip DC may not actually store data. The memory chip MC may be electrically connected to at least a portion of the circuit elements included in the logic circuit region  50  by a bump. In an exemplary embodiment of the present inventive concept, the bump may be a micro-bump. 
     Next, referring to  FIG.  3   , an image sensor  3  according to an exemplary embodiment of the present inventive concept may include a first layer  70  and a second layer  80 . The first layer  70  includes a sensing region SA in which a plurality of pixels PX are provided, a control logic region LC in which elements for driving the plurality of pixels PX are provided, and a first pad area PA 1  provided around the control logic area LC. The first pad area PA 1  includes a plurality of upper pads PAD to which a memory chip MC provided in the second layer  80  can be connected through a via. The second layer  80  may include the memory chip MC, a dummy chip DC and a protective layer EN sealing the memory chip MC and the dummy chip DC. 
       FIGS.  4  and  5    are plan views illustrating a pixel array included in an image sensor according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG.  4   , a pixel array  100  may include a plurality of pixels  110 . The plurality of pixels  110  may be arranged in a matrix form along a plurality of rows and columns on an X-Y plane. An isolation region  120  may be formed between the plurality of pixels  110  to prevent cross-talk. The isolation region  120  may include an insulating material such as an oxide, and may be firmed by a deep trench isolation (DTI) process. Sidewalls of the isolation region  120  adjacent to the plurality of pixels  110  may be formed of a material having high reflectance. 
     Each of the plurality of pixels  110  may include a photodiode for receiving light to generate an electric charge, and a plurality of semiconductor elements that convert the electric charge generated by the photodiode into an electrical signal. For example, each of the plurality of pixels  110  may include a first photodiode  111  and a second photodiode  112 . The first photodiode  111  and the second photodiode  112  may be arranged adjacent to each other on the X-Y plane. For example, the first photodiode  111  and the second photodiode  112  may be disposed at the same level in a direction along a Z-axis. 
     In an exemplary embodiment of the present inventive concept, the first photodiode  111  may have a light receiving area larger than the second photodiode  112 . Therefore, the second photodiode  112  may be more easily saturated than the first photodiode  111 . In an exemplary embodiment of the present inventive concept, the first photodiode  111  may be used for general image processing, while the second photodiode  112  may be used for accurately detecting an external light source in which a flicker phenomenon occurs. The second photodiode  112  may also be used for image processing to increase dynamic range. When the image sensor according to an exemplary embodiment of the present inventive concept is applied to an autonomous vehicle or the like, it is possible to accurately detect a signal lamp, a head lamp or tail lamp of a nearby vehicle, or the like, which uses a light emitting diode (LED) in which a flicker phenomenon occurs. 
     The second photodiode  112  may have a light receiving area smaller than that of the first photodiode  111 . This way, the second photodiode  112  can be more easily saturated. In an exemplary embodiment of the present inventive concept, a means for preventing saturation of the second photodiode  112 , or a means for generating accurate image data despite the saturation of the second photodiode  112  may be provided to prevent erroneous sensing of light of a signal lamp and/or a nearby vehicle using an LED. 
     The arrangement of the first photodiode  111  and the second photodiode  112  is not necessarily limited to that illustrated in  FIG.  4   , and may be variously modified. In the pixel array  100 A according to the embodiment illustrated in  FIG.  5   , in the four pixels  110 A adjacent to each other, the first photodiodes  111 A may be disposed adjacent to each other. The second photodiode  112 A has a light receiving area smaller than that of the first photodiode  111 A and the isolation region  120 A may be provided between the pixels  110 A to prevent crosstalk. 
       FIG.  6    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG.  6   , a pixel circuit  200  according to an exemplary embodiment of the present inventive concept may include a first pixel circuit  210  and a second pixel circuit  220 . The first pixel circuit  210  may output an electrical signal using an electric charge generated by a first photodiode PD 1 , and the second pixel circuit  220  may output an electrical signal using an electric charge generated by a second photodiode PD 2 . The first photodiode PD 1  has a light receiving area larger than that of the second photodiode PD 2 . 
     The first pixel circuit  210  may include a first reset transistor RX 1 , a first transfer transistor TX 1 , a driving transistor DX, and a selection transistor SX. The first photodiode PD 1  may be connected to a first floating diffusion FD 1  via the first transfer transistor TX 1 . The first reset transistor RX 1  may be connected to a supply or pixel voltage VDD. 
     The first transfer transistor TX 1  may transfer the electric charge accumulated in the first photodiode PD 1  to the first floating diffusion FD 1 , based on a first transfer control signal TG 1  transferred from a row driver. The first photodiode PD 1  may generate electrons as a main charge carrier. The driving transistor DX may operate as a source follower buffer amplifier by the electric charge accumulated in the first floating diffusion FD 1 . The driving transistor DX may amplify the electric charge accumulated in the first floating diffusion FD 1  to generate an electric signal, and transfer the electric signal to the selection transistor SX. 
     The selection transistor SX may be operated by a selection control signal SEL input by the row driver, and may perform switching and addressing operations. When the selection control signal SEL is applied from the row driver, a voltage corresponding to the electric signal may be output to a column line Col connected to the selection transistor SX. The voltage may be detected by a column driver and a read-out circuit, connected to the column line Col. The column driver and the read-out circuit may detect a reset voltage in a state in which no electric charge is accumulated in the first floating diffusion FD 1 , and detect a pixel voltage in a state in which the electric charge is transferred to the first floating diffusion FD 1 . In an exemplary embodiment of the present inventive concept, the image sensor may generate an image data by calculating a difference between the reset voltage and the pixel voltage. 
     The second photodiode PD 2  may be connected to an overflow transistor OX and a second transfer transistor TX 2  of the second pixel circuit  220 . The second photodiode PD 2  may generate electrons as a main charge carrier, in a similar manner to the first photodiode PD 1 . The electric charge generated by the second photodiode PD 2  may move to a second floating diffusion FD 2 , when the second transfer transistor TX 2  is turned on. The second photodiode PD 2  may generate an electric charge in response to light when the second transfer transistor TX 2  is turned off. Each time the second transfer transistor TX 2  is turned on, the electric charge generated by the second photodiode PD 2  may be accumulated in the second floating diffusion FD 2 . The overflow transistor OX may be connected to a supply or pixel voltage VDD. 
     In an exemplary embodiment of the present inventive concept, the overflow transistor OX may be used to prevent saturation of the second photodiode PD 2 . The overflow transistor OX may prevent saturation of the second photodiode PD 2 , by repeatedly turning on and off to remove at least a portion of the electric charge venerated by the second photodiode PD 2 . The repeated switching of the overflow transistor OX on and off may occur over a predetermined period of time. The second transfer transistor TX 2  may be turned on, while the overflow transistor OX is turned off, to transfer the electric charge generated by the second photodiode PD 2  to the second floating diffusion FD 2 . To prevent an unintended reset of the second floating diffusion FD 2 , the second transfer transistor TX 2  and the overflow transistor OX may not be turned on at the same time. A second reset transistor RX 2  may be turned off, while the electric charge of the second photodiode PD 2  is moved to the second floating diffusion FD 2 . This way, the electric charge generated in the first photodiode PD 1  and the electric charge generated in the second photodiode PD 2  are not combined. 
     In the embodiment illustrated in  FIG.  6   , the first photodiode PD 1  and the second photodiode PD 2  may share the column line Col. Therefore, while the first pixel voltage generated using the electric charge of the first photodiode PD 1  is output to the column line Col, the second photodiode PD 2  is separated from the column line Col. In the embodiment illustrated in  FIG.  6   , when the first pixel voltage is output to the column line Col, at least one of the second reset transistor RX 2  and the second transfer transistor TX 2  may be turned off to separate the second photodiode PD 2  from the column line Col. The first transfer transistor TX 1  may be turned on to accumulate the electric charge of the first photodiode PD 1  in the first floating diffusion FD 1 . This way, a first pixel voltage is generated using the electric charge of the first photodiode PD 1 , and is output to the column line Col. 
     Similarly, when a second pixel voltage corresponding to the electric charge of the second photodiode PD 2  is output to the column line Col, the first photodiode PD 1  may be separated from the column line Col. In the embodiment illustrated in  FIG.  6   , when the second pixel voltage is output to the column line Col, the first transfer transistor TX 1  may be turned off to separate the first photodiode PD 1  from the column line Col. The second transfer transistor TX 2  and the second reset transistor RX 2  may be turned on to connect the first floating diffusion FD 1  and the second floating diffusion FD 2 . This way, a second pixel voltage is generated using the electric charge of the second photodiode PD 2 , and is output to the column line Col. The electric charge of the second photodiode PD 2  may be accumulated in the first floating diffusion FD 1  and the second floating diffusion FD 2 , and the driving transistor DX may convert the electric charge to a voltage. 
     In an exemplary embodiment by the driving transistor DX, the second photodiode PD 2  may be used to sense an external light source in which a flicker phenomenon occurs, or to increase a dynamic range of an image sensor. To increase the dynamic range of the image sensor, when the first pixel voltage generated from the electric charge of the first photodiode PD 1  is output over a plurality of times, the second pixel voltage generated from the electric charge of the second photodiode PD 2  may be output once. 
     The first photodiode PD 1  may have a relatively larger area than the second photodiode PD 2 . In an exemplary embodiment by the driving transistor DX, an image expressing an external light source in which a flicker phenomenon occurs may be generated using an electric charge generated by the second photodiode PD 2 . In addition, an electric charge generated by the first photodiode PD 1  may be used for general image processing. In addition, the dynamic range, the image quality, and the like of the image sensor may be increased by adjusting an exposure time in which each of the first photodiode PD 1  and the second photodiode PD 2  receives light. The following description will be made with reference to  FIG.  7   . 
       FIG.  7    is a view illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept. In an exemplary embodiment of the present inventive concept.  FIG.  7    may be a timing diagram for illustrating operations of image sensors in different modes of operation. 
     First, while a dynamic range of an image sensor may be increased, an image expressing an external light source in which a flicker phenomenon occurs may be generated by an operation according to an exemplary embodiment of the present inventive concept illustrated in  FIG.  7   . Referring to  FIG.  7   , the operation of the image sensor according to an exemplary embodiment of the present inventive concept may start, as a first reset transistor RX 1  and a second reset transistor RX 2  are turned on to reset voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 . The first reset transistor RX 1  may be turned on by a first reset control signal RG 1 , and the second reset transistor RX 2  may be turned on by a second reset control signal RG 2 . 
     When the voltages of the first and second floating diffusions FD 1  and FD 2  are reset, a second transfer transistor TX 2  and an overflow transistor OX may be alternately turned on and off, and an electric charge generated by a second photodiode PD 2  may be accumulated in the second floating diffusion FD 2 . The overflow transistor OX may be turned on and off using an overflow control signal OG, and the second transfer transistor TX 2  may be turned on and off by a second transfer control signal TG 2 . The second reset transistor RX 2  may maintain a turned-off state, such that the electric charge accumulated in the second floating diffusion FD 2  is not moved (e.g., leaked) to the first floating diffusion FD 1 . In addition, the first reset transistor RX 1  may maintain a turned on state such that the voltage of the first floating diffusion FD 1  is sufficiently reset. 
     In an exemplary embodiment of the present inventive concept illustrated in  FIG.  7   , the electric charge generated by the second photodiode PD 2  may be accumulated in the second floating diffusion FD 2  over n times (where n is a natural number). Referring to  FIG.  7   , the second photodiode PD 2  may be exposed to light during a plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) to generate an electric charge. The electric charge generated by the second photodiode PD 2  may be accumulated in the second floating diffusion FD 2 , each time the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) elapse. Each of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) may be set as an amount of time the second photodiode PD 2  can be exposed without being saturated. According to the present embodiment, the electric charge generated by the second photodiode PD 2  may be transferred to the second floating diffusion FD 2  immediately. 
     The number of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) and the respective lengths of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) may be determined in consideration of an operating frequency and a duty ratio of a commercial LED, or the like. For example, when the operating frequency of the LED is about 100 Hz and the duty ratio is about 10%, the total sum of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) may be 10 msec or less, and a number n, e.g., the number of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn), may be 10 or more. By setting the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) as described above, at least one of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) may be overlapped with a turn-on time of the LED. Therefore, the first photodiode PD 1  may generate the electric charge in response to light of a turned on LED, and may accurately detect light of an LED operating in a pulse-width-modulation manner. 
     The image sensor may turn the second reset transistor RX 2  off and turn the selection transistor SX on to detect a first reset voltage, before, the electric charge generated by the second photodiode PD 2  is transferred to the second floating diffusion FD 2  during a period of the last n th  time. The column driver and the read-out circuit connected to the column line Col of the pixel circuit may include a sampling circuit for detecting a voltage of the column line Col. The sampling circuit may detect the first reset voltage during a first sampling time t 1  and a reset voltage detecting signal SHR has a high (HIGH) logic value during the first sampling time t 1 . 
     In one example, the reset voltage detected by the sampling circuit during the first sampling time t 1  may be the voltage of the first floating diffusion FD 1 . The first floating diffusion FD 1  and the second floating diffusion FD 2  may be reset together, since the first and second reset transistors RX 1  and RX 2  are simultaneously turned on at the beginning of operation. Therefore, the voltage of the first floating diffusion FD 1  detected during the first sampling time t 1  may be selected as the first reset voltage. 
     When the electric charge generated by the second photodiode PD 2  is accumulated at the n th  time in the second floating diffusion FD 2 , the image sensor may turn the second reset transistor RX 2  on and the electric charge accumulated in the second floating diffusion FD 2  may be shared with the first floating diffusion FD 1 . At the same time, the image sensor may turn the selection transistor SX on to detect the first pixel voltage generated from the electric charge of the second photodiode PD 2  through the column line Col. The sampling circuit may detect the first pixel voltage during a second sampling time t 2  in which the pixel voltage detecting signal SHS has a high logic value. The image sensor may calculate a difference between the first reset voltage and the first pixel voltage detected in each of the first sampling time t 1  and the second sampling time t 2  to generate first raw data. The first raw data may be data corresponding to the electric charges generated by the second photodiode PD 2  when the second photodiode PD 2  is exposed to light during the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn). 
     In addition, while the electric charges generated by the second photodiode PD 2  are accumulated in the second floating diffusion FD 2  over a plurality of times, the first transfer transistor TX 1  may be sequentially turned on and off. For example, when the first transfer transistor TX 1  is turned on, the first reset transistor RX 1  may be turned on to remove the electric charge present in the first photodiode PD 1 , and to reset the first floating diffusion FD 1 . The first transfer transistor TX 1  may then be turned off, such that the first photodiode PD 1  and the first floating diffusion FD 1  are separated from each other. The first transfer transistor TX 1  may be turned off for at least a portion of the plurality of times in which the electric charge of the second photodiode PD 2  accumulates in the second floating diffusion FD 2 . While the first transfer transistor TX 1  is turned off, the first photodiode PD 1  may be exposed to light and generate an electric charge. 
     For example, the first photodiode PD 1  may be exposed to light during a first exposure time de 1  to generate the electric charge. The first photodiode PD 1  may have a larger area than the second photodiode PD 2  such that the first exposure time de 1  is longer than each of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn) in which the second photodiode PD 2  is exposed to light. For example, the first exposure time de 1  may be set to be longer than the sum of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn). 
     During a third sampling time t 3  in which the reset voltage detecting signal SHR has a high logic value, before the first exposure time de 1  ends, the image sensor may detect the voltage of the first floating diffusion FD 1  as the second reset voltage. Referring to  FIG.  7   , the first reset transistor RX 1  may be turned on to reset the first floating diffusion FD 1 , before the third sampling time t 3 . The third sampling time t 3  may be a time within the first exposure time de 1 . When the first exposure time de 1  ends, the first transfer transistor TX 1  may be turned on to transfer the electric charge accumulated in the first photodiode PD 1  to the first floating diffusion FD 1 . 
     The image sensor may detect a second pixel voltage corresponding to the electric charge transferred to the first floating diffusion FD 1  during a fourth sampling time t 4 , after the first transfer transistor TX 1  is turned off. The image sensor may generate second raw data for generating an image by calculating a difference between the second reset voltage and the second pixel voltage detected in each of the third sampling time t 3  and the fourth sampling time t 4 . When the fourth sampling time t 4  ends, the first transfer transistor TX 1  may be turned on to transfer an electric charge existing in the first photodiode PD 1  to the first floating diffusion FD 1 . In this case, the electric charge of the first photodiode PD 1  may be removed. Thereafter, the first reset transistor RX 1  may be turned on to reset the first floating diffusion FD 1 . 
     In addition, the image sensor may expose the first photodiode PD 1  to light during a second exposure time de 2  that is shorter than the first exposure time de 1 , after the fourth sampling time t 4  elapses, and may detect a voltage of the first floating diffusion FD 1  as the third reset voltage, during a fifth time t 5  included in the second exposure time de 2 . Referring to  FIG.  7   , the first reset transistor RX 1  may be turned on to reset the first floating diffusion FD 1 , before the fifth time t 5  starts. For example, the second exposure time de 2  may be set to be shorter than the sum of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn). 
     When the second exposure time de 2  ends, the first transfer transistor TX 1  may be turned on to apply an electric charge generated by the first photodiode PD 1  during the second exposure time de 2  to the first floating diffusion FD 1 . The image sensor may detect the voltage of the first floating diffusion FD 1  as the third pixel voltage for a sixth time t 6 , after the first transfer transistor TX 1  is turned off. The image sensor may calculate a difference between the third reset voltage and the third pixel voltage detected in each of the fifth time t 5  and the sixth time t 6  to generate third raw data for generating an image. 
     In an exemplary embodiment of the present inventive concept, the image sensor may combine the first to third raw data obtained in each of the plurality of pixels to obtain a single image. As described above, the first to third raw data may be data obtained by exposing the first and second photodiodes PD 1  and PD 2  to light for different exposure times. Therefore, the first to third raw data may be combined to obtain a single image, to increase a dynamic range characteristic of the image. 
     In an exemplary embodiment of the present inventive concept explained referring to  FIG.  7   , it is assumed that the first raw data corresponding to an intermediate exposure time may be obtained using the second photodiode PD 2 , and the second and third raw data corresponding to a long exposure time and a short exposure time may be obtained using the first photodiode PD 1 . However, the present inventive concept is not limited thereto. For example, in various alternative embodiments, the second photodiode PD 2  may be used to obtain raw data corresponding to a long exposure time or a short exposure time. 
     In addition, the first raw data may be used as data for accurately reflecting a light source such as an LED, etc., in which a flicker phenomenon occurs, in an image. In an exemplary embodiment of the present inventive concept, the second photodiode PD 2  may be prevented from being saturated by using the overflow transistor OX, such that light of a light source such as an LED is accurately detected, even when the surrounding illuminance is relatively low. In addition, the first photodiode PD 1  and the second photodiode PD 2  are disposed in one pixel to use the electric charge of the second photodiode PD 2  for generating a general image, and to use the first photodiode PD 1  for detecting light of a light source such as an LED. Therefore, the frame rate of the image does not have to be sacrificed to detect the light of the light source such as the LED. 
       FIGS.  8  and  9    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept. 
       FIG.  8    is a view illustrating an operation of a general image sensor. In this embodiment, an LED may operate in a pulse-width-modulation PWM manner. Therefore, as illustrated in  FIG.  8   , the LED may operate according to a period T having a turn-on time T on  and a turn-off time T off . 
     First, referring to a first case (case 1) of  FIG.  8   , whether or not light emitted from the LED is detected by the image sensor may be determined, according to whether the exposure time of the photodiodes included in the image sensor to light is overlapped with the turn-on T on  of the LED. For example, since the first case (case 1) is a case in which the image sensor is exposed to a high illuminance environment, the exposure time of the photodiode in the first case (case 1) may be set short. A first exposure time ex 1  in the first case (case 1) may overlap the turn-on time T on  of the LED, and thus, a light of the LED may be accurately detected by using an electric charge generated in the photodiode during the first exposure time ex 1 . 
     In addition, in the first case (case 1), a second exposure time ex 2  may not overlap the turn-on time T on  of the LED. The duty ratio representing the ratio of the turn-on time T on  to the whole period T in the PWM manner for driving the LED may not be 100%. Therefore, in the first case (case 1) in which the first and second exposure times ex 1  and ex 2  are set to be short, the exposure times of the photodiode may not overlap with the turn-on time T on  of the LED as illustrated by the second exposure time ex 2 . Therefore, the light of the LED may not be accurately detected by the image produced by the electric charge generated in the photodiode during the second exposure time ex 2 . 
     Next, a second ease (case 2) of  FIG.  8    may be a case in which the image sensor is exposed to a low illuminance environment. Therefore, as illustrated in  FIG.  8   , the exposure time of the photodiode may be set to be long. In the second case (case 2), since the photodiode is exposed for a long period of time, the photodiode may be easily saturated. As a result, the light of the LED may not be accurately detected. 
       FIG.  9    is a view illustrating an operation of the image sensor according to an exemplary embodiment of the present inventive concept. As described above, the image sensor according to an exemplary embodiment of the present inventive concept may include a plurality of pixels, and each of the plurality of pixels may include a first photodiode and a second photodiode. The second photodiode may have a small area as compared with the first photodiode, and may be used for detecting a light source in which a flicker phenomenon occurs, such as an LED. Similar to the embodiment illustrated in  FIG.  8   , the LED may operate in a pulse-width-modulation manner, and have a turn-on time T on  and a turn-off time T off  within one period T. 
     Referring to  FIG.  9   , the exposure time of the second photodiode PD 2  may be shorter than the turn-on time of the LED. As described above with reference to  FIG.  7   , the second photodiode PD 2  may not be saturated due to on/off switching operations of the second transfer transistor TX 2  and the overflow transistor OX, and thus, the second photodiode PD 2  may be exposed to light over a plurality of times to generate an electric charge. An electric charge generated by the second photodiode PD 2  may accumulate in the second floating diffusion FD 2 , every time the exposure time is ended. Therefore, saturation of the second photodiode PD 2  may be prevented, regardless of the illuminance of the external environment to be captured by the image sensor. The second photodiode PD 2  may be exposed to light for a short exposure time over a plurality of times, such that the turn-on time T on  of the LED and the exposure time of the second photodiode PD 2  do not deviate from each other. Therefore, LED light in which a flicker phenomenon occurs may be accurately detected. 
       FIGS.  10  to  12    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept.  FIGS.  10  to  12    may be timing diagrams illustrating an operation of an image sensor having the pixel circuit according to the embodiment illustrated in  FIG.  6   . 
     Referring to  FIG.  10   , an operation of an image sensor according to an exemplary embodiment of the present inventive concept may start, such that the first reset transistor RX 1  and the second reset transistor RX 2  are turned on to reset voltages of the first floating diffusion FD 1  and second the floating diffusion FD 2 . When the voltages of the first and second floating diffusions FD 1  and FD 2  are reset, the second transfer transistor TX 2  and the overflow transistor OX may be alternately turned on and off, such that an electric charge generated by the second photodiode PD 2  may be accumulated in the second floating diffusion FD 2 . The second reset transistor RX 2  may maintain a turned-off state such that the electric charge accumulated in the second floating diffusion FD 2  is not leaked. In addition, the first reset transistor RX 1  may maintain a turned on state such that the voltage of the first floating diffusion FD 1  may be sufficiently reset. 
     While the electric charge is accumulated in the second floating diffusion FD 2 , the first transfer transistor TX 1  may be turned off, and the first photodiode PD 1  may generate an electric charge. The first photodiode PD 1  may generate the electric charge during a first exposure time de 1 , and then, move the electric charge to the first floating diffusion FD 1  in response to a turn-on operation of the first transfer transistor TX 1 . 
     In the embodiment illustrated in  FIG.  10   , a controller may obtain a first pixel voltage using the electric charge accumulated in the second floating diffusion FD 2  during a first sampling time t 1 . The controller may generate first raw data using a difference between the first pixel voltage obtained during the first sampling time t 1  and the reset voltage obtained during a second sampling time t 2 . The controller may also generate second raw data using a difference between the reset voltage obtained during the second sampling time t 2  and the second pixel voltage obtained during a third sampling time t 3 . The electric charge of the first photodiode PD 1  may be transferred to the first floating diffusion FD 1  by the first transfer transistor TX 1 , turned on between the second sampling time t 2  and the third sampling time t 3 . The controller may use the electric charge of the first floating diffusion FD 1  to obtain the second pixel voltage. 
     In the embodiment illustrated in  FIG.  10   , a reset voltage for generating the first raw data, and a reset voltage for generating the second raw data may be shared. The controller may not separately detect the reset voltage before acquiring the first pixel voltage, and may generate the first raw data using the reset voltage detected after acquiring the first pixel voltage. 
     In addition, the controller may expose the first photodiode PD 1  during the second exposure time de 2 , after acquiring the second pixel voltage and rescuing the first photodiode PD 1 . A third pixel voltage may be detected during a fifth time t 5  by the electric charge generated in the first photodiode PD 1  during the second exposure time de 2 . The controller may generate third raw data using a difference between the reset voltage and the third pixel voltage obtained during the fourth sampling time t 4 , prior to the fifth time t 5 . The reset voltage obtained by the controller during the fourth sampling time t 4  may be different from the reset voltage obtained by the controller during the second sampling time t 2 . The controller may generate a single image using the first raw data, the second raw data, and the third raw data. 
     Next, referring to  FIG.  11   , an operation of an image sensor according to an exemplary embodiment of the present inventive concept may start such that a first reset transistor RX 1  and a second reset transistor RX 2  are turned on to reset voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 . In the embodiment illustrated in  FIG.  11   , a controller may obtain a first reset voltage from the first floating diffusion FD 1  during a first sampling time t 1 , after the first reset transistor RX 1  is turned off. When the first sampling time t 1  elapses, a second transfer transistor TX 2  and an overflow transistor OX may be alternately turned on and off, such that an electric charge generated by the second photodiode PD 2  is accumulated to the second floating diffusion FD 2 . 
     For example, in the embodiment illustrated in  FIG.  11   , the controller may acquire the first reset voltage, before the second photodiode PD 2  is exposed to light to generate an electric charge. The first reset voltage may be stored a memory connected to the image sensor, and the controller may obtain first raw data by calculating a difference between the first pixel voltage and the first reset voltage acquired during a second sampling time t 2 . In one example, the first reset voltage may be stored in a line memory. 
     An exemplary embodiment of the present inventive concept illustrated in  FIG.  12    may utilize a second photodiode PD 2  for accurately detecting light of a light source in which a flicker phenomenon occurs. Referring to  FIG.  12   , and similar to the embodiment explained with reference to  FIG.  7   , after rescuing voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 , an electric charge generated in the second photodiode PD 2  may be accumulated in the second floating diffusion FD 2  over a plurality of times. At this time, when the electric charge is accumulated in the second floating diffusion FD 2 , a second reset transistor RX 2  may be turned off to separate the first floating diffusion FD 1  and the second floating diffusion FD 2 , and a first photodiode PD 1  may be exposed to light to generate an electric charge. For example, the first photodiode PD 1  may generate the electric charge during a predetermined exposure time de. 
     The reset voltage and the pixel voltage detected from the first floating diffusion FD 1  in each of a first sampling time t 1  and a second sampling time t 2  may be used to generate first raw data. The first raw data may be used to accurately detect light of a light source in which a flicker phenomenon occurs. The second reset transistor RX 2  may be turned on, during the second sampling time t 2  or prior to the second sampling time t 2 , and the electric charge generated by the second photodiode PD 2  and accumulated in the second floating diffusion FD 2  may be shared with the first floating diffusion FD 1 . 
     In addition, when the second sampling time t 2  elapses, the first reset transistor RX 1  may be turned on to reset the first floating diffusion FD 1 , and a reset voltage and a pixel voltage may be detected at a third sampling time t 3  and a fourth sampling time t 4 , respectively. The pixel voltage detected at the fourth sampling time t 4  may be a voltage of the first floating diffusion FD 1  corresponding to an amount of electric charge accumulated in the first photodiode PD 1  during the exposure time de. An image sensor may generate the second raw data by calculating a difference between the reset voltage and the pixel voltage detected at the third sampling time t 3  and the fourth sampling time t 4 , respectively. The second raw data may be used to generate a general image. In an example, when the image sensor generates the first raw data once, the second raw data may be generated a plurality of times. Therefore, image frame rate degradation may be significantly decreased due to the generation of the first raw data. 
     In addition, the embodiment illustrated in  FIG.  12    may utilize the second photodiode PD 2  to increase the dynamic range of the image. In this case, the first raw data may be image data generated during a short exposure time, and the second raw data may be image data generated during a long exposure time. A controller of the image sensor may increase the dynamic range of the image by generating a single image using the first raw data and the second raw data. 
       FIG.  13    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept. 
     A pixel circuit  300  according to an exemplary embodiment of the present inventive concept illustrated in  FIG.  13    may include a first pixel circuit  310  and a second pixel circuit  320 . The first pixel circuit  310  may output an electrical signal using an electric charge generated by a first photodiode PD 1 , and the second pixel circuit  320  may output an electrical signal using an electric charge generated by a second photodiode PD 2 . A configuration of transistors included in each of the first pixel circuit  310  and the second pixel circuit  320  may be similar to the embodiment illustrated in  FIG.  6   . 
     Unlike the embodiment illustrated in  FIG.  6   , in the embodiment illustrated in  FIG.  13   , a second reset transistor RX 2  may be connected between a first floating diffusion FD 1  and a second floating diffusion FD 2 . For example, the second floating diffusion FD 2  may be connected to a first reset transistor RX 1 , the second reset transistor RX 2 , and a second transfer transistor TX 2 . Hereinafter, an operation of the pixel circuit  300  according to the embodiment illustrated in  FIG.  13    will be described with reference to  FIGS.  14  to  17   . 
       FIGS.  14  to  17    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept. 
     According to the embodiments illustrated in  FIGS.  14  to  17   , a dynamic range of an image sensor may be increased, and an image accurately representing an external light source, in which a flicker phenomenon occurs, may be generated. Referring to  FIG.  14   , an operation of the image sensor according to an exemplary embodiment of the present inventive concept may start such that a first reset transistor RX 1  and a second reset transistor RX 2  are turned on to reset voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 . The first reset transistor RX 1  may be turned on by a first reset control signal RG 1 , and the second reset transistor RX 2  may be turned on by a second reset control signal RG 2 . 
     When the voltages of the first and second floating diffusions FD 1  and FD 2  are reset, a second transfer transistor TX 2  and an overflow transistor OX may be alternately turned on and off, such that an electric charge generated by the second photodiode PD 2  is accumulated in the second floating diffusion FD 2 . The overflow transistor OX may be turned on and off using an overflow control signal OG, and the second transfer transistor TX 2  may be turned on and off by a second transfer control signal TG 2 . Since the second floating diffusion FD 2  is disposed between the first reset transistor RX 1  and the second reset transistor RX 2 , the first reset transistor RX 1  and the second reset transistor RX 2  may be kept in a turned-off state, while the electric charge is accumulated in the second floating diffusion FD 2 . 
     To generate first raw data corresponding to an amount of the electric charge generated by the second photodiode PD 2 , the image sensor may detect a first reset voltage and a first pixel voltage at a first sampling time t 1  and a second sampling time t 2 , respectively. Each of the first sampling time t 1  and the second sampling time t 2  may occur before and after the electric charge generated by the second photodiode PD 2  is accumulated in the second floating diffusion FD 2  at n th  time. In addition, since the electric charge accumulated in the second floating diffusion FD 2  is to be converted to the voltage in a driving transistor DX through the first floating diffusion FD 1 , the second reset transistor RX 2  may be turned on during the second sampling time t 2 . 
     In the pixel circuit  300  according to the embodiment illustrated in  FIG.  13   , the second floating diffusion FD 2  may be a node between the first reset transistor RX 1  and the second reset transistor RX 2 . Therefore, while the electric charge generated by the second photodiode PD 2  is accumulated in the second floating diffusion FD 2  over a plurality of times, the first reset transistor RX 1  can maintain a turned-off state and the first photodiode PD 1  may not be exposed to light to generate an electric charge. After the second sampling time t 2  elapses, the first transfer transistor TX 1 , the first reset transistor RX 1 , and the second reset transistor RX 2  may be turned on together to reset the first photodiode PD 1  and the first floating diffusion FD 1 . 
     When the first photodiode PD 1  and the first floating diffusion FD 1  are reset, the image sensor may turn the first transfer transistor TX 1  off, and expose the first photodiode PD 1  to light during a first exposure time de 1  to generate an electric charge. Before the first exposure time de 1  ends, the image sensor may turn the first and second reset transistors RX 1  and RX 2  off, and detect the second reset voltage from the first floating diffusion FD 1 . The second reset voltage may be detected during a third sampling time t 3  in which the reset voltage detecting signal SHR has a high logic value. For example, the first exposure time de 1  may be a time longer than the sum of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn). 
     When the first exposure time de 1  elapses, the image sensor may turn the first transfer transistor TX 1  on to transfer the electric charge accumulated in the first photodiode PD 1  to the first floating diffusion FD 1 . The image sensor may detect a voltage of the first floating diffusion FD 1  as a second pixel voltage during a fourth sampling time t 4 , after the first transfer transistor TX 1  is turned off. The image sensor may generate second raw data for generating an image by calculating a difference between the second reset voltage and the second pixel voltage detected at each of the third sampling time t 3  and the fourth sampling time t 4 . 
     When the fourth sampling time t 4  ends, the image sensor may expose the first photodiode PD 1  to light during a second exposure time de 2 , and detect a third reset voltage and a third pixel voltage at a fifth sampling time t 5  and a sixth sampling time t 6 , respectively. The second exposure time de 2  may be shorter than the sum of the plurality of times (d 1 , d 2 , . . . , dn- 1 , dn). The image sensor may generate third raw data necessary for generating an image by calculating a difference between the third reset voltage and the third pixel voltage detected at the fifth sampling time t 5  and the sixth sampling time t 6 , respectively. 
     In an exemplary embodiment of the present inventive concept, the image sensor may combine the first to third raw data obtained in each of the plurality of pixels to obtain a single image. As described above, the first to third raw data may be data obtained by exposing the first and second photodiodes PD 1  and PD 2  to light during different exposure times. Therefore, the first to third raw data may be combined to obtain a single image, to increase a dynamic range characteristic of the image. 
     In addition, the first raw data may be used to accurately reflect a light source such as an LED, etc., in which a flicker phenomenon occurs, in an image. In an exemplary embodiment of the present inventive concept, the second photodiode PD 2  may be prevented from being saturated due to the overflow transistor OX, such that light of a light source such as an LED may be accurately detected even when the surrounding illuminance is relatively low. In addition, the second photodiode PD 2  may be used for detecting light of a light source such as an LED in one pixel, and thus, general image data may be generated using the first photodiode PD 1 . Therefore, the frame rate does not have to be sacrificed to detect the light of the image. 
     Unlike the embodiment illustrated in  FIG.  14   , in an exemplary embodiment of the present inventive concept illustrated in  FIG.  15   , a first reset voltage may not be detected, when an electric charge of a second photodiode PD 2  is accumulated in a second floating diffusion FD 2 . A controller may obtain a first pixel voltage during a first sampling time t 1 , after the electric charge generated by the second photodiode PD 2  is accumulated at an n th  time in the second floating diffusion FD 2 . The controller may calculate a difference between a reset voltage obtained during a second sampling time t 2  and the first pixel voltage to obtain first raw data. 
     During a third sampling time t 3 , the controller may also obtain the second pixel voltage by an electric charge generated in a first photodiode PD 1  during a first exposure time de 1 . The controller may calculate a difference between the reset voltage obtained during the second sampling time t 2  and the second pixel voltage to obtain second raw data. For example, in the embodiment illustrated in  FIG.  15   , there may be one reset voltage used to acquire each of the first raw data and the second raw data. 
     In an exemplary embodiment of the present inventive concept illustrated in  FIG.  16   , unlike the embodiment illustrated in  FIG.  14   , during a first sampling time t 1 , before an electric charge of a second photodiode PD 2  is accumulated in the second floating diffusion FD 2 , a controller may obtain a first reset voltage. The controller may store the first reset voltage obtained during the first sampling time t 1  in a separate memory, and then, calculate a difference between a first pixel voltage obtained during a second sampling time t 2  and the first reset voltage to generate first raw data. The method of generating second raw data and third raw data may be the same as described with reference to  FIGS.  14  and  15   . 
     Next, an exemplary embodiment of the present inventive concept illustrated in  FIG.  17    may use a second photodiode PD 2  only to accurately detecting light of a light source in which a flicker phenomenon occurs. Referring to  FIG.  17   , after voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2  are reset, an electric charge generated by a second photodiode PD 2  may be accumulated in the second floating diffusion FD 2  over a plurality of times. The electric charge accumulated in the second floating diffusion FD 2  may be shared by the first floating diffusion FD 1 , as a second reset transistor RX 2  is turned on. 
     A first reset voltage and a first pixel voltage detected from the first floating diffusion FD 1  at each of the first sampling time t 1  and the second sampling time t 2  may be used to generate first raw data. The first raw data may be used to accurately detect light of a light source in which a flicker phenomenon occurs. 
     In addition, when the second sampling time t 2  ends, a first reset transistor RX 1  and a second reset transistor RX 2  may be turned on to reset voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2 . In this case, a reset voltage and a pixel voltage may be detected at a third sampling time t 3  and a fourth sampling time t 4 , respectively. The pixel voltage detected at the fourth sampling time t 4  may be a voltage of the first floating diffusion FD 1  corresponding to an amount of an electric charge accumulated in the first photodiode PD 1  during an exposure time de. The image sensor may generate second raw data by calculating a difference between the reset voltage and the pixel voltage detected at the third sampling time t 3  and the fourth sampling time t 4 , respectively. The second raw data may be used to generate general image data. As an example, when the image sensor generates the first raw data once, the second raw data may be generated over a plurality of times. Therefore, image frame rate degradation may be significantly decreased due to the generation of the first raw data. 
       FIG.  18    is a circuit diagram illustrating a pixel circuit included in an image sensor according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG.  18   , a pixel circuit  400  according to an exemplary embodiment of the present inventive concept may include a first pixel circuit  410  and a second pixel circuit  420 . The first pixel circuit  410  may process an electric charge generated in a first photodiode PD 1 , and the second pixel circuit  420  may process an electric charge generated in a second photodiode PD 2 . 
     The first pixel circuit  410  may include a first floating diffusion FD 1 , a first reset transistor RX 1 , a first transfer transistor TX 1 , a driving transistor DX, a selection transistor SX, and the like. The second pixel circuit  420  may include a second floating diffusion FD 2 , a second reset transistor RX 2 , a second transfer transistor TX 2 , an overflow transistor OX, a storage capacitor SC, a switch element SW, and the like. The operation of the active elements included in each of the first pixel circuit  410  and the second pixel circuit  420  may be controlled by a controller included in the image sensor. The storage capacitor SC may be connected to a supply or pixel voltage VDD. 
     In operation of the pixel circuit  400 , the first pixel circuit  410  and the second pixel circuit  420  may share at least a portion of the circuit elements. For example, the second pixel circuit  420  may use the driving transistor DX and the selection transistor SX to output a pixel voltage corresponding to an electric charge generated by the second photodiode PD 2 . The first pixel circuit  410  also may use the second reset transistor RX 2  and the second floating diffusion FD 2  to control an electric charge generated by the first photodiode PD 1  or a capacitance of the pixel. 
     In the embodiment illustrated in  FIG.  18   , the second pixel circuit  420  may include a storage capacitor SC for storing the electric charge generated by the second photodiode PD 2 . The storage capacitor SC may be a metal-insulator-metal (MIM) capacitor or an active capacitor. The storage capacitor SC may store an electric charge in response to an amount of the electric charge generated in the second photodiode PD 2  and an operation of the second transfer transistor TX 2 . The switch element SW may be connected between the storage capacitor SC and the second floating diffusion FD 2 , and the electric charge of the storage capacitor SC may be transferred to the second floating diffusion FD 2  according to on/off operation of the switch element SW. An output of the second circuit  420  may be connected to a node N between the driving transistor DX and the first reset transistor RX 1 , for example. A channel of the first transfer transistor TX 1 , the switch element SW or the second transfer transistor TX 2  includes indium gallium zinc oxide (IGZO), for example. 
     The second photodiode PD 2  may have a small light receiving area as compared with the first photodiode PD 1 , and therefore may be saturated more easily. In the embodiment illustrated in  FIG.  18   , the electric charge of the second photodiode PD 2  may be removed by using the overflow transistor OX, or the saturation of the second photodiode PD 2  may be prevented by transferring the electric charge of the second photodiode PD 2  to the storage capacitor SC. The image sensor including the pixel circuit  400  according to the embodiment illustrated in  FIG.  18    may use the electric charge generated in each of the first photodiode PD 1  and the second photodiode PD 2  to increase the dynamic range, and thus, may accurately capture a light source such as an LED, in which a flicker phenomenon occurs. 
       FIGS.  19 ,  20 A and  20 B  are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept. 
     First,  FIG.  19    may be a timing diagram for explaining the operation of the image sensor having the pixel circuit  400  according to the embodiment illustrated in  FIG.  18   . Referring to  FIG.  19   , the operation of the image sensor according to an exemplary embodiment of the present inventive concept may start, such that the first and second reset transistors RX 1  and RX 2  are turned on to reset voltages of the first and second floating diffusions FD 1  and FD 2 . The first reset transistor RX 1  may be turned on by a first reset control signal RG 1 , and the second reset transistor RX 2  may be turned on by a second reset control signal RG 2 . At this time, the electric charge of the first photodiode PD 1  may be removed by turning on the first transfer transistor TX 1 . 
     After the first transfer transistor TX 1  is turned off, the first photodiode PD 1  may be exposed to light during the first exposure time de 1 . Then, the selection transistor SX may be turned on by the selection control signal SEL to detect the reset voltage and the pixel voltage. When the selection transistor SX is turned on, the first reset voltage and the first pixel voltage may be sequentially detected during the first time D 1 . In one example, the sampling circuit of the controller may detect the first reset voltage during a first sampling time t 1  at which a reset voltage detection signal SHR has a high logic value. The controller may also detect the first pixel voltage during a second sampling time t 2  at which a pixel voltage detection signal SHS has a high logic value. The first transfer transistor TX 1  may be turned on and off between the first sampling time t 1  and the second sampling time t 2 , such that an electric charge at the first photodiode PD 1  may move to the first floating diffusion FD 1  during the first exposure time de 1 . 
     In the embodiment illustrated in  FIG.  19   , during the second time D 2  after the first time D 1 , the image sensor may detect the pixel voltage and the reset voltage from the first photodiode PD 1  once again. As illustrated in  FIG.  19   , when the second time D 2  starts, the first reset transistor RX 1  is turned off and the second reset transistor RX 2  is turned on. Therefore, the sum of turned on capacitances of the first floating diffusion FD 1 , the second floating diffusion FD 2 , and the second reset transistor RX 2  may correspond to a floating diffusion of the first photodiode PD 1 . As a result, in the second time D 2 , the electric charge of the first photodiode PD 1  may be stored in the floating diffusion having a larger area than a floating diffusion in the first time D 1 . Therefore, the conversion gain of the pixel during the second time D 2  may be less than the conversion gain of the pixel during the first time D 1 . 
     The sampling circuit of the image sensor may detect the second pixel voltage and the second reset voltage at each of the third sampling time t 3  and the fourth sampling time t 4 . For example, at the second time D 2 , the pixel voltage may be detected before the reset voltage. The second pixel voltage may correspond to the electric charge generated in the first photodiode PD 1  during the first exposure time de 1  and the second exposure time de 2 . The second exposure time de 2  may be shorter than the first exposure time de 1 . 
     When the second pixel voltage is detected, the controller may turn the first reset transistor RX 1  on and off to reset the voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2 , and may then detect the second reset voltage. In this case, to compensate for the coupling effect, the second reset transistor RX 2  may be turned off when the first reset transistor RX 1  is turned on. Referring to  FIG.  19   , at least a portion of the turn-on time of the first reset transistor RX 1  and the turn-off time of the second reset transistor RX 2  may overlap each other within the second time D 2 . 
     In the embodiment illustrated in  FIG.  19   , image data may be generated using the reset voltage and the pixel voltage detected under conversion gain conditions different from each other. Therefore, saturation of the first photodiode PD 1  and the first floating diffusion FD 1  may be prevented, and an optimized image may be provided to the user, irrespective of the illuminance of the environment in which the image sensor operates. In general, the capacitance of the first photodiode PD 1  may be determined in accordance with the high illuminance condition that can easily saturate the first photodiode PD 1 . In the embodiment illustrated  FIG.  19   , before reading the pixel voltage and the reset voltage at the second time D 2 , the second reset transistor RX 2  may be turned on to connect the first floating diffusion FD 1  and the second floating diffusion FD 2 , and not to saturate the first floating diffusion FD 1  by the electric charge of the first photodiode PD 1 . Therefore, the electric charge generated in the first photodiode FD 1  may be sufficiently accumulated in the first floating diffusion FD 1  and the second floating diffusion FD 2 . In addition, an image may be generated by using amounts of the electric charge more than in the first photodiode PD 1 , and thus, the saturation of the pixel may be prevented. In addition, in the operation according to the embodiment illustrated in  FIG.  19   , the second photodiode PD 2  may not be used. 
       FIGS.  20 A and  20 B  are provided to explain operations of a first photodiode PD 1  and a floating diffusion  FIGS.  20 A and  20 B  are views illustrating first and second photodiodes PD 1  and PD 2 , at a first time D 1  having a high conversion gain condition and at a second time D 2  having a low conversion gain condition, respectively. 
     First, referring to  FIG.  20 A , an electric charge generated in the first photodiode PD 1  may be transferred to the first floating diffusion FD 1  in the first time D 1 . Since the second reset transistor RX 2  is turned off during the first time D 1 , the electric charge may be accumulated only in the first floating diffusion FD 1 . A capacitance of the first photodiode PD 1  may be determined in consideration of a high conversion gain condition as illustrated in  FIG.  20 A . Therefore, a capacitance of the first floating diffusion FD 1  may be similar to a capacitance of the first photodiode PD 1 . 
     Next, referring to  FIG.  20 B , at the second time D 2 , the second reset transistor RX 2  may be turned on, and then, a turned on capacitance of the reset transistor RX 2 , as well as the first floating diffusion FD 1  and the second floating diffusion FD 2 , may be used as a floating diffusion. Therefore, amounts of electric charge exceeding the capacitance of the first photodiode PD 1  may be accumulated in the floating diffusion, and reflected in the pixel voltage through the driving transistor DX. For example, according to the embodiment described with reference to  FIGS.  19 ,  20 A and  20 B , image data may be generated using amounts of an electric charge that are more than the capacitance of the first photodiode PD 1 . Therefore, saturation of pixels may be prevented, and the quality of the image data may be increased at the same time. 
       FIGS.  21  to  27    are views illustrating an operation of an image sensor according to an exemplary embodiment of the present inventive concept.  FIGS.  21  to  27    are provided to illustrate different modes of operation of the image sensor. The image sensor may have a pixel circuit  400  according to the embodiment illustrated in  FIG.  18   . 
       FIGS.  21  and  22    are views illustrating an operation mode in which the dynamic range of the image sensor may be increased by using the first photodiode PD 1  and the second photodiode PD 2 . Referring to  FIG.  21   , in the operation mode for increasing the dynamic range, the first reset transistor RX 1  and the second reset transistor RX 2  may be turned on to reset voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2 . In addition, while the first reset transistor RX 1  and the second reset transistor RX 2  are turned on, the first transfer transistor TX 1  may be turned on and off to remove the electric charge from the first photodiode PD 1 . The first exposure time de 1  may start, when the first transfer transistor TX 1  is turned off. 
     When the first exposure time de 1  elapses and the first transfer transistor TX 1  is turned on, the electric charge generated in the first photodiode PD 1  during the first exposure time de 1  may be transferred to the first floating diffusion FD 1 . A sampling circuit of the controller may detect the first reset voltage and the first pixel voltage in each of the first sampling time t 1  and the second sampling time t 2  of the first time D 1 . The first sampling time t 1  may be a time before the electric charge of the first photodiode PD 1  is transferred to the first floating diffusion FD 1 , and the second sampling time t 2  may be a time after the electric charge of the first photodiode PD 1  is transferred to the first floating diffusion FD 1 . At least a portion of the first time D 1  including the first sampling time t 1  and the second sampling time t 2  may overlap the first exposure time de 1 . The controller may generate first raw data for generating the image data using the difference between the first reset voltage and the first pixel voltage. 
     The controller may separate the first floating diffusion FD 1  and the second floating diffusion FD 2  by turning off the second reset transistor RX 2  during the first time D 1 . Therefore, the electric charge of the first photodiode PD 1  may be accumulated only in the first floating diffusion FD 1 . When the first time D 1  elapses, the controller may turn on the second reset transistor RX 2  to reset voltage of the first floating diffusion FD 1 . 
     Referring to  FIG.  21   , a second exposure time de 2  may start during the first exposure time de 1 . The second exposure time de 2  may start, such that a second transfer transistor TX 2  and a switch element SW are turned on and off to reset a second floating diffusion FD 2 , a storage capacitor SC, and a second photodiode PD 2 . The second exposure time de 2  may be shorter than the first exposure time de 1 . 
     A second pixel voltage corresponding to an electric charge generated by the second photodiode PD 2  during the second exposure time de 2  may be detected in the second time D 2 . To detect the second pixel voltage, a first reset transistor RX 1  may be turned off to separate the second floating diffusion FD 2  from a power supply node. Further, a second reset transistor RX 2  may be turned on to connect a first floating diffusion FD 1  and the second floating diffusion FD 2 . 
     When the second exposure time de 2  elapses, the second transfer transistor TX 2  and the switch element SW may be turned on to transfer the electric charge of the second photodiode PD 2  to the first floating diffusion FD 1  and the second floating diffusion FD 2 . Then, during a third sampling time t 3  in the second time D 2 , a sampling circuit may detect the second pixel voltage. 
     When the second pixel voltage is detected, a controller of an image sensor may turn on the first reset transistor RX 1  to reset voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2 , and may detect a second reset voltage during a fourth sampling time t 4 . The controller may calculate the difference between the second pixel voltage and the second reset voltage to generate second raw data. The first reset transistor RX 1  may be turned off during the fourth sampling time t 4  to detect the second reset voltage, and may be turned back on after the second reset voltage is detected. 
     When the second reset voltage is detected, the controller may expose a first photodiode PD 1  to light during a third exposure time de 3 . The third exposure time de 3  may be shorter than the second exposure time de 2 , and the controller may turn on the first reset transistor RX 1 , the second reset transistor RX 2 , and the first transfer transistor TX 1  to reset the voltage of the first floating diffusion FD 1 , before the start of the third exposure time de 3 . 
     The controller may obtain the third reset voltage during a fifth sampling time t 5  in the third exposure time de 3 . When the third exposure time de 3  ends, the controller may transfer the electric charge of the first photodiode PD 1  to the first floating diffusion FD 1 , and may then acquire a third pixel voltage during a sixth sampling time t 6 . During a third time D 3  in which the controller acquires the third reset voltage and the third pixel voltage, the second reset transistor RX 2  may be turned off to separate the first floating diffusion FD 1  and the second floating diffusion FD 2 . The controller may calculate the difference between the third reset voltage and the third pixel voltage to obtain third raw data. 
     The controller may generate image data using the first raw data, the second raw data, and the third raw data. Since the second exposure time de 2  is shorter than the first exposure time de 1 , but longer than the third exposure time de 3 , the first to third raw data may correspond to an electric charge generated during exposure times different from each other. The controller may combine the first to third raw data to generate the image data, such that a dynamic range of the image sensor is increased and quality of the image is increased. 
     Referring to  FIG.  22   , a portion of operations may be the same as the embodiment illustrated in  FIG.  21   , but an operation of an overflow transistor OX may be different. A default state of the overflow transistor OX in the embodiment illustrated in  FIG.  21    may be off. A default state of the overflow transistor OX in the embodiment illustrated in  FIG.  22    may be on. In the embodiment illustrated in  FIG.  22   , the overflow transistor OX may maintain a turn-on state by default, and may be turned off during a portion of time corresponding to the second exposure time de 2 . The overflow transistor OX may be turned off during at least the second exposure time de 2 , since the second photodiode PD 2  is exposed to light to generate an electric charge during the second exposure time de 2 . 
     Next, the operation of the image sensor according to various exemplary embodiments of the present inventive concept will be described with reference to  FIGS.  23  to  27   . In embodiments illustrated in  FIGS.  23  to  27   , an image sensor may increase dynamic range, and may generate an image that accurately captures an external light source in which the flicker phenomenon appears, using a second photodiode PD 2 . 
     Referring to  FIG.  23   , a controller of an image sensor may turn on a first reset transistor RX 1  and a second reset transistor RX 2  to reset voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 . The controller may also control a second photodiode PD 2  to generate an electric charge by alternately turning an overflow transistor OX and a second transfer transistor TX 2  on and off. The second photodiode PD 2  may generate the electric charge during a second exposure time de 2  over a plurality of times. A switch element SW may be turned off while the overflow transistor OX and the second transfer transistor TX 2  are alternately turned on and off. Therefore, the electric charge of the second photodiode PD 2  may not be transferred to the second floating diffusion FD 2 , and may be stored in a storage capacitor SC. 
     While the second photodiode PD 2  generates an electric charge, the controller may perform a shutter operation for the first photodiode PD 1  by turning the first transfer transistor TX 1  on and off. Since the switch element SW is turned off, the shutter operation for the first photodiode PD 1  may not affect the second photodiode PD 2 . When the shutter operation is completed, the first photodiode PD 1  may generate an electric charge during a first exposure time de 1 . 
     In the embodiment illustrated in  FIG.  23   , the controller may control the second reset transistor RX 2  to change an area of a floating diffusion, and may detect a first sub-pixel voltage and a second sub-pixel voltage corresponding to the electric charge of the first photodiode PD 1 . In other words, the controller may detect a pixel voltage corresponding to the electric charge of the first photodiode PD 1  over two periods of time. Therefore, the first sub-pixel voltage and the second sub-pixel voltage may be detected under conversion gain conditions different from each other. By changing the area of the floating diffusion to detect the first sub-pixel voltage and the second sub-pixel voltage, a quality of an image may not be deteriorated due to the saturation of the first photodiode PD 1  under a high illuminance condition. The controller may obtain the first sub-pixel voltage and the second sub-pixel voltage in each of a first sub-time DS 1  and a second sub-time DS 2  of a first time D 1 . 
     For example, when the first sub-time DS 1  starts during the first exposure time de 1 , the controller may turn off the second reset transistor RX 2  to separate the first floating diffusion FD 1  from the second floating diffusion FD 2 , and obtain a first sub-reset voltage from the first floating diffusion FD 1  during a first sampling time t 1 . When the first sampling time t 1  elapses, the controller may turn the first transfer transistor TX 1  on to transfer an electric charge generated by the first photodiode PD 1  during the first exposure time de 1  to the first floating diffusion FD 1 , and may detect the first sub-pixel voltage during a second sampling time t 2 . 
     Next, when the second sub-time DS 2  starts, the controller may turn the first reset transistor RX 1  off and turn the second reset transistor RX 2  on to increase an area of the floating diffusion of the pixel and to lower the conversion gain. Therefore, a larger amount of an electric charge may be stored in the floating diffusion of the pixel. The controller may turn on the first transfer transistor TX 1  to store the electric charge of the first photodiode PD 1  in the first floating diffusion FD 1 , the second floating diffusion FD 2 , the turned on second reset transistor RX 2 , and the like. 
     The controller may detect the second sub-pixel voltage during a third sampling time t 3 , and detect a second sub-reset voltage during a subsequent fourth sampling time t 4 . The first reset transistor RX 1  may be turned on between the third sampling time t 3  and the fourth sampling time t 4  such that voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2  are reset. As described above with reference to  FIG.  19   , an operation of temporarily turning off the second reset transistor RX 2  may be further performed to offset a coupling effect. 
     The controller may generate first raw data using a difference between the first sub-reset voltage and the first sub-pixel voltage and a difference between the second sub-reset voltage and the second sub-pixel voltage. The first raw data may be image data corresponding to an electric charge generated by the first photodiode PD 1  during the longest first exposure time de 1 . 
     At a second time D 2 , subsequent to the first time D 1 , the controller may detect a second pixel voltage corresponding to the electric charge of the second photodiode PD 2 . At the second time D 2 , the second reset transistor RX 2  may be turned on and the first reset transistor RX 1  may be turned off, to connect the first floating diffusion FD 1  and the second floating diffusion FD 2 . An electric charge generated by the second photodiode PD 2  during the second exposure time de 2  and stored in the storage capacitor SC may be transferred to the first floating diffusion FD 1  and the second floating diffusion FD 2  in response to a turn-on operation of the switch element SW. A portion of the electric charge generated by the second photodiode PD 2  may be stored in the turned on second reset transistor RX 2 . For example, the switch element SW may be turned on after the last second exposure time de 2  has elapsed. In the embodiment illustrated in  FIG.  23   , although the switch element SW is turned on, together with the second transfer transistor TX 2  the switch element SW may be turned on prior to or subsequent to a turn on operation of the second transfer transistor TX 2 . 
     While the switch element SW is maintained in a turned on state, the controller may turn off the second transfer transistor TX 2  and turn on the overflow transistor OX to remove the electric charge of the second photodiode PD 2 . The controller may also detect the second pixel voltage during a fifth sampling time t 5 , and may detect the second reset voltage during a sixth sampling time t 6  subsequent to the fifth sampling time t 5 . Between the fifth sampling time t 5  and the sixth sampling time t 6 , the controller may turn on the first reset transistor RX 1  to reset voltages of the first floating diffusion FD 1  and the second floating diffusion FD 2 . 
     The controller may calculate a difference between the second pixel voltage and the second reset voltage to generate second raw data. The second raw data may be data corresponding to an electric charge generated in the second photodiode PD 2  during the second exposure time de 2 , which is shorter than the first exposure time de 1 . In other words, the second raw data may be data corresponding to an intermediate exposure time. Further, since the second photodiode PD 2  is controlled to generate an electric charge by setting the second exposure time de 2  over a plurality of times, an external light source such as an LED, or the like, which generates the flicker phenomenon, may be captured accurately using the second raw data. The length and the number of times of the second exposure time de 2  may be determined in consideration of an operating frequency and a duty ratio of the external light source such as an LED, or the like. 
     When entering a third time D 3  after the second time D 2 , the controller may expose the first photodiode PD 1  to light during a third exposure time de 3 . The third exposure time de 3  may be shorter than the second exposure time de 2 . The first reset transistor RX 1  may be turned off and the second reset transistor RX 2  may be turned on during the third time D 3 , and the controller may obtain a third reset voltage during a seventh sampling time t 7  and a third pixel voltage during an eighth sampling time t 8 , in sequence. The controller may obtain third raw data using the difference between the third reset voltage and the third pixel voltage. 
     The controller may generate image data using the first raw data, the second raw data, and the third raw data. Since the first to third raw data may be obtained using the electric charges generated by the first and second photodiodes PD 1  and PD 2  during the different exposure times de 1  to de 3 , a dynamic range of an image sensor may be increased by combining the first to third raw data. Further, the second photodiode PD 2  may be controlled to generate an electric charge in the second exposure time de 2  a plurality of times. Therefore, the external light source in which a flicker phenomenon occurs may also be accurately captured. 
     In an exemplary embodiment of the present inventive concept illustrated in  FIG.  24   , an image sensor may operate similarly to the embodiment illustrated in  FIG.  23   . During a first time D 1 , a controller may detect a first reset voltage and a first pixel voltage once to generate first raw data corresponding to a first exposure time de 1 . Therefore, in the embodiment illustrated in  FIG.  24   , an integration time of a unit in which an overflow transistor OX and a second transfer transistor TX 2  are alternately turned on and off may be shorter than that in the embodiment illustrated in  FIG.  23   . In addition, an operation in each of a second time D 2  and a third time D 3  may be similar to that in the embodiment illustrated in  FIG.  23   . 
     In the embodiment illustrated in  FIG.  25   , an image sensor may operate similarly to that of the embodiment illustrated in  FIG.  24   . A first exposure time de 1  may start earlier than a second exposure time de 2 . For example, in the embodiment illustrated in  FIG.  25   , a shutter operation for a first photodiode PD 1  may be performed prior to a shutter operation for a second photodiode PD 2 . An integration time of a unit in the embodiment illustrated in  FIG.  25    may be the same as or shorter than that in the embodiment illustrated in  FIG.  24   . 
     In addition, in the embodiment illustrated in  FIG.  25   , an operation in a third time D 3  may be different from those in the embodiments illustrated in  FIGS.  23  and  24   . Referring to  FIG.  25   , a first reset transistor RX 1  may be turned on and a second reset transistor RX 2  may be turned off during the third time D 3 . Therefore, an electric charge generated by the first photodiode PD 1  during a third exposure time de 1  may be accumulated only in a first floating diffusion FD 1 , and thus, the controller may acquire a third pixel voltage at a relatively high conversion gain condition. 
     In the embodiments described with reference to  FIGS.  23  to  25   , the second photodiode PD 2  may generate an electric charge by switching the overflow transistor OX and the second transfer transistor TX 2  a plurality of times. The ratio and number of turn-on times of the overflow transistor OX and the second transfer transistor TX 2  may be determined in consideration of the operating frequency and the duty ratio of the external light source in which the flicker phenomenon occurs. Further, in other exemplary embodiments of the present inventive concept, the second exposure time at which the second photodiode PD 2  generates the electric charge may be set long enough to accurately capture the external light source in which the flicker phenomenon appears. The following description will be made with reference to  FIGS.  26  and  27   . 
     Referring first to  FIG.  26   , an operation of an image sensor may start such that a controller turns on a first reset transistor RX 1  and a second reset transistor RX 2  to reset voltages of a first floating diffusion FD 1  and a second floating diffusion FD 2 . The controller may then perform a shutter operation to reset a second photodiode PD 2  and a storage capacitor SC by turning on a second transfer transistor TX 2  and a switch element SW. When the shutter operation is completed, the second photodiode PD 2  may generate an electric charge during a second exposure time de 2 . The electric charge generated by the second photodiode PD 2  during the second exposure time de 2  may be stored in the storage capacitor SC by an overflow phenomenon. 
     During the second exposure time de 2 , the first photodiode PD 1  may also generate an electric charge. The first photodiode PD 1  may generate an electric charge during a first exposure time de 1 , and the controller may detect voltages used to generate first row data during a first time D 1 . For example, the first time D 1  may include a first sub-time DS 1  and a second sub-time DS 2 , and the controller may detect a first sub-reset voltage and a first sub-pixel voltage at each of a first sampling time t 1  and a second sampling time t 2  of the first sub-time DS 1 . In addition, the controller may detect the second sub-pixel voltage and the second sub-reset voltage at each of a third sampling time t 3  and a fourth sampling time t 4  of the second sub-time DS 2 . For example, the operation of the image sensor at the first time D 1  may be similar to that described in the embodiment of  FIG.  23   . The operation of the image sensor at a third time D 3  may be similar to that described in the embodiment of  FIG.  23   . 
     The second exposure time de 2  may end in a second time D 2  subsequent to the first time D 1 . The second exposure time de 2  may end by turning on the second transfer transistor TX 2 . The switch element SW may also be turned on, together with the second transfer transistor TX 2 . As the switch element SW is turned on, an electric charge stored in the storage capacitor SC during the second exposure time de 2  may move to the floating diffusion. Since the second reset transistor RX 2  is turned on during the second time D 2 , the electric charge of the storage capacitor SC may move to the first floating diffusion FD 1  and the second floating diffusion FD 2 . A portion of the electric charge of the storage capacitor SC may be stored in a turned on capacitance of the second reset transistor RX 2 . 
     The controller may detect the second pixel voltage during a fifth sampling time t 5 , and detect the second reset voltage during a sixth sampling time t 6  in the second time D 2 . The second exposure time de 2  may be determined in consideration of the operating frequency of the light source in which a flicker phenomenon occurs. For example, the second exposure time de 2  may be longer than the inverse number of the operation frequency of the light source in which a flicker phenomenon occurs, e.g., the operation period. Therefore, the light source may be accurately captured despite the flicker phenomenon. 
     In the embodiment illustrated in  FIG.  26   , the controller may generate first to third raw data using a difference between the reset voltages and pixel voltages obtained in each of the first to third times D 1  to D 3 . The second raw data may also be used as data for detecting the light source in which a flicker phenomenon appears. The controller may combine the first to third raw data and conversion gain conditions generated at different exposure times de 1  to de 3 , to generate image data. Therefore, dynamic range of the image sensor may be increased. 
     An operation of an image sensor according to an exemplary embodiment of the present inventive concept illustrated in  FIG.  27    may be similar to the embodiment illustrated in  FIG.  26   . At a first time D 1 , a controller may detect only a first reset voltage and a first pixel voltage only once, without changing an area of a floating diffusion. In addition, in a second time D 2  a second transfer transistor TX 2  may be turned on and off. Therefore, an electric charge that is not transferred to a storage capacitor SC due to an overflow phenomenon may be transferred to the storage capacitor SC. A switch element SW may be turned on later. Other operations may be the same as those described for the embodiment of  FIG.  27   . 
       FIG.  28    is a block diagram illustrating an electronic device including an image sensor according to an exemplary embodiment of the present inventive concept. 
     A computer device  1000  according to the embodiment illustrated in  FIG.  28    may include a display  1010 , an image sensor  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 unit, a power supply unit, and the like. Among the components illustrated in  FIG.  28   , the port  1050  may be a device in which the computer device  1000  is provided for communicating with a video card, a sound card, a memory card, a universal serial bus (USB) device, and the like. The computer device  1000  may be a desktop computer, a laptop computer, a smartphone, a tablet personal computer (PC), a smart wearable device, and the like. 
     The processor  1040  may perform specific operations, commands, tasks, and the like. The processor  1040  may be a central processing unit or a microprocessor unit, and may be connected to the display  1010 , the image sensor  1020 , the memory device  1030 , as well as to other units connected the port  1050 , through a bus  1060 . 
     The memory  1030  may be storage medium for storing data, or multimedia data for operating the computer device  1000 . The memory  1030  may include a volatile memory, such as a random access memory, or a non-volatile memory, such as a flash memory. The memory  1030  may also include a solid state drive, a hard disk drive, or an optical drive as a storage unit. The computer device  1000  may comprise 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, etc., to be provided to a user. 
     The image sensor  1020  may be mounted on a package substrate and connected to the processor  1040  by the bus  1060 , or other communications means. The image sensor  1020  may be used in the computer device  1000  in the form of various embodiments described with reference to  FIGS.  1  to  27   . 
     According to an exemplary embodiment of the present inventive concept, an electric charge generated by a first photodiode and a second photodiode included in each of a plurality of pixels of an image sensor may be used to accurately detect light from an external light source having a flicker phenomenon. Further, an exposure time of the first photodiode and the second photodiode may be controlled in different times. Therefore, a dynamic range of an image sensor may be increased, and quality of an image generated by the image sensor may be increased. 
     While the present inventive concept has been shown and described with reference to exemplary embodiments thereof, it will be apparent to those skilled in the art that modifications and variations could be made thereto without departing from the scope of the present inventive concept as defined by the appended claims.