Passive type image sensor and method including first and second anti-blooming transistors discharging electric charges while integrating electric charges

A passive type image sensor and a method for operating the same. The passive type image sensor includes a photoelectric conversion section configured to receive light and integrate electric charges; a transfer section configured to transmit the integrated electric charges; an output section configured to received integrated electric charges from the transfer section and amplify and output the amplified electric charges; and an electric charge discharging section configured to discharge the electric charges flowing from the photoelectric conversion section to a common node through the transfer section while integrating the electric charge integration in the photoelectric conversion section.

The present application claims priority to Korean Patent Application No. 10-2010-0078962 (filed on Aug. 16, 2010) which hereby incorporated by reference in its entirety.

BACKGROUND

An image sensor may sense light emitted from a subject and convert the sensed light to an electrical value. The image sensors may be classified as charge coupled device (CCDs) image sensors and complementary metal oxide semiconductor (CMOS) image sensors.

The CCD image sensor may include MOS capacitors that store and transfer electric charge carriers. The CCD image sensor has drawbacks such as complicated driving method, high power consumption, and large number of mask steps. Accordingly, it is difficult to dispose a signal processing circuit inside a CCD chip.

The CMOS image sensor may include a plurality of unit pixels each including a photodiode (PD) and a MOS transistor. The CMOS image sensor may form an image by detecting a signal in a switching method.

The CMOS image sensor has advantages such as low manufacturing cost, low power consumption, and relatively easy integration with a peripheral circuit chip. The CMOS image sensor may be manufactured using the CMOS manufacturing technique. Accordingly, it is relatively easy to integrate the CMOS image sensor with a peripheral system that amplifies and processes signals. Due to easy integration, a manufacture cost thereof is relatively low. Further, a processing speed is fast, and power consumption is as low as about one percent of the CCD image sensor.

Pixels of an image sensor may be divided into a passive type and an active type. The passive type pixel has no buffer that serves as an active element. That is, a photodiode generates an electrical signal and transmits the generated signal along a vertical line when a horizontal selection signal is activated. Accordingly, the passive type pixel cannot perform a signal amplification function or a signal reduction minimization function. The active type pixel has a buffer that serves as an active element. The buffer has characteristics of high input impedance and low output impedance. Due to such characteristics of the buffer, signal reduction operation may be minimized even when an electrical signal is supplied with high output impedance. Further, a signal can be easily transferred without any loss even when a receiving terminal receives an electric signal with the input impedance.

A unit pixel of a typical passive type image sensor may include a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a select transistor. The photodiode receives light and generates photocharges. The transfer transistor transfers the photocharges generated by the photodiode. The reset transistor sets a node potential to a desired value and emits the photocharges in order to perform a reset operation. The drive transistor serves as a source follower buffer amplifier. The select transistor performs addressing through switching.

A passive type image sensor, however, has disadvantages in which an output signal of the unit pixel is reduced in size by about 20% due to the body effect of the drive transistors.

In order to eliminate such a drawback, many studies have been made to exclude a drive transistor from a unit pixel of a passive type image sensor.

U.S. Pat. No. 6,975,356 discloses a passive type image sensor having no drive transistor.

FIG. 1is a circuit diagram that illustrates a passive type image sensor of U.S. Pat. No. 6,975,356.

As illustrated inFIG. 1, the image sensor includes transfer gates10,12,14, and16and photodiodes2,4,6, and8in photosensitive sections of respective pixels. A source electrode of each transfer gate is respectively connected to a cathode of corresponding photodiode. The image sensor further includes source junction capacitors18,20,22, and24of floating diffusion regions between drain electrodes of the transfer gates10,12,14, and16and source electrodes of horizontal selection switches26,28,30, and32. Such source junction capacitors18,20,22, and24are used as detection capacitance.

Gate electrodes of the transfer gates10and12are connected to a transfer gate control line62. Gate electrodes of the transfer gates14and16are connected to a transfer gate control line70. Gate electrodes of the horizontal selection switches26and28are connected to a horizontal selection line64. Gate electrodes of the horizontal selection switches30and32are connected to a horizontal selection line72.

The image sensor further includes reset switches34,36,38, and40for charging the detection capacitors18,20,22, and24to a reset level. The reset switches34,36,38, and40are respectively connected to the detection capacitors18,20,22, and24. Drain electrodes of the reset switches34and36are connected to a reset voltage supply line58to which a reset voltage is supplied. Source electrodes of the reset switches34and36are respectively connected to the detection capacitors18and20, and gate electrodes of the reset switches34and36are connected to a reset control signal line60. Drain electrodes of the reset switches38and40are connected to a reset voltage supply line66to which the reset voltage is supplied. Source electrodes of the reset switches38and40are respectively connected to the detection capacitors22and24. The gate electrodes of the reset switches38and40are connected to a reset control signal line68.

The horizontal selection switches26and30are connected to a vertical selection line54. The horizontal selection switches28and32are connected to a vertical selection line56. Electric charge amplifiers41and43are respectively connected to the vertical selection lines54and56.

The passive type image sensor ofFIG. 1may cause blooming effect. That is, the passive type image sensor performs light integration operation to integrate photocharges generated by the photodiodes2,4,6, and8. When the photocharges exceed a predetermined threshold value during the light integration operation, the photocharges leak through the transfer gates10,12,14, and16and the horizontal selection switches26,28,30, and32. Such leaked photocharges adversely affect adjacent pixels and it causes a blooming effect.

FIG. 2is a circuit diagram that illustrates an image sensor configured to prevent a blooming effect, and particularly, an active type image sensor disclosed in U.S. Pat. No. 7,385,272.

As illustrated inFIG. 2, a transfer gate220is disposed between a cathode of a photodiode210and a floating diffusion region240. A gate of a source follower260and a current transport terminal of a reset transistor230are connected to the floating diffusion region240. Another current transport terminal of the reset transistor230is coupled to a current transport terminal of the source follower260. A voltage selection circuit110is coupled to the transfer gate220.

The active type image sensor ofFIG. 2sets a voltage of the gate electrode of the transfer gate220to an intermediate value while the photodiode210integrates photocharges in order to prevent the blooming effect. Accordingly, the active type image sensor needs a voltage selection circuit110. Such hardware requirement not only significantly increases a cost of the image sensor but also reduces a fill factor of the image sensor.

SUMMARY

Embodiments relate to a passive type image sensor excluding a drive transistor and a method for operating the same.

In accordance with embodiments, a passive type image sensor includes at least one of the following: a photoelectric conversion section configured to receive light and integrate electric charges; a transfer section configured to transmit the integrated electric charges; an output section configured to received integrated electric charges from the transfer section and amplify and output the amplified electric charges; and an electric charge discharging section configured to discharge the electric charges flowing from the photoelectric conversion section to a common node through the transfer section while integrating the electric charge integration in the photoelectric conversion section.

The electric charge discharging section may include: a first electric charge discharge switching element connected to the output section; and a second electric charge discharge switching element connected to a common node that is disposed between the first electric charge discharge switching element and the transfer section.

The electric charge discharging section may discharge the electric charges when the first electric charge discharge switching element is turned off and the second electric charge discharge switching element is turned on.

The first electric charge discharge switching element and the second electric charge discharge switching element may include at least one transistor. The transistor of the second electric charge discharge switching element may be connected in a diode-connection fashion. The output section may include: a reset switching element configured to reset the photoelectric conversion section, and an amplification element configured to amplify the electric charges from the photoelectric conversion section. The photoelectric conversion section may include a photodiode, and the transfer section and the electric charge discharging section include at least one transistor.

In accordance with embodiments, a passive type image sensor includes at least one of the following: a first photosensitive pixel, a second photosensitive pixel, an output section, and an electric charge discharging section.

The first photosensitive pixel includes a first photoelectric conversion section for receiving light and integrating electric charges and a first transfer section for transmitting the integrated electric charges to a common node. The second photosensitive pixel includes a second photoelectric conversion section for receiving light and integrating electric charges and a second transfer section for transmitting the integrated electric charges to the common node.

The output section amplifies the electric charges from the first photoelectric conversion section and/or the second photoelectric conversion section and outputs amplified electric charges through the common node.

The electric charge discharging section discharges the electric charges flowing from the first photoelectric conversion section or the second photoelectric conversion section to the common node through the transfer section while integrating the electric charges in the first photoelectric conversion section or the second photoelectric conversion section.

The electric charge discharging section may include a first electric charge discharge switching element connected to the output section, and a second electric charge discharge switching element connected to the common node between the first electric charge discharge switching element and the transfer section.

The electric charge discharging section may discharge the electric charges when the first electric charge discharge switching element is turned off and the second electric charge discharge switching element is turned on.

The first electric charge discharge switching element and the second electric charge discharge switching element may include at least one transistor. The second electric charge discharge switching element may be connected in a diode connection fashion.

The output section may include: a reset switching element configured to reset the first photoelectric conversion section and the second photoelectric conversion section; and an amplification element configured to amplify the electric charges from the first photoelectric conversion section and the second photoelectric conversion section.

The first photoelectric conversion section and the second photoelectric conversion section may include photodiodes, and the first transfer section, the second transfer section, and the electric charge discharging section include at least one transistor.

In accordance with embodiments, a method of operating a passive type image sensor including a photoelectric conversion element, a transfer transistor for transmitting electric charges in the photoelectric conversion element to a common node, a first anti-blooming transistor connected to the common node, a second anti-blooming transistor having a first terminal connected to the common node, and an output section connected to a second terminal of the second anti-blooming transistor, the method including one of the following: integrating electric charges in the photoelectric conversion element, discharging a current flowing through the transfer transistor while integrating the electric charges in the photoelectric conversion element by activating the first anti-blooming transistor and inactivating the second anti-blooming transistor, outputting a voltage on the common node as a reference voltage, and outputting electric charges in the photoelectric conversion element as a sensing voltage by inactivating the first anti-blooming transistor, activating the second anti-blooming transistor, and activating the transfer transistor.

The method may further include resetting the photoelectric conversion element through the output section before integrating the electric charges in the photoelectric conversion element.

The output section may include: a reset switching element for resetting the photoelectric conversion element; and an amplification element for amplifying the electric charges from the photoelectric conversion element.

In accordance with embodiments, the passive type image sensor excludes a drive transistor. Since no body effect is induced, an output signal of the unit pixel is not reduced.

When electric charges are integrated excessively during the electric charge integration operation of the photoelectric conversion element, the excessive electric charges are discharged according to the embodiments of the present invention. That is, anti-blooming effect is induced. Further, to the image sensor according to the embodiments does not require additional hardware such as the voltage selection circuit for inducing such anti-blooming effect. Accordingly, a manufacturing cost thereof does not increase. Moreover, since the image sensor according to the embodiments includes multiple photoelectric conversion elements coupled to the common node, the fill factor is enhanced.

DESCRIPTION

Hereinafter, an image sensor in accordance with embodiments and a method for operating the same will be described with reference to the annexed drawings.

ExampleFIG. 3is a diagram that illustrates a passive type image sensor in accordance with embodiments.

As illustrated in exampleFIG. 3, the passive type image sensor may include a first photosensitive pixel310, a second photosensitive pixel320, an electric charge discharging section330, and an output section340. The first photosensitive pixel310may include a first photoelectric conversion section311and a first transfer section313. The first photoelectric conversion section311may receive light and integrate electric charges. The first transfer section313may perform an operation of transferring the integrated electric charges to a common node or an operation of preventing the electric charges from being transferred to the common node during the charge integration.

The second photosensitive pixel320may include a second photoelectric conversion section321and a second transfer section323. The second photoelectric conversion section321may receive light and integrate electric charges. The second transfer section323may perform an operation of transferring the integrated electric charges to the common node or an operation of preventing the electric charges from being transferred to the common node during the charge integration. The first transfer section313and the second transfer section323may output the electric charges integrated in the first photoelectric conversion section311and the second photoelectric conversion321at different timings, respectively.

The electric charge discharging section330may discharge the electric charges flowing from the first photoelectric conversion section311and/or the second photoelectric conversion section321to the common node through the first transfer section313and/or the second transfer section323during the electric charge integration operation in which the first photoelectric conversion section311and/or the second photoelectric conversion section321receive light and integrate the electric charges, thereby inducing the anti-blooming effect.

The electric charge discharging section330may include a first electric charge discharge switching element331and a second electric charge discharge switching element332. The first electric charge discharge switching element331may transfer or block the electric charges from the first transfer section313and the second transfer section323to the output section340based on a switching state thereof. The second electric charge discharge switching element333may selectively discharge the electric charges transferred from the first transfer section313and the second transfer section323to the common node based on its switching state.

The output section340may reset the first photoelectric conversion section311and the second photoelectric conversion section321. Furthermore, the output section340may amplify the electric charges from the first photoelectric conversion section311and the second photoelectric conversion section321through the common node and the electric charge discharging section330and output the amplified electric charges.

As illustrated in exampleFIG. 3, a passive type image sensor includes a pair of photoelectric conversion sections311,321and a pair of transfer sections313,323coupled to the common node. However, embodiments are not limited thereto. For example, an image sensor in accordance with embodiments may include more than three photoelectric conversion sections and more than three transfer sections coupled to the common node.

As illustrated in exampleFIG. 3, reference alphabet CLM represents a vertical selection line of the image sensor.

Hereinafter, a method of operating the passive type image sensor illustrated in exampleFIG. 3will be described. For convenience of description and ease of understanding, a description will be made in connection with five time periods. It would be understood that two or more adjacent time periods may be unified as a single time period at the time of driving.

First Time Period

An electric charge transfer operation of the first transfer section313and/or the second transfer section323is activated. Further, an electric charge discharging operation of the electric charge discharging section330is inactivated, and a reset operation of the output section340is activated. When the electric charge discharging operation is inactivated, a switching control operation is performed. In the switching control operation, the first electric charge discharge switching element331transfers electric charges and the second electric charge discharge switching element333does not transfer electric charges.

During the first time period, the integrated electric charges of the first photoelectric conversion section311or/and the second photoelectric conversion section321pass through the first transfer section313or/and the second transfer section323and the first electric charge discharge switching element331. Then, the electric charges are discharged to the output section340. As a result, the first photoelectric conversion section311or/and the second photoelectric conversion section321are reset. That is, the first photosensitive pixel310and the second photosensitive pixel320are refreshed.

Second Time Period

An electric charge blocking operation of the first transfer section313or/and the second transfer section323is activated. Further, an electric charge integration operation of the first photoelectric conversion section311or/and the second photoelectric conversion section321is activated, and an electric charge discharging operation of the electric charge discharging section330is activated. Here, the electric charge discharging operation may be an anti-blooming operation. When the electric charge discharging operation is activated, a switching control operation is performed. In this switching control operation, the first electric charge discharge switching element331blocks the electric charges, and the second electric charge discharge switching element333discharges the electric charges.

During the second time period, the first photoelectric conversion section311or/and the second photoelectric conversion section321integrate photocharges. If the integrated photocharges of the first photoelectric conversion section311or/and the second photoelectric conversion section321exceed a predetermined threshold value, the excessive electric charges may leak to the common node through the first transfer second313or/and the second transfer section323. In more detail, the excessive electric charges may leak to the common node even when the transport function of the first transfer section313or/and the second transfer section323is inactivated, or even when the electric charge blocking operation is performed.

However, the leakage electric charges flow from the common node toward the second electric charge discharge switching element333because the electric charge discharging operation of the electric charge discharging section330is activated. Then, the leakage electric charges are discharged through the second electric charge discharge switching element333. Therefore, the anti-blooming effect is induced.

Third Time Period

An electric charge blocking operation of the first transfer section313or/and the second transfer section323remains activated, and an electric charge discharging operation of the electric charge discharging section330is inactivated.

The third time period is to prepare outputting the photocharges integrated in the first photoelectric conversion section311or/and the second photoelectric conversion section321. During the third time period, the first photoelectric conversion section311or/and the second photoelectric conversion section321can continue to integrate photocharges.

Fourth Time Period

The electric charge blocking operation of the first transfer section313or/and the second transfer section323remains activated, and an electric charge discharging operation of the electric charge discharging section330remains inactivated.

The fourth time period is a period that the output section340outputs a voltage at the common node as a reference voltage.

Fifth Time Period

The electric charge transfer operation of the first transfer section313or/and the second transfer section323is activated, and an electric charge discharging operation of the electric charge discharging section330remains inactivated.

During the fifth time period, the photocharges integrated in the first photoelectric conversion section311or/and the second photoelectric conversion section321are transferred to the common node. Then, the photocharges are transferred to the output section340through the first electric charge discharge switching element331.

Sixth Time Period

The electric charge transfer operation of the first transfer section313or/and the second transfer section323is inactivated, and an electric charge discharging operation of the electric charge discharging section330remains inactivated.

During the sixth time period, the output section340amplifies the voltage corresponding to the photocharges transferred from the first photoelectric conversion section311or/and the second photoelectric conversion section321to the common node and outputs the amplified voltage as a sensing voltage.

After outputting the amplified voltage from the output section340, sampling may be performed using the reference voltage and the sensing voltage output from the output section340. The reason for reading the sensing voltage during the fifth time period is to avoid a voltage variation error that may occur due to the switching operation of the first transfer section311or/and the second transfer section321.

ExampleFIG. 4illustrates a circuit diagram that illustrates a photoelectric conversion section and a transfer section in a passive type image sensor according to embodiments of the invention.

The first photoelectric conversion section311and the second photoelectric conversion section321may include one or more photodiodes. The first and second transfer sections313and323may include more than one transistor. For example, the first and second photoelectric conversion sections311and321according to the embodiment include a single photodiode, and the first transfer section313and the second transfer section323include a single transistor, as illustrated inFIG. 4.

As illustrated in exampleFIGS. 3 and 4, the first photoelectric conversion section311may include a first photodiode PD1. The first transfer section313may include a first transfer transistor TX1. The first transfer transistor TX1may include a source connected to a cathode of the first photodiode PD1, a gate connected to a first transfer gate control line TG1, and a drain connected to the common node.

The second photoelectric conversion section321may include a second photodiode PD2. The second transfer section323may include a second transfer transistor TX2. The second transfer transistor TX2may include a source connected to a cathode of the second photodiode PD2, a gate connected to a second transfer gate control line TG2, and a drain connected to the common node.

As illustrated in exampleFIG. 5illustrates a circuit diagram of a first electric charge discharge switching element of an electric charge discharging section in a passive type image sensor in accordance with embodiments.

The first electric charge discharge switching element331may include one or more transistors. For example, the first electric charge switching element331in accordance with embodiments includes a single transistor, as illustrated in exampleFIG. 5.

As illustrated in exampleFIGS. 3 and 5, the first electric charge discharge switching element331may include a first anti-blooming transistor S1which has a source connected to the common node, a gate connected to a first anti-blooming control line CLR1, and a drain connected to a vertical selection line CLM.

ExampleFIG. 6is a circuit diagram that illustrates a second electric charge discharge switching element of an electric charge discharging section in a passive type image sensor in accordance with embodiments.

The second electric charge discharge switching element333may include one or more transistors. For example, the second electric discharge switching element333according to the embodiments includes a single transistor, as illustrated in exampleFIG. 6.

As illustrated in exampleFIGS. 3 and 6, the second electric charge discharge switching element333may include a second anti-blooming transistor S2. The second anti-blooming transistor S2may include a source, a drain, and a gate. The source is connected to the common node and the drain and the gate are commonly connected to a second anti-blooming control line CLR2. That is, the second anti-blooming transistor S2may be connected between the common node and the second anti-blooming control line CLR2in a diode-connection fashion. In such configuration, the number of metal wires can be reduced. Accordingly, the fill factor thereof is enhanced.

ExampleFIG. 7is a circuit diagram that illustrates a passive type image sensor in accordance with embodiments.

As illustrated in exampleFIG. 7, the passive type image sensor may include a first photodiode PD1, a second photodiode PD2, a first transfer transistor TX1, a second transfer transistor TX2, a first anti-blooming transistor S1, a second anti-blooming transistor S2, an amplifier OP, a reset transistor RX, and a feedback capacitor Ct. The first and second photodiode PD1and PD2may serve as a photoelectric conversion element that receives light and generates photocharges. The first and second transfer transistors TX1and TX2may transfer or block the photocharges integrated in the first and second photodiodes PD1and PD2according to the switch states thereof. The first anti-blooming transistor S1may transfer the electric charges at the common node to the amplifier OP as an output element.

The first anti-blooming transistor S1may block the electric charges transferred to amplifier OP at the common node. The second anti-blooming transistor S2may serve as a switching element. The second anti-blooming transistor S2may discharge the electric charges at the common node according to a switching state thereof. The amplifier OP may amplify and output a current output through the second anti-blooming transistor S2. The reset transistor RX serves as a reset switching element. The reset transistor RX may reset the first photodiode PD1and the second photodiode PD2. The feedback capacitor Ct may adjust the gain of the amplifier OP.

The first transfer transistor TX1may include a source connected to a cathode of the first photodiode PD1, a gate connected to a first transfer gate control line, and a drain connected to the common node. The second transfer transistor TX2may include a source connected to a cathode of the second photodiode PD2, a gate connected to the second transfer gate control line, and a drain connected to the drain of the first transfer transistor TX1and the common node.

The first anti-blooming transistor S1may include a source connected to the common node, a gate connected to the first anti-blooming control line, and a drain connected to the vertical selection line CLM. The second anti-blooming transistor S2may include a source, a drain, and a gate. The source is connected to the common node. The drain and gate commonly are connected to the second anti-blooming control line.

As illustrated in exampleFIG. 7, the image sensor includes two photodiodes PD1and PD2and two transfer transistors TX1and TX2coupled to the common node. However, the present invention is not limited thereto. An image sensor in accordance with embodiments may include three or more photodiodes and transfer transistors coupled to the common node.

The first photodiode PD1and the first transfer transistor TX1may form a first photosensitive pixel. Further, the second photodiode PD2and the second transfer transistor TX2may constitute a second photosensitive pixel. The first and second photosensitive pixels share the first anti-blooming transistor S1and the second anti-blooming transistor S2. Thus, the fill factor is enhanced compared to an image sensor including first and second anti-blooming transistors S1and S2separately connected to the first and second photosensitive pixels.

ExampleFIG. 8is a signal waveform diagram for describing a method of operating a passive type image sensor in accordance with embodiments. Hereinafter, the method of operating a passive type image sensor in accordance with embodiments will be described with reference to exampleFIGS. 7 and 8. For convenience of description and ease of understanding, a description will be made in connection a plurality of time periods, i.e., time points a to g as illustrated in exampleFIG. 8. It should be noted, however, that two or more adjacent time periods may be unified as a single time period at the time of driving.

As illustrated in exampleFIG. 8, reference alphabet Vbit denotes an input voltage of the amplifier OP, reference alphabet Vcpt denotes a voltage of the first photodiode PD1, and reference alphabet Vout denotes an output voltage of the amplifier OP. The operation of the passive type image sensor will be described focusing on the first photosensitive pixel.

Time Period a-b

The first transfer transistor TX1and the first anti-blooming transistor S1are turned on. The second anti-blooming transistor S2is turned off, and the reset transistor RX is turned on. During the a-b time period, the integrated electric charges of the first photodiode PD1or/and the second photodiode PD2pass through the first transfer transistor TX1and the first anti-blooming transistor S1.

Then, the electric charges are discharged through the reset transistor RX. Accordingly, the first photodiode PD1or/and the second photodiode PD2are reset. That is, global reset is performed. The global reset resets the first photodiode PD1or/and the second photodiode PD2before the first or/and second photodiode PD1and PD2read the pixel value because the common node serves as a floating node.

Time Period b-c

The first transfer transistor TX1and the first anti-blooming transistor S1are turned off. The second anti-blooming transistor S2is turned on, and the reset transistor RX remains turned on. During the b-c time period, the first photodiode PD1integrates the photocharges. If the integrated photocharges exceed the predetermined threshold value of the first transfer transistor TX1, the excessive electric charge may leak to the common node through the first transfer transistor TX1although the first transfer transistor TX1is turned off.

The leakage electric charges flow, however, from the common node to the second anti-blooming transistor S2because the second anti-blooming transistor S2is turned on. Then, the leakage electric charges are discharged through the second anti-blooming transistor S2. Accordingly, the anti-blooming effect is induced. Here, when a negative potential is applied to the gate of the first anti-blooming transistor S1through the first anti-blooming control line although the first anti-blooming transistor S1is turned off, the electric charge blocking effect may be improved. Accordingly, the anti-blooming effect may be further improved.

Time Period c-d

The first transfer transistor TX1remains turned off, and the first anti-blooming transistor S1is turned on. The second anti-blooming transistor S2is turned off, and the reset transistor RX remains turned on. The c-d time period is to prepare outputting the photocharges integrated in the first photodiode PD1or/and the second photodiode PD2. During the c-d time period, the first photodiode PD1may continue to integrate photocharges.

Time Period d-e

The first transfer transistor TX1remains turned off, and the first anti-blooming transistor S1remains turned on. The second anti-blooming transistor S2remains turned off, and the reset transistor RX is turned off. During the d-e time period, the feedback capacitor Ct is activated so as to read the output of the photocharges integrated in the first photodiode PD1. Furthermore, the amplifier OP outputs the voltage at the common node as the reference voltage. At this time, if the first transfer transistor TX1remains turned off, the voltage of the vertical selection line CLM is fixed. Accordingly, the output value of the amplifier OP is fixed too.

Time Period e-f

The first transfer transistor TX1is turned on, and the first anti-blooming transistor S1remains turned on. The second anti-blooming transistor S2remains turned off, and the reset transistor RX remains turned off. During the e-f time period, the integrated photocharges are transferred from the first photodiode PD1to the amplifier OP through the first anti-blooming transistor S1.

Time Period f-g

The first transfer transistor TX1is turned off, and the first anti-blooming transistor S1remains turned on. The second anti-blooming transistor S2remains turned off, and the reset transistor RX remains turned off. During The f-g time period, the voltage corresponding to the output of the photocharges integrated in the first photodiode PD1is read. Then, the amplifier OP amplifies the voltage at the common node and outputs the amplified voltage as the sensing voltage.

Thereafter, sampling may be performed using the reference voltage and the sensing voltage after outputting the amplified voltage. In order to prevent the voltage variation error, the sampling is performed in the f-g time period rather than the e-f time period. The voltage variation error may occur due to the switching operation of the first transfer transistor TX1.

As the dashed dot line of exampleFIG. 8indicates, the first anti-blooming transistor S1may remain in an off state during the e-f time period. For example, the first anti-blooming transistor S1may be turned off before the first transfer transistor TX1is turned on, and the first anti-blooming transistor S1may be turned on after the first transfer transistor TX1is turned off. In this case, the photocharges integrated in the first photodiode PD1are transferred to the common node. At this time, the common node is used as the floating diffusion region. Thus, the photocharges at the common node are transferred and output to the amplifier OP.

An operation of the second photosensitive pixel including the second photodiode PD2and the second transfer transistor TX2is the same as or similar to the operation of the first photosensitive pixel. The first transfer transistor TX1and the second transfer transistor TX2are controlled at different timings to respectively output the photocharges integrated in the first photodiode PD1and the second photodiode PD2. Thus, a detailed description on the operation of the second photosensitive pixel will be omitted.

ExampleFIG. 9is a circuit diagram that illustrates a unit pixel in a passive type image sensor according to embodiments of the invention.

As illustrated in exampleFIG. 9, a unit pixel of the passive type image sensor may include a first transfer transistor TX1, a first anti-blooming transistor S1, a second anti-blooming transistor S2. The first transfer transistor TX1may include a source connected to a cathode of the first photodiode PD1and a gate connected to the first transfer gate control line. The first anti-blooming transistor S1may include a source commonly connected to the drain of the first transfer transistor TX1and the source of the second anti-blooming transistor S2, a gate connected to the first anti-blooming control line, and a drain connected to the vertical selection line. The second anti-blooming transistor S2may include a source connected to the drain of the first transfer transistor TX1and the source of the first anti-blooming transistor S1, and a drain and a gate commonly connected to the second anti-blooming control line.

Unlike the unit pixel of the passive type image sensor of exampleFIG. 7, the passive type image sensor of exampleFIG. 9excludes the second photodiode PD2and the second transfer transistor TX2. That is, the passive type image sensor of exampleFIG. 9may perform anti-blooming although the passive type image sensor includes a single photodiode and a single transistor coupled to the common node.