WO2007139254A1

WO2007139254A1 - Image sensor and method for sensing signal thereof

Info

Publication number: WO2007139254A1
Application number: PCT/KR2006/004174
Authority: WO
Inventors: Jawoong Lee; Dae Sung Min
Original assignee: Pixelplus Co., Ltd.
Priority date: 2006-05-25
Filing date: 2006-10-16
Publication date: 2007-12-06
Also published as: KR20070113607A; KR100790585B1

Abstract

A CMOS image sensor pixel which comprises a multi-functional charge sensing unit and a method for sensing a signal thereof. Wherein a voltage of a n-type diffusion region of a charge sensing node is highly boosted even in a low driving voltage and a signal voltage is coupled out with a coupling capacitor without forming ohmic contact in the n-type diffusion region of the charge sensing node.

Description

IMAGE SENSORAND METHOD FOR SENSING SIGNAL THEREOF

Technical Field

The present invention generally relates to a Complementary Metal-Oxide- Semiconductor (CMOS) image sensor and a method for sensing a signal thereof, and more specifically, to a technology of a structure, a function and an operating method of a multi-functional signal charge sensing unit in the CMOS image sensor.

Background of the Invention Generally, an image sensor is a device configured to convert an external optical image signal into an electric image signal. Specifically, a CMOS image sensor is fabricated with a CMOS manufacturing technology. Each pixel of the CMOS image sensor changes light signals radiated from the corresponding part of an object for photography into electrons with a photodiode, and converts the accumulated charge into voltage signals.

The CMOS image sensor includes a Floating Diffusion (FD) sensing node as a charge sensing node in order to sense the signal charge amount. A charge sensing unit includes a reset switch for resetting the FD sensing node and a reset voltage source combined with the FD sensing node. Fig. 1 is a circuit diagram illustrating a unit pixel of a conventional image sensor.

The unit pixel of the CMOS image sensor comprises a photodiode 1, a transfer gate 5, a charge sensing unit 6 and a signal amplifier 12. The charge sensing unit 6 includes a FD sensing node 9, a reset switch 10 and a reset voltage source VR.

A capacitor 8 of Fig. 1 includes a junction capacitor and peripheral parasite capacitors in the FD sensing node 9. A signal voltage of the capacitor 8 is transmitted into an input terminal of the signal amplifier 12 through a conducting line ohmic- contacted to a n-type diffusion region of the FD sensing node 9.

Fig. 2 is a cross-sectional and circuit diagram illustrating the unit pixel of Fig. 1. The unit pixel comprises a pinned photodiode PPD and n-channel MOSFETs. The pinned photodiode includes a n-type diffusion region 3 formed in a p-type epitaxial layer P-EPI, a p+ region 4 formed in the n-type diffusion region 3 and a P-well 2 formed at a sidewall of the n-type diffusion region 3 and the p+ region 4. The charge sensing unit 6 includes a P-well 7 formed in the p-type epitaxial layer P-EPI and the FD sensing node 9 formed in the P-well 7. Hereinafter, the operation of the unit pixel of the conventional CMOS image sensor is described.

When the transfer gate 5 is turned off, the photodiode PPD changes light signals into electrons to store the electrons in the n-type diffusion region 3. The photodiode PPD absorbs light for a charge integration time before the transfer gate 5 is turned on, and changes the light into electrons.

A voltage of the n-type diffusion region of the FD sensing node 9 is reset into a voltage higher than the voltage of the n-type diffusion region 3 of the photodiode PPD with the reset switch 10 and the reset voltage source VR.

When the transfer gate 5 is turned on, signal electrons accumulated in the n-type diffusion region 3 of the photodiode PPD are moved into the n-type diffusion region of the FD sensing node 9 by a voltage difference between the photodiode PPD and the n- type diffusion region of the FD sensing node 9. That is, signal electrons are moved from the photodiode PPD into the FD sensing node 9 for a transferring time so that the voltage of the n-type diffusion region of the FD sensing node 9 is changed. The voltage change is transmitted into the input terminal of the signal amplifier 12 through the conducting line ohmic-contacted to the n-type diffusion region of the FD sensing node 9.

An output signal of the signal amplifier 12 is directly connected to a signal line of a pixel array or connected to a signal line through an addressing switch for connecting/disconnecting the output signal of the signal amplifier 12. A directly connected structure without the addressing switch is used when the on/off states of the signal amplifier 12 can be controlled in the operating method of the pixel.

After the electrons transfer from the photodiode PPD into the FD sensing node 9, the n-type diffusion region 3 of the photodiode PPD is depleted. The transfer gate 5 is turned off, and the photodiode PPD changes light signals into electrons to store the electrons in the n-type diffusion region 3. The above-described operation of the unit pixel is repeated to read all image signals of the whole subject for photography.

However, the conventional CMOS image sensor has four problems as follows.

The CMOS image sensor uses low pixel driving voltage unlike a charge coupled device (CCD) image sensor. When electrons are moved from the photodiode PPD into the FD sensing node 9, the voltage difference between the n-type diffusion regions 3 of the photodiode PPD and the FD sensing node 9 is insufficient, so that it is difficult to move all electrons into the FD sensing node 9. In order to move all electrons from the n- type diffusion region 3 of the photodiode PPD into that of the FD sensing node 9, the voltage of the n-type diffusion region of the FD sensing node 9 is required to be higher even in the low driving voltage.

In the conventional CMOS image sensor, the FD sensing node 9 is connected to the input terminal of the signal amplifier 12 through an ohmic-contacted conducting line.

For the direct ohmic contact of the conducting line to the diffusion region of the FD sensing node 9, the doping concentration of the diffusion region should be high. However, a large dark current in the FD sensing node 9 occurs at physical defects generated from the ohmic contact process. The dark current of the ohmic-contacted FD sensing node 9 causes a severe error whenever the signal electrons are stored in the FD sensing node 9 for a long time. As a result, a sensing node having a smaller dark current than that of the ohmic-contacted FD sensing node 9 is required.

For example, a global shuttering of the image sensor is performed to store signal electrons in the FD sensing node 9 for a long time.

In the conventional CMOS image sensor, the FD sensing node 9 is directly connected to the signal amplifier 12 with the conducting line. So, if a voltage of the input terminal of the signal amplifier 12 is externally changed, the signal charge stored in the FD sensing node 9 is also changed so that the stored information is lost. That is, the voltage of the input terminal of the signal amplifier 12 in the conventional CMOS image sensor can not be externally changed to perform the next operation of the pixel without loss of the stored information in the FD sensing node 9. Since the incident light into the FD sensing node 9 generates an error in the operation of the pixel, it is necessary to block the light. As a result, a charge sensing unit having a structure for effectively blocking an incident light into the charge sensing node is required.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Technical Subject

In order to overcome the above-described shortcomings, the present invention has the following objects.

Various embodiments of the present invention is directed at boosting a voltage of a n-type diffusion region of a charge sensing node even in a low pixel driving voltage to transfer all signal electrons from a n-type diffusion region of a photodiode into the n-type diffusion region of the charge sensing node and to increase the maximum charge storage capacity of the charge sensing node.

Various embodiments of the present invention is directed at transmitting a signal voltage of the charge sensing node to an input terminal of a signal amplifier with a coupling capacitor, without forming ohmic contact to the n-type diffusion region of the charge sensing node in order to reduce a dark current of the sensing node than that of the conventional ohmic-contacted sensing node.

Various embodiments of the present invention is directed at coupling the signal voltage to the signal amplifier with a coupling capacitor, thereby facilitating the operation of the pixel that externally changes a voltage of the input terminal of the signal amplifier without loss of the signal charge stored in the charge sensing node .

Various embodiments of the present invention is directed at making an electrode of the coupling capacitor included in the charge sensing unit serve as an optical blocking mask, thereby effectively blocking the incident light into the charge sensing node.

Technical Solution

According to an embodiment of the present invention, an image sensor comprises: a photodiode, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to absorb incident light and generate charged particles so as to store the particles in the first diffusion region; a floating diffusion sensing node, which includes a second floating diffusion region having a second conductive type formed in a second semiconductor region having a first conductive type, configured to store the charged particles transferred from the first diffusion region of the photodiode in the second diffusion region; a transfer gate configured to transfer the charged particles stored in the first diffusion region into the second diffusion region; a reset switch, connected between a reset voltage source and the second diffusion region, configured to reset a voltage of the second diffusion region; a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier; the signal amplifier having the input terminal connected to the second electrode and configured to transmit a voltage signal corresponding to a voltage of the second electrode into a signal line of a pixel array; and a multi-functional switch, which has one terminal connected in common to the second electrode and the input terminal of the signal amplifier and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier.

According to an embodiment of the present invention, an image sensor comprises: a floating diffusion sensing node, which includes a first floating diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to store externally transferred charged particles in the first diffusion region; a reset switch, connected between a reset voltage source and the first diffusion region, configured to reset a voltage of the first diffusion region; a coupling capacitor having a first electrode connected to the first diffusion region and a second electrode connected to a multi-functional switch; and the multi-functional switch, which has one terminal connected to the second electrode and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode.

According to an embodiment of the present invention, an image sensor comprises: a photodiode, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to absorb incident light and generate charged particles so as to store the particles in the first diffusion region; a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region having a second conductive type formed in a second semiconductor region having a first conductive type, a third floating diffusion region having a first conductive type formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store the charged particles transferred from the first diffusion region in the second diffusion region; a transfer gate configured to transfer the charged particles stored in the first diffusion region into the second diffusion region; a reset switch, connected between a reset voltage source and the second diffusion region, configured to reset a voltage of the second diffusion region; a signal amplifier, which has an input terminal connected to the second electrode, configured to transmit a voltage signal corresponding to a voltage of the second electrode into a signal line of a pixel array; and a multi-functional switch, which has one terminal connected to the second electrode and the input terminal of the signal amplifier and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier.

According to an embodiment of the present invention, an image sensor comprises: a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, a second floating diffusion region having a first conductive type formed in the first diffusion region and a coupling capacitor having the second diffusion region as a first electrode and an electrode formed over the second diffusion region as a second electrode, configured to store externally transferred charged particles in the first diffusion region; a reset switch, connected between a reset voltage source and the first diffusion region, configured to reset a voltage of the first diffusion region; and a multi-functional switch, which has one terminal connected to the second electrode and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode.

According to an embodiment of the present invention, a method for sensing a signal of an image sensor, which comprises a floating diffusion sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi-functional switch connected between a variable voltage source and the second electrode, comprises the steps of: applying a voltage of the variable voltage source to the second electrode by turning on the multi-functional switch so as to fix the second electrode at a first voltage; applying a voltage of the reset voltage source to the second diffusion region by turning on the reset switch so as to reset the second diffusion region and turning off the reset switch; changing a voltage of the variable voltage source to rise a voltage of the second electrode as a second voltage higher than the first voltage, thereby boosting a voltage of the second diffusion region, and setting the input terminal of the signal amplifier at the second voltage to read an output signal of the signal amplifier; turning off the multi-functional switch and turning on a transfer gate so as to move an electron stored in the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; and sensing the amount of change of the output voltage of the signal amplifier.

According to an embodiment of the present invention, a method for sensing a signal of an image sensor, which comprises a floating diffusion sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi-functional switch connected between a variable voltage source and the second electrode, comprises the steps of: turning on the transfer gate and moving the electrons stored in the first diffusion region into the second diffusion region with the reset switch turned off, thereafter turning off the transfer gate; setting a voltage of the input terminal of the signal amplifier at a first voltage with the variable voltage source with the multi-functional switch turned on, and reading an output signal of the signal amplifier; turning off the multi-functional switch, thereafter discharging electrons of the second diffusion region by turning on the reset switch; and sensing the amount of change of an output voltage of the signal amplifier.

According to an embodiment of the present invention, a method for sensing a signal of an image sensor, which comprises a sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi- functional switch connected between a variable voltage source and the second electrode, comprises the steps of: setting a voltage of the input terminal of the signal amplifier at a first voltage with the multi-functional switch and the variable voltage source when the reset switch is turned off, and reading an output signal of the signal amplifier; turning off the multi-functional function, thereafter turning on the transfer gate and moving electrons from the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; sensing the amount of change of an output voltage of the signal amplifier; and repeating from the setting-a-first-voltage step to the sensing-the-change-amount step while the reset switch is turned off.

According to an embodiment of the present invention, a method for sensing a signal of an image sensor, which comprises a photodiode configured to absorb light and generate electrons so as to store the electrons in a first diffusion region, a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region, a third floating diffusion region formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store electrons transferred from the first diffusion region in the second diffusion region, a reset switch connected between a reset voltage source and the second diffusion region, a signal amplifier having an input terminal connected to the second electrode, and a multifunctional switch configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier, comprises the steps of: applying a voltage of the variable voltage source to the second electrode by turning on the multi-functional switch so as to fix the second electrode at a first voltage; applying a voltage of the reset voltage source to the second diffusion region by turning on the reset switch so as to reset the second diffusion region and turning off the reset switch; changing a voltage of the variable voltage source to raise a voltage of the second electrode as a second voltage higher than the first voltage, thereby boosting a voltage of the second diffusion region, and setting the input terminal of the signal amplifier at the second voltage to read an output signal of the signal amplifier; turning off the multi-functional switch and turning on a transfer gate so as to move an electron stored in the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; and sensing the amount of change of the output voltage of the signal amplifier.

According to an embodiment of the present invention, a method for sensing a signal of an image sensor, which comprises a photodiode configured to absorb light and generate electrons so as to store the electron in a first diffusion region, a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region, a third floating diffusion region formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store electrons transferred from the first diffusion region in the second diffusion region, a reset switch connected between a reset voltage source and the second diffusion region, a signal amplifier having an input terminal connected to the second electrode, and a multi- functional switch configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier, comprises the steps of: turning on the transfer gate and moving the electron stored in the first diffusion region into the second diffusion region with the reset switch turned off, thereafter turning off the transfer gate; setting a voltage of the input terminal of the signal amplifier at a first voltage with the variable voltage source and the multi-functional switch turned on, and reading an output signal of the signal amplifier; turning off the multi-functional switch, thereafter discharging electrons of the second diffusion region by turning on the reset switch; and sensing the amount of change of the output voltage of the signal amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a circuit diagram illustrating a unit pixel of a conventional image sensor. Fig. 2 is a cross-sectional and circuit diagram illustrating the unit pixel of Fig. 1. Fig. 3 is a circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention.

Fig. 4 is a cross-sectional and circuit diagram illustrating the unit pixel of Fig. 3. Fig. 5 is a cross-sectional and circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention. Fig. 6 is an expanded cross-sectional and circuit diagram illustrating a signal charge sensing node of Fig. 5.

Fig. 7 is a cross-sectional and circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention.

Fig. 8 is a cross-sectional and circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention.

Preferred Embodiments

The present invention will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like units.

Fig. 3 is a circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention.

The unit pixel comprises a pinned photodiode PPD, a transfer gate 24, a charge sensing unit 25 and a signal amplifier 35. The charge sensing unit 25 includes a floating diffusion sensing node 28, a reset switch 29, a reset voltage source VR, a coupling capacitor CC, a multi-functional switch 33 and a variable voltage source VC.

The capacitor 27 of Fig. 3 includes a junction capacitor of the floating diffusion sensing node 28 and peripheral parasitic capacitors. A signal voltage of the capacitor 27is transmitted into an input terminal of the signal amplifier 35 through the coupling capacitor CC.

Fig. 4 is a cross-sectional and circuit diagram illustrating the unit pixel of Fig. 3.

The unit pixel comprises the pinned photodiode PPD and n-channel MOS field effect transistors (MOSFETs).

The pinned photodiode PPD includes a n-type diffusion region 22 formed in a p- type epitaxial layer P-EPI, a p+ region 23 formed in the n-type diffusion region 22, and a P-well 21 formed at a sidewall of the n-type diffusion region 22 and the p+ region 23. Although the n-type diffusion region 22 is formed in the p-type epitaxial layer P-EPI in the embodiment, a p-type substrate P-SUB can be formed instead of the p-type epitaxial layer P-EPI. The charge sensing unit 25 includes a P-well 26 formed in the p-type epitaxial layer P-EPI, and a n-type diffusion region 28 enclosed by the P-well 26.

An insulating layer 34 is formed over the P-well 26. The transfer gate 24 formed over the insulating layer 34 connects a n-type diffusion region of the floating diffusion sensing node 28 to the n-type diffusion region 22 of the pinned photodiode PPD. The coupling capacitor CC has a first electrode 31 connected to the n-type diffusion region of the floating diffusion sensing node 28, and a second electrode 32 connected to an input terminal of the signal amplifier 35. The multi-functional switch 33 is connected between the variable voltage source VC and the second electrode 32 of the coupling capacitor CC. Hereinafter, functions and connections of each component are described. The pinned photodiode PPD changes incident lights into electrons to store the electrons in the n-type diffusion region 22. The transfer gate 24 moves the electrons stored in the n-type diffusion region 22 of the pinned photodiode PPD into the floating diffusion sensing node 28 of the charge sensing unit 25. The floating diffusion sensing node 28 stores the electrons transferred through the transfer gate 24 in the n-type diffusion region.

The reset switch 29 formed over the insulating layer 34 has one terminal connected to the n-type diffusion region of the floating diffusion sensing node 28, and the other terminal connected to the reset voltage source VR so that the reset switch 29 discharges electronic charge stored in the n-type diffusion region of the floating diffusion sensing node 28. The reset switch 29 can be formed of a field effect transistor (FET) or a transfer gate structure. The coupling capacitor CC has the first electrode 31 connected to the n-type diffusion region of the floating diffusion sensing node 28, and the second electrode 32 connected to the multi-functional switch 33 and the input terminal of the signal amplifier 35.

The multi-functional switch 33 has one terminal connected in common to the second electrode 32 of the coupling capacitor CC and the input terminal of the signal amplifier 35, and the other terminal connected to the variable voltage source VC. The multi-functional switch 33 can be formed of a FET. The reset voltage source VR and the variable voltage source VC are located outside the pixel, and connected to the corresponding nodes of each pixel. A signal voltage of the floating diffusion sensing node 28 is transmitted into the input terminal of the signal amplifier 35 connected to the second electrode 32 of the coupling capacitor CC.

An output signal of the signal amplifier 35 is directly connected to a signal line of a pixel array or connected to the signal line through a switch for connecting/disconnecting the output signal of the signal amplifier 35. A directly connected structure without the switch is used when the on/off states of the signal amplifier 35 can be controlled in the operating method of the pixel.

The charge sensing unit 25 included in the pixel can be shared by the photodiode or the signal amplifier 35 of other pixels.

Hereinafter, the operating process of the image sensor according to the embodiment of the present invention is described.

When the transfer gate 24 is turned off, the pinned photodiode PPD changes light signal into electrons to store the electrons in the n-type diffusion region 22. That is, the pinned photodiode PPD absorbs light and accumulates photo-generated electrons during the integration time before the transfer gate 24 is turned on.

With the transfer gate 24 is turned off, a voltage of the n-type diffusion region of the floating diffusion sensing node 28 is reset into a voltage higher than the voltage of the n-type diffusion region 22 of the photodiode PPD. The resetting procedure is as follows. The variable voltage source VC is set to a voltage level VCL, and the multi-functional switch 33 is turned on to fix the second electrode 32 of the coupling capacitor CC at a voltage level VCL. In this embodiment, the voltage level VCL is set at a ground voltage level. Thereafter the reset switch 29 is turned on. The voltage of the n-type diffusion region of the floating diffusion sensing node 28, that is, the voltage of the capacitor 27 is reset to a reset voltage VRN. The reset voltage VRN is determined by characteristics of the reset voltage source VR and the reset switch 29.

Thereafter, the reset switch 29 is turned off and the variable voltage source VC is raised from a voltage level VCL to a voltage level VCH. In this embodiment, the voltage VCH is set at a pixel driving power voltage level. Through the coupling capacitor CC, the capacitor 27 of the floating diffusion sensing node 28 is charged, and the voltage of the capacitor 27 rises from the reset voltage VRN to VRN + α x (VCH -

VCL). The constant α has positive value less than unity. That is, the voltage of the n- type diffusion region of the floating diffusion sensing node 28 is boosted by α x (VCH -

VCL). As a result, it is easy to transfer the electrons stored in the pinned photodiode PPD into the floating diffusion sensing node 28. Also, the maximum charge amount which can be stored in the n-type diffusion region of the floating diffusion sensing node

28 is increased. After the boosting operation, the multi-functional switch 33 is turned off.

When the transfer gate 24 is turned on, signal electrons accumulated in the n- type diffusion region 22 of the pinned photodiode PPD are transferred into the n-type diffusion region of the floating diffusion sensing node 28 by a voltage difference between the n-type diffusion region 22 of the pinned photodiode PPD and the n-type diffusion region of the floating diffusion sensing node 28. As a result, the n-type diffusion region 22 of the pinned photodiode PPD is depleted. As the signal electrons are moved from the pinned photodiode PPD into the floating diffusion sensing node 28, the voltage of the n-type diffusion region of the floating diffusion sensing node 28 is changed.

The voltage change of the n-type diffusion region of the floating diffusion sensing node 28, that is, the voltage change of the capacitor 27 of the floating diffusion sensing node 28 is transmitted into the input terminal of the signal amplifier 35 through the coupling capacitor CC. The transfer gate 24 is again turned off, and the pinned photodiode PPD again begins to accumulate the photo-generated electrons in the n-type diffusion region 22. The above-described operation of the unit pixel is repeated to read all image signals of the whole object for photography.

The charge sensing unit 25 is characterized in that the voltage change of the n- type diffusion region of the floating diffusion sensing node 28 is transmitted into the input terminal of the signal amplifier 35 through coupling capacitor CC. Although a voltage of the second electrode 32 of the coupling capacitor CC or the input terminal of the signal amplifier 35 is changed into a required value with the multi-functional switch 33 and the variable voltage source VC, the number of signal electrons stored in the n-type diffusion region of the floating diffusion sensing node 28, that is, the charge amount of the n-type diffusion region is preserved.

With the above characteristic of the charge sensing unit 25, the voltage of the input terminal of the signal amplifier 35 can be intentionally changed for the next operation of the pixel with the multi-functional switch 33 and the variable voltage source VC, preserving the signal charge in the floating diffusion sensing node 28. For example, in the CMOS image sensor according to the embodiment of the present invention a signal can be sensed by the following method unlike general CMOS image sensors.

Like the operation process of the above-described CMOS image sensor, electrons of the n-type diffusion region 22 of the pinned photodiode PPD are transferred into the n-type diffusion region of the floating diffusion sensing node 28.

The voltage of the input terminal of the signal amplifier 35 is set at a voltage level VL larger than an input threshold voltage of the signal amplifier 35 with the multifunctional switch 33 and the variable voltage source VC.

When the multi-functional switch 33 is turned off and the reset switch 29 is turned on, the signal electrons stored in the n-type diffusion region of the floating diffusion sensing node 28 are discharged. The voltage of the second electrode 32 of the coupling capacitor CC or the voltage of the input terminal of the signal amplifier 35 becomes higher than a voltage VL so that the charge amount of the signal electrons discharged from the n-type diffusion region of the floating diffusion sensing node 28 can be measured. As far as the reset operation is not performed with the reset switch 29, the voltage of the input terminal of the signal amplifier 35 can be set with the multifunctional switch 33 and the variable voltage source VC without loss of the signal electrons in the n-type diffusion sensing node 28. When the transfer gate 24 is turned on without precedent reset operation, the electrons transferred from the pinned photodiode PPD can be added with those stored previously in the n-type diffusion region of the floating diffusion sensing node 28.

Hereinafter, the function of the coupling capacitor CC in the charge sensing unit 25 in the pixel is described. The coupling capacitor CC boosts the voltage of the n-type diffusion region of the floating diffusion sensing node 28 by the linked operation of the reset switch 29, the reset voltage source VR, the multi-functional switch 33 and the variable voltage source VC.

Also, the coupling capacitor CC transmits the signal voltage of the n-type diffusion region of the floating diffusion sensing node 28 into the input terminal of the signal amplifier 35.

Hereinafter, the functions of the multi-functional switch 33 and the variable voltage source VC in the charge sensing unit 25 are explained.

A discharging path is provided to the floating structure of the second electrode 32 of the coupling capacitor CC and the input terminal of the signal amplifier 35 to discharge undesired charge, thereby preventing malfunction of devices.

The voltage of the n-type diffusion region of the floating diffusion sensing node 28 is boosted by the linked operation of the reset switch 29, the reset voltage source VR, and the coupling capacitor CC. Additionally, using the multi-functional switch 33 and the variable voltage source VC, the voltage of the input terminal of the signal amplifier 35 can be set to a required initial value. This setting operation enables various ways to sense the signal charge. It can also control the on/off state of the signal amplifier 35 when the signal amplifier 35 is source-follower structured. The operation of the whole pixels except that of the charge sense unit 25 is not explained because it is similar to the well-known operation of the conventional pixels.

Fig. 5 is a cross-sectional and circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention.

The unit pixel comprises a pinned photodiode PPD, a transfer gate 44, a charge sensing unit 45 and a signal amplifier 55. The charge sensing unit 45 includes a capacitor combined floating layer floating diffusion (CCFLFD) sensing node 46, a reset switch 53, a reset voltage source VR, a multi-functional switch 54 and a variable voltage source VC.

Fig. 6 is an expanded cross-sectional and circuit diagram illustrating a signal charge sensing node of Fig. 5.

The pinned photodiode PPD includes a n-type diffusion region 42 formed in a p- type epitaxial layer P-EPI, a p+ region 43 formed in the n-type diffusion region 42, and a P-well 41 formed at a sidewall of the n-type diffusion region 42 and the p+ region 43.

A CCFLFD sensing node 46 includes a P-well 47 or a p-type diffusion region formed in the P-type epitaxial layer P-EPI, and a n-type floating diffusion region 48 enclosed by the P-well within the P-well 46. A floating p-layer 50 is formed in the n- type diffusion region 48 so as to be enclosed by the n-type diffusion region 48, and an dielectric layer 52 is formed between the second electrode 51 of the coupling capacitor

CC and the floating p-layer 50. The floating p-layer 50 is formed not to contact with the p-well 47 or a p-type diffusion region which encloses the n-type diffusion region 48. That is, the floating p- layer 50 is enclosed by the n-type diffusion region 48 so that only the upper portion is exposed to be connected to the dielectric layer 52.

The floating p-layer 50 serves as a first electrode of the coupling capacitor CC. A second electrode 51 of the coupling capacitor CC is formed over the dielectric layer 52.

That is, the coupling capacitor CC consists of the floating p-layer 50 as a first electrode, dielectric layers 52, and the second electrode 51 over the dielectric layer 52. Additional area is not required to form the second electrode 51 because the second electrode 51 of the coupling capacitor CC is formed over the floating p-layer 50, thereby reducing the area of pixels which are additionally required to form the coupling capacitor CC.

The thickness of the dielectric layer 52 of the coupling capacitor CC is so thin that the second electrode 51 of the coupling capacitor CC may serve as an optical blocking mask for effectively blocking light incident to the CCFLFD sensing node 46.

Also, the floating p-layer 50 connects the coupling capacitor CC in the CCFLFD sensing node 46 to the n-type diffusion region 48 without ohmic contact. As a result, physical defects generated for forming ohmic contact are reduced, and in turn a dark current is decreased. Moreover, generation of electrons in the semiconductor surface is inhibited by the floating p-layer 50 to reduce the dark current and noise of the CCFLFD sensing node 46. This functional aspect of the floating p-layer 50 is similar to that of the p+ region 43 in the pinned photodiode PPD.

A junction capacitor of a pn junction Jl between the floating p-layer 50 and the n-type diffusion region 48 is called CFP. The coupling capacitor CC and the junction capacitor CFP are serially connected in a circuit.

The n-type diffusion region 48 stores signal electrons. A capacitor CFD includes a junction capacitor of a np junction J2 between the n-type diffusion region 48 and the p-well 47 and peripheral parasitic capacitors. Whenever electrons flow into and out from the n-type diffusion region 48, the voltage of the n-type diffusion region 48 is changed, and the voltage change is coupled to another units through the junction capacitor CFP and the coupling capacitor CC. A dielectric layer 52a is formed over the P-well 47. The transfer gate 44 formed over the dielectric layer 52a serves as a switch for connecting the n-type diffusion region 48 of the CCFLFD sensing node 46 to the n-type diffusion region 42 of the pinned photodiode PPD.

Hereinafter, functions and connections of each component of Figs. 5 and 6 are described in detail.

The pinned photodiode PPD changes incident lights into electrons to store the electrons in the n-type diffusion region 42. The transfer gate 44 moves the electrons stored in the photodiode PPD into the CCFLFD sensing node 46 of the charge sensing unit 45. The CCFLFD sensing node 46 stores the electrons transferred from the pinned photodiode PPD in the n-type diffusion region 48.

A dielectric layer 52b is formed over the P-well 47. The reset switch 53 formed over the dielectric layer 52b has one terminal connected to the n-type diffusion region 48 of the CCFLFD sensing node 46, and the other terminal connected to the reset voltage source VR. The reset switch 53 discharges the electric charge stored in the n-type diffusion region 48 of the CCFLFD sensing node 46, and resets a voltage of the n- diffusion region 48. The multi-functional switch 54 has one terminal connected to the second electrode 51 of the coupling capacitor CC and the input terminal of the signal amplifier 55, and the other terminal connected to the variable voltage source VC. The multi-functional switch 54 transmits a voltage of the variable voltage source VC to the second electrode 51 of the coupling capacitor CC. The reset voltage source VR and the variable voltage source VC are located outside of the pixel, and connected to the corresponding nodes of each pixel by conducting liness. An output voltage of the CCFLFD sensing node 46 is transmitted to the input terminal of the signal amplifier 55 which is connected to the second electrode 51 of the coupling capacitor CC.

The input terminal of the signal amplifier 55 is connected to the second electrode 51 of the coupling capacitor CC and the one terminal of the multi-functional switch 54. An output terminal of the signal amplifier 55 is directly connected to a signal line of a pixel array, or is connected to the signal line through a switch for turning on/off an output signal of the signal amplifier 55. A directly connected structure without the switch is used when the on/off states of the signal amplifier 35 can be regulated in the operating method of the pixel.

Hereinafter, the operating process of the image sensor according to the embodiment of the present invention is described. When the transfer gate 44 is turned off, the pinned photodiode PPD changes light signals into electrons to store the electrons in the n-type diffusion region 42. So, the pinned photodiode PPD absorbs light and accumulates the photo-generated electrons during integration time before the transfer gate 44 is turned on.

With the transfer gate 44 turned off, a voltage of the n-type diffusion region 48 of the CCFLFD sensing node 46 is reset into a voltage higher than the voltage of the n- type diffusion region 42 of the pinned photodiode PPD. The resetting procedure is as follows. The variable voltage source VC is set to a voltage level VCL, and the multifunctional switch 54 is turned on to fix the second electrode 51 of the coupling capacitor CC at a voltage level VCL. Thereafter the reset switch 53 is turned on. The voltage of the n-type diffusion region 48 of the CCFLFD sensing node 46 is reset to a reset voltage VRN. The reset voltage VRN is obtained by characteristics of the reset voltage source VR and the reset switch 53.

After the reset switch 53 is turned off, and the variable voltage source VC is risen from a voltage level VCL to a voltage level VCH. Through the coupling capacitor CC and the junction capacitor CFP of Jl junction which are connected serially, the junction capacitor CFD of J2 junction is charged, and the voltage of the n-type diffusion region 48 of the CCFLFD sensing node 46 is risen from the reset voltage VRN to VRN + α x (VCH - VCL), α is a positive constant less than unity. That is, the voltage of the n- type diffusion region 48 of the CCFLFD sensing node 46 is boosted by α x (VCH - VCL) from the reset voltage VRN to VRN + α x (VCH - VCL).

As a result, it is easy to transfer the signal electrons stored in the pinned photodiode PPD into the CCFLFD sensing node 46 because the voltage of the n-type diffusion region 48 of the CCFLFD sensing node 46 is risen to the higher level. Also, the maximum charge amount which can be stored in the n-type diffusion region 48 is increased. After the boosting operation, the multi-functional switch 54 is turned off.

When the transfer gate 44 is turned on, signal electrons accumulated in the n- type diffusion region 42 of the pinned photodiode PPD are moved into the n-type diffusion region 48 of the CCFLFD sensing node 46 by a voltage difference between the n-type diffusion region 42 of the pinned photodiode PPD and the n-type diffusion region 48 of the CCFLFD sensing node 46. As the signal electrons are moved from the pinned photodiode PPD into the CCFLFD sensing node 46 for the charge transfer period, the voltage of the n-type diffusion region 48 of the CCFLFD sensing node 46 is changed.

The voltage change of the n-type diffusion region 48 is transmitted into the input terminal of the signal amplifier 55 through the coupling capacitor CC. The diffusion region42 of the pinned photodiode PPD is depleted at the end of the transferring. The transfer gate 44 is again turned off, and the pined photodiode PPD again begins to accumulate the photo-generated electrons in the n-type diffusion region 42. The above-described operation of the unit pixel is repeated to read all image signals of the whole subjects for photography. The charge sensing unit 45 is characterized in that the voltage change of the n- type diffusion region 48 of the CCFLFD sensing node 46 is transmitted into the input terminal of the signal amplifier 55 through coupling capacitor CC. Although the voltage of the second electrode 51 of the coupling capacitor CC or the input terminal of the signal amplifier 55 is intentionally changed into a random value with the multi-functional switch 54 and the variable voltage source VC, the number of signal electrons stored in the n-type diffusion region 48 of the CCFLFD sensing node 46 is preserved.

When the charge sensing unit 45 includes the CCFLFD sensing node 46, the present invention has the following merits.

The floating p-layer 50 is formed and used as a first electrode of the coupling capacitor CC. So, the ohmic contact is not formed to connect the CCFLFD sensing node 46 to the coupling capacitor CC with a conducting line. The doping concentration of the n-type diffusion region 48 of the CCFLFD sensing node 46 can be reduced lower than that for ohmic contact. AS a result, physical defects generated from the doping process can be reduced, and the dark current generated from the sensing node can be decreased. The floating p-layer 50 is formed in an interface region of the n-type diffusion region 48, thereby reducing the dark current caused by electrons in the interface region with the oxide dielectric layer 52.

The coupling capacitor CC is formed over the floating diffusion structure, thereby reducing the area of additional pixels for forming the coupling capacitor CC. The second electrode 51 of the coupling capacitor CC formed over the CCFLFD sensing node 46. The thickness of the dielectric layer 52 is thin(~nm). So, if the second electrode 51 is made of opaque materials it can effectively block the light incident to the sensing node. The opaque electrodes include metal electrodes such as copper and aluminum, or poly suicide electrodes. That is, the second electrode 51 which covers the CCFLFD sensing node 46 in its very close distance serves as an optical blocking mask for effectively blocking the incident light into the CCFLFD sensing node 46.

Fig. 7 is a cross-sectional and circuit diagram illustrating a unit pixel of an image sensor according to an embodiment of the present invention. The unit pixel of Fig. 7 is different from that of Fig. 5 in that the lower region of the transfer gate 44 between the pinned photodiode PPD and the P-well 47 is formed with a p-type diffusion region 60. The other configurations and operations of Fig. 7 are substantially similar to those of Fig. 5. The p-type region 60, which has a different doping concentration from that of the P-well 47, is formed between the pinned photodiode PPD and the P-well 46 to improve the transmission efficiency of the transfer gate 44.

In the unit pixel of Fig. 8, the signal amplifier 55 is composed of a source follower (SF) amplifier 70.

The SF amplifier 70 includes an active transistor 71 and a constant current source 72 connected through a signal line SL. Without the reset voltage source VR, one terminal of the reset switch 53 is combined with a drain of the SF transistor 71, and a pixel driving voltage source VDD is used as a reset voltage source. Industrial Applicability

As described above, the present invention has the following effects. Even in a low driving voltage required in a CMOS image sensor, a voltage of a n-type diffusion region of a charge sensing node is boosted so that it is easy to transfer signal electrons from a photodiode into the charge sensing node. Also, the maximum charge amount stored in the charge sensing node is increased.

Without ohmic contact in the n-type diffusion region of the charge sensing node, a signal voltage of the charge sensing node is transmitted into an input terminal of a signal amplifier through a coupling capacitor, thereby reducing a dark current of the sensing node in comparison with a conventional charge sensing node.

As a voltage change of the charge sensing node is coupled to the input terminal of the signal amplifier through coupling capacitor, a voltage of the input terminal of the signal amplifier is externally changed into a required value in order to perform the next pixel operation without loss of signal charge stored in the charge sensing node are preserved.

An electrode of the coupling capacitor serves as an optical blocking mask close to the charge sensing node, thereby effectively blocking an incident light into the charge sensing node. While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and described in detail herein. However, it should be understood that the invention is not limited to the particular forms disclosed. Rather, the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined in the appended claims.

Claims

What is Claimed is:

1. An image sensor comprising: a photodiode, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to absorb incident light and generate charged particles so as to store the particles in the first diffusion region; a floating diffusion sensing node, which includes a second floating diffusion region having a second conductive type formed in a second semiconductor region having a first conductive type, configured to store the charged particles transferred from the first diffusion region of the photodiode in the second diffusion region; a transfer gate configured to transfer the charged particles stored in the first diffusion region into the second diffusion region; a reset switch, connected between a reset voltage source and the second diffusion region, configured to reset a voltage of the second diffusion region; a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier; the signal amplifier having the input terminal connected to the second electrode and configured to transmit a voltage signal corresponding to a voltage of the second electrode into a signal line of a pixel array; and a multi-functional switch, which has one terminal connected in common to the second electrode and the input terminal of the signal amplifier and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier.

2. The image sensor according to claim 1, wherein the first conductive type has an opposite polarity to that of the second conductive type.

3. The image sensor according to claim 1, wherein the first semiconductor region includes one of an epitaxial layer having the first conductive type and a substrate having the first conductive type.

4. The image sensor according to claim 1, wherein the second semiconductor region is a diffusion region having the first conductive type formed in the first semiconductor region.

5. The image sensor according to claim 1, wherein the photodiode is a pinned photodiode further comprising a diffusion region having a first conductive type formed in the first diffusion region.

6. The image sensor according to claim 1, wherein the multi-functional switch includes a FET.

7. The image sensor according to claim 1, wherein the reset switch includes one of a FET and a transfer gate structure.

8. The image sensor according to claim 1, wherein the multi-functional switch forms a discharging path in order to prevent the second electrode of the coupling capacitor and the input terminal of the signal amplifier from being electrically a floating structure.

9. The image sensor according to claim 1, wherein the multi-functional switch sets voltages of the second electrode of the coupling capacitor and the input terminal of the signal amplifier to be at predetermined values.

10. The image sensor according to claim 1, wherein the signal amplifier further comprises a switch connected between an output terminal of the signal amplifier and the signal line of the pixel array.

11. The image sensor according to claim 1 , wherein the signal amplifier is a source follower amplifier.

12. The image sensor according to claim 1 or 11, wherein a driving voltage source of the signal amplifier is used as the reset voltage source.

13. The image sensor according to claim 1, wherein the signal amplifier is turned on/off by the multi-functional switch and the variable voltage source.

14. The image sensor according to claim 1, wherein two or more pixels share at least one of the floating diffusion sensing node, the reset switch, the coupling capacitor, the multi-functional switch and the signal amplifier.

15. An image sensor comprising : a floating diffusion sensing node, which includes a first floating diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to store externally transferred charged particles in the first diffusion region; a reset switch, connected between a reset voltage source and the first diffusion region, configured to reset a voltage of the first diffusion region; a coupling capacitor having a first electrode connected to the first diffusion region and a second electrode connected to a multi-functional switch; and the multi-functional switch, which has one terminal connected to the second electrode and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode.

16. The image sensor according to claim 15, wherein the first conductive type has an opposite polarity to that of the second conductive type.

17. An image sensor comprising: a photodiode, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, configured to absorb incident light and generate charged particles so as to store the particle in the first diffusion region; a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region having a second conductive type formed in a second semiconductor region having a first conductive type, a third floating diffusion region having a first conductive type formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store the charged particles transferred from the first diffusion region in the second diffusion region; a transfer gate configured to transfer the charged particles stored in the first diffusion region into the second diffusion region; a reset switch, connected between a reset voltage source and the second diffusion region, configured to reset a voltage of the second diffusion region; a signal amplifier, which has an input terminal connected to the second electrode, configured to transmit a voltage signal corresponding to a voltage of the second electrode into a signal line of a pixel array; and a multi-functional switch, which has one terminal connected to the second electrode and the input terminal of the signal amplifier and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier.

18. The image sensor according to claim 17, wherein the first conductive type has an opposite polarity to that of the second conductive type.

19. The image sensor according to claim 17, wherein the first semiconductor region includes one of an epitaxial layer having the first conductive type and a substrate having the first conductive type.

20. The image sensor according to claim 17, wherein the second semiconductor region is a diffusion region having the first conductive type formed in the first semiconductor region.

21. The image sensor according to claim 17, wherein the photodiode is a pinned photodiode further comprising a diffusion region having the first conductive type formed in the first diffusion region.

22. The image sensor according to claim 17, wherein the coupling capacitor comprises: the first electrode formed of the third diffusion region; an dielectric layer formed over the first electrode; and the second electrode formed over the dielectric layer.

23. The image sensor according to claim 17, wherein the third diffusion region does not contact with the second semiconductor region having the first conductive type and the third diffusion region is floating in the second diffusion region.

24. The image sensor according to claim 17, wherein the doping concentration of the second diffusion region and the third diffusion region is set to be lower than that for ohmic contact.

25. The image sensor according to claim 17, wherein the multi-functional switch includes a FET.

26. The image sensor according to claim 17, wherein the reset switch includes one of a FET and a transfer gate structure.

27. The image sensor according to claim 17, wherein the multi-functional switch forms a discharging path in order to prevent the second electrode of the coupling capacitor and the input terminal of the signal amplifier from being electrically floating.

28. The image sensor according to claim 17, wherein the multi-functional switch sets voltages of the second electrode of the coupling capacitor and the input terminal of the signal amplifier to be at predetermined values.

29. The image sensor according to claim 17, wherein the signal amplifier further comprises a switch connected between an output terminal of the signal amplifier and the signal line of the pixel array.

30. The image sensor according to claim 17, wherein the signal amplifier is a source follower amplifier.

31. The image sensor according to claim 17 or 30, wherein a driving voltage source of the signal amplifier is used as the reset voltage source.

32. The image sensor according to claim 17, wherein the signal amplifier is turned on/off by the multi-functional switch and the variable voltage source.

33. The image sensor according to claim 17, wherein two or more pixels share at least one of the floating diffusion sensing node, the reset switch, the coupling capacitor, the multi-functional switch and the signal amplifier.

34. The image sensor according to claim 17, wherein the second electrode formed of opaque materials serves as a light blocking mask for blocking incident light into the second diffusion region.

35. An image sensor comprising: a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a first diffusion region having a second conductive type formed in a first semiconductor region having a first conductive type, a second floating diffusion region having a first conductive type formed in the first diffusion region and a coupling capacitor having the second diffusion region as a first electrode and an electrode formed over the second diffusion region as a second electrode, configured to store the externally transferred charged particles in the first diffusion region; a reset switch, connected between a reset voltage source and the second diffusion region, configured to reset a voltage of the second diffusion region; and a multi-functional switch, which has one terminal connected to the second electrode and the other terminal connected to a variable voltage source, configured to apply a voltage of the variable voltage source to the second electrode.

36. The image sensor according to claim 35, wherein the first conductive type has an opposite polarity to that of the second conductive type.

37. A method for sensing a signal of an image sensor, which comprises a floating diffusion sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi-functional switch connected between a variable voltage source and the second electrode, the method comprising the steps of: applying a voltage of the variable voltage source to the second electrode by turning on the multi-functional switch so as to fix the second electrode at a first voltage; applying a voltage of the reset voltage source to the second diffusion region by turning on the reset switch so as to reset the second diffusion region and turning off the reset switch; changing a voltage of the variable voltage source to rise a voltage of the second electrode as a second voltage higher than the first voltage, thereby boosting a voltage of the second diffusion region, and setting the input terminal of the signal amplifier at the second voltage to read an output signal of the signal amplifier; turning off the multi-functional switch and turning on a transfer gate so as to move an electron stored in the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; and sensing the amount of change of the output voltage of the signal amplifier.

38. The method according to claim 37, wherein a voltage of the second diffusion region is boosted by αx(second voltage - first voltage) (0 < α < 1).

39. The method according to claim 37, wherein the first voltage is at a ground voltage level and the second voltage is at a pixel driving voltage level.

40. A method for sensing a signal of an image sensor, which comprises a floating diffusion sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi-functional switch connected between a variable voltage source and the second electrode, the method comprising the steps of: turning on the transfer gate and moving the electrons stored in the first diffusion region into the second diffusion region with the reset switch turned off , thereafter turning off the transfer gate; setting a voltage of the input terminal of the signal amplifier at a first voltage with the variable voltage source with the multi-functional switch turned on, and reading an output signal of the signal amplifier; turning off the multi-functional switch, thereafter discharging electrons of the second diffusion region by turning on the reset switch; and sensing the amount of change of an output voltage of the signal amplifier.

41. The method according to claim 40, wherein the first voltage is set to be higher than an input threshold voltage of the signal amplifier.

42. A method for sensing a signal of an image sensor, which comprises a sensing node configured to store electrons transferred from a first diffusion region of a photodiode in a second diffusion region, a coupling capacitor having a first electrode connected to the second diffusion region and a second electrode connected to an input terminal of a signal amplifier, a reset switch connected between a reset voltage source and the second diffusion region, the signal amplifier having the input terminal connected to the second electrode, and a multi-functional switch connected between a variable voltage source and the second electrode, the method comprising the steps of: setting a voltage of the input terminal of the signal amplifier at a first voltage with the multi-functional switch and the variable voltage source when the reset switch is turned off, and reading an output signal of the signal amplifier; turning off the multi-functional switch, thereafter turning on the transfer gate and moving electrons from the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; sensing the amount of change of the output voltage of the signal amplifier; and repeating from the setting-a-first- voltage step to the sensing-the-amount-of- change step while the reset switch is turned off.

43. A method for sensing a signal of an image sensor, which comprises a photodiode configured to absorb light and generate electrons so as to store the electrons in a first diffusion region, a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region, a third floating diffusion region formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store electrons transferred from the first diffusion region in the second diffusion region, a reset switch connected between a reset voltage source and the second diffusion region, a signal amplifier having an input terminal connected to the second electrode, and a multi-functional switch configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier, the method comprising the steps of: applying a voltage of the variable voltage source to the second electrode by turning on the multi-functional switch so as to fix the second electrode at a first voltage; applying a voltage of the reset voltage source to the second diffusion region by turning on the reset switch as to reset the second diffusion region and turning off the reset switch; changing a voltage of the variable voltage source to raise a voltage of the second electrode as a second voltage higher than the first voltage, thereby boosting a voltage of the second diffusion region, and setting the input terminal of the signal amplifier at the second voltage to read an output signal of the signal amplifier; turning off the multi-functional switch and turning on a transfer gate so as to move an electron stored in the first diffusion region into the second diffusion region, thereafter turning off the transfer gate; and sensing the amount of change of the output voltage of the signal amplifier.

44. The method according to claim 43, wherein a voltage of the second diffusion region is boosted by αx(second voltage - first voltage) (0 < α < 1).

45. The method according to claim 43, wherein the first voltage is at a ground voltage level and the second voltage is at a pixel driving power voltage level.

46. A method for sensing a signal of an image sensor, which comprises a photodiode configured to absorb light and generate electrons so as to store the electrons in a first diffusion region, a Capacitor Combined Floating Layer Floating Diffusion (CCFLFD) sensing node, which includes a second diffusion region, a third floating diffusion region formed in the second diffusion region and a coupling capacitor having the third diffusion region as a first electrode and an electrode formed over the third diffusion region as a second electrode, configured to store electrons transferred from the first diffusion region in the second diffusion region, a reset switch connected between a reset voltage source and the second diffusion region, a signal amplifier having an input terminal connected to the second electrode, and a multi-functional switch configured to apply a voltage of the variable voltage source to the second electrode and the input terminal of the signal amplifier, the method comprising the steps of: turning on the transfer gate and moving the electron stored in the first diffusion region into the second diffusion region with the reset switch turned off, thereafter turning off the transfer gate; setting a voltage of the input terminal of the signal amplifier at a first voltage with the variable voltage source and the multi-functional switch turned on, and reading an output signal of the signal amplifier; turning off the multi-functional function, thereafter discharging electrons of the second diffusion region by turning on the reset switch; and sensing the amount of change of the output voltage of the signal amplifier.

47. The method according to claim 46, wherein the first voltage is set to be higher than an input threshold voltage of the signal amplifier.