CN104010142B - Active pixel and imaging sensor and its control sequential - Google Patents

Abstract

The invention discloses an active pixel, an image sensor and its control timing. The active pixel includes a first photosensitive element placed in a semiconductor substrate, a transfer transistor located between the first photosensitive element and a floating node, and a connection transistor connected to the floating node. The first reset transistor also includes a source follower transistor connected to the floating node, a row selection transistor and a column bit line, and the active pixel also includes a second photosensitive element, which is connected to the floating node through an auxiliary capacitor, and the first photosensitive element is connected to the floating node through an auxiliary capacitor. The second photosensitive element is also connected with a second reset transistor. The capacitance at the floating node can be automatically adjusted according to the incident light on the image sensor and the pixel; compared with the low lighting environment, in the high lighting environment, the capacitance at the floating node increases so that the signal saturation capacity of the floating node increases, Then the dynamic range of the image sensor is improved, and the signal-to-noise ratio is also increased.

Description

Active pixels and image sensors and their control timing

technical field

The invention relates to an image sensor, in particular to an active pixel, an image sensor and its control sequence.

Background technique

Image sensors have been widely used in digital cameras, mobile phones, medical devices, automobiles and other applications. In particular, the rapid development of the technology for manufacturing CMOS (Complementary Metal Oxide Semiconductor) image sensors has made people have higher requirements for the output image quality of the image sensor.

In the prior art, fixed capacitors are generally used at floating nodes of CMOS image sensor pixels. As shown in FIG. 1 , an active pixel with four transistors in a CMOS image sensor is used, which is also called a 4T active pixel in the field. The components of a 4T active pixel include: a photodiode 101 , a transfer transistor 102 , a reset transistor 103 , a source follower transistor 104 and a row selection transistor 105 . The photodiode 101 receives the incident light from the outside, generates a photoelectric signal, turns on the transmission transistor 102, transmits the photoelectric signal to the floating node FD (Floating Diffusing) and then turns off the transmission transistor 102, and the photoelectric signal is detected by the source follower transistor 104, and simultaneously turns on the row The selection transistor 105 reads the signal through the column bit line 106 . Wherein, the amount of photoelectric signal generated in the photodiode 101 is proportional to the amount of incident light, and the signal detected by the transistor 104 at the floating node FD is also proportional to the amount of light.

The photoelectric response of the image sensor in the above prior art is linear, which is called a linear sensor in the art. The light range detected by the linear sensor is small, especially in a high-light environment, it is impossible to recognize the physical information, and it cannot collect all the signals from the change of the dark light environment to the strong light environment, which is called a small dynamic range in the industry, thereby reducing The output image quality of the sensor is improved.

Contents of the invention

The purpose of the present invention is to provide an active pixel and an image sensor with an automatic adjustment function of the capacitance at the floating node and its control sequence, so as to solve the problem that the existing technology cannot collect all signals from the dark light environment to the strong light environment , to expand the dynamic range of the image sensor and pixels.

The purpose of the present invention is achieved through the following technical solutions:

The active pixel of the present invention includes a first photosensitive element placed in a semiconductor substrate, a transfer transistor located between the first photosensitive element and a floating node, a first reset transistor connected to the floating node, and a first reset transistor connected to the floating node. A source follower transistor, a row selection transistor and a column bit line, the active pixel also includes a second photosensitive element, the second photosensitive element is connected to the floating node through an auxiliary capacitor, and the second photosensitive element is also connected to a first photosensitive element Two reset transistors;

The second photosensitive element is used to test the amount of incident light;

The second reset transistor is used to clear the charge in the potential well of the second photosensitive element before the exposure starts;

The auxiliary capacitor is used to automatically adjust its size according to the amount of light received by the second photosensitive element.

In the image sensor of the present invention, several active pixels are arranged in a matrix in the vertical and horizontal directions, and the active pixels are the above-mentioned active pixels.

The control timing of the above-mentioned active pixels of the present invention comprises the steps of:

Firstly, simultaneously turn on the transfer transistor, the first reset transistor and the second reset transistor, and clear the charges in the potential wells of the first photosensitive element and the second photosensitive element;

Then, the transfer transistor, the first reset transistor and the second reset transistor are turned off, and the first photosensitive element and the second photosensitive element start to expose;

At the end of the exposure, after removing the charge of the floating node, transferring the photoelectric charge in the first photosensitive element to the floating node;

The capacitance of the floating node is automatically adjusted by controlling the source-drain potential of the auxiliary capacitor through the amount of incident light received by the second photosensitive element.

It can be seen from the above-mentioned technical solution provided by the present invention that the active pixel and the image sensor and their control timing provided by the embodiment of the present invention, since the active pixel also includes a second photosensitive element, the second photosensitive element communicates with the floating node through the auxiliary capacitor connected, the second photosensitive element is also connected with a second reset transistor, which can automatically adjust the capacitance at the floating node according to the amount of incident light on the image sensor and the pixel; compared with the low lighting environment, in the high lighting environment, the floating node The capacitance of the floating node is increased to increase the signal saturation capacity of the floating node, which improves the dynamic range of the image sensor and also increases the signal-to-noise ratio.

Description of drawings

1 is a schematic diagram of a four-transistor (4T) active pixel of a CMOS image sensor in the prior art;

2 is a schematic diagram of a four-transistor (4T) active pixel of a CMOS image sensor in an embodiment of the present invention;

3 is a schematic diagram of timing control of 4T active pixel work in an embodiment of the present invention;

FIG. 4 is a schematic diagram of the relationship between the capacitance of the floating node in the active pixel and the amount of incident light in an embodiment of the present invention.

FIG. 5 is a schematic diagram of an image sensor using active pixels in an embodiment of the present invention.

Detailed ways

The embodiments of the present invention will be further described in detail below.

Active pixel of the present invention, its preferred embodiment is:

It includes a first photosensitive element placed in a semiconductor substrate, a transfer transistor located between the first photosensitive element and a floating node, a first reset transistor connected to the floating node, a source follower transistor connected to the floating node, and a row selection transistor. and a column bit line, the active pixel further includes a second photosensitive element, the second photosensitive element is connected to the floating node through an auxiliary capacitor, and the second photosensitive element is also connected to a second reset transistor;

The second photosensitive element is used to test the amount of incident light;

The second reset transistor is used to clear the charge in the potential well of the second photosensitive element before the exposure starts;

The auxiliary capacitor is used to automatically adjust its size according to the amount of light received by the second photosensitive element.

The first photosensitive element and the second photosensitive element include any one or more of the following elements: photodiodes, PIN photodiodes, partial PIN photodiodes or polysilicon gate photodiodes.

The auxiliary capacitor is a transistor capacitor that can work as a MOS transistor.

The source and drain terminals of the auxiliary capacitor are connected to the second photosensitive element, and the gate is connected to the floating node.

In the image sensor of the present invention, several active pixels are arranged in a matrix in the vertical and horizontal directions, and its preferred specific implementation is:

The active pixels are the above-mentioned active pixels.

The control timing of the above-mentioned active pixels of the present invention, its preferred specific implementation mode is:

Include steps:

Firstly, simultaneously turn on the transfer transistor, the first reset transistor and the second reset transistor, and clear the charges in the potential wells of the first photosensitive element and the second photosensitive element;

Then, the transfer transistor, the first reset transistor and the second reset transistor are turned off, and the first photosensitive element and the second photosensitive element start to expose;

At the end of the exposure, after removing the charge of the floating node, transferring the photoelectric charge in the first photosensitive element to the floating node;

The capacitance of the floating node is automatically adjusted by controlling the source-drain potential of the auxiliary capacitor through the amount of incident light received by the second photosensitive element.

In the CMOS image sensor, in order to obtain high-quality images, the present invention starts from improving the photoelectric response properties of 4T pixels, compresses the photoelectric response sensitivity curve in high-illumination environments, and increases the photoelectric charge saturation capacity at the floating node FD of the pixel. Delaying the saturation time of pixels increases the dynamic range of the sensor. For example, in a low lighting environment, the capacitance of the floating node FD is 1.2fF, and the voltage swing of the floating node FD is 1V, then the charge saturation capacity is 7491, and the corresponding light quantity when it is just saturated is Q ₁ ; if in a high lighting environment When the capacitance of FD increases to 2fF, the charge saturation capacity increases to 12484, and the corresponding illumination amount when it is just saturated is Q ₂ ; thus the illumination range that the sensor pixels can detect increases to the original 1.67 (Q ₂ /Q ₁ = 12484/7491 = 1.67) times, that is, the dynamic range is expanded to 1.67 times of the original. Image sensor pixels working in this way can detect more physical details in high-light environments, thereby improving the image quality output by the sensor.

Specific examples:

As shown in FIG. 2, some components are added on the basis of the four-transistor pixel shown in FIG. , the second reset transistor 202 can perform the operation of clearing the charge in the second photodiode 201 before the exposure starts; the capacitance C _A represents the auxiliary capacitance of the auxiliary transistor 203, and the capacitance C _A includes the device source-drain overlap capacitance C _V and the device gate There are two parts of oxygen capacitor C _X ; the capacitor CF represents the parasitic capacitance of the floating node FD other than the capacitor CA; the total capacitance C _FD of FD is equal to the sum of the capacitor _{C A and} the capacitor _{C F.}

In this embodiment, PIN-type photodiodes are used in the pixels, and photoelectric charges are N-type electrons, and all transistors are N-type CMOS devices. In Fig. 2, marked TX is the gate of the transfer transistor 102, RX is the gate of the first reset transistor 103, SX is the gate of the row selection transistor 105, AUX is the gate of the second reset transistor 202, and Vdd is Power supply voltage, 106 is the column bit line as a channel for outputting photoelectric signals. Wherein, if the gate is at a high voltage, such as Vdd, it means that the transistor is turned on; if the gate is at a low voltage, such as gnd, it means that the transistor is turned off.

The schematic diagram of pixel timing control for implementing this embodiment is shown in FIG. 3 . The operation completed by the timing 301 is to remove electrons in the potential wells of the first photodiode 101 and the second photodiode 201 at the same time before the exposure starts, and the potential wells rise. High, it is called reset photodiode; the specific operation is to set TX, RX and AUX from low level to high level at the time t ₀ , turn on transistors 102, 103 and 202, and keep SX low level at the same time, T ₀₁ After the time period, at time _t1 , TX, RX and AUX are simultaneously set to low level, transistors 102, 103 and 202 are turned off, SX is kept at low level, the operation of sequence 301 is completed, and the first and second photodiodes start to expose at the same time.

The operation completed in the second sequence 302 is to clear the electrons at the FD before the end of the exposure, and the FD potential rises, which is called resetting the FD; the specific operation is to set RX from low level to high level at time _t2 , Turn on the transistor 103 and keep TX, AUX and SX at low level at the same time. After _T23 period, set RX to low level at time _t3 , and keep TX, AUX and SX at low level, and the operation of sequence 302 is completed. After the timing operation 302 is completed, the operation of reading the reset signal is completed within the time period _t3 and _t4 , which will not be described in detail here.

The operation completed in the third sequence 303 is to transmit the photoelectrons generated by the first photodiode 101 to the FD; the specific operation is to set TX and SX from low level to high level at time t4, and turn _on the transistor 102 and 105, while RX and AUX keep low level, after _T45 period, set TX and SX back to low level, turn off transistors 102 and 105, the operation of sequence 303 is completed, and the exposure of the first photodiode 101 ends. Wherein, in the later period of _T45 , the photoelectric signal detected by the source follower transistor 104 is read out by the readout circuit through the column bit line 106 and recorded. At the end of the exposure period, the amount of photoelectrons generated in the second photodiode 201 is proportional to the amount of incident light, and the amount of photoelectrons controls the potential at the source and drain SD of the transistor 203 .

The key to implementing this embodiment is that the total FD capacitance C _FD can automatically adjust its capacitance according to the amount of incident light at time t ₃ , as shown in FIG. 4 . If the amount of light is less than E ₁ , the electrons generated in the photodiode 201 are less, the potential of the source and drain terminals of the auxiliary capacitance transistor 203 is high, this transistor works in the cut-off region, and the capacitance CA has only a C _V component, then C _min =C _F +C _V If the amount of light is greater than E ₂ , more electrons are generated in the photodiode 201, which reduces the source-drain potential of the auxiliary capacitor transistor 203, and the transistor 203 works in a strong inversion region, and the capacitor CA includes two parts, C _V and C _X , then C _max ＝C _F +C _V +C _X ; if the amount of light is between E ₁ and E ₂ , the transistor 203 works in the weak inversion region, and the capacitance CA gradually increases from C _V to C _V +C _X . FD capacitance increases, and under a fixed voltage swing, the electron saturation capacity will also increase by the same multiple. Here, assuming that the number of electrons generated by the photodiode 101 is greater than the FD electron saturation capacity, the dynamic range of the pixel can be increased to the original The multiple (M) of can be expressed as:

M＝C _max /C _min

＝(C _F +C _V +C _X )/(C _F +C _V )

＝1+C _X /(C _F +C _V )

As we all know, when MOS transistors work in the strong inversion region, C _X is much larger than C _V . If C _F is also designed reasonably, it can be assumed that _{C X} is 1.2fF, CF is 1fF, and C _V is 0.2fF. Then the dynamics of the pixel The range can be increased up to 2 times the original. With the increase of the dynamic range of the pixel, the pixel collects more detailed information in the high-light environment, thus improving the quality of the output image of the sensor.

The above-mentioned active pixels can be used in a sensor array 403 of a CMOS image sensor, as shown in FIG. 5 . FIG. 5 specifically shows a CMOS image sensor formed according to the present invention, including a pixel array control circuit 401 , a pixel matrix array 403 , a processing circuit, a memory element and an input and output circuit 404 , and a logic AND gate circuit 402 . Each component is formed on a separate silicon substrate and integrated on a separate chip using standard CMOS manufacturing processes.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person familiar with the technical field can easily conceive of changes or changes within the technical scope disclosed in the present invention. Replacement should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (3)

1. An active pixel, comprising a first photosensitive element placed in a semiconductor substrate, a transfer transistor located between the first photosensitive element and a floating node, a first reset transistor connected to the floating node, and a first reset transistor connected to the floating node A source follower transistor, a row selection transistor, and a column bit line, wherein the active pixel further includes a second photosensitive element, the second photosensitive element is connected to the floating node through an auxiliary capacitor, and the second photosensitive element The element is also connected with a second reset transistor; The second photosensitive element is used to test the amount of incident light; The second reset transistor is used to clear the charge in the potential well of the second photosensitive element before the exposure starts; The auxiliary capacitor is used to automatically adjust its size according to the amount of light received by the second photosensitive element; The first photosensitive element and the second photosensitive element include any one or more of the following elements: photodiodes, PIN photodiodes, part of PIN photodiodes or polysilicon gate photodiodes; The auxiliary capacitor is a transistor capacitor that can be used as a MOS transistor; The source and drain terminals of the auxiliary capacitor are connected to the second photosensitive element, and the gate is connected to the floating node. 2. An image sensor, a plurality of active pixels are arranged in a matrix in the vertical and horizontal directions, characterized in that the active pixels are the active pixels according to claim 1. 3. A method for controlling the timing of active pixels according to claim 1, characterized in that it comprises the steps of: Firstly, simultaneously turn on the transfer transistor, the first reset transistor and the second reset transistor, and clear the charges in the potential wells of the first photosensitive element and the second photosensitive element; Then, the transfer transistor, the first reset transistor and the second reset transistor are turned off, and the first photosensitive element and the second photosensitive element start to expose; At the end of the exposure, after removing the charge of the floating node, transferring the photoelectric charge in the first photosensitive element to the floating node; The capacitance of the floating node is automatically adjusted by controlling the source-drain potential of the auxiliary capacitor through the amount of incident light received by the second photosensitive element.