CN1791184A - Active pixel sensor and image sensing module with simplified transistor structure - Google Patents

Abstract

Compared with the traditional active pixel sensor with a 3T or 4T structure, the active pixel sensor has better light transmittance and light sensing capability because the transistor structure is simplified.

Description

Active pixel sensor and image sensing module with simplified transistor structure

technical field

The present invention relates to a pixel sensor, in particular to an active pixel sensor made with a complementary metal-oxide semiconductor (Complementary Metal-Oxide Semiconductor; CMOS) process and an image sensing module with the pixel sensors.

Background technique

At present, the types of image sensing devices, in addition to the long-developed CCD image sensing technology, also have CMOS image sensing technology made of complementary metal oxide semiconductor (CMOS), and CMOS image sensing technology can be applied in addition to The optical mouse is also widely used in various aspects including digital cameras, videophones, third-generation mobile phone systems, etc., so it has become the mainstream of image technology development.

The existing CMOS image sensing technology is composed of a CMOS image sensor (Image sensor) in the form of a pixel array (Pixel Array), and the pixel array is composed of many pixel (Pixel) units arranged, and the general pixel units can be mainly divided into Passive Pixel Sensor or Active Pixel Sensor are introduced as follows:

As shown in FIG. 1 , a passive pixel sensor 8 is shown, which has a photodiode (Photo diode) 81 and a transistor (Transfer gate) 82. Since there is only one transistor, it can be referred to as a passive pixel sensor 8 with a 1T structure. The sensing principle is that the photodiode 81 is excited by the external light 7 and correspondingly generates photocharges, and the switching of the transistor 82 can be controlled through a column of output signals 801. When the transistor 82 is turned on (ON), each pixel sensor 8 A row of output signals 802 is output, and finally the output signals 802 of each row are read, and then the overall image signal of the pixel array can be obtained through processes such as signal amplification and analog-to-digital conversion.

However, in the passive pixel sensor 8 with 1T structure, since its induction signal is extremely small, and its readout line is prone to generate parasitic capacitance, resulting in high noise output, it has not been widely used, so there is an active development of pixel sensors.

As shown in FIG. 2 , an active image sensor module 9 is illustrated, which has a pixel array 91 , a column addressing circuit 92 , a row decoder 93 , a timing generating circuit 94 and an amplifier 95 . Wherein, the pixel array 91 has M*N pixel sensors 911; the column addressing circuit 92 is used to buffer and output the column selection signal 901 and the reset signal 902 to control the pixel sensors 911 of each column, so as to continuously receive The external light (not shown) is converted into charge output and updated; the row decoder 93 receives the row scan signal 903 of each row of charge output for decoding, and then outputs it from the amplifier 95; as for the timing generation circuit 94, it is for the column search The column selection signal 901 of the address circuit 92, the output of the reset signal 902, and the timing control of the row scanning signal 903 of the row decoder 93.

As shown in Figures 3 and 4, the two types of current active pixel sensors are shown respectively. They are divided into pixel sensors 3 and 4 with 3T structure and 4T structure respectively according to the number of transistors they have, and are introduced as follows:

As shown in FIG. 3 , a pixel sensor 3 with a 3T structure has a photodiode PD ₃ and three transistors. These transistors are respectively called a reset (Reset) transistor T ₃₁ and a source follower ( Source follower) transistor T ₃₂ and a row selection (Row select) transistor T ₃₃ .

The principle of operation of the pixel sensor 3 is that when the transistor _T31 receives a high-level reset signal 301, the transistor _T31 is turned on (ON), and the photocharge collected by the photodiode PD ₃ last time is cleared; then , the transistor T ₃₁ receives a low-level reset signal 301 to make it in an OFF state, and the photodiode PD ₃ is still excited by the external light 7 to generate photocharges correspondingly, convert the photocharges into voltage outputs, and start The discharge reduces the _value of VA; then the transistor _T32 shifts the voltage level, and the transistor _T33 is turned on to receive a column selection signal 302 and output it. In this way, the row of each pixel sensor 3 can be read each time The output signal 303 is then input with the reset signal 301 to update the stored charge to continuously obtain new signals.

Wherein, the way to turn on the transistor _T31 is to input a high logic level reset signal 301 to the gate of the transistor _T31 to give the photodiode _PD3 a reverse bias, so that a depletion region will be formed in the photodiode _PD3 (Depletion Region), photons can be absorbed in the depletion region by illumination to generate current.

When the transistor T ₃₁ is turned off during the exposure period, the photodiode PD ₃ generates an electron-hole pair that is proportional to the intensity of the light source due to the photodiode PD 3 being irradiated by the external light 7, and the electron-hole pair will move to the photodiode PD ₃ respectively. The two terminals of the transistor T33 are turned on to reduce the reverse bias voltage, and when the transistor _T33 is turned on, a termination exposure voltage _V2 can be obtained from its source; thereafter, the transistor _T33 is still turned on, and the transistor _T31 is turned on again, An initial exposure voltage V ₁ is also obtained from the source of the transistor T ₃₃ , and can be converted into an actual exposure voltage value by subtracting the termination exposure voltage V ₂ from the initial exposure voltage V ₁ .

Since the pixel sensor 3 of the 3T structure updates and reads the signal in the manner of resetting the transistor _T31 , and since the transistor _T32 can prevent the readout row output signal 303 from interfering with V _A , it is reset with the passive pixel sensor 8 Compared with the case where the signal 801 is directly reset, the noise is lower.

As shown in FIG. 4 , the pixel sensor 4 with a 4T structure has a photodiode PD ₄ , a capacitor C _f and four transistors. These transistors are called a transfer transistor T ₄₁ , a reset transistor T ₄₂ , and A source follower transistor T ₄₃ and a column selection transistor T ₄₄ , wherein the functions of the transistors T ₄₂ , T ₄₃ and T ₄₄ are similar to those of the transistors T ₃₁ , T ₃₂ and T ₃₃ of the pixel sensor 3 with a 3T structure ,No longer.

The difference between the 4T structure and the 3T structure is that the pixel sensor 4 of the 4T structure has an extra transistor T ₄₁ and a capacitor C _f , and the photodiode PD ₄ is connected to one end of the transistor T ₄₁ , and the other end of the transistor T ₄₁ is connected to the transistor T ₄₂ , T ₄₃ and capacitor C _f are connected in parallel; in addition, the structure of the photodiode PD ₄ of the 4T structure is different from the photodiode PD ₃ of the 3T structure. Because it is a PNP structure, the photodiode PD ₄ has the function of clamping the voltage, which can make The initial exposure voltage of the photodiode PD ₃ can be maintained without being disturbed by noise, so the pixel sensor 4 with 4T structure has a better anti-noise effect.

When the pixel sensor 4 is in operation, a high logic signal V _DD is input to the gate of the transistor T ₄₁ to give the photodiode PD ₄ an appropriate reverse bias voltage, so that a depletion region will be formed in the photodiode PD ₄ , because The photodiode PD ₄ has a PNP structure, which makes the depletion region larger due to the increase in voltage, and finally causes the depletion region to occupy the entire N-type region, so the voltage cannot re-enter and has the effect of clamping the voltage. Photons are absorbed in the depletion region to generate charge accumulation.

When calculating the exposure of the pixel sensor 4, the transistor T ₄₁ and the transistor T ₄₂ are first turned on, so that the photodiode PD ₄ is clamped at a fixed voltage level, and then the transistor T ₄₁ is turned off, so that the photodiode PD ₄ Exposure begins by receiving external light 7; when data is to be read out, the transistor _T42 is turned on to charge the level of the capacitor C _f to an initial fixed level V _B ', and the level is first read out as The horizontal output signal 403 of V _B '; then, the transistor _T41 is turned on, and the photodiode PD ₄ is transmitted from the transistor _T41 to the capacitor C _f through the design of the charge well depth, and since the capacitor C _f can accumulate voltage, therefore A new level V _B ” can be obtained, and the level V _B ′-V _B ” is the exposure voltage value. In this way, by inputting the reset signal 401 to the transistor _T42 to make it continuously updated, and by the voltage level shifting effect of the transistor _T43 , the output column selection signal 402 turns on the transistor _T44 , so that the transistor T44 can be turned on every time. The row output signal 303 of each pixel sensor 3 is read, and then the reset signal 301 is input to update the stored charge to continuously obtain new signals.

Overall, current active pixel sensors have the following disadvantages:

1. In the 3T structure, since the photodiode PD ₃ is electrically connected to the source of the transistor _T31 , and its reverse bias voltage is reset through the transistor _T31 , therefore, when the transistor _T31 or the photodiode When the PD ₃ has leakage current, it will affect the voltage levels of the initial exposure voltage V ₁ and the termination exposure voltage V ₂ . In addition, the voltage drop caused by the body effect (Body effect) caused by the reset transistor T ₃₁ and the attenuation of the barrier potential of the reverse bias voltage will also reduce the initial exposure voltage V ₁ and relatively reduce the image dynamics. range, which affects image quality.

2. In the 4T structure, the transistor T ₄₁ is mainly used to isolate the photodiode PD ₄ and reset the source of the transistor T ₄₂ to reduce the leakage current, but since 4 transistors are required, the aperture ratio (Aperture ratio), that is The light transmittance ratio will decrease, this is because in the unit area of the pixel sensor 4, if there are more transistors, the proportion of its area will increase, resulting in a relatively lower proportion of the area that can transmit light, so that the entire pixel array can transmit light area is also reduced.

3. The characteristics of the transistor itself are prone to KTC noise, which is difficult to eliminate.

4. When light is irradiated on the photodiode, the depletion region of the PN junction in the photodiode will separate electron-hole pairs to generate photocurrent, and light of different wavelengths will be absorbed at the PN junction at different depths. For example: short-wavelength light will be absorbed in the depletion region close to the surface, and long-wavelength light will be absorbed in the deeper depletion region. Since the depletion region of the PN junction is mostly in the deeper part of the photodiode, compared with For red light with a longer wavelength, the current pixel sensor has poor detection sensitivity for blue light with a shorter wavelength.

Brief description of the drawings

Below in conjunction with accompanying drawing and embodiment the present invention is described in detail:

FIG. 1 is a circuit diagram illustrating a current passive pixel sensor;

FIG. 2 is a circuit block diagram illustrating a current active image sensing module having a pixel array composed of a plurality of active pixel sensors;

FIG. 3 is a circuit diagram illustrating one of two types of current active pixel sensors: a pixel sensor with a 3T structure;

FIG. 4 is a circuit diagram illustrating another type of current active pixel sensor: a pixel sensor with a 4T structure;

5 is a circuit diagram illustrating the first preferred embodiment of the active pixel sensor with simplified transistor structure of the present invention;

FIG. 6 is a schematic diagram illustrating the element structure of the reset diode in the first preferred embodiment, which is a p-type potential well formed in the n-type doped region of the photosensitive element 21;

7 is a schematic diagram illustrating an image sensing module of the present invention, which has a plurality of active pixel sensors with simplified transistor structures;

8 is a circuit diagram illustrating a second preferred embodiment of the active pixel sensor with simplified transistor structure of the present invention;

Fig. 9 is a schematic diagram illustrating that the photosensitive element in the second preferred embodiment is a PNPN triple-well structure.

Detailed ways

The foregoing and other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of two preferred embodiments with reference to the accompanying drawings.

As shown in FIG. 5 , it is a first preferred embodiment of the active pixel sensor 2 with a simplified transistor structure in the present invention. This preferred embodiment includes a photosensitive (Photosensitive) element 21, a reset diode 22, a first transistor 23, a second transistor 24 .

Wherein, the photosensitive element 21 and the reset diode 22 are photodiode (Photo diode) structures, which are suitable for converting an external light 7 into an electrical signal during an exposure period; the photosensitive element 21 is connected with the reset diode 22 are connected in parallel, and control the start and end of the exposure period according to the voltage level of a reset signal 201; the first transistor 23 is used to amplify and convert the electrical signal into an output signal. In this preferred embodiment, the first transistor 23 A transistor 23 is a source follower (Source follower), which has the characteristics of large input impedance, small output impedance, large current gain, and voltage gain close to 1. The first transistor 23 mainly uses the drain of the NMOS transistor to receive a high-level voltage V _DD , the gate is coupled to the photosensitive element 21 and the reset diode 22 , and the source is coupled to the second transistor 24 .

The second transistor 24 is electrically connected to the first transistor 23 and controls whether to send the row output signal 203 according to the logic level of a column selection signal 202 . Wherein, the second transistor 24 is an NMOS transistor in this preferred embodiment, its drain is electrically connected to the source of the first transistor 23, its gate is used to receive the column selection signal 202, and its source is used to send the row selection signal 202. output signal 203 .

The active pixel sensor 2 operates in the following manner: first, a high-level reset signal 201 is input from the anode of the reset diode 22 to give the photosensitive element 21 a reverse bias, so that the PN junction in the photosensitive element 21 will form In the depletion region, light can be used to absorb photons in the depletion region to generate current. In addition, because the reset diode 22 itself is also a photodiode, it will also generate current due to light, and an initial state can be obtained at the output terminal S _out exposure voltage V ₁ .

Then, a reset signal 201 of a low voltage level is input, and the photosensitive element 21 and the reset diode 22 start to be exposed, and receive the irradiation of the external light 7 to generate a charge that is proportional to the intensity of the light source; when the gate of the second transistor 24 pole, when the column selection signal 202 of a high logic level is obtained and turned on, then through the amplification of the first transistor 23, a terminal exposure voltage _V2 output is obtained from the output terminal S _out , if the initial exposure voltage The voltage difference is obtained by subtracting the two voltages of V ₁ and the termination exposure voltage V ₂ , that is, one exposure cycle is completed.

As shown in Figure 6, the element structure of the reset diode 22 is to form a p-type potential well in the n-type doped region of the photosensitive element 21, so the depletion region formed by it can absorb more photons, and then The photosensitivity of the photosensitive element 21 is increased, and the initial exposure voltage V ₁ is relatively increased, thereby increasing the image dynamic range and image sensitivity.

As shown in FIG. 7 , it is an image sensing module 200, wherein the image sensing module 200 has a plurality of active pixel sensors 2, and each pixel sensor 2 is arranged in a matrix, and the pixel sensors 2 of each column share the transmission reset The transmission line of the signal 201; the pixel sensors 2 of each column also share the transmission line for transmitting the column selection signal 201, and these transmission lines are coupled and connected with the gates of the second transistors 24; as for the pixel sensors 2 of each row, they share the transmission line output signal 203 The signal is output by the transmission line, as for its control method, please refer to the description in Figure 2.

As shown in FIG. 8 , it is the second preferred embodiment of the active pixel sensor 5 with simplified transistor structure of the present invention. This preferred embodiment includes a photosensitive element 51, a source follower transistor 52 and a column selection transistor 53, wherein, in addition to The element structure of the photosensitive element 51 is different from that of the first preferred embodiment, because other source follower transistors 52 and column selection transistor 53 components, and reset signal 501, column selection signal 502 and the action principle of the row output signal are all the same as The first transistor 23 and the second transistor 24 described in the first preferred embodiment are the same, so they will not be repeated here.

As shown in FIG. 9 , the display photosensitive element 51 is a PNPN triple well (Triple well) structure. In addition to reducing the cross talk between the pixel sensors, the depletion region formed by the PNPN junction inside it, It can increase the absorption efficiency of photons, thereby increasing the photosensitivity of the photosensitive element 51, relatively increasing the initial exposure voltage V ₁ , increasing the dynamic range of the image, and improving the image sensitivity. The photosensitive element 51 can be easily charged.

To sum up the above, the active pixel sensors 2 and 5 with a simplified transistor structure in the present invention have the following advantages over the commonly used pixel sensors with 3T and 4T structures:

1. Since in the active pixel sensor 2, the reset diode 22 is formed by the p-type potential well generated in the n-type doped region of the photosensitive element 21, a type of PNP structure will be formed, through the built-in PN The conduction of the diode can easily give an appropriate reverse bias to the photodiode, and therefore can eliminate the body effect of the transistor in the 3T and 4T structure, and increase the initial exposure voltage V ₁ , increase the dynamic range of the image, and improve the image sensitivity .

2. Since the reset diode 22 is used instead of the transistor, the KTC noise generated when the general transistor is turned on/off can be avoided.

3. Since the reset diode 22 of the active pixel sensor 2 is formed in the photosensitive element 21, the active pixel sensor 5 only needs one set of photosensitive elements 51, and each active pixel sensor 2, 5 only has two Transistors 23, 24 and 52, 53, so the light transmittance per unit area will be much higher than the light transmittance per unit area of pixels with 3T and 4T structures, that is, the light-sensitive area of the pixel 2 is effectively increased and further improved its sensitivity.

4. Since the PN junction 12 of the photosensitive element 21 is closer to the surface of the active pixel sensor 2 , it can sense blue light with a shorter wavelength more effectively and thus is more sensitive.

Claims (6)

1, a kind of active pixel sensor of simplifying transistor structure is characterized in that: this element sensor comprises:

One light-sensitive element in order to after the sensitization light signal being converted to an electric signal, and upgrades this electric signal after reading a reset signal;

One the first transistor is electrically connected this light-sensitive element, is in order to this electric signal is amplified and be converted to an output signal; And

One transistor seconds is electrically connected this first transistor, and complies with the output of this output signal of selection signal controlling.

2, the active pixel sensor of simplifying transistor structure as claimed in claim 1 is characterized in that:

This light-sensitive element is the photodiode of a PNPN three gesture well constructions.

3, the active pixel sensor of simplifying transistor structure as claimed in claim 1 is characterized in that:

This element sensor also comprises a replacement diode, and this replacement diode is in order to export this reset signal to upgrade the electric signal of this light-sensitive element.

4, the active pixel sensor of simplifying transistor structure as claimed in claim 3 is characterized in that:

This replacement diode is a photodiode.

5, the active pixel sensor of simplifying transistor structure as claimed in claim 3 is characterized in that:

This replacement diode is the p type gesture well that produces in the n of this light-sensitive element type doped region and the class positive-negative-positive structure that forms.

6, a kind of image sensing module, comprise a plurality of active pixel sensors, respectively this element sensor is the matrix form arrangement, and the transmission line of shared this reset signal of transmission of element sensor of each row, the element sensor of each row also coupled in common connects one in order to transmit the row transmission line of this array selecting signal, element sensor of each row then coupled in common connect delegation's transmission line with signal in order to transmit this line output signal, it is characterized in that:

Respectively this element sensor has a light-sensitive element, a first transistor and a transistor seconds, wherein, this light-sensitive element is converted to an electric signal with a light signal after in order to sensitization, and after reading a reset signal, upgrade this electric signal, this the first transistor is electrically connected this light-sensitive element, be in order to this electric signal being amplified and be converted to an output signal, this transistor seconds is electrically connected this first transistor, and according to the output of array selecting signal control delegation output signal.

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