Differential active pixel circuit based on double-gate thin film transistor

Abstract

The invention discloses a differential active pixel circuit based on a double-gate thin film transistor. The differential active pixel circuit comprises a photoelectric device, a first transistor, a second transistor and a third transistor, the second transistor is connected with the photoelectric device, the first transistor and the third transistor; the photoelectric device is connected with the first external power supply and the first transistor; the first transistor is grounded; the second transistor is grounded; the third transistor is connected with a second external power supply; the joint of the second transistor and the third transistor is an output end. The second transistor in the differential active pixel circuit works as a pixel internal amplifier, the sensitivity of the pixel circuit under the condition of weak light is improved, and the problems that in the prior art, a passive pixel PPS cannot achieve pixel internal amplification, consequently, the signal-to-noise ratio is low, and the application scene requirement of low-dose X-ray imaging cannot be met are solved through the second transistor.

Description

Technical Field

[0001] The present invention relates to the technical field of differential active pixel technology, and in particular to a differential active pixel circuit based on a dual-gate thin film transistor. Background Art

[0002] The most widely used architecture in medium and large format X-ray flat panel imagers is the passive pixel sensor (PPS), which consists of a detector and a readout switch.

[0003] Passive pixel PPS has the advantages of being compact and suitable for high-resolution imaging, but external circuits, such as external charge amplifiers, are required to read low passive pixel PPS output signals. These circuits generate additional noise and reduce the minimum readable sensor input signal. Moreover, since passive pixel PPS only transmits the detected light-sensing signal (i.e., electrons) to the column-shared readout circuit without signal amplification, passive pixel PPS is affected by electronic noise, resulting in a low signal-to-noise ratio, which cannot meet the application scenario requirements of low-dose X-ray imaging. Summary of the invention

[0004] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a differential active pixel circuit based on a dual-gate thin-film transistor, which can solve the problem that the passive pixel PPS in the prior art cannot achieve intra-pixel amplification, resulting in its low signal-to-noise ratio and cannot meet the application scenario requirements of low-dose X-ray imaging.

[0005] The present invention provides a differential active pixel circuit based on a dual-gate thin film transistor, comprising: A photoelectric device, a first transistor, a second transistor and a third transistor; The second transistor is connected to the photoelectric device, the first transistor, and the third transistor; The photoelectric device is connected to a first external power source and the first transistor; The first transistor is grounded; The second transistor is grounded; The third transistor is connected to a second external power supply; The connection point between the second transistor and the third transistor is an output terminal.

[0006] Compared with the prior art, the beneficial effects of a differential active pixel circuit based on a dual-gate thin-film transistor of the present invention are as follows: in the differential active pixel circuit, the second transistor works as an intra-pixel amplifier, thereby improving the sensitivity of the pixel circuit under weak light conditions. The second transistor solves the problem that the passive pixel PPS in the prior art cannot achieve intra-pixel amplification, resulting in a low signal-to-noise ratio and an inability to meet the application scenario requirements of low-dose X-ray imaging.

[0007] Optionally, the photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; An anode of the photodiode is connected to the first transistor and the second transistor.

[0008] Optionally, the first transistor is a dual-gate thin film transistor; The drain of the first transistor is connected to the bottom gate of the first transistor, the photoelectric device, and the second transistor; and the source of the first transistor is grounded.

[0009] Optionally, the first transistor is a dual-gate thin film transistor; The drain of the first transistor is connected to the top gate of the first transistor, the photoelectric device, and the second transistor; and the source of the first transistor is grounded.

[0010] Optionally, the second transistor is a dual-gate thin film transistor; the third transistor is a dual-gate thin film transistor; The bottom gate of the second transistor is connected to the optoelectronic device and the drain of the first transistor; The drain of the second transistor is grounded; the top gate of the third transistor is connected to the top gate of the second transistor; the bottom gate of the third transistor is connected to the drain of the third transistor and the source of the second transistor; the source of the third transistor is connected to the second external power supply; and the source of the second transistor is the output terminal.

[0011] Optionally, the second transistor is a dual-gate thin film transistor; the third transistor is a dual-gate thin film transistor; The top gate of the second transistor is connected to the photoelectric device and the drain of the first transistor; The drain of the second transistor is grounded; the bottom gate of the third transistor is connected to the bottom gate of the second transistor; the top gate of the third transistor is connected to the drain of the third transistor and the source of the second transistor; the source of the third transistor is connected to the second external power supply; and the source of the second transistor is the output terminal.

[0012] Optionally, the photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; An anode of the photodiode is connected to the drain of the first transistor and the bottom gate of the second transistor.

[0013] Optionally, the photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; The anode of the photodiode is connected to the drain of the first transistor and the top gate of the second transistor.

[0014] Optionally, the optoelectronic device is composed of a photoconductive material.

[0015] Optionally, the second external power supply is twice the output voltage of the output terminal. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of the first embodiment; Figure 2 It is a schematic diagram of the second embodiment; Figure 3 It is a schematic diagram of the third embodiment; Figure 4 It is a schematic diagram of the fourth embodiment; Figure 5 It is a schematic diagram of the sixth embodiment; Figure 6 It is a schematic diagram of the pixel unit in the sixth embodiment. Detailed Embodiments

[0017] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention.

[0018] When the following description relates to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. On the contrary, they are only examples of devices and methods consistent with some aspects of the present application as detailed in the appended claims. In the description of the present application, it should be understood that the terms "first", "second", "third", etc. are only used to distinguish similar objects, and do not have to be used to describe a specific order or sequence, nor can they be understood as indicating or implying relative importance. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0019] Embodiment 1 Refer to Figure 1 As shown, this embodiment discloses a differential active pixel circuit based on a double-gate thin-film transistor, which is placed inside the pixel unit. That is, this embodiment includes: an optoelectronic device, a first transistor TFT 1 , a second transistor TFT 2 and a third transistor TFT 3 ; the second transistor TFT2 is connected to the optoelectronic device, the first transistor TFT 1 , the third transistor TFT 3 ; the optoelectronic device is connected to the first external power supply , first transistor TFT 1 Connection: First transistor TFT 1 grounded; the second transistor TFT2 is grounded; the third transistor TFT 3 With a second external power supply Connect the second transistor TFT2 and the third transistor TFT 3 The connection point is the output terminal.

[0020] In this embodiment, the photoelectric device is a photodiode PD, and the first transistor TFT 1 is a double-gate thin film transistor, the second transistor TFT2 is a double-gate thin film transistor, and the third transistor TFT 3 It is a dual-gate thin film transistor.

[0021] The cathode of the photodiode PD is connected to the first external power supply Connect, first external power supply Used to provide working power for the photodiode PD; the first transistor TFT 1 The drain of the photodiode PD and the anode of the first transistor TFT 1 The bottom gate of the second transistor TFT2 is connected; the source of the first transistor TFT1 is grounded; the drain of the second transistor TFT2 is grounded; the third transistor TFT 3 The top gate of the third transistor TFT is connected to the top gate of the second transistor TFT2; 3 The bottom gate of the third transistor TFT 3 The drain of the second transistor TFT2 is connected to the source of the third transistor TFT. 3 The source of the second external power supply The source of the second transistor TFT2 is an output terminal, which is used to output a signal.

[0022] The anode of the photodiode PD is connected to the first transistor TFT 1 The drain electrode of the first transistor TFT 1 The bottom gate of the second transistor TFT 2 The bottom gate of the first transistor TFT is connected to form a sensing part in the differential active pixel circuit; 1 The drain electrode and the first transistor TFT 1 The bottom gate connection, that is, the first transistor TFT 1 The diode connection is adopted, and the first transistor TFT 1 The source is grounded.

[0023] The first transistor TFT 1 The top gate is connected to the peripheral circuit, and the first transistor TFT is controlled by the peripheral circuit. 1 The top gate voltage VTG1 , top gate voltage V TG1 Used to control the first transistor TFT 1 The working area of ​​the first transistor TFT 1 Working in the subthreshold region, it effectively broadens the dynamic response range of the differential active pixel circuit and realizes the photocurrent to the first transistor TFT 1 The drain-source voltage V DS1 The logarithmic transformation of is as follows:

[0024] in, The first transistor TFT 1 The drain-source voltage, The first transistor TFT 1 The threshold voltage, is the photocurrent, The first transistor TFT 1 The gate-source voltage is equal to the threshold voltage ( = ) when the output current is is the Boltzmann constant, is the Kelvin temperature, q is the charge of the electron, and its value is 1.6021892×10 -19 C, is a parameter of the first transistor TFT1 related to its subthreshold swing SS, and .

[0025] The second transistor TFT2 works as an in-pixel amplifier in the differential active pixel circuit, thereby improving the sensitivity of the differential active pixel circuit under weak light conditions, and thus can expand the dynamic response range by reducing the detection lower limit.

[0026] The top gate of the second transistor TFT2 is connected to the peripheral circuit, and the top gate voltage V of the second transistor TFT2 is controlled by the peripheral circuit. TG2 , top gate voltage V TG2 It is used to adjust the working area of ​​the second transistor TFT2, so that the second transistor TFT2 works in the subthreshold area, so that the second transistor TFT2 has the function of exponential amplification; the output current output by the source of the second transistor TFT2 The drain-source voltage of the first transistor TFT1 The mathematical relationship between them is as follows:

[0027] in, is the output current output from the source of the second transistor TFT2, The gate-source voltage of the second transistor TFT2 is equal to the threshold voltage ( = ) when the output current is is the drain-source voltage of the first transistor TFT1, is the threshold voltage of the second transistor TFT2, q is the charge of the electron, and its value is 1.6021892×10 -19 C, is a parameter of the second transistor TFT2 related to its subthreshold swing SS, and .

[0028] The third transistor TFT 3 The top gate is connected to the peripheral circuit, and the third transistor TFT is controlled by the peripheral circuit. 3 The top gate voltage V TG3 , top gate voltage V TG3 Used to control the third transistor TFT 3 The working area of ​​the third transistor TFT 3 Works in the subthreshold region, so that the third transistor TFT 3 Working as a differential pair, it can reduce the dark current in the differential active pixel circuit and the uneven influence caused by the process. 3 The width-to-length ratio of is consistent with the width-to-length ratio of the second transistor TFT2. For the double-gate thin film transistor in the subthreshold region, its output current is:

[0029] in, is the gate-source voltage of the transistor, is the threshold voltage of the transistor, is the drain-source voltage of the transistor, The gate-source voltage of the transistor is equal to the threshold voltage ( = ) when the output current is is the Boltzmann constant, is the Kelvin temperature, q is the charge of the electron, and its value is 1.6021892×10 -19 C, is a parameter of the transistor related to its subthreshold swing SS, and .

[0030] For the second transistor TFT2 and the third transistor TFT 3 For example, the drain-source voltage of the second transistor TFT2 is , the third transistor TFT 3 The drain-source voltage is , so by making the second external power supply Output voltage twice, that is , that is, to maintain the drain-source voltage of the second transistor TFT2 and the drain-source voltage of the third transistor TFT 3 The drain-source voltage is consistent.

[0031] The gate-source voltage of the second transistor TFT2 and the gate-source voltage of the third transistor TFT 3 The gate-source voltage of the second transistor TFT2 and the third transistor TFT 3 The top gates of the second transistor TFT2 and the third transistor TFT 3 The top gate voltage remains the same.

[0032] For the bottom gate of the second transistor TFT2 and the third transistor TFT 3 The bottom gate of the third transistor TFT 3 The bottom gate of the second transistor TFT2 and the source of the third transistor TFT 3 The source of the third transistor TFT 3 The bottom gate of the second transistor TFT2 is short-circuited with the output terminal; the bottom gate of the second transistor TFT2 is used as the sensing terminal, and its initial value is 0, which is the dark state. When light enters, the bottom gate voltage of the second transistor TFT2 increases. Therefore, in the dark state, the bottom gate of the second transistor TFT2 is also short-circuited with its source; the gate-source current of the second transistor TFT2 is and the third transistor TFT 3 The gate-source current In the opposite direction, according to Kirchhoff's current law, the output current at the output end can be obtained , the specific calculation formula is as follows:

[0033] in, is the output current at the output terminal, is the gate-source current of the second transistor TFT2, The third transistor TFT 3 of gate-source current.

[0034] Through the second transistor TFT2 and the third transistor TFT 3 , that is, the dark state current can be reduced, and only the light response current is retained. The signal-to-noise ratio and dynamic range of the differential active pixel circuit are improved, and the second transistor TFT 2 and the third transistor TFT 3 Placing them in adjacent positions during layout design can minimize process unevenness.

[0035] Embodiment 2 See also Figure 2As shown, this embodiment discloses a second differential active pixel circuit based on a dual-gate thin film transistor, which is different from the first embodiment in that: the first transistor TFT 1 The bottom gate and top gate are interchanged.

[0036] Specifically: the first transistor TFT 1 The top gate of the first transistor TFT 1 The drain of the photodiode PD, the anode of the second transistor TFT 2 Bottom gate connection.

[0037] The first transistor TFT 1 The bottom gate of the transistor TFT is connected to the peripheral circuit, and the peripheral circuit controls the first transistor TFT 1 The bottom gate voltage V BG1 , bottom gate voltage V BG1 Used to control the first transistor TFT 1 The working area of ​​the first transistor is TFT 1 It operates in the subthreshold region, thereby effectively widening the dynamic response range of the differential active pixel circuit.

[0038] The differential active pixel circuit of this embodiment can realize the same function as the differential active pixel circuit of the first embodiment, but the top gate voltage VTG1 and the bottom gate voltage VBG1 have different effects on the first transistor TFT. 1 The control capability of the threshold voltage is different, which is specifically reflected in that the control coefficient γ of the gate voltage to the threshold voltage is related to different manufacturing processes. The larger γ is, the stronger the control capability is. Therefore, compared with the control capability of the differential active pixel circuit in embodiment 1, the control capability of the differential active pixel circuit in this embodiment will change.

[0039] Embodiment 3 See also Figure 3 As shown, this embodiment discloses a third differential active pixel circuit based on a dual-gate thin film transistor, which differs from the first embodiment in that the bottom gate and the top gate of the second transistor TFT2 are interchanged, and the third transistor TFT 3 The bottom gate and top gate are interchanged.

[0040] Specifically, the top gate of the second transistor TFT2 and the anode of the photodiode PD, the first transistor TFT 1 The drain of the second transistor TFT2 is connected to the bottom gate of the third transistor TFT 3 The bottom gate connection of the third transistor TFT 3 The top gate and the third transistor TFT 3 The drain of the second transistor TFT2 is connected.

[0041] The bottom gate of the second transistor TFT2 is connected to the peripheral circuit, and the bottom gate voltage V of the second transistor TFT2 is controlled by the peripheral circuit. BG2 , bottom gate voltage V BG2 It is used to adjust the working area of ​​the second transistor TFT2 so that the second transistor TFT2 works in the subthreshold area, thereby enabling the second transistor TFT2 to have an exponential amplification function.

[0042] The third transistor TFT 3 The bottom gate is connected to the peripheral circuit, and the third transistor TFT is controlled by the peripheral circuit. 3 The bottom gate voltage V BG3 , bottom gate voltage V BG3 Used to control the third transistor TFT 3 The working area of ​​the third transistor TFT 3 Works in the subthreshold region.

[0043] The differential active pixel circuit of this embodiment can realize the same function as the differential active pixel circuit of the first embodiment, but the top gate voltage VTG2 and the bottom gate voltage VBG2 have different control capabilities on the threshold voltage of the second transistor TFT2, and the top gate voltage VTG3 and the bottom gate voltage VBG3 have different control capabilities on the threshold voltage of the third transistor TFT 3 The control capability of the threshold voltage is different, which is specifically reflected in that the control coefficient γ of the gate voltage to the threshold voltage is related to different manufacturing processes. The larger γ is, the stronger the control capability is. Therefore, compared with the control capability of the differential active pixel circuit in embodiment 1, the control capability of the differential active pixel circuit in this embodiment will change.

[0044] Embodiment 4 See also Figure 4 As shown, this embodiment discloses a fourth differential active pixel circuit based on a dual-gate thin film transistor, which is different from the first embodiment in that: the first transistor TFT 1 The bottom gate and the top gate of the second transistor TFT2 are interchanged, and the bottom gate and the top gate of the third transistor TFT 3 The bottom gate and top gate are interchanged.

[0045] Specifically: the first transistor TFT 1 The top gate of the first transistor TFT 1 The drain of the photodiode PD is connected to the anode of the photodiode PD and the bottom gate of the second transistor TFT2. The top gate of the second transistor TFT2 is connected to the anode of the photodiode PD and the first transistor TFT 1 The drain of the second transistor TFT2 is connected to the bottom gate of the third transistor TFT 3 The bottom gate connection of the third transistor TFT 3 The top gate and the third transistor TFT 3The drain of the second transistor TFT2 is connected.

[0046] The first transistor TFT 1 The bottom gate of the transistor TFT is connected to the peripheral circuit, and the peripheral circuit controls the first transistor TFT 1 The bottom gate voltage V BG1 , bottom gate voltage V BG1 Used to control the first transistor TFT 1 The working area of ​​the first transistor TFT 1 It works in the subthreshold region, thereby effectively broadening the dynamic response range of the differential active pixel circuit.

[0047] The bottom gate of the second transistor TFT2 is connected to the peripheral circuit, and the bottom gate voltage V of the second transistor TFT2 is controlled by the peripheral circuit. BG2 , bottom gate voltage V BG2 It is used to adjust the working area of ​​the second transistor TFT2 so that the second transistor TFT2 works in the subthreshold area, thereby enabling the second transistor TFT2 to have an exponential amplification function.

[0048] The third transistor TFT 3 The bottom gate is connected to the peripheral circuit, and the third transistor TFT is controlled by the peripheral circuit. 3 The bottom gate voltage V BG3 , bottom gate voltage V BG3 Used to control the third transistor TFT 3 The working area of ​​the third transistor TFT 3 Works in the subthreshold region.

[0049] The differential active pixel circuit of this embodiment can realize the same function as the differential active pixel circuit of the first embodiment, but the top gate voltage VTG2 and the bottom gate voltage VBG2 have different control capabilities on the threshold voltage of the second transistor TFT2, and the top gate voltage VTG3 and the bottom gate voltage VBG3 have different control capabilities on the threshold voltage of the third transistor TFT 3 The control capability of the threshold voltage is different, which is specifically reflected in that the control coefficient γ of the gate voltage to the threshold voltage is related to different manufacturing processes. The larger γ is, the stronger the control capability is. Therefore, compared with the control capability of the differential active pixel circuit in embodiment 1, the control capability of the differential active pixel circuit in this embodiment will change.

[0050] Embodiment 5 This embodiment discloses a fifth differential active pixel circuit based on a dual-gate thin film transistor, which differs from the first embodiment in that the optoelectronic device is composed of a photoconductive material; the photoconductive material includes but is not limited to halides, calcium, titanium, and ore.

[0051] The differential active pixel circuit of this embodiment can realize the same function as the differential active pixel circuit of the first embodiment.

[0052] Embodiment 6 The difference between this embodiment and the first embodiment is that this embodiment is placed outside the pixel unit and connected to multiple columns of pixel units. Each column of pixel units and the reset transistor form a Figure 5 The inter-row shared amplifier circuit shown in Figure 6 As shown, each pixel unit includes a photodiode PD and a switch transistor TFT, which can greatly reduce the pixel area and thus improve the resolution.

[0053] The pixel units of each column (Pixel 1.1, Pixel 2.1, Pixel m.1 in the first column, Pixel 1.2, Pixel 2.2, Pixel m.2, ... in the second column) are connected in series so that multiple pixel units can be connected to only one differential active pixel circuit, so that multiple pixel units can share one differential active pixel circuit. After each row of pixel units is read, the gate voltage V reset By resetting the sensor node, the next row of pixel units can be read, so that the differential active pixel circuit of the present invention has strong controllability, low dark current, high signal-to-noise ratio, wide dynamic range, high gain, and is suitable for inter-row shared amplifier circuits, which solves the problem that the passive pixel circuit cannot achieve intra-pixel amplification, resulting in its low signal-to-noise ratio and inability to meet the application scenario requirements of low-dose X-ray imaging.

[0054] The present invention uses a differential active pixel circuit composed of dual-gate thin film transistors to make the preparation process fully compatible with the panel process, reduce production costs, and improve circuit integration; and the second transistor and the third transistor are designed as differential pairs to differentiate the current, which can reduce dark current and weaken the influence of unevenness in the manufacturing process; the same amplifier circuit can be shared between rows to minimize the unevenness between rows, and the area of ​​a single pixel unit is greatly reduced, which can improve spatial resolution; further, it can also achieve the goal of minimizing noise and eliminating the influence of noise, such as thermal noise and flicker noise, while reducing dark current, widening the circuit dynamic range, and improving gain.

[0055] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A differential active pixel circuit based on a dual-gate thin film transistor, characterized in that: include: A photoelectric device, a first transistor, a second transistor and a third transistor; The second transistor is connected to the photoelectric device, the first transistor, and the third transistor; The photoelectric device is connected to a first external power source and the first transistor; The first transistor is grounded; The second transistor is grounded; The third transistor is connected to a second external power supply; The connection point between the second transistor and the third transistor is an output terminal.

2. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 1, characterized in that: The photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; An anode of the photodiode is connected to the first transistor and the second transistor.

3. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 1, characterized in that: The first transistor is a dual-gate thin film transistor; The drain of the first transistor is connected to the bottom gate of the first transistor, the photoelectric device, and the second transistor; and the source of the first transistor is grounded.

4. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 1, characterized in that: The first transistor is a dual-gate thin film transistor; The drain of the first transistor is connected to the top gate of the first transistor, the photoelectric device, and the second transistor; and the source of the first transistor is grounded.

5. A differential active pixel circuit based on a dual-gate thin film transistor according to claim 3 or 4, characterized in that: The second transistor is a dual-gate thin film transistor; the third transistor is a dual-gate thin film transistor; The bottom gate of the second transistor is connected to the optoelectronic device and the drain of the first transistor; The drain of the second transistor is grounded; the top gate of the third transistor is connected to the top gate of the second transistor; the bottom gate of the third transistor is connected to the drain of the third transistor and the source of the second transistor; the source of the third transistor is connected to the second external power supply; and the source of the second transistor is the output terminal.

6. A differential active pixel circuit based on a dual-gate thin film transistor according to claim 3 or 4, characterized in that: The second transistor is a dual-gate thin film transistor; the third transistor is a dual-gate thin film transistor; The top gate of the second transistor is connected to the photoelectric device and the drain of the first transistor; The drain of the second transistor is grounded; the bottom gate of the third transistor is connected to the bottom gate of the second transistor; the top gate of the third transistor is connected to the drain of the third transistor and the source of the second transistor; the source of the third transistor is connected to the second external power supply; and the source of the second transistor is the output terminal.

7. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 5, characterized in that: The photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; An anode of the photodiode is connected to the drain of the first transistor and the bottom gate of the second transistor.

8. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 6, characterized in that: The photoelectric device is a photodiode; The cathode of the photodiode is connected to the first external power supply; An anode of the photodiode is connected to the drain of the first transistor and the top gate of the second transistor.

9. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 1, characterized in that: The optoelectronic device is composed of a photoconductive material.

10. The differential active pixel circuit based on a dual-gate thin film transistor according to claim 1, characterized in that: The second external power supply is twice the output voltage of the output terminal.

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