Patent Publication Number: US-2023142239-A1

Title: Capacitive sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of pending U.S. patent application Ser. No. 17/228,883, filed Apr. 13, 2021 and entitled “CAPACITIVE SENSOR”, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure is related to an electronic device, and in particular it is related to a capacitive sensor using stray-capacitive sensing. 
     DESCRIPTION OF THE RELATED ART 
     The two major capacitive sensing methods are self-capacitive sensing and mutual-capacitive sensing. Self-capacitive sensing may be a good solution for fingerprint sensing because a large capacitance (signal) changed will be expected due to vertical electrical field, but it needs capacitance to voltage conversion for sensing. On the other hand, mutual-capacitive sensing has a voltage output which will be simplified for circuit design, but large capacitance (signal) changed will not be expected due to lateral electrical field for fingerprint sensing. 
     In addition, self-capacitive sensing has capacitance to voltage converter in a pixel, which is obstacle for high dots per inch (dpi). Another method is charge transfer from the capacitance to fingerprint (ridge or valley) to a converter outside of active area through sensing lines. Capacitive loading of sensing lines make scan speed slower and noise immunity worse. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In order to resolve the issue described above, the present disclose provides an electronic device applicable to sense a fingerprint. The capacitive sensor includes a first electrode and at least one second electrode. The first electrode includes at least one opening. The at least one second electrode is disposed on the first electrode. The at least one second electrode covers the at least one opening of the first electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be more fully understood by reading the subsequent detailed description with references made to the accompanying figures. It should be understood that the figures are not drawn to scale in accordance with standard practice in the industry. In fact, it is allowed to arbitrarily enlarge or reduce the size of components for clear illustration. This means that many special details, relationships and methods are disclosed to provide a complete understanding of the disclosure. 
         FIG.  1    is a schematic diagram of a sensor pixel array with scan lines in row and sensing lines in column in accordance with some embodiments of the disclosure. 
         FIG.  2    is a top view of one capacitive sensor in the sensor pixel array in  FIG.  1    in accordance with some embodiments of the disclosure. 
         FIG.  3 A  is a schematic diagram of the capacitive senor in  FIG.  2    interacting with a fingerprint in accordance with some embodiments of the disclosure. 
         FIG.  3 B  is an equivalent circuit of the capacitive senor in  FIG.  3 A  in accordance with some embodiments of the disclosure. 
         FIG.  4 A  is a schematic diagram of a capacitive sensor interacting with a fingerprint in accordance with some embodiments of the disclosure. 
         FIG.  4 B  is an equivalent circuit of the capacitive sensor in  FIG.  4 A  in accordance with some embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     In order to make the above purposes, features, and advantages of some embodiments of the present disclosure more comprehensible, the following is a detailed description in conjunction with the accompanying drawing. 
     Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will understand, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. It is understood that the words “comprise”, “have” and “include” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Thus, when the terms “comprise”, “have” and/or “include” used in the present disclosure are used to indicate the existence of specific technical features, values, method steps, operations, units and/or components. However, it does not exclude that more technical features, numerical values, method steps, work processes, units, components, or any combination of the above can be added. 
     The directional terms used throughout the description and following claims, such as: “on”, “up”, “above”, “down”, “below”, “front”, “rear”, “back”, “left”, “right”, etc., are only directions referring to the drawings. Therefore, the directional terms are used for explaining and not used for limiting the present disclosure. Regarding the drawings, the drawings show the general characteristics of methods, structures, and/or materials used in specific embodiments. However, the drawings should not be construed as defining or limiting the scope or properties encompassed by these embodiments. For example, for clarity, the relative size, thickness, and position of each layer, each area, and/or each structure may be reduced or enlarged. 
     When the corresponding component such as layer or area is referred to “on another component”, it may be directly on this another component, or other component(s) may exist between them. On the other hand, when the component is referred to “directly on another component (or the variant thereof)”, any component does not exist between them. Furthermore, when the corresponding component is referred to “on another component”, the corresponding component and the another component have a disposition relationship along a top-view/vertical direction, the corresponding component may be below or above the another component, and the disposition relationship along the top-view/vertical direction are determined by an orientation of the device. 
     It will be understood that when a component or layer is referred to as being “connected to” another component or layer, it can be directly connected to this another component or layer, or intervening components or layers may be presented. In contrast, when a component is referred to as being “directly connected to” another component or layer, there are no intervening components or layers presented. 
     The electrical connection or coupling described in this disclosure may refer to direct connection or indirect connection. In the case of direct connection, the endpoints of the components on the two circuits are directly connected or connected to each other by a conductor line segment, while in the case of indirectly connected, there are switches, diodes, capacitors, inductors, resistors, other suitable components, or a combination of the above components between the endpoints of the components on the two circuits, but the intermediate component is not limited thereto. 
     The words “first”, “second”, “third”, “fourth”, “fifth”, and “sixth” are used to describe components, they are not used to indicate the priority order of or advance relationship, but only to distinguish components with the same name. 
     It should be noted that the technical features in different embodiments described in the following can be replaced, recombined, or mixed with one another to constitute another embodiment without departing from the spirit of the present disclosure. 
       FIG.  1    is a schematic diagram of a sensor pixel array with scan lines in row and sensing lines in column in accordance with some embodiments of the disclosure. As shown in  FIG.  1   , the sensor pixel array in  FIG.  1    includes nine capacitive sensors, such as a capacitive sensor (m−1, n−1), a capacitive sensor (m−1, n), a capacitive sensor (m−1, n+1), a capacitive sensor (m, n−1), a capacitive sensor (m,n), a capacitive sensor (m, n+1), a capacitive sensor (m+1, n−1), a capacitive sensor (m+1, n), and a capacitive sensor (m+1, n+1), but the present disclosure is not limited thereto. The capacitive sensors may be fingerprint sensors for sensing a fingerprint, but is not limited thereto. The following may take the fingerprint sensors as an example. 
     Each of the nine capacitive sensors is electrically connected to one of sensing lines and one of scan lines respectively. For example, the capacitive sensor (m−1, n−1) is electrically connected to a sensing line SSL(m−1) and a scan line SL(n−1). The capacitive sensor (m−1, n) is electrically connected to the sensing line SSL(m−1) and a scan line SL(n). The capacitive sensor (m−1, n+1) is electrically connected to the sensing line SSL(m−1) and a scan line SL(n+1). The capacitive sensor (m, n−1) is electrically connected to a sensing line SSL(m) and the scan line SL(n−1). The capacitive sensor (m,n) is electrically connected to the sensing line SSL(m) and the scan line SL(n). The capacitive sensor (m, n+1) is electrically connected to the sensing line SSL(m) and the scan line SL(n+1). The capacitive sensor (m+1, n−1) is electrically connected to a sensing line SSL(m+1) and the scan line SL(n−1). The capacitive sensor (m+1, n) is electrically connected to the sensing line SSL(m+1) and the scan line SL(n). The capacitive sensor (m+1, n+1) is electrically connected to the SSL sensing line (m+1) and the scan line SL(n+1). 
     When a finger of a user touches the sensor pixel array, for example, a portion of the finger of the user is placed above the capacitive sensor (m,n) in a sensing period, and the voltage on the scan line SL(n) may be pulled high at the sensing period, the capacitance variance between a ridge of the fingerprint and the capacitive sensor (m,n), or a valley of the fingerprint and the capacitive sensor (m,n) can be converted into voltage variance by the capacitive sensor (m,n). 
     In the present disclosure, the sensor pixel array in  FIG.  1    is applied in an electronic device. The electronic device can be any suitable type device, such as a touch display device, an antenna device, a tiled device, a sensing device, a flexible device, etc., but is not limited thereto. The electronic device described in the present disclosure is a touch display device with touch and display functions, and the display device may include liquid crystal (LC), light-emitting diode (LED), quantum dots (QDs), fluorescence, phosphor, other suitable materials or a combination of the above materials, but is not limited thereto. The light-emitting diode may include organic light-emitting diode (OLED), inorganic light-emitting diode, micro-LED, mini-LED, quantum dot light-emitting diode (QLED, QDLED), other suitable materials or a combination of the above materials, but is not limited thereto. The tiled device may be, for example, a tiled display device or a tiled antenna device, but is not limited thereto. In addition, the display device in the electronic device may be a color display device or a monochrome display device, and the shape of the electronic device may be rectangular, circular, polygonal, a shape with curved edges, or other suitable shapes. In addition, the electronic device described below uses, as an example, the sensing of a touch through an embedded touch device, but the touch-sensing method is not limited thereto, and another suitable touch-sensing method can be used provided that it meets all requirements. 
       FIG.  2    is a top view of one capacitive sensor in the sensor pixel array in  FIG.  1    in accordance with some embodiments of the disclosure. Taking the capacitive sensor (m,n) as an example, the capacitive sensor (m,n) includes an electrode  200  and an electrode  202 . The electrode  202  is disposed on the electrode  200 . The electrode  200  includes an opening  210 , which is present at the center of the electrode  200 , but is not limited thereto. As shown in  FIG.  2   , the electrode  202  covers the opening  210  of the electrode  200  from the top view. In other words, the size of the electrode  202  is larger than that of the opening  210  of the electrode  200 . 
     In some embodiments, the electrode  202  is electrically connected to the sensing line SSL(m) and the scan line SL(n) through at least one switch, such as a transistor (for example a thin-film transistor, TFT), which is disposed in a dotted circle marked in  FIG.  2   . In some embodiments, the at least one switch is in a circuit layer (not shown) disposed under the electrode  200 . The present disclosure uses the “transistor” as a driving switch, as an example for description. 
       FIG.  3 A  is a schematic diagram of the fingerprint senor in  FIG.  2    interacting with a fingerprint in accordance with some embodiments of the disclosure. As shown in  FIG.  3 A , the capacitive senor (m,n) further includes a conductive layer  300 , which is disposed under the electrode  200 . In other words, the electrode  200  is disposed between the electrode  202  and the conductive layer  300 . In some embodiments, the electrode  200  and the electrode  202  may be transparent, for example, which comprise indium tin oxide (ITO), indium zinc oxide (IZO), other suitable materials or combinations of the foregoing materials, but the present disclosure is not limited thereto. The conductive layer  300  may comprise metal, ITO, other suitable materials, or combinations of the foregoing materials based on the application of the capacitive senor (m,n), but the present disclosure is not limited thereto. For example, if the capacitive senor (m,n) is applied on a display with backlights, the conductive layer  300  may be transparent. If the capacitive senor (m,n) is applied on a keyboard for fingerprint detecting, the conductive layer  300  may comprise metal or other materials. In some embodiments, a voltage Vr, which may be a direct current (DC) voltage, such as a ground voltage, is provided on the conductive layer  300 . 
     The capacitive senor (m,n) may further include an insulating layer  310 , an insulating layer  312 , and an insulating layer  314 . As shown in  FIG.  3 A , the insulating layer  310  may be disposed between the electrode  200  and the conductive layer  300 . In detail, the insulating layer  310  may be disposed between the above-mentioned circuit layer and the electrode  200  or the insulating layer  310  may be one layer in the above-mentioned circuit layer. The insulating layer  312  may be disposed between the electrode  200  and the electrode  202 . The insulating layer  314  may be disposed between the electrode  202  and the fingerprint  302 . The material of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314  may include such as silicon oxide (SiO x ), silicon nitride (SiN y ), silicon oxynitride (SiO x N y ), polymethylmetacrylate (PMMA), other suitable insulating material or a combination thereof, but the present disclosure is not limited thereto. In some embodiments, the material of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314  may be the same, or may be different from each other, but the present disclosure is not limited thereto. In some embodiments, the thickness of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314  may be, for example 2500A, but the present disclosure is not limited thereto. In some embodiments, the thickness of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314  may be different from each other. For example, the insulating layer  310  is thicker than the insulating layer  312 , and the insulating layer  312  is thicker than the insulating layer  314 . In some embodiment, the insulating layer  314  is thicker than the insulating layer  312 , and the insulating layer  312  is thicker than the insulating layer  310 . In some embodiment, the insulating layer  312  is thicker than the insulating layer  310 , and the insulating layer  312  is thicker than the insulating layer  314 , but the present disclosure is not limited thereto. 
     In some embodiments, the capacitive senor (m,n) further includes a common electrode  304 . The common electrode  304  may surround the electrode  202 , and the voltage Vr may be also provided on the common electrode  304  for shielding an electrical field generated from the other nearby components, for example the capacitive sensor (m, n−1), the capacitive sensor (m−1, n), the capacitive sensor (m, n+1), or the capacitive sensor (m+1, n) in  FIG.  1   , to lower the interference from the nearby capacitive sensors. 
     As shown in  FIG.  3 A , a capacitor Cts is formed between the conductive layer  300  and the electrode  200 . A capacitor Crs is formed between the conductive layer  300  and the electrode  202 . A capacitor Ctr is formed between the electrode  200  and the electrode  202 . A capacitor Cf is formed between the electrode  202  and the fingerprint  302 . 
       FIG.  3 B  is an equivalent circuit of the fingerprint senor in  FIG.  3 A  in accordance with some embodiments of the disclosure. As shown in  FIG.  3 B , a sensing signal Vtx, which may be a clock signal, is provided on the electrode  200 . The capacitor Cts is disposed between the voltage Vr (that is, the conductive layer  300 ) and the electrode  200 . The capacitor Crs is disposed between the voltage Vr and the electrode  202 . The capacitor Ctr is disposed between the electrode  200  and the electrode  202 . The capacitor Cf is disposed between the electrode  202  and the fingerprint  302 . In some embodiments, the electrode  200  can be seen as a transmitter (Tx) of the capacitive sensor (m,n), because the sensing signal Vtx is transmitted from the electrode  200 . The electrode  202  can be seen as a receiver (Rx) of the capacitive sensor (m,n), because an output voltage Vrx(m,n) can be received from the electrode  202 . 
     In some embodiments, the capacitor Cts may be 7.76 femto-Farad(fF), the capacitor Crs may be 13.47 fF, and the capacitor Ctr may be 150.52 fF, but they are not limited thereto. The capacitor Cf between the electrode  202  and the ridge of the fingerprint  302  may be 430.30 fF. However, the capacitor Cf between the electrode  202  and the valley of the fingerprint  302  may be 0.18 fF. Since the output voltage Vrx(m,n) is inversely proportional to the capacitance between the electrode  202  and the fingerprint  302 , the output voltage Vrx(m,n) corresponding to the ridge of the fingerprint  302  is less than that corresponding to the valley of the fingerprint  302 . 
     For example, when the sensing signal Vtx with amplitude of 15V is applied to the electrode  200 , the output voltage Vrx(m,n) may be 3.8V corresponding to the ridge of the fingerprint  302  with depth of 0 um (assuming that the ridge of the fingerprint  302  is directly contacted with the insulating layer  314 ), and the output voltage Vrx(m,n) may be 13.75V corresponding to the valley of the fingerprint  302  with depth of 100 um (assuming that the depth of the valley of the fingerprint  302  is 100 um). 
     As shown in  FIG.  3 B , the capacitive senor (m,n) may further include a control circuit  320 . The sensing line SSL(m) may be electrically connected to the electrode  202  through the control circuit  320 . In some embodiments, the control circuit  320  may include a transistor  322 , a transistor  324 , and a transistor  326 , but the present disclosure is not limited thereto. The transistor  322  may have a first terminal received the voltage Vr, a second terminal electrically connected to the electrode  202 , and a gate terminal received a reset signal Reset. The transistor  324  may have a first terminal received the sensing signal Vtx, a second terminal electrically connected to a first terminal of the transistor  326 , and a gate terminal of the transistor  324  may be electrically connected to the electrode  202 . The transistor  326  may have a second terminal electrically connected to the sensing line SSL(m), and a gate terminal of the transistor  326  may be electrically connected to the scan line SL(n). 
     The reset signal Reset defines non-sensing periods, and the sensing signal Vtx defines sensing periods. In other words, the reset signal Reset may be pulled high and the sensing signal Vtx may be pulled low during the non-sensing periods. The reset signal Reset may be pulled low and the sensing signal Vtx may be pulled high during the sensing periods. During the sensing periods, for example, when the sensing signal Vtx is pulled high, the voltage on the scan line SL(n) is also pulled high, the transistor  322  may be turned off and the transistor  326  may be turned on, the magnitude of the current passing through the transistor  324  and the transistor  326  can be determined according to the output voltage Vrx(m,n). In other words, the higher the output voltage Vrx(m,n) is, the larger magnitude of the current passing through the transistor  324  and the transistor  326  to the sensing line SSL(m). Therefore, a voltage corresponding to the ridge of the fingerprint  302  is present on the sensing line SSL(m) based on the current flowing through the transistor  324  and the transistor  326  during the sensing periods. Similarly, a voltage corresponding to the valley of the fingerprint  302  is also present on the sensing line SSL(m) based on the current flowing through the transistor  324  and the transistor  326  during the sensing periods. In some embodiments, the reset signal Reset defines reset periods. During a reset period, the reset signal Reset is pulled high and the sensing signal Vtx is pulled low, then output voltage Vrx(m,n) is initialized with the voltage Vr by the transistor  322 . 
     In some embodiments, a voltage read-out circuit  330  receives the voltages corresponding to the ridge and/or the valley of the fingerprint  302  from the sensing line SSL(m), and converts the voltages into digital signals. The voltage read-out circuit  330 , for example, may include a transistor  332  and an analog-to-digital converter (ADC)  334 . The transistor  332  may have a first terminal electrically connected to the sensing line SSL(m), and a second terminal received the voltage Vr. A gate terminal of the transistor  332  receives the reset signal Reset, and an input of the ADC  334  is electrically connected to the sensing line SSL(m). During the sensing periods, when the reset signal Reset is pulled low, the transistor  332  is turned off, the voltages corresponding to the ridge and/or the valley of the fingerprint  302  from the sensing line SSL(m) is converted into digital signals by the ADC  334 . In some embodiments, during the reset period, the reset signal Reset is pulled high, the transistor  332  is turned on, then the sensing line SSL(m) is initialized with the voltage Vr by the transistor  332 . 
     In some embodiments, the capacitive sensor (m,n) may further include a current read-out circuit (not shown) for replacing the voltage read-out circuit  330 . The current read-out circuit, for example, may include a transistor, an op amplifier, an ADC, and a capacitor. The first input port of the op amplifier may be electrically connected to the sensing line SSL(m), the second input port of the op amplifier may receive the voltage Vr, and the output of the op amplifier may be electrically connected to the input of the ADC. The capacitor may be electrically connected between the sensing line SSL(m) and the output of the op amplifier. The transistor may be electrically connected between the sensing line SSL(m) and the output of the op amplifier. During the sensing periods, when the transistor is turned off, the currents corresponding to the ridge and/or the valley of the fingerprint  302  from the sensing line SSL(m) is converted into voltages by the capacitor and the op amplifier. The voltages will be converted into digital signals by the ADC. In some embodiments, the voltage read-out circuit  330  and the current read-out circuit may be disposed outside the capacitive sensor (m,n), for example, they are disposed in other function chip. In some embodiments, during the reset period, the transistor is turned on, the capacitor is initialized, then the output of the op amplifier has the voltage Vr as an initial value. 
     In some embodiments, when the current read-out circuit is used to replace the voltage read-out circuit  330 , a p-type transistor can be used as the transistor  324  in the control circuit  320 . When the voltage read-out circuit  330  is used, a n-type transistor can be used as the transistor  324  in the control circuit  320 . 
       FIG.  4 A  is a schematic diagram of a capacitive sensor interacting with a fingerprint in accordance with some embodiments of the disclosure. As shown in  FIG.  4 A , the capacitive senor (m,n) includes a conductive layer  400 , an electrode  402 , an electrode  404 , and an electrode  406 . The electrode  404  is disposed on the electrode  402 . The electrode  404  covers the opening of the electrode  402 . In other words, the size of the electrode  404  is larger than that of the opening of the electrode  402 . The electrode  406  is disposed in the opening of the electrode  402 . In some embodiments, the electrode  406  is substantially coplanar with the electrode  402 , but the present disclosure is not limited thereto. 
     The conductive layer  400  is disposed under the electrodes  402  and  406 . In other words, the electrodes  402  and  406  are disposed between the conductive layer  400  and the electrode  404 . The electrode  404  is disposed between the electrode  402  and a fingerprint  420 . In some embodiments, a voltage Vr, which may be a direct current (DC) voltage, such as a ground voltage, is provided on the conductive layer  400 . In some embodiments, the electrode  400  and the electrode  402  may be transparent, for example, which comprise ITO, indium zinc oxide (IZO), other suitable materials or combinations of the foregoing materials, but the present disclosure is not limited thereto. The conductive layer  400  may comprise metal, ITO, other suitable materials, or combinations of the foregoing materials based on the application of the capacitive senor (m,n), but the present disclosure is not limited thereto. 
     The capacitive senor (m,n) in  FIG.  4 A  further includes an insulating layer  430 , an insulating layer  432 , and an insulating layer  434 . The materials of the insulating layer  430 , the insulating layer  432 , and the insulating layer  434  may be the same as those of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314 , thus the present disclosure does not repeat them again. In some embodiments, the thickness of the insulating layer  430 , the insulating layer  432 , and the insulating layer  434  may be the same as that of the insulating layer  310 , the insulating layer  312 , and the insulating layer  314 , thus the present disclosure does not repeat it again. 
     In some embodiments, the capacitive senor (m,n) in  FIG.  4 A  further includes a common electrode  408  and/or a common electrode  410 . The common electrode  408  may surround the electrode  404 , and the electrode  410  may surround the electrode  406 . The voltage Vr may be also provided on the common electrodes  408  for shielding to lower the interference from the nearby capacitive sensors, for example, the capacitive sensor (m, n−1), the capacitive sensor (m−1, n), the capacitive sensor (m, n+1), or the capacitive sensor (m+1, n) in  FIG.  1   . The voltage Vr may be also provided on the common electrodes  410  for shielding to lower the interference between the electrode  402  and the electrode  406 . 
     As shown in  FIG.  4 A , a capacitor Cts&#39; is formed between the conductive layer  400  and the electrode  402 . A capacitor Crs&#39; is formed between the conductive layer  400  and the electrode  406 . A capacitor Cfs&#39; is formed between the conductive layer  400  and the electrode  404 . A capacitor Ctf&#39; is formed between the electrode  402  and the electrode  404 . A capacitor Cfr′ is formed between the electrode  406  and the electrode  404 . A capacitor Cf′ is formed between the electrode  404  and the fingerprint  420 . 
       FIG.  4 B  is an equivalent circuit of the capacitive sensor in  FIG.  4 A  in accordance with some embodiments of the disclosure. As shown in  FIG.  4 B , a sensing signal Vtx, which is a clock signal, is provided on the electrode  402 . The capacitor Cts&#39; is disposed between the voltage Vr (that is, the conductive layer  400 ) and the electrode  402 . The capacitor Ctf&#39; is disposed between the electrode  402  and the electrode  404 . The capacitor Cfr′ is disposed between the electrode  404  and the electrode  406 . The capacitor Cfs&#39; is disposed between the voltage Vr and the electrode  404 . The capacitor Crs&#39; is disposed between the voltage Vr and the electrode  406 . The capacitor Cf′ is disposed between the electrode  404  and the fingerprint  420 . 
     In some embodiments, the electrode  402  can be seen as a transmitter (Tx) of the capacitive sensor (m,n) in  FIG.  4 A , because the sensing signal Vtx is transmitted from the electrode  402 . The electrode  406  can be seen as a receiver (Rx) of the capacitive sensor (m,n) in  FIG.  4 A , because an output voltage Vrx(m,n) can be received from the electrode  406 . 
     In some embodiments, the capacitor Cts&#39; may be 7.76 fF, the capacitor Ctf&#39; may be 150.52 fF, and the capacitor Cfr′ may be 141.66 fF. The capacitor Crs&#39; may be 3.45 fF. The capacitor Cfs&#39; may be 10.01 fF, but they are not limited thereto. The capacitor Cf′ between the electrode  404  and the ridge of the fingerprint  420  may be 430.30 fF. However, the capacitor Cf′ between the electrode  404  and the valley of the fingerprint  420  may be 0.18 fF. Since the output voltage is inversely proportional to the capacitance between the electrode  404  and the fingerprint  420 , the output voltage Vrx(m,n) corresponding to the ridge of the fingerprint  420  may be less than that corresponding to the valley of the fingerprint  420 . 
     For example, when the sensing signal Vtx with amplitude of 15V is applied to the electrode  402 , the output voltage Vrx(m,n) may be 3.71V corresponding to the ridge of the fingerprint  420  with depth of 0 um (assuming that the ridge of the fingerprint  420  is directly contacted with the insulating layer  434 ), and the output voltage Vrx(m,n) may be 13.43V corresponding to the valley of the fingerprint  420  with depth of 100 um (assuming that the depth of the valley of the fingerprint  420  is 100 um). 
     As shown in  FIG.  4 B , the capacitive senor (m,n) further includes a control circuit  440 . The sensing line SSL(m) may be electrically connected to the electrode  406  through the control circuit  440 . In some embodiments, the control circuit  440  includes a transistor  442 , a transistor  444 , and a transistor  446 . The transistor  442  may have a first terminal received the voltage Vr, a second terminal electrically connected to the electrode  406 , and a gate terminal received a reset signal Reset. The transistor  444  may have a first terminal received the sensing signal Vtx, a second terminal electrically connected to a first terminal of the transistor  446 , and a gate terminal of the transistor  444  may be electrically connected to the electrode  406 . The transistor  446  may have a second terminal electrically connected to the sensing line SSL(m), and a gate terminal of the transistor  446  may be electrically connected to the scan line SL(n). 
     Similarly, the reset signal Reset is pulled low and the sensing signal Vtx is pulled high during the sensing periods. During the sensing periods, when the sensing signal Vtx is pulled high, the voltage on the scan line SL(n) is also pulled high, the transistor  442  is turned off and the transistor  446  is turned on, the magnitude of the current passing through the transistor  444  and the transistor  446  can be determined according to the output voltage Vrx(m,n). In other words, the higher the output voltage Vrx(m,n) is, the larger magnitude of the current passing through the transistor  444  and the transistor  446  to the sensing line SSL(m). Therefore, a voltage corresponding to the ridge of the fingerprint  420  is present on the sensing line SSL(m) based on the current flowing through the transistor  444  and the transistor  446  during the sensing periods. Similarly, a voltage corresponding to the valley of the fingerprint  420  is also present on the sensing line SSL(m) based on the current flowing through the transistor  444  and the transistor  446  during the sensing periods. In some embodiments, before the sensing period, the reset signal Reset is pulled high and the sensing signal Vtx is pulled low in the reset period, then the output voltage Vrx(m,n) is initialized with the voltage Vr by the transistor  422 . 
     In some embodiments, a voltage read-out circuit  450  receives the voltages corresponding to the ridge and/or the valley of the fingerprint  420  from the sensing line SSL(m), and converts the voltages into digital signals. The voltage read-out circuit  450  may include a transistor  452  and an analog-to-digital converter (ADC)  454 . The transistor  452  may have a first terminal electrically connected to the sensing line SSL(m) and a second terminal received the voltage Vr. A gate terminal of the transistor  452  receives the reset signal Reset, and an input of the ADC  454  is electrically connected to the sensing line SSL(m). During the sensing periods, when the reset signal is pulled low, the transistor  452  is turned off, the voltages corresponding to the ridge and/or the valley of the fingerprint  420  from the sensing line SSL(m) is converted into digital signals by the ADC  454 . In some embodiments, during the reset period before the sensing periods, the reset signal (Reset) is pulled high, the transistor  452  is turned on, then the sensing line SSL(m) is initialized with the voltage Vr by the transistor  452 . 
     In some embodiments, the capacitive sensor (m,n) in  FIG.  4 A  further includes a current read-out circuit (not shown) for replacing the voltage read-out circuit  450 . The current read-out circuit includes a transistor, an op amplifier, an ADC, and a capacitor. The first input port of the op amplifier may be electrically connected to the sensing line SSL(m), the second input port of the op amplifier may receive the voltage Vr, and the output of the op amplifier may be electrically connected to the input of the ADC. The capacitor may be electrically connected between the sensing line SSL(m) and the output of the op amplifier. The transistor may be electrically connected between the sensing line SSL(m) and the output of the op amplifier. During the sensing periods, when the transistor is turned off, the currents corresponding to the ridge and/or the valley of the fingerprint  420  from the sensing line SSL(m) is converted into voltages by the capacitor and the op amplifier. The voltages will be converted into digital signals by the ADC. In some embodiments, the voltage read-out circuit  450  and the current read-out circuit may be disposed outside the capacitive sensor (m,n), for example, they are disposed in other function chip. In some embodiments, during the reset period before the sensing periods, the transistor is turned on, the capacitor is initialized, then the output of the op amplifier has the voltage Vr as an initial value. 
     In some embodiments, when the current read-out circuit is used to replace for the voltage read-out circuit  450 , a p-type transistor can be used as the transistor  444  in the control circuit  440 . When the voltage read-out circuit  440  is used, an n-type can be used as the transistor  444  in the control circuit  440 . 
     In some embodiments, each of the control circuit  320  in  FIG.  3 B  and the control circuit  440  in  FIG.  4 B  may include one transistor (not shown), wherein the gate terminal of the transistor may be electrically connected to the scan line SL(n), a first terminal of the transistor may be electrically connected to the electrode  202  or the electrode  406 , and a second terminal of the transistor may be electrically connected to the sensing line SSL(m). In some embodiments, the capacitive sensor (m,n) with the control circuit  320  in  FIG.  3 B  or with the control circuit  440  in  FIG.  4 B  can be called as an active pixel. The capacitive sensor (m,n) with the control circuit having one transistor can be called as a passive pixel. 
     Stray-capacitive (stray-cap) sensing is provided for the capacitive sensors in  FIG.  1   ,  FIG.  2   ,  FIGS.  3 A and  3 B , and  FIGS.  4 A and  4 B . Stray-cap sensing enables a large capacitance change which is the same as self-cap sensing and a simple read-out circuit which is the same as mutual-cap sensing. The simple sensor structure with the simple readout circuit enables large, flex and/or fast scan frames per second (FPS) with utilizing in-plane-switching liquid crystal (IPS-LC) array process and/or thinner system with no extra device such as light source for optical FPS. 
     The embodiments of the present disclosure are disclosed above, but they are not used to limit the scope of the present disclosure. A person skilled in the art can make some changes and retouches without departing from the spirit and scope of the embodiments of the present disclosure. Therefore, the scope of protection in the present disclosure shall be deemed as defined by the scope of the attached claims.