Patent Publication Number: US-2023161032-A1

Title: Ultrasonic fingerprint identification circuit, driving method thereof, and display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/019,821, filed on Sep. 14, 2020, which claims priority to Chinese Patent Application No. 202010558026.3, filed on Jun. 18, 2020. All of the above-mentioned patent applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of display technology, and particularly, to an ultrasonic fingerprint identification circuit, a driving method thereof, and a display device. 
     BACKGROUND 
     In recent years, with the rapid development of technology, display devices with biometric functions have gradually emerged in our lives and work, and the fingerprint identification technology is widely used in the areas such as unlocking and secure payment by virtue of unique identities of fingerprints. In addition, for an ultrasonic fingerprint identification technology, due to strong penetration of ultrasonic waves, the fingerprints can still be identified even when there are water stains or dirty stains on a surface of the finger, and the technology attracts a lot of attention. 
     However, regarding the existing designs of the ultrasonic fingerprint identification circuit, a large number of wirings are arranged in the circuit, and the reading of signal will be interfered by coupling capacitance generated between the wirings during a transmission process, thereby resulting in poor identification precision. 
     SUMMARY 
     In view of this, embodiments of the present disclosure provide an ultrasonic fingerprint identification circuit, a driving method thereof, and a display device, which improve the precision of fingerprint identification. 
     In a first aspect, the present disclosure provides an ultrasonic fingerprint identification circuit, comprising a plurality of fingerprint identification units, each of the plurality of fingerprint identification units comprising: an ultrasonic fingerprint identification sensor electrically connected to a first node, wherein the ultrasonic fingerprint identification sensor is configured to: convert a first electrical signal to an ultrasonic signal and radiate the ultrasonic signal towards a finger, and convert an ultrasonic signal reflected by the finger to a second electrical signal and transmit the second electrical signal to the first node; a control module electrically connected to a composite signal line, the first node, and one first control signal line of a plurality of first control signal lines, wherein the control module is configured to: provide a reset potential to the first node in response to a first level provided by the one of the plurality of first control signal lines, and provide a pull-up potential to the first node in response to a first level provided by the composite signal line; and a reading module electrically connected to the first node, a reading signal line, and one second control signal line of a plurality of second control signal lines, wherein the reading module is configured to read a detection signal of the first node in response to a first level provided by the one second control signal line, wherein one of the plurality of first control signal lines electrically connected to one of the plurality of fingerprint identification units is reused as one of the plurality of second control signal lines electrically connected to another one of the plurality of fingerprint identification units. 
     In a second aspect, the present disclosure provides a driving method of an ultrasonic fingerprint identification circuit, the driving method being used to drive the ultrasonic fingerprint identification circuit according to claim  1 , a driving cycle of each of the plurality of fingerprint identification units of the ultrasonic fingerprint identification circuit comprising a preparation period, a pull-up period, and a reading period. The driving method comprises: in the preparation period, converting, by the ultrasonic fingerprint identification sensor, the first electrical signal to the ultrasonic signal and radiating the ultrasonic signal towards the finger, providing, by the one first control signal line of a plurality of first control signal lines, the first level, and transmitting, by the control module, the second level provided by the composite signal line to the first node; in the pull-up period, converting, by the ultrasonic fingerprint identification sensor, the ultrasonic signal reflected by the finger to the second electrical signal and transmitting the second electrical signal to the first node, providing, by the composite signal line, the first level, and transmitting, by the control module, the first level provided by the composite signal line to the first node, to pull up the potential of the first node; and in the reading period, providing, by one of the plurality of second control signal lines, the first level, and reading, by the reading module, the detection signal of the first node. 
     In a third aspect, the present disclosure provides a display device, including: a display panel having a display area, wherein the display area comprises a main display area and a fingerprint identification area; the ultrasonic fingerprint identification circuit according to the first aspect, wherein the ultrasonic fingerprint identification circuit is disposed in the fingerprint identification area; and a processor electrically connected to the reading signal line and configured to identify fingerprints based on a signal read by the reading signal line. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order to explain technical solutions of embodiments of the present disclosure, accompanying drawings used in the embodiments are briefly described below. The drawings described below merely illustrate some of the embodiments of the present disclosure. Based on these drawings, those skilled in the art can obtain other drawings. 
         FIG.  1    is a structural schematic diagram of a fingerprint identification circuit provided by an embodiment of the present disclosure; 
         FIG.  2    is a timing diagram corresponding to a circuit structure shown in  FIG.  1   ; 
         FIG.  3    is another timing diagram corresponding to the circuit structure shown in  FIG.  1   ; 
         FIG.  4    is a structural schematic diagram of an ultrasonic fingerprint identification circuit provided by another embodiment of the present disclosure; 
         FIG.  5    is a timing diagram corresponding to a circuit structure shown in  FIG.  4   ; 
         FIG.  6    is another timing diagram corresponding to the circuit structure shown in  FIG.  4   ; 
         FIG.  7    is a schematic diagram of a circuit structure of a fingerprint identification unit provided by an embodiment of the present disclosure; 
         FIG.  8    is a structural schematic diagram of an ultrasonic fingerprint identification circuit corresponding to the fingerprint identification unit shown in  FIG.  7   ; 
         FIG.  9    is a graph of leakage currents of a diode and a transistor TFT-D provided by an embodiment of the present disclosure; 
         FIG.  10    is another graph of leakage current of a diode and a transistor TFT-D provided by an embodiment of the present disclosure; 
         FIG.  11    illustrates graphs of relation of a thickness of an amorphous silicon layer with an operating current, a leakage current, a conduction voltage and electron mobility; 
         FIG.  12    is a structural schematic diagram of a second transistor provided by an embodiment of the present disclosure; 
         FIG.  13    is a structural schematic diagram of a film layer of a second transistor provided by an embodiment of the present disclosure; 
         FIG.  14    is another structural schematic diagram of a film layer of a second transistor provided by an embodiment of the present disclosure; 
         FIG.  15    is a schematic diagram of another circuit structure of a fingerprint identification unit provided by an embodiment of the present disclosure; 
         FIG.  16    is a structural schematic diagram of an ultrasonic fingerprint identification circuit corresponding to the fingerprint identification unit shown in  FIG.  15   ; 
         FIG.  17    is a schematic diagram of yet another circuit structure of a fingerprint identification unit provided by an embodiment of the present disclosure; 
         FIG.  18    is another structural schematic diagram of a fingerprint identification unit provided by an embodiment of the present disclosure; 
         FIG.  19    is a structural schematic diagram of a film layer of a third transistor provided by an embodiment of the present disclosure; 
         FIG.  20    is a structural schematic diagram of a fingerprint identification unit provided by another embodiment of the present disclosure; 
         FIG.  21    is a timing diagram corresponding to a circuit structure shown in  FIG.  20   ; 
         FIG.  22    is a structural schematic diagram of an ultrasonic fingerprint identification sensor provided by an embodiment of the present disclosure; 
         FIG.  23    is a flowchart of a driving method provided by an embodiment of the present disclosure; 
         FIG.  24    is a waveform diagram of an electrical signal converted from an ultrasonic signal and subjected to a wave chopping according to an embodiment of the present disclosure; 
         FIG.  25    is another flowchart of a driving method provided by an embodiment of the present disclosure; 
         FIG.  26    is another timing diagram provided by an embodiment of the present disclosure; and 
         FIG.  27    is a structural schematic diagram of a display device provided by an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In order to explain technical solutions of the present disclosure, the embodiments of the present disclosure are described in detail with reference to the drawings. 
     It should be understood that the described embodiments are merely some of the embodiments of the present disclosure rather than all of the embodiments. All other embodiments obtained by those skilled in the art shall fall into the protection scope of the present disclosure. 
     The terms used in the embodiments of the present disclosure are merely for the purpose of describing particular embodiments, but not intended to limit the present disclosure. Unless otherwise noted in the context, the singular form expressions “a”, “an”, “the” and “said” used in the embodiments and appended claims of the present disclosure also represent a plural form. 
     It should be understood that the term “and/or” describes three relations of the associated objects. For example, A and/or B may indicate three cases: only A exists; A and B exist concurrently; only B exists. In addition, a character “/” herein generally indicates that the associated objects are in an “or” relationship. 
       FIG.  1    is a structural schematic diagram of a fingerprint identification circuit provided by an embodiment of the present disclosure. As shown in  FIG.  1   , the ultrasonic fingerprint identification circuit includes a plurality of fingerprint identification units  1 . Each of the fingerprint identification units  1  includes an ultrasonic fingerprint identification sensor  2 , a control module  3  and a reading module  4 . The ultrasonic fingerprint identification sensor  2  is electrically connected to a first node N 1  to convert an electrical signal into an ultrasonic signal, radiate the ultrasonic signal towards a finger, convert the ultrasonic signal reflected by the finger into an electrical signal, and transmit the electrical signal to the first node N 1 . The control module  3  is electrically connected to a composite signal line cp, a first control signal line reset and the first node N 1 , to provide a reset potential to the first node N 1  in response to a first level provided by the first control signal line reset, and provide a pull-up potential to the first node N 1  in response to a first level provided by the composite signal line cp. The reading module  4  is electrically connected to a second control signal line read, the first node N 1  and a reading signal line read line, to read a detection signal of the first node N 1  in response to a first level provided by the second control signal line read. 
     The first control signal line reset electrically connected to one of the fingerprint identification units  1  is reused as the second control signal line read electrically connected to another one of the fingerprint identification units  1 . For the purpose of distinction, in  FIG.  1   , read1, reset1, and cp1 respectively denote the first control signal line reset, the second control signal line read, and the composite signal line cp corresponding to one of the fingerprint identification units  1 , and read2, reset2, and cp2 respectively denote the first control signal line reset, the second control signal line read, and the composite signal line cp corresponding to another one of the fingerprint identification units  1 . 
     It should be noted that the above first level and second level are correspond to high and low levels. If the first level is a high level, the second level is a low level; and if the first level is a low level, the second level is a high level. In the embodiments of the present disclosure, for example, the first level is a high level and the second level is a low level. 
     For example, a driving cycle of one frame of the fingerprint identification unit  1  includes a preparation period, a pull-up period, and a reading period. In the preparation period, the ultrasonic fingerprint identification sensor  2  converts the electrical signal for fingerprint identification into the ultrasonic signal, and radiates the ultrasonic signal towards the finger, and the control module  3  provides the reset potential to the first node N 1  to maintain the stable potential of the first node N 1 . In the pull-up period, the ultrasonic fingerprint identification sensor  2  converts the ultrasonic signal reflected by the finger into the electrical signal and transmits the electrical signal to the first node N 1 , and the control module  3  provides the pull-up potential to the first node N 1 , to pull up the potential of the first node N 1 . In the reading period, the reading module  4  reads the detection signal of the first node N 1 . 
     Further, the detection signal of the first node N 1  may include an initial voltage and a detection voltage, and correspondingly, the reading period may include an initial voltage reading period and a detection voltage reading period. The initial voltage reading period is prior to the preparation period, and the detection voltage reading period is latter than the pull-up period. 
     As an example, the reading period includes the initial voltage reading period and the detection voltage reading period, in order to explain a driving principle of the fingerprint identification unit in combination with  FIG.  2   .  FIG.  2    is a timing diagram corresponding to a circuit structure shown in  FIG.  1   . 
     The driving cycle of one frame of the fingerprint identification unit  1  includes an initial voltage reading period t 1 , a preparation period t 2 , a pull-up period t 3 , and a detection voltage reading period t 4 . In order to facilitate understanding, respective periods corresponding to the first fingerprint identification unit  1  (i.e., the fingerprint identification unit labeled with the read1, the reset1, and the cp1) shown in  FIG.  1    are represented by t1_1 to t4_1 in  FIG.  2   ; and respective periods corresponding to the second fingerprint identification unit  1  (i.e., the fingerprint identification unit labeled with the read2, the reset2, and the cp2) shown in  FIG.  1    are represented by t1_2 to t4_2 in  FIG.  2   . 
     In the initial voltage reading period t 1 , the second control signal line read provides a first level, and the reading module  4  reads an initial voltage V 1  at the first node N 1 . 
     In the preparation period t 2 , a detection signal line Rbias outputs an electrical signal (such as a Rbias signal shown in  FIG.  2   ) for fingerprint identification to the ultrasonic fingerprint identification sensor  2 , the ultrasonic fingerprint identification sensor  2  converts the electrical signal into the ultrasonic signal and radiates it towards the finger, the first control signal line reset provides a first level, the control module  3  transmits a second level, which is provided by the composite signal line cp, to the first node N 1 , i.e., to provide a reset potential to the first node N 1 . In this way, the first node N 1  is maintained at a stable low potential, preventing the potential of the first node N 1  from being interfered by the ultrasonic signal converted by the ultrasonic fingerprint identification sensor  2 . 
     In the pull-up period t 3 , the ultrasonic fingerprint identification sensor  2  converts the ultrasonic signal reflected by the finger into an electrical signal and transmits it to the first node N 1 . The electrical signal converted by the ultrasonic signal is a signal fluctuating between the high and low potentials, and thus the composite signal line cp provides a first level, the control module  3  transmits the first level provided by the composite signal line cp to the first node N 1 , i.e., to provide a pull-up potential to the first node N 1 . In this way, the pull-up potential and the low potential of the electric signal are superimposed to pull up the potential of the first node N 1 , and to perform wave-chopping on the electric signal converted by the ultrasonic signal. 
     It should be noted that the electrical signal converted from the ultrasonic signal is a signal fluctuating between the high and low potentials, and the highest potential of the electrical signal is used for subsequent detection and identification. Therefore, the pull-up potential only pulls up the low potential of the electrical signal to a reasonable extent and does not cover the original highest potential of the electrical signal. 
     In the detection voltage reading period t 4 , the second control signal line read provides a first level, and the reading module  4  reads the detection voltage V 2  at the first node N 1 . 
     Furthermore, a processor can identify valleys and ridges of the fingerprint by reading V 1  and V 2  in a time division manner and by determining a difference between V 1  and V 2 . 
     Based on the above driving principle, in the embodiment of the present disclosure, it is assumed that the ultrasonic fingerprint identification circuit includes n fingerprint identification units  1 , if the first control signal line reset electrically connected to one of the n fingerprint identification units  1  is reused as the second control signal line read electrically connected to another one of the n fingerprint identification units  1 , as long as all of the n fingerprint identification units  1  can work normally, the number of the first control signal lines reset and the second control signal lines read that are initially required to be provided can be reduced from 2n to n+1, thereby greatly reducing the number of required control signal lines. On the one hand, the space occupied by the control signal lines can be reduced and saved for the design space of the ultrasonic fingerprint identification circuit. On the other hand, coupling between the control signal lines, and coupling between the control signal lines and other wirings can be reduced to lower interference of coupling capacitance on the read signal and to improve accuracy of the read signal, thereby improving accuracy of fingerprint identification. 
     In addition, it can be understood that, when reading the voltage at the first node N 1 , there will inevitably be a noise signal in the read signal. However, in the embodiment of the present disclosure, since the reading period includes the initial voltage reading period t 1  and the detection voltage reading period t 4 , in the driving cycle of one frame, the voltage at the first node N 1  can be read twice in a time division manner respectively at the initial voltage reading period t 1  and the detection voltage reading period t 4 . Further, the valleys and ridges of the fingerprint are identified based on a difference between the two voltages. Therefore, even if the read signal contains the noise signal, the noise signal can be eliminated by subtracting the two voltages, which reduces the influence of the noise signal on the detection accuracy and effectively enhances a signal-to-noise ratio, thereby further improving the accuracy of the fingerprint identification. 
     Further, based on the above structure, applicant has found through research that when the initial voltage at the first node N 1  is read in the initial voltage reading period t 1 , the initial voltage at the first node N 1  of different fingerprint identification units  1  may be different; the slight difference between the fingerprint valley and the fingerprint ridge is likely to be covered by the difference of the initial signal if the respective initial voltages are read directly to subsequently perform differencing based on the initial voltage and the detection voltage, thereby causing inaccuracy of the identification. In view of this, in the embodiment of the present disclosure, as shown in  FIG.  3   , which is another timing diagram corresponding to the circuit structure shown in  FIG.  1   , the driving cycle further includes an overall resetting period t 0  prior to the initial voltage reading period t 1 . In the overall resetting period to, the first control signal line reset, the second control signal line read, and the composite signal line cp respectively provide the first level, the control module  3  transmits the first level provided by the composite signal line cp to the first node N 1 , to perform overall resetting on the first node N 1 . In this way, before the beginning of the driving cycle of each frame, the voltages of the first nodes N 1  in the respective fingerprint identification units  1  are uniformly reset to a high potential, so as to allow the respective fingerprint identification units  1  to read uniform magnitudes of the initial voltages, thereby preventing the difference in initial voltage from affecting the accuracy of the identification. 
       FIG.  4    is another structural schematic diagram of the ultrasonic fingerprint identification circuit provided by an embodiment of the present disclosure. In the embodiment as shown in  FIG.  4   , a plurality of fingerprint identification unit groups  5  is arranged along a first direction, each fingerprint identification unit group  5  includes multiple fingerprint identification units  1  arranged along a second direction, and the first direction intersects the second direction. The fingerprint identification units  1  in the same fingerprint identification unit group  5  are electrically connected to the same first control signal line reset and the same second control signal line read. Moreover, for any two adjacent fingerprint identification unit groups  5 , the first control signal line reset corresponding to one fingerprint identification unit group  5  is reused as the second control signal line read corresponding to the other one fingerprint identification unit group  5 . 
     Compared with the example in which each fingerprint identification unit  1  is provided with one first control signal line reset and one second control signal line read correspondingly, in the above structure, the fingerprint identification units  1  located in the same fingerprint identification unit group  5  are correspondingly provided with one first control signal line reset and one second control signal line read, and further, by reusing the first control signal line reset corresponding to one fingerprint identification unit group  5  to the second control signal line read corresponding to the adjacent fingerprint identification unit group  5 , the number of the control signal lines required to be provided for all the fingerprint identification units  1  can be greatly reduced. In this way, the space occupied by the control signal lines is further reduced, and the interference of the coupling capacitance generated by the control signal lines on the read signal is further reduced. 
     It should be noted that according to a different output frequencies of the first control signal and the second control signal, for example, when the output frequencies of the first control signal and the second control signal are relatively high, a timing diagram corresponding to the circuit structure shown in  FIG.  4    is illustrated in  FIG.  5   . In this case, the second high level of the read1 signal is flush with the first high level in the read4 signal. For example, when the output frequencies of the first control signal and the second control signal are relatively low, another timing diagram corresponding to the circuit structure shown in  FIG.  4    is illustrated in  FIG.  6   . In this case, the second high level in the read1 signal is flush with the first high level in the read3 signal. The output frequencies of the first control signal and the second control signal can be set according to actual requirements, which are not specifically limited in the embodiments of the present disclosure. 
     Further referring to  FIG.  4   , the first control signal line reset and the second control signal line read are alternately arranged in a gap between two adjacent fingerprint identification unit groups  5 . Only one of the first control signal line reset and the second control signal line read is provided in the gap between any two adjacent fingerprint identification unit groups  5 . In addition, the first control signal line reset corresponding to the i-th fingerprint identification unit group  5  is provided in a gap between the i-th fingerprint identification unit group  5  and the (i+1)-th fingerprint identification unit group  5 . With such configuration, when the first control signal line reset corresponding to the i-th fingerprint identification unit group  5  is reused as the second control signal line read of the (i+1)-th fingerprint identification unit group  5 , the first control signal line reset corresponding to the i-th fingerprint identification unit group  5  is provided in the gap, the first control signal line reset can be electrically connected to the fingerprint identification units  1  in the i-th and (i+1)-th fingerprint identification unit groups  5  more conveniently, which reduces complexity of the wirings. 
       FIG.  7    is a schematic diagram of a circuit structure of a fingerprint identification unit provided by an embodiment of the present disclosure, and  FIG.  8    is a structural schematic diagram of an ultrasonic fingerprint identification circuit corresponding to the fingerprint identification unit shown in  FIG.  7   . In the embodiment as shown in  FIG.  7    and  FIG.  8   , the control module  3  includes a first transistor M 1  and a second transistor M 2 . A gate of the first transistor M 1  is electrically connected to the first control signal line reset, a first terminal of the first transistor M 1  is electrically connected to the first node N 1 , and a second terminal of the first transistor M 1  is electrically connected to the composite signal line cp. A gate and a second terminal of the second transistor M 2  are electrically connected to the composite signal line cp, and a first terminal of the second transistor M 2  is electrically connected to the first node N 1 . 
     For example, in combination with the timing shown in  FIG.  3   , in the overall resetting period to, the first control signal line reset, the second control signal line read, and the composite signal line cp each provides the first level, the first transistor M 1  is conducted under the first level provided by the first control signal line reset, the second transistor M 2  is conducted under an action of the first level provided by the composite signal line cp, the first level provided by the composite signal line cp is transmitted to the first node N 1  via the conducted first transistor M 1  and the second transistor M 2 , to perform the overall resetting on the first node N 1 , so that the magnitudes of the initial voltages corresponding to the respective fingerprint identification units  1  are uniform, and the difference in the initial voltage is prevented from affecting the identification accuracy. 
     With reference to the timing shown in  FIG.  3   , in the preparation period t 2 , the first control signal line reset provides the first level, the second control signal line read and the composite signal line cp provide the second level, the first transistor M 1  is conducted under the first level provided by the first control signal line reset, and the second level provided by the composite signal line cp is transmitted to the first node N 1  through the conducted first transistor M 1 , so that the first node N 1  is maintained at a stable low potential and thus is not interfered by ultrasonic signals. 
     In combination with the timing shown in  FIG.  3   , in the pull-up period t 3 , the first control signal line reset and the second control signal line read provide the second level, the composite signal line cp provides the first level, the second transistor M 2  is conducted under the first level provided by the composite signal line cp, and the gate and the second terminal of the second transistor M 2  are electrically connected to each other, so that the second transistor M 2  has unidirectional conductivity, the first level provided by the composite signal line cp is transmitted to the first node N 1  via the conducted second transistor M 2 , and the high potential of the first level is used to pull up the potential of the first node N 1 . 
     In addition, it should be noted that in one solution in the research, a diode is adopted to pull up the potential of the first node N 1 . That is, an anode of the diode is electrically connected to the first node N 1 , and a cathode of the diode is electrically connected to the composite signal line cp. However, since a leakage current of the diode is relatively great when the diode is turned off, when reading the voltage of the first node N 1 , the leakage current of the diode may cause loss of charges of the first node N 1  and may affect the stability of the potential of the first node N 1 . For example, for the ultrasonic fingerprint identification circuit that includes a large number of the fingerprint identification units  1 , distances between the processor and some of the fingerprint identification units  1  are relatively large, and thus, when the signal read by these fingerprint identification units  1  is transmitted to the processor, a transmission path is relatively long, which may result in a significant signal attenuation. In addition to the influence of the leakage current on the potential of the first node N 1 , the signal of these fingerprint identification units  1  may be unreadable in severe cases, thereby affecting the identification. 
     In view of the above, applicant has found through research that, when the gate and the first terminal of the transistor are electrically connected to each other, or when the gate and the second terminal of the transistor are electrically connected to each other (the transistor having the electrically connected gate and the first terminals or the transistor having the electrically connected gate and the second electrodes are referred to as the transistors TFT-D hereinafter), the leakage current of the transistor TFT-D that is turned off is very small, and can be 2 to 3 orders of magnitude smaller than the leakage current of the diode that is turned off, with reference to Table 1. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 NTFT 
                 NTFT 
                 NTFT 
                 NTFT 
                 NTFT 
                 NTFT 
                 PTFT 
               
               
                   
                 (W3L3) 
                 (W4L3) 
                 (W8L3) 
                 (W4.5L4) 
                 (W5.5L4) 
                 (W2.5L3) 
                 (W6L4.2) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 I off   
                 3.45E−12 
                 4.42E−12 
                 5.50E−12 
                 3.85E−12 
                 4.53E−12 
                 2.50E−12 
                 5.01−12 
               
               
                   
               
            
           
         
       
     
       FIG.  9    is a graph of leakage currents of a diode and a transistor TFT-D provided by an embodiment of the present disclosure, and  FIG.  10    is another graph of leakage current of a diode and a transistor TFT-D provided by an embodiment of the present disclosure. As shown in  FIG.  9    and  FIG.  10   , Ioff of the ordinate in  FIG.  9    is the leakage current, and the ordinate in  FIG.  10    indicates Ioff′=lg|Ioff|, when the diode has a width of 6 μm and a length of 3 μm, under a reverse bias voltage of −5V, the leakage current of the diode is −1.06E-09A. Referring to  FIG.  10   , when the transistor TFT-D also has a width of 6 μm and a length of 3 μm, the Ioff′ corresponding to the transistor TFT-D is much smaller than the Ioff′ of the diode, and correspondingly, the leakage current of the transistor TFT-D in the turn-off state is much smaller than the leakage current of the diode in the turn-off state. 
     In view of the above, in an embodiment of the present disclosure, as the gate and the second terminal of the second transistor M 2  are electrically connected to each other, the second transistor M 2  can be in a unidirectional conduction state when it is conducted, and in the detection voltage reading period t 4 , when the second transistor M 2  is turned off under the second level provided by the composite signal line cp, the leakage current of the second transistor M 2  in the turn-off state is very small, and thus the loss of the charges of the first node N 1  is also very small. In this way, the influence of the leakage current on the potential of the first node N 1  is reduced, and the potential at the first node N 1  is more stable, thereby improving the accuracy of the reading of the voltage at the first node N 1 . 
     In an embodiment, the first transistor M 1  is a low-temperature polysilicon transistor. When the first transistor M 1  is set to be the low-temperature polysilicon transistor, the first transistor M 1  has a relatively fast response speed, the resetting of the first node N 1  in the overall resetting period t 0  can be performed faster, and thus the resetting of the first node N 1  is more complete. 
     In an embodiment, the second transistor M 2  is a low-temperature polycrystalline oxide transistor or an amorphous silicon transistor. Compared with silicon-based transistors, the low-temperature polycrystalline oxide transistor has no p-channel inverse state, and thus the second transistor M 2  can have a relatively low leakage current in the turn-off state when the second transistor M 2  is set to be the low-temperature polycrystalline oxide transistor. The amorphous silicon transistor, due to a low mobility of the amorphous silicon, has very low conductivity, and thus the second transistor M 2  can also have a relatively low leakage current when in the turn-off state when the second transistor M 2  is the amorphous silicon transistor. 
     Further, when the second transistor M 2  is the amorphous silicon transistor, the leakage current of the second transistor M 2  is positively correlated with the film thickness of the amorphous silicon layer, i.e., positively correlated with the film thickness of an active layer, according to an equation: 
     
       
         
           
             
               Ioff 
               = 
               
                 
                   W 
                   L 
                 
                 ⁢ 
                 q 
                 ⁢ 
                 
                   
                     k 
                     r 
                   
                 
                 ⁢ 
                 
                   T 
                   
                     3 
                     2 
                   
                 
                 ⁢ 
                 
                   
                     e 
                     
                       
                         - 
                         ε 
                       
                       / 
                       2 
                       ⁢ 
                       kT 
                     
                   
                   ( 
                   
                     
                       μ 
                       ⁢ 
                       e 
                     
                     + 
                     
                       μ 
                       ⁢ 
                       p 
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   V 
                   d 
                 
                 ⁢ 
                 d 
               
             
             , 
           
         
       
     
     where w is a width of the channel, L is a length of the channel, q is electron charge, k is the Boltzmann constant, k/r is charge density, V d  is a source-drain voltage of the transistor, d is a film thickness of an amorphous silicon layer, c is an optical band gap, t is a temperature, and μe and μp are the mobility of electrons and holes respectively.  FIG.  11    illustrates graphs of relationship of the thickness of the amorphous silicon layer with an operating current, the leakage current, a conduction voltage and electron mobility, when the thickness of the active layer of the second transistor M 2  is L1, L1 is set to satisfy: 200 Å≤L1≤1000 Å, in order to ensure that the second transistor M 2  has a relatively low leakage current. 
     In an embodiment, in order to ensure that the second transistor M 2  has a relatively low leakage current and that the first transistor M 1  has a relatively high response speed, a width-to-length ratio of the second transistor M 2  is smaller than a width-to-length ratio of the first transistor M 1 . 
       FIG.  12    is a structural schematic diagram of a second transistor provided by an embodiment of the present disclosure. In the embodiment shown in  FIG.  12   , the second transistor M 2  includes a first sub-transistor M 21  and a second sub-transistor M 22  arranged in series, gates of the first sub-transistor M 21  and the second sub-transistor M 22  are electrically connected to each other, a second terminal of the second sub-transistor M 22  is electrically connected to a first terminal of the first sub-transistor M 21 , a first terminal of the second sub-transistor M 22  is electrically connected to the gate of the second sub-transistor M 22  and the composite signal line cp, and a second terminal of the first sub-transistor M 21  is electrically connected to the first node N 1 . As the second transistor M 2  includes two sub-transistors connected in series, the second transistor M 2  in the layout has an increased length, which reduces the width-to-length ratio of the second transistor M 2  and thus reduces the leakage current of the second transistor M 2  in the turn-off state. 
     In a manufacturing process of an existing transistor, doped regions of the active layer of the transistor are all formed by high-concentration process doping, and thus relatively large leakage current may exist between the first terminal and the second terminal of the transistor.  FIG.  13    is a structural schematic diagram of a film layer of the second transistor provided by an embodiment of the present disclosure. In the embodiment of the present disclosure shown in  FIG.  13   , the second transistor M 2  includes an active layer  6 , the active layer  6  includes a channel region  7 , a first heavily doped region  8  and a second heavily doped region  9 ; a first terminal s of the second transistor M 2  is electrically connected to the first heavily doped region  8 , and a second terminal d of the second transistor M 2  is electrically connected to the second heavily doped region  9  and a gate g; a first lightly doped region  10  is provided between the first heavily doped region  8  and the channel region  7 , a second lightly doped region  11  is provided between the second heavily doped region  9  and the channel region  7 . The lightly doped regions are provided to balance a doping concentration of the doped region, so as to reduce the leakage current generated between the first terminal and the second terminal and reduce the leakage current of the second transistor M 2 . 
       FIG.  14    is a structural schematic diagram of another film layer of the second transistor provided by an embodiment of the present disclosure. In the embodiment shown in  FIG.  14   , a film layer where the second transistor M 2  is located includes an active layer  6 , a gate layer  12 , and a source-drain layer  13  arranged in sequence. The gate g of the second transistor M 2  is located in the gate layer  12 , a first terminal (not shown) and a second terminal d of the second transistor M 2  are located in the source-drain layer  13 . In a direction perpendicular to a plane where the active layer  6  is located, an orthographic projection of the source-drain layer  13  covers the active layer  6 , and thus the source-drain layer  13  is used to shield the active layer  6  and to prevent ambient light or light reflected by the finger from irradiating on the active layer  6  and accelerating flowing of carriers. In this way, an increase in the leakage current of the second transistor M 2  can be avoided. 
       FIG.  15    is a schematic diagram of another circuit structure of the fingerprint identification unit provided by the embodiment of the present disclosure, and  FIG.  16    is a structural schematic diagram of an ultrasonic fingerprint identification circuit corresponding to the fingerprint identification unit shown in  FIG.  15   . In the embodiment shown in  FIG.  15    and  FIG.  16   , the control module  3  includes a third transistor M 3  and a switch unit  14 , a first terminal of the third transistor M 3  is electrically connected to the first node N 1 , and a second terminal of the third transistor M 3  is electrically connected to the composite signal line cp; a gate of the third transistor M 3  is electrically connected to the second terminal of the third transistor M 3  through the switch unit  14 , and the switch unit  14  is also electrically connected to the first control signal line reset. 
     For example, in combination with the timing shown in  FIG.  3   , in the overall resetting period to, the first control signal line reset, the second control signal line read, and the composite signal line cp each provide the first level, and the switch unit  14  drives the third transistor M 3  to be conducted under the first level provided by the first control signal line reset, and the first level provided by the composite signal line cp is transmitted to the first node N 1  through the conducted third transistor M 3 , so as to perform the overall resetting on the first node N 1 . In the preparation period t 2 , the first control signal line reset provides the first level, and the switch unit  14  drives the third transistor M 3  to be conducted under the first level provided by the first control signal line reset, and thus the second level provided by the composite signal line cp is transmitted to the first node N 1  through the conducted third transistor M 3 , to maintain the second node at a stable low potential and to prevent the second node from being interfered by the ultrasonic signals. In the pull-up period t 3 , the composite signal line cp provides the first level, and the switch unit  14  drives the third transistor M 3  to be conducted under the second level provided by the first control signal line reset, such that the first level provided by the composite signal line cp is transmitted to the first node N 1  through the conducted third transistor M 3 , so as to pull up the potential at the first node N 1 . 
     Further referring to  FIG.  15   , the switch unit  14  includes a fourth transistor M 4  and a diode D, a gate of the fourth transistor M 4  is electrically connected to the first control signal line reset, a first terminal of the fourth transistor M 4  is electrically connected to the gate of the third transistor M 3 , and a second terminal of the fourth transistor M 4  is electrically connected to the second terminal of the third transistor M 3 . In combination with the timing shown in  FIG.  3   , the fourth transistor M 4  is conducted in the pull-up period t 3  under the second level provided by the first control signal line reset, so as to transmit the first level provided by the composite signal line cp to the gate of the third transistor M 3  and to drive the third transistor M 3  to be conducted. An anode of the diode D is electrically connected to the first control signal line reset, and a cathode of the diode D is electrically connected to the gate of the third transistor M 3 . The fourth transistor M 4  is of an opposite type of the third transistor M 3 . Combined with the timing shown in  FIG.  3   , the diode D is conducted in the overall resetting period t 0  and the preparation period t 2  under the first level provided by the first control signal line reset, to transmit the high level provided by the first control signal line reset to the gate of the third transistor M 3 , so as to drive the third transistor M 3  to be conducted. Through cooperation of the fourth transistor M 4  and the diode D, the third transistor M 3  can be in a corresponding turn-on or turn-off state in each period, which ensures normal operation of the circuit. 
       FIG.  17    is a schematic diagram of still another circuit structure of the fingerprint identification unit provided by the embodiment of the present disclosure. In the embodiment shown in  FIG.  17   , the switch unit  14  includes a fifth transistor M 5  and a sixth transistor M 6 , the fifth transistor M 5  and the third transistor M 3  are of opposite types, and the sixth transistor M 6  and the third transistor M 3  are of the same type. A gate of the fifth transistor M 5  is electrically connected to the first control signal line reset, a first terminal of the fifth transistor M 5  is electrically connected to the gate of the third transistor M 3 , and a second terminal of the fifth transistor M 5  is electrically connected to the second terminal of the third transistor M 3 . In combination with the timing shown in  FIG.  3   , the fifth transistor M 5  is used to be conducted in the pull-up period t 3  under the second level provided by the first control signal line reset, to transmit the first level provided by the composite signal line cp to the gate of the third transistor M 3 , and to drive the third transistor M 3  to be conducted. A gate and a first terminal of the sixth transistor M 6  are electrically connected to the first control signal line reset, and a second terminal of the sixth transistor M 6  is electrically connected to the gate of the third transistor M 3 . In combination with the timing shown in  FIG.  3   , the sixth transistor M 6  is used to be conducted in the overall resetting period t 0  and the preparation period t 2  under the first level provided by the first control signal line reset, to transmit the high level provided by the first control signal line reset to the gate of the third transistor M 3 , and to drive the third transistor M 3  to be conducted. Through the cooperation of the fifth transistor M 5  and the sixth transistor M 6 , the third transistor M 3  can be in a corresponding turn-on or turn-off state in each period, which ensures normal operation of the circuit. 
     In addition, since the gate and the first terminal of the sixth transistor M 6  are electrically connected, in combination with the previous discussion regarding the transistor TFT-D, the sixth transistor M 6  has a relatively low leakage current when in the turn-off state, thereby ensuring the stability of the gate potential of the third transistor M 3 . Moreover, the sixth transistor M 6  can be a low-temperature polysilicon transistor and has a relatively fast response speed. 
     In an embodiment, the third transistor M 3  is a low-temperature polycrystalline oxide transistor or an amorphous silicon transistor, to ensure that the third transistor M 3  has a relatively low leakage current in the turn-off state. 
     Further, in combination with the discussion regarding the second transistor M 2  in the above embodiments, when the third transistor M 3  is an amorphous silicon transistor, the thickness of the amorphous silicon layer of the third transistor M 3  is L2, and L2 is set to satisfy: 200 Å≤L2≤1000 Å, in order to ensure that the third transistor M 3  has a relatively low leakage current. 
       FIG.  18    is another structural schematic diagram of the fingerprint identification unit provided by the embodiment of the present disclosure. In the embodiment shown in  FIG.  18   , the third transistor M 3  includes a third sub-transistor M 31  and a fourth sub-transistor M 32  arranged in series, gates of the third sub-transistor M 31  and the fourth sub-transistor M 32  are electrically connected, a second terminal of the fourth sub-transistor M 32  is electrically connected to a first terminal of the third sub-transistor M 31 , a first terminal of the fourth sub-transistor M 32  is electrically connected to the composite signal line cp, and a second terminal of the third sub-transistor M 31  is electrically connected to the first node N. As the third transistor M 3  has two sub-transistors connected in series, the third transistor M 3  in the layout design can have an increased length, which reduces the width-to-length ratio of the third transistor M 3  and thus reduces the leakage current of the third transistor M 3  in the turn-off state. 
       FIG.  19    is a structural schematic diagram of the film layer of the third transistor provided by an embodiment of the present disclosure. In the embodiment shown in  FIG.  19   , the third transistor M 3  includes an active layer  6 , the active layer  6  includes a channel region  7 , a first heavily doped region  8  and a second heavily doped region  9 ; the first terminal of the third transistor M 3  is electrically connected to the first heavily doped region  8 , and the second terminal of the third transistor M 3  is electrically connected to the second heavily doped region  9 ; a first lightly doped region  10  is provided between the first heavily doped region  8  and the channel region  7 , a second lightly doped region  11  is provided between the second heavily doped region  9  and the channel region  7 . The lightly doped regions are used to balance a doping concentration of the doped region, so as to reduce the leakage current generated between the first terminal and the second terminal, thereby reducing the leakage current of the third transistor M 3 . 
     It should be noted that, in combination with  FIG.  15   , since the diode D is a unidirectional conduction structure, in the initial state, if the gate potential of the third transistor M 3 , i.e., a potential of the cathode of the diode D is uncertain, then the diode D may be in a reverse bias state, such that the first level provided by the first control signal line reset cannot be transmitted to the gate of the third transistor M 3 , thereby resulting in a failure of the circuit. 
       FIG.  20    is another structural schematic diagram of the fingerprint identification unit provided by the embodiment of the present disclosure. In the embodiment of the present disclosure shown in  FIG.  20   , the control module  3  also includes a reset unit  15 , and the reset unit  15  is electrically connected to the third control signal line cl, the reset signal line Rst and the gate of the third transistor M 3 , for transmitting the reset signal provided by the reset signal line Rst to the gate of the third transistor M 3  under an action of an effective level provided by the third control signal line cl, to reset the gate of the third transistor M 3 , and the effective level is the first level or the second level. 
       FIG.  21    is a timing diagram corresponding to the circuit structure shown in  FIG.  20   . For example, as shown in  FIG.  21   , the driving cycle also includes a node resetting period t 0 ′ prior to the preparation period. It should be noted that, when the driving cycle includes the initial voltage reading period t 1 , the node resetting period t 0 ′ is prior to the initial voltage reading period t 1 . In the node resetting period t 0 ′, by resetting the cathode of the diode D using the reset unit  15 , the cathode of the diode D can be maintained at a stable low potential, to avoid reverse biasing the diode D, ensure the reliability of the subsequent operation of the diode D, and improve the stability of the circuit operation. 
     Further referring to  FIG.  20   , the reset unit  15  includes a seventh transistor M 7 , a gate of the seventh transistor M 7  is electrically connected to the third control signal line cl, a first terminal of the seventh transistor M 7  is electrically connected to the reset signal line Rst, and a second terminal of the seventh transistor M 7  is electrically connected to the gate of the third transistor M 3 . When the seventh transistor M 7  is an N-type transistor, the above effective level is the first level, and when the seventh transistor M 7  is a P-type transistor, the above effective level is the second level. In combination with the timing shown in  FIG.  21   , the seventh transistor M 7  is conducted under the first level or the second level provided by the third control signal line cl, and the reset signal provided by the reset signal line Rst is transmitted to the gate of the third transistor M 3  via the conducted seventh transistor M 7 , so as to reset the gate of the third transistor M 3  and the cathode of the diode D. 
     It should be noted that, in combination with  FIG.  15   , in other optional embodiments of the present disclosure, the reset unit  15  may not be provided, and before the initial voltage reading period t 1 , the first control signal line reset and the composite signal line cp are directly allowed to provide the second level, such that the fourth transistor M 4  is conducted under the second level provided by the first control signal line reset, so as to transmit the second level provided by the composite signal line cp to the gate of the third transistor M 3  and to reset the gate of the third transistor M 3  and the cathode of the diode D. 
     In an embodiment, referring to  FIG.  7   ,  FIG.  15    and  FIG.  17   , the reading module  4  includes an eighth transistor M 8  and a ninth transistor M 9 , a gate of the eighth transistor M 8  is electrically connected to the first node N 1 , and a first terminal of the eighth transistor M 8  is electrically connected to a fixed potential signal line VDD; a gate of the ninth transistor M 9  is electrically connected to the second control signal line read, a first terminal of the ninth transistor M 9  is electrically connected to a second terminal of the eighth transistor M 8 , and a second terminal of the ninth transistor M 9  is electrically connected to the reading signal line read line. 
     For example, during the pull-up period t 3 , the ultrasonic fingerprint identification sensor  2  converts the ultrasonic signal reflected by the finger into an electrical signal, which is a signal fluctuating between high and low potentials. Since the potential of the first node N 1  is the gate potential of the eighth transistor M 8 , the potential of the first node N 1  can control the conduction state of the eighth transistor M 8 . In order to prevent the low potential of the electrical signal from affecting the conduction state of the eighth transistor M 8  and resulting in unstable operation of the eighth transistor M 8 , the potential of the first node N 1  is pulled up to a reasonable extent by using the control module  3 . Thus, in the detection voltage reading period t 4 , the gate potential of the eighth transistor M 8  satisfies: Vgs&gt;Vth, and Vds&gt;Vgs-Vth. In this case, the eighth transistor M 8  is in a saturated state, and according to saturation characteristics of the transistor, it can be known that a source-drain current Ids of the eighth transistor M 8  is independent of a source-drain voltage Vds and can be only increased when the gate-source voltage Vgs increases. In this regard, when the detection voltage of the first node N 1  is relatively high, the Vgs of the eighth transistor is relatively large, and correspondingly, a current transferred from the eighth transistor M 8  to the ninth transistor M 9  is also relatively large, and thus the signal intensity read by the reading signal line read line and configured to feed back the detection voltage of the first node N 1  is also relatively large. When the detection voltage of the first node N 1  is relatively low, the Vgs of the eighth transistor M 8  is relatively small, and correspondingly, the current transferred from the eighth transistor M 8  to the ninth transistor M 9  is relatively small, which results in the small read signal intensity configured to feed back the detection voltage of the first node N 1 , thereby obtaining the magnitude of the detection voltage at the first node N 1  according to the read signal intensity. 
     In an embodiment, referring to  FIG.  7   ,  FIG.  15    and  FIG.  17   , the fingerprint identification unit  1  further includes a storage capacitor C, a first plate of the storage capacitor C is electrically connected to the first node N 1 , and a second plate of the storage capacitor C is electrically connected to the fixed potential signal line VDD, so that the storage capacitor C is used to stabilize the potential of the first node N 1  in the pull-up period t 3 . Moreover, in the circuit structure shown in  FIG.  7   , in the pull-up period t 3 , the unidirectionally conducted second transistor M 2  can further keep the charge of the storage capacitor C until the reading is completed, thereby further improving the stability of the potential at the first node N 1  during the reading process. 
       FIG.  22    is a structural schematic diagram of an ultrasonic fingerprint identification sensor provided by an embodiment of the present disclosure. In the embodiment shown in  FIG.  22   , the ultrasonic fingerprint identification sensor  2  includes: a first electrode  16  electrically connected to the detection signal line Rbias, a second electrode  17  disposed opposite to the first electrode  16  and electrically connected to the first node N 1 , and an ultrasonic material layer  18  located between the first electrode  16  and the second electrode  17 . For example, in the preparation period t 2 , the detection signal line Rbias outputs an electrical signal for identification to the first electrode  16  of the ultrasonic fingerprint identification sensor  2 , and the ultrasonic material layer  18  converts the electrical signal into the ultrasonic signal and radiates it towards the finger; in the pull-up period t 3 , the ultrasonic signal reflected by the finger is reflected back, and the ultrasonic material layer  18  converts the reflected ultrasonic signal into an electrical signal and transmits it to the first node N 1  through the second electrode  17 , thereby achieving the normal operation of the ultrasonic fingerprint identification sensor  2 . 
     The embodiment of the present disclosure also provides a driving method of the ultrasonic fingerprint identification circuit, for driving the above ultrasonic fingerprint identification circuit. With reference to  FIG.  1   , the driving cycle of the fingerprint identification unit  1  of the ultrasonic fingerprint identification circuit includes a preparation period, a pull-up period, and a detection voltage reading period.  FIG.  23    is a flowchart of a driving method provided by an embodiment of the present disclosure. As shown in  FIG.  23   , the driving method includes the following steps. 
     Step S 1 : in the preparation period, the ultrasonic fingerprint identification sensor  2  converts an electrical signal into an ultrasonic signal and radiates it towards the finger, the first control signal line reset provides the first level, the control module  3  transmits the second level provided by the composite signal line cp to the first node N 1 , that is, to provide the reset potential to the first node N 1 , so that the first node N 1  is maintained at a stable low potential. 
     Step S 2 : in the pull-up period, the ultrasonic fingerprint identification sensor  2  converts the ultrasonic signal reflected by the finger into an electrical signal and transmits it to the first node N 1 , the composite signal line cp provides the first level, and the control module  3  transmits the first level provided by the composite signal line cp to the first node N 1 , to pull up the potential of the first node N 1 , that is, to perform wave-chopping on the electric signal converted by the ultrasonic signal. 
     It should be noted that the electrical signal converted by the ultrasonic signal is a signal fluctuating between the high and low potentials, and the highest potential in the electrical signal is used for subsequent detection and identification, thus pull-up potential will only pull up the low potential in the electrical signal to a reasonable extent, the pulled-up potential will not cover an original highest potential in the electrical signal, and the waveform diagram of the electrical signal converted from the ultrasonic signal after the wave-chopping is as shown in  FIG.  24   . 
     Step S 3 : in the reading period, the second control signal line read provides the first level, and the reading module  4  reads the detection signal of the first node N 1 . 
     In the fingerprint identification circuit driven by the above driving method, the first control signal line reset electrically connected to one fingerprint identification unit  1  can be reused as the second control signal line read electrically connected to another fingerprint identification unit  1 , thereby greatly reducing the number of the control signal lines required to be provided. On the one hand, the space occupied by the control signal lines can be reduced and saved for the design space of the ultrasonic fingerprint identification circuit; and on the other hand, the coupling between the control signal lines and the coupling between the control signal lines and other wirings can also be reduced, so as to reduce interference of coupling capacitance on the read signal and improve accuracy of the read signal, thereby improving accuracy of the fingerprint identification. 
     Further, in combination with  FIG.  2   , the reading period includes an initial voltage reading period t 1  prior to the preparation period t 2 , and a detection voltage reading period t 4  latter than the pull-up period t 3 . That is, the driving cycle of the fingerprint identification unit  1  of the ultrasonic fingerprint identification circuit includes an initial voltage reading period t 1 , a preparation period t 2 , a pull-up period t 3 , and a detection voltage reading period t 4 .  FIG.  25    is another flowchart of the driving method provided by the embodiment of the present disclosure. As shown in  FIG.  25   , Step S 3  includes the following steps. 
     Step S 31 : in the initial voltage reading period t 1 , the second control signal line read provides the first level, and the reading module  4  reads the initial voltage V 1  of the first node N 1 . 
     Step S 32 : in the detection voltage reading period t 4 , the second control signal line read provides the first level, and the reading module  4  reads the detection voltage of the first node N 1 . 
     Furthermore, the processor is configured to identify of fingerprint valleys and the fingerprint ridges by determining a difference between V 1  and V 2  according to the V 1  and V 2  read in a time-division manner. 
     Using the above reading method, in the driving cycle of one frame, the voltage of the first node N 1  is read twice in a time division manner respectively in the initial voltage reading period t 1  and the detection voltage reading period t 4 , and the valleys and ridges of the fingerprint are determined based on a difference between the two voltages. Therefore, even if the read signal contains the noise signal, the noise signal can be eliminated by differencing the two voltages, thereby reducing the influence of the noise signal on the detection accuracy and effectively enhancing a signal-to-noise ratio. 
     In an embodiment, referring to  FIG.  3   , the driving cycle further includes an overall resetting period t 0  prior to the initial voltage reading period t 1 . In combination with the timing shown in  FIG.  3   , in the overall resetting period to, the first control signal line reset, the second control signal line read, and the composite signal line cp each provide the first level, and the control module  3  transmits the first level provided by the composite signal line cp to the first node N 1 , in order to reset the first node N 1 . 
       FIG.  26    is another timing diagram provided by an embodiment of the present disclosure. In the embodiment of the present disclosure, as shown in  FIG.  26   , the overall resetting is performed in each frame of the driving cycle, so that the voltages of the first nodes N 1  in the respective fingerprint identification units  1  in each frame are uniformly reset to a high potential, and magnitudes of the initial voltages read by the respective fingerprint identification units  1  in each frame in the initial voltage reading period t 1  are uniform, thereby avoiding the impact of the difference of the initial voltage on the identification accuracy. 
     In an embodiment, referring to  FIG.  7   , the control module  3  includes a first transistor M 1  and a second transistor M 2 . A gate of the first transistor M 1  is electrically connected to the first control signal line reset, a first terminal of the first transistor M 1  is electrically connected to the first node N 1 , and a second terminal of the first transistor M 1  is electrically connected to the composite signal line cp. A gate and a second terminal of the second transistor M 2  is respectively electrically connected to the composite signal line cp, and a first terminal of the second transistor M 2  is electrically connected to the first node N 1 . 
     Based on the above structure, in combination with the timing shown in  FIG.  3   , in the preparation period t 2 , the process of transmitting, by the control module  3 , the second level provided by the composite signal line cp to the first node N 1  includes: in the preparation period t 2 , the first control signal line reset provides a first level, the composite signal line cp provides a second level, the first transistor M 1  is conducted under the first level, the second level provided by the composite signal line cp is transmitted to the first node N 1  through the conducted first transistor M 1 , such that the first node N 1  is maintained at a stable low potential, thereby preventing the first node N 1  from being interfered by ultrasonic signals. 
     In combination with the timing shown in  FIG.  3   , in the pull-up period t 3 , the process of transmitting, by the control module  3 , the first level provided by the composite signal line cp to the first node N 1  to pull up the potential of the first node N 1  includes: in the pull-up period t 3 , the first control signal line reset provides the second level, the composite signal line cp provides the first level, the second transistor M 2  is conducted under the first level, the first level provided by the composite signal line cp is transmitted to the first node N 1  via the conducted second transistor M 2 , so as to pull up the potential of the first node N 1  with the first level. 
     Since the gate and the second terminal of the second transistor M 2  are electrically connected to each other, the second transistor M 2  can be in a unidirectional conduction state when it is conducted, and in the detection voltage reading period t 4 , when the second transistor M 2  is turned off under the second level provided by the composite signal line cp, the leakage current of the second transistor M 2  in the turn-off state is very small, and thus the loss of the charge of the first node N 1  is also very small, thereby reducing the influence of the leakage current on the potential of the first node N 1 , improving the stability of the potential of the first node N 1 , and improving the accuracy of the read voltage of the first node N 1 . 
     In an embodiment, referring to  FIG.  15    again, the control module  3  includes: a third transistor M 3 , a fourth transistor M 4  and a diode D. A first terminal of the third transistor M 3  is electrically connected to the first node N 1 , and a second terminal of the third transistor M 3  is electrically connected to the composite signal line cp. A gate of the fourth transistor M 4  is electrically connected to the first control signal line reset, a first terminal of the fourth transistor M 4  is electrically connected to the gate of the third transistor M 3 , and a second terminal of the fourth transistor M 4  is electrically connected to the second terminal of the third transistor M 3 . A type of the fourth transistor M 4  is opposite to that of the third transistor M 3 . An anode of the diode D is electrically connected to the first control signal line reset, and a cathode of the diode D is electrically connected to the gate of the third transistor M 3 . 
     Based on the above structure, in combination with the timing shown in  FIG.  3   , in the preparation period t 2 , the process of transmitting, by the control module  3 , the second level provided by the composite signal line cp to the first node N 1  includes: in the preparation period t 2 , the first control signal line reset provides a first level, the composite signal line cp provides a second level, the diode D is conducted under the second level to allow the third transistor M 3  to be conducted under the first level provided by the first control signal line reset, so as to transmit the second level provided by the composite signal line cp to the first node N 1 . In this way, the second node is maintained at a stable low potential and thus is prevented from being interfered by ultrasonic signals. 
     In combination with the timing shown in  FIG.  3   , in the pull-up period t 3 , the process of transmitting, by the control module  3 , the first level provided by the composite signal line cp to the first node N 1  to pull up the potential of the first node N 1  includes: in the pull-up period t 3 , the first control signal line reset provides the second level, the composite signal line cp provides the first level, the fourth transistor M 4  is conducted under the second level to allow the third transistor M 3  to be conducted under the first level provided by the composite signal line cp, so as to transmit the first level provided by the composite signal line cp to the first node N 1 , thereby pulling up the potential of the first node N 1 . 
     Through cooperation of the fourth transistor M 4  and the diode D, the third transistor M 3  can be in a corresponding turn-on or turn-off state in each period, such that the first node N 1  can accurately receive the required potential in each period, ensuring the normal operation of the circuit. 
     In an embodiment, referring to  FIG.  19   , the control module  3  includes: a third transistor M 3 , a fifth transistor M 5 , and a sixth transistor M 6 . A first terminal of the third transistor M 3  is electrically connected to the first node N 1 , and a second terminal of the third transistor M 3  is electrically connected to the composite signal line cp. A gate of the fifth transistor M 5  is electrically connected to the first control signal line reset, a first terminal of the fifth transistor M 5  is electrically connected to the gate of the third transistor M 3 , and a second terminal of the fifth transistor M 5  is electrically connected to the second terminal of the third transistor M 3 . The fifth transistor M 5  and the third transistor M 3  are of opposite types. A gate and a first terminal of the sixth transistor M 6  are electrically connected to the first control signal line reset, and a second terminal of the sixth transistor M 6  is electrically connected to the gate of the third transistor M 3 . The sixth transistor M 6  is of the same type as the third transistor M 3 . 
     Based on the above structure, in combination with the timing shown in  FIG.  3   , in the preparation period t 2 , the process of transmitting, by the control module  3 , the second level provided by the composite signal line cp to the first node N 1  includes: in the preparation period t 2 , the first control signal line reset provides a first level, the composite signal line cp provides a second level, and the sixth transistor M 6  is conducted under the second level to allow the third transistor M 3  to be conducted under the first level provided by the first control signal line reset, so as to transmit the second level provided by the composite signal line cp to the first node N 1 . Thus, the second node is maintained at a stable low potential and is prevented from being interfered by ultrasonic signals. 
     In combination with the timing shown in  FIG.  3   , in the pull-up period t 3 , the process of transmitting, by the control module  3 , the first level provided by the composite signal line cp to the first node N 1  to pull up the potential of the first node N 1  includes: in the pull-up period t 3 , the first control signal line reset provides the second level, the composite signal line cp provides the first level, and the fifth transistor M 5  is conducted under the second level to allow the third transistor M 3  to be conducted under the first level provided by the composite signal line cp, so as to transmit the first level provided by the composite signal line cp to the first node N 1 , thereby pulling up the potential of the first node N 1 . 
     Through the cooperation of the fifth transistor M 5  and the sixth transistor M 6 , the third transistor M 3  can be in a corresponding on turn-or turn-off state in each period, such that the first node N 1  can accurately receive the required potential in each period, ensuring the normal operation of the circuit. In addition, since the gate and the first terminal of the sixth transistor M 6  are electrically connected to each other, in combination with the previous discussion regarding the transistor TFT-D, the sixth transistor M 6  has a relatively low leakage current when it is in the turn-off state, thereby ensuring the stability of the gate potential of the third transistor M 3 . 
     Further, referring to  FIG.  20    and  FIG.  21   , the control module  3  further includes a reset unit  15 , and the reset unit  15  is electrically connected to the third control signal line cl, the composite signal line cp, and the gate of the third transistor M 3 . 
     Based on the above structure, in combination with the timing shown in  FIG.  21   , the driving cycle further includes a node resetting period t 0 ′ prior to the initial voltage reading period t 1 . In the node resetting period t 0 ′, the third control signal line cl provides an effective level, the reset unit  15  transmits the reset signal provided by the composite signal line cp to the gate of the third transistor M 3  to reset the gate of the third transistor M 3 , and the effective level is the first level or the second level. By using the reset unit  15  to reset the gate of the third transistor M 3  and the cathode of the diode D, the cathode of the diode D can be maintained at a stable low potential, and thus the diode D is avoided to be in a reverse bias state that may be caused by the instability of the node voltage, thereby ensuring the reliability of the subsequent operation of the diode D, and improving the stability of the circuit operation. 
     In an embodiment, referring to  FIG.  7   ,  FIG.  15    and  FIG.  17   , the reading module  4  includes an eighth transistor M 8  and a ninth transistor M 9 . A gate of the eighth transistor M 8  is electrically connected to the first node N 1 , and a first terminal of the eighth transistor M 8  is electrically connected to the fixed potential signal line. A gate of the ninth transistor M 9  is electrically connected to the second control signal line read, a first terminal of the ninth transistor M 9  is electrically connected to a second terminal of the eighth transistor M 8 , and a second terminal of the ninth transistor M 9  is electrically connected to the reading signal line read line. 
     Based on the above structure, in combination with the timing shown in  FIG.  3   , in the initial voltage reading period t 1 , the process of reading, by the reading module  4 , the initial voltage of the first node N 1  includes: in the initial voltage reading period t 1 , the second control signal line read provides the first level, and the ninth transistor M 9  is conducted under the first level, to transmit a signal for feeding back a magnitude of the initial voltage of the first node N 1  to the reading signal line read line. 
     In combination with the timing shown in  FIG.  3   , in the detection voltage reading period t 4 , the process of reading, by the reading module  4 , the detection voltage of the first node N 1  includes: in the detection voltage reading period t 4 , the eighth transistor M 8  is located in a saturation region under the potential of the first node N 1 , the second control signal line read provides the first level, and the ninth transistor M 9  is conducted under the first level, to transmit a signal for feeding back the magnitude of the detection voltage of the first node N 1  to the reading signal line read line. 
     It should be noted that, in the pull-up period t 3 , the control module  3  pulls up the potential of the first node N 1  to a reasonable extent, and in the detection voltage reading period t 4 , the gate potential of the eighth transistor M 8  satisfies: Vgs&gt;Vth, and Vds&gt;Vgs-Vth. In this case, the eighth transistor M 8  is in a saturated state, and according to the saturation characteristics of the transistor, it can be known that a source-drain current Ids of the eighth transistor M 8  is independent of a source-drain voltage Vds and will be only increased when the gate-source voltage Vgs increases. In this regard, when the detection voltage of the first node N 1  is relatively high, the Vgs of the eighth transistor is relatively large, and correspondingly, a current transferred from the eighth transistor M 8  to the ninth transistor M 9  is also relatively large. Thus, the signal intensity that is read by the reading signal line read line and used to feed back the detection voltage of the first node N 1  is relatively large. When the detection voltage of the first node N 1  is relatively low, the Vgs of the eighth transistor is relatively small, and correspondingly, the current transmitted from the eighth transistor M 8  to the ninth transistor M 9  is relatively small. Thus, the read signal intensity used to feed back the detection voltage of the first node N 1  is also relatively small, so as to obtain the magnitude of the detection voltage of the first node N 1  according to the read signal intensity. 
     In an embodiment, referring to  FIG.  7   ,  FIG.  15    and  FIG.  17   , the fingerprint identification unit  1  further includes a storage capacitor C, a first plate of the storage capacitor C is electrically connected to the first node N 1 , and a second plate of the storage capacitor C is electrically connected to the fixed potential signal line VDD. The driving method further includes: in the pull-up period t 3 , the charges of the first node N 1  are stored by using the storage capacitor C, thereby stabilizing the potential of the first node N 1  with the storage capacitor C. In the circuit structure shown in  FIG.  7   , in the pull-up period t 3 , the unidirectionally conducted second transistor M 2  can further keep the charges of the storage capacitor C until the reading is completed, thereby further improving the stability of the potential of the first node N 1  during the reading process. 
     The embodiment of the present disclosure also provides a display device.  FIG.  27    is a structural schematic diagram of the display device provided by the embodiment of the present disclosure. As shown in  FIG.  27   , the display device includes: a display panel  100 , the above ultrasonic fingerprint identification circuit  200 , and a processor  300 . A display area of the display panel includes a main display area  101  and a fingerprint identification area  102 . The ultrasonic fingerprint identification circuit is provided in the fingerprint identification area  102 . The processor is electrically connected to the reading signal line read line (not shown) in the ultrasonic fingerprint identification circuit  200 , and configured to identify fingerprints according to the signal read by the reading signal line read line. The specific structure of the ultrasonic fingerprint identification circuit  200  has been described in detail in the foregoing embodiments, which will not be repeated herein. The display device shown in  FIG.  26    is an example, and the display device may be any electronic device with a display function, such as a mobile phone, a tablet computer, a notebook computer, an electronic paper book, or a television. 
     Since the display device provided by the embodiment of the present disclosure includes the above ultrasonic fingerprint identification circuit  200 , the display device can greatly reduce the number of the control signal lines required by the ultrasonic fingerprint identification circuit  200 , thereby saving the design space for the ultrasonic fingerprint identification circuit and reducing the interference of the coupling capacitance generated by the control signal lines on the read signal; and the display device can also lower the influence of the noise signal on the detection accuracy, thereby increasing the signal-to-noise ratio and effectively improving the accuracy of fingerprint identification. 
     The above are only preferred embodiments of the present disclosure, but not intended to limit the present disclosure. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present disclosure shall be included in the protection scope of the present disclosure. 
     It should be noted that the above various embodiments are only used to illustrate, but not to limit the technical solutions of the present disclosure. Although the present disclosure has been described in detail with reference to the above various embodiments, those skilled in the art can modify the technical solutions described in the above various embodiments, or equivalently replace some or all of the technical features, without departing from the scope of the technical solutions of the various embodiments of the present disclosure.