Biometric identification device comprising an array of ultrasonic transducers and a capacitive sensing wake-up circuit

Abstract

Biometric identification device comprising an array of ultrasonic transducers and a capacitive detection wake-up circuit The present description relates to a biometric identification device (100) comprising at least: - an array of ultrasonic transducers, each of the ultrasonic transducers comprising first and second electrodes (110, 112) forming, at least in a standby mode of the biometric identification device, an electrical capacitance; - a wake-up circuit (126) configured to detect, when the biometric identification device is in the standby mode, a variation of an overall electrical capacitance value formed by at least a portion of the ultrasonic transducers. Figure for abstract: Fig. 4

Description

Title of the invention: Biometric identification device comprising an array of ultrasonic transducers and a capacitive detection wake-up circuit Technical field

[0001] The present disclosure relates to the field of biometric identification devices based on capture and detection by ultrasonic transduction. Prior art

[0002] A biometric identification device makes it possible to verify or determine the identity of a user of the device based on a measurement of one or more biometric characteristics of the user such as a fingerprint, the shape of the face, the pattern of the iris, the pattern of the retina, etc. The different techniques for carrying out measurements of one or other of these biometric characteristics each have advantages and disadvantages according to different criteria, including in particular: more or less significant error rate, ease or not of theft of the template with which the measurements are compared, possibility of detecting or not an imitation, proof or not of life, ease and comfort of use, size of the sensor required, energy consumption required, etc.

[0003] When the biometric identification device is in standby mode, its awakening is triggered by the approach of a finger to the capture surface of the device. In the case of a biometric identification device equipped with an ultrasonic sensor, this approach is regularly verified by the sensor via an emission and reception of ultrasounds in order to detect the presence of the finger above or on the capture surface. However, the emission and reception of ultrasounds require a certain quantity of electrical energy, which can impact for example the autonomy and the lifetime of the battery of the device. Summary of the invention

[0004] There is a need to propose a biometric identification device which does not have at least some of the drawbacks of existing solutions.

[0005] One embodiment overcomes all or part of the drawbacks of known solutions and proposes a biometric identification device comprising at least:

[0006] - an array of ultrasonic transducers, each of the ultrasonic transducers comprising first and second electrodes forming, at least in a standby mode of the biometric identification device, an electrical capacity;

[0007] - a wake-up circuit configured to detect, when the identification device biometric is in sleep mode, a variation of an electrical capacity value global formed by at least part of the ultrasonic transducers.

[0008] According to a particular embodiment, the ultrasonic transducers are of the PMUT or CMUT type.

[0009] According to a particular embodiment:

[0010] - the first electrodes of the ultrasonic transducers of the same line of the matrix are electrically coupled to each other, and / or

[0011] - the second electrodes of the ultrasonic transducers of the same column of the matrix are electrically coupled to each other.

[0012] According to a particular embodiment, the biometric identification device further comprises at least one first analog front-end circuit electrically coupled to the first electrodes and at least one second analog front-end circuit electrically coupled to the second electrodes, each of the first and second analog front-end circuits being configured to convert an electrode output electrical voltage of the ultrasonic transducers into an output electrical current.

[0013] According to a particular embodiment, the wake-up circuit is configured to detect the variation in the value of the overall electrical capacitance formed by at least a portion of the ultrasonic transducers by detecting a variation in the output electrical current of at least one of the first and second analog front-end circuits.

[0014] According to a particular embodiment, the first electrodes of the ultrasonic transducers are arranged between the second electrodes of the ultrasonic transducers and a capture surface of the biometric identification device, and the wake-up circuit is configured to detect the variation of the output electric current of the first analog front-end circuit or of at least one of the first analog front-end circuits.

[0015] According to a particular embodiment:

[0016] - the first electrodes of ultrasonic transducers of the same line of the matrix are electrically coupled to the first analog front-end circuit or to one of the first analog front-end circuits, and / or

[0017] - the second electrodes of ultrasonic transducers of the same column of the matrix are electrically coupled to the second analog front-end circuit or to one of the second analog front-end circuits.

[0018] According to a particular embodiment:

[0019] - the first analog front-end circuit or at least one of the first front-end circuits analog comprises a first multiplexer whose inputs are electrically coupled to first electrodes of ultrasonic transducers belonging to different rows of the matrix, and / or

[0020] - the second analog front-end circuit or at least one of the second circuits analog front-ends comprises a second multiplexer whose inputs are electrically coupled to second electrodes of ultrasonic transducers belonging to different columns of the matrix.

[0021] According to a particular embodiment, each of the first and second analog front-end circuits comprises an analog-to-digital converter such that the output electrical currents delivered by the first and second analog front-end circuits are digital signals, and the wake-up circuit is part of a digital circuit whose inputs are electrically coupled to outputs of the first and second analog front-end circuits.

[0022] According to a particular embodiment, the biometric identification device further comprises at least one circuit for polarizing the ultrasonic transducers, configured to apply the same electrical polarization potential to the first and second electrodes of each of the ultrasonic transducers.

[0023] According to a particular embodiment, the biasing circuit is configured such that the applied biasing electrical potential includes a periodically repeated amplitude variation.

[0024] According to a particular embodiment, the biometric identification device comprises several polarization circuits, each forming part of one of the first and second analog front-end circuits.

[0025] According to a particular embodiment, the wake-up circuit is configured to take the biometric identification device out of sleep mode when a variation in the overall electrical capacitance value formed by at least a portion of the ultrasonic transducers is detected. Brief description of the drawings

[0026] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0027] - [Fig.l] and [Fig.2] schematically represent an identification device biometric according to a particular embodiment;

[0028] - [Fig.3] schematically represents an example of a matrix of transducers with ultrasound of a biometric identification device according to a particular embodiment;

[0029] - [Fig.4] schematically represents an example of the embodiment of a part of a biometric identification device according to a particular embodiment;

[0030] - [Fig.5] represents an example of a polarization signal applied to ultrasonic transducer electrodes of a biometric identification device according to a particular embodiment. Description of the embodiments

[0031] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0032] For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been shown and are detailed. In particular, various elements (ultrasonic transducer array, control circuit, data processing circuit, wake-up circuit, analog front-end circuits, polarization circuit, etc.) and various steps implemented (image acquisition, processing of acquired images, details of calculations performed, etc.) are not detailed. Those skilled in the art will be able to implement these elements in detail from the functional description given here.

[0033] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be connected by means of one or more other elements.

[0034] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures in a normal position of use.

[0035] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

[0036] Throughout the description, the terms "row" and "column" are used considering an arbitrary orientation given to the described biometric identification device, these terms being able to be interchanged depending on the orientation of the device.

[0037] A biometric identification device 100 according to a particular embodiment is described below in connection with FIGS. 1 to 5.

[0038] In the example described, the device 100 is configured to exit a standby mode by detecting the presence of at least one finger 101 of the user near a capture surface 102 of the device 100 and to implement, when the device 100 is not in standby mode, a biometric identification from a fingerprint capture of at least one finger 101 of the user of the device 100. In the remainder of the description, it is considered that the device 100 is configured to exit the standby mode standby and to carry out biometric identification from a single finger 101 of the user. The device 100 comprises the capture surface 102 on which this finger 101 of the user of the device 100 is intended to be placed during biometric identification.

[0039] According to an exemplary embodiment, the device 100 comprises at least one matrix of ultrasonic transducers 104, a control circuit 106, and a data processing circuit 108. In the schematic example shown in [Fig. 2], the control circuit 106 can be electrically coupled to the matrix of ultrasonic transducers 104 and thus be able to transmit to this matrix 104 signals controlling the emission of ultrasound for the measurements to be carried out, and also be able to receive electrical measurement signals transmitted by the matrix of ultrasonic transducers 104 following the reception of echoes of the emitted ultrasound. The control circuit 106 can also be coupled to the data processing circuit 108 in order to transmit to the data processing circuit 108 the measurement results obtained from the matrix of ultrasonic transducers 104.

[0040] The array of ultrasound transducers 104 may be configured to perform an image acquisition from which a biometric identification is performed by the device 100. The number of ultrasound transducers of the array 104 may depend on the dimensions of the capture surface 102 and / or the dimensions of each ultrasound transducer and / or the desired resolution.

[0041] In the described embodiment, the device 100 may be configured to perform biometric identification via an acquisition of at least one surface image of the user's finger 101 placed on the capture surface 102 of the device 100 and in contact therewith, i.e. an acquisition of at least one fingerprint image formed of ridges and valleys present on the surface of the skin of the finger 101 placed on the capture surface 102. In the device 100, the acquisition of a surface image of the user's finger 101 placed on the capture surface 102 and in contact therewith is based on the fact that the ultrasound emitted by the transducer circuit 104 is reflected more significantly against the air present in the valleys of the fingerprint than against the ridges of the print.

[0042] The matrix 104 may for example comprise transducers of the CMUT type (Capacitive Micromachined Ultrasonic Transducer) or of the PMUT type (Piezoelectric Micromachined Ultrasonic Transducer). Alternatively, the matrix 104 may comprise ultrasonic transducers of another type.

[0043] In the exemplary embodiment described herein, the control circuit 106 may be configured to provide the ultrasonic transducer array 104 with signals electrical excitation signals causing the emission of ultrasonic waves by the ultrasonic transducers of the matrix 104, and for receiving electrical response signals generated by the transducers of the matrix 104 under the effect of the reception of ultrasonic waves reflected by the finger 101 present on the capture surface 102 or in the vicinity thereof.

[0044] In the described embodiment, the data processing circuit 108 may be configured to analyze and process the electrical response signals received by the control circuit 106 and sent by the ultrasonic transducer array 104. The processing circuit 108 may comprise, for example, at least one microprocessor coupled to at least one memory for processing the received data.

[0045] An exemplary embodiment of the ultrasonic transducer array 104 is shown schematically in [Fig. 3]. In this example, each of the transducers of the array 104 comprises a first electrode 110 and a second electrode 112. In addition, in this example, the first electrodes 110 of the transducers arranged on the same line of the array 104 are electrically coupled to each other. Furthermore, in the example of [Fig. 3], the first electrodes 110 of the transducers arranged on the same line of the array 104 are formed by the same portion of electrically conductive material extending in a direction parallel to the X axis visible in [Fig. 3]. Alternatively, it is possible for the first electrodes 110 to be arranged in a configuration different from that shown in [Fig. 3].

[0046] In the example described, the second electrodes 112 of the transducers arranged on the same column of the matrix 104 are electrically coupled to each other. Furthermore, in the example of [Fig. 3], the second electrodes 112 of the transducers arranged on the same column of the matrix 104 are formed by the same portion of electrically conductive material extending in a direction parallel to the Y axis visible in [Fig. 3]. Alternatively, it is possible for the second electrodes 112 to be arranged in a configuration different from that shown in [Fig. 3].

[0047] In the example described, the matrix 104 is such that the first electrodes 110 are arranged between the second electrodes 112 and the capture surface 102. In other words, the first electrodes 110 of the ultrasonic transducers of the matrix 104 are arranged on the side of the capture surface 102 of the device 100.

[0048] In the example of [Fig. 3], each ultrasonic transducer of the matrix 104 also comprises a transduction element 114 arranged between the first and second electrodes 110, 112 of the transducer. The material(s) of this transduction element depend on the technology used to produce the matrix 104, this transduction element corresponding for example to a portion of piezoelectric material in the case of PMUT type transducers.

[0049] The device 100 further comprises one or more first front-end circuits logic circuits 116, or first AFEs, coupled to the first electrodes 110, and one or more second analog front-end circuits 118, or second AFEs, coupled to the second electrodes 112. Each of the AFEs 116, 118 can implement different functions from the signals delivered on the electrodes 110, 112 of the transducers: amplification, filtering, analog-digital conversion, etc. In [Fig.4], which schematically represents part of the elements and circuits of an exemplary embodiment of the device 100, two first AFEs 116 and a second AFE 118 are represented.

[0050] Each of the first and second AFEs 116, 118 comprises at least one electrical capacitance 120, corresponding for example to a capacitor, and is configured to convert an output voltage of the ultrasonic transducers, for example obtained on at least one electrode of the transducers coupled to the AFE, into an output current. In the example of [Fig.4], each of the capacitances 120 has a value called Cafe and the output voltages are called OUT1, OUT2 and OUT3. The Cafe values ​​of the capacitances 120 of the AFEs 116, 118 may be different or similar to each other. These values ​​may in particular be different to weight the response obtained with a certain coefficient according to the position of the line in the matrix (principle called "apodization" in English).

[0051] In the example described, the first electrodes 110 of the transducers arranged on the same row of the matrix 104 are coupled to the first AFE 116 (when the device 100 comprises a single first AFE 116) or to one of the first AFEs 116 of the device 100. In addition, in this example, the second electrodes 112 of the transducers arranged on the same column of the matrix 104 are coupled to the second AFE 118 (when the device 100 comprises a single second AFE 118) or to one of the second AFEs 118 of the device 100.

[0052] In a particular configuration, it is possible for first electrodes 110 of transducers arranged on different lines of the matrix 104 to be coupled to the same first AFE 116. In this case, this first AFE 116, or each of these first AFE 116, may comprise a first multiplexer (not visible in FIGS. 1 to 5) whose inputs are electrically coupled to these first electrodes 110. When the device 100 comprises a single first AFE 116, this single first AFE 116 may comprise a first multiplexer whose inputs are electrically coupled to the first electrodes 110 of all the transducers of the matrix 104.

[0053] Similarly, it is possible that second electrodes 112 of transducers arranged on different columns of the matrix 104 are coupled to the same second AFE 118. In this case, this second AFE 118, or each of these second AFE 118, may comprise a second multiplexer (not visible in FIGS. 1 to 5) whose inputs are electrically coupled to these second electrodes 112. When the device 100 comprises a single second AFE 118, this single second AFE 118 may comprise a second multiplexer whose inputs are electrically coupled to the second electrodes 112 of all the transducers of the matrix 104.

[0054] In the example described, the device 100 further comprises at least one circuit 122 for polarizing the ultrasonic transducers, configured to apply, for example, the same electrical polarization potential to the first and second electrodes 110, 112 of each of the ultrasonic transducers of the matrix 104. Thus, each transducer of the matrix 104 forms an electrical capacitance provided with terminals to which the same electrical polarization potential is applied. This potential may be different between the two armatures (row and column) in ultrasonic operation. Setting the rows and columns to the same potential may be used for capacitive detection only.

[0055] In the example of [Fig.4], a single bias circuit 122 is shown and electrically coupled to all of the first and second electrodes 110, 112 of the transducers of the matrix 104. Furthermore, the electrical bias signal is also applied to the input of the AFEs 116, 118 and serves as a reference electrical potential for detecting a variation in the overall electrical capacitance formed at least by the transducers.

[0056] Alternatively, it is possible for the device 100 to comprise one or more polarization circuits, each forming part of one of the first and second AFEs 116, 118, for example. In this case, it is possible for the different polarization circuits to apply the same electrical polarization potential to the first and second electrodes 110, 112 of all the transducers of the matrix 104.

[0057] In the example described, the bias signal is applied to the first and second electrodes 110, 112 through coupling resistors 124. Other alternatives to this example are possible.

[0058] In the example described, in order to be able to detect a variation in a global electrical capacitance value formed by at least a portion of the transducers of the matrix 104, this variation being the consequence of the approach of at least one finger close to the matrix 104, the biasing circuit 122 is configured to apply a biasing signal including a periodically repeated variation, for example a periodically repeated pulse. The global electrical capacitance formed by at least a portion of the transducers of the matrix corresponds to the sum of the electrical capacitances formed by these transducers to which is added the electrical capacitance formed by the finger(s) located close to the matrix 104 and which forms an additional electrical capacitance coupled between the ground and the electrical capacitances of the transducers.The variation in the overall electrical capacitance is due to the addition of this electrical capacitance formed by the finger(s) to that of the trans- . drivers.

[0059] [Fig. 5] schematically represents an example of such an electrical bias voltage including an amplitude variation dVbias relative to the value of the DC component of the electrical bias voltage. Although represented only once in [Fig. 5], the amplitude variation dVbias can be repeated periodically. The value of the amplitude variation dVbias is for example less than 1 V. The frequency with which this amplitude variation is present in the bias signal is for example between 10 kHz and 1 MHz. The value of the DC component of the bias signal is for example between 0 V and several tens of volts.

[0060] The device 100 further comprises a wake-up circuit 126 configured to detect, when the device 100 is in a standby mode, the variation in the overall electrical capacitance value formed by at least a portion of the transducers of the matrix 104. In the example described, this detection is carried out, by the wake-up circuit 126, via the detection of a variation in at least one output signal of at least one of the AFEs 116, 118, this variation being representative of a presence of at least one finger in the vicinity of the matrix 104. In the example described, the approach of a finger in the vicinity of a portion of the matrix 104 can be assimilated, from the electrical point of view, as an addition of an electrical capacitance coupled on one side to the ground and on the other to the capacitances formed by the transducers present in this portion of the matrix 104.In the example described, given that the first electrodes 110 are arranged between the second electrodes 112 and the capture surface 102, the detection circuit 126 is configured to detect the variation of the output currents OUT of at least a portion of the first AFEs 116 expressed according to the equation: .

[0061] [Math.l] OUT - x dVbias L to I

[0062] In the example described, given that the same electrical polarization potential is applied to the first and second electrodes 110, 112 and that each transducer of the matrix 104 forms a capacitance across which the same polarization potential is applied, only a variation in current referenced to the ground, and therefore due to the presence of a finger near the capture surface 102 in the present case, is transmitted at the output of the AFEs 116, 118. In the absence of a finger near the matrix 104, the variations in amplitude dVbias of the polarization signal do not generate any current at the output of the AFEs 116, 118. On the other hand, in the presence of a finger near the matrix 104, the pulses of the applied polarization voltage generate current pulses at the output of the AFEs 116, 118.

[0063] When the wake-up circuit 126 detects such a current variation, the circuit alarm clock 126 may be configured to transmit to one or more other circuits of the device 100 a signal controlling the exit from sleep mode of the device 100.

[0064] According to an exemplary embodiment, each of the AFEs 116, 118 may comprise an analog-to-digital converter such that the output currents delivered by the AFEs 116, 118 are digital signals. In this case, the wake-up circuit 126 may be part of a digital circuit whose inputs are coupled to outputs of the first AFEs 116.

[0065] In the example described, the matrix 104 is such that the first electrodes 110 are arranged between the second electrodes 112 and the capture surface 102. In other words, the first electrodes 110 of the ultrasonic transducers of the matrix 104 are arranged on the side of the capture surface 102 of the device 100. The detection of a current variation representative of the presence of a finger near the detection surface 102 can in this case be carried out by detecting a variation in the output current delivered by the first AFE(s) 116. Alternatively, when the device 100 comprises several first AFE(s) 116, it is possible for the wake-up circuit 126 to detect the presence of a finger near the detection surface 102 by detecting a variation in the output current delivered by only a portion of the first AFE(s) 116, for example the one or those coupled to the transducers located towards the center of the detection surface 102.

[0066] According to another variant, it is possible for the wake-up circuit 126 to detect the presence of a finger near the detection surface 102 by detecting a variation in the output current delivered by the second AFE 118 (when the device 100 comprises a single second AFE 118) or by at least a portion of the second AFE 118, or by detecting current variations on the outputs of at least a portion of the first and second AFE(s) 116, 118.

[0067] The detection of a variation in the output current of the AFEs 116, 118, that is to say the verification of the current delivered at the output of the first AFEs 116 and / or the second AFEs 118, can be carried out continuously or at time intervals which may or may not be regular.

[0068] Thus, the device 100 proposes to use the matrix of ultrasonic transducers 104 as a capacitive sensor to detect the approach of an element, for example at least one finger, near the capture surface 102. In the device 100, the reception circuit formed by the AFEs 116, 118 can therefore be seen as forming a low voltage transducer detecting variations in electrical capacitance. The same electrical polarization potential applied to the two electrodes 110, 112 of the transducers of the matrix 104 and the use of this polarization as a reference in the AFEs 116, 118 makes it possible to form a receiver which is only sensitive to the approach of an electrical capacitance coupled to the ground, such as a human finger, to proximity to matrix 104.

[0069] Thanks to the capacitive detection implemented to detect the presence of at least one finger near the capture surface 102 when the device 100 is in standby mode, it is not necessary to carry out regular transmission / reception of ultrasound to carry out this detection, which represents a significant source of energy saving.

[0070] The device 100 can implement, outside of standby mode, a biometric identification of the user of the device 100. This biometric identification can be carried out by implementing the steps described in a non-detailed manner below.

[0071] First of all, the device 100 carries out an acquisition of at least one surface image of the finger 101 present on the capture surface 102, this acquisition comprising for example the emission of a series of ultrasonic signals in the form of pulses by the matrix 104, then the reception of the echoes by the matrix 104 and the processing of the responses obtained to obtain the surface image of the finger 101, that is to say the image of the fingerprint of the finger 101. The acquisition of the surface image of the finger 101 may also include the implementation of other steps not detailed here: filtering of the response signal obtained, envelope detection, logarithmic compression, etc. The data relating to the acquired image are for example stored in a memory of the device 100 or in an external memory of the device 100 for example connected to the device 100 by a communication link.

[0072] From the previously acquired surface image, the data processing circuit 108 can determine or extract minutiae from the fingerprint 101 obtained on this image. This determination of minutiae can be carried out by one or more image processing algorithms not described in detail here and known to those skilled in the art. Each of the minutiae extracted from the surface image can be characterized (type of minutiae, position in the plane of the surface image, orientation of the minutiae, orientation relative to the other minutiae, etc.).

[0073] The characteristics of the minutiae can then be compared with expected characteristics of minutiae, for example previously determined during a preliminary step of user enrollment, to confirm or not the identity of the user. This comparison can include a score calculation whose value depends on the correlations between on the one hand the characteristics of the minutiae previously determined and on the other hand the expected characteristics of minutiae to confirm or not the identity of the user. The score obtained can then be compared with a threshold value in order to evaluate the correspondence between the biometric measurement carried out and the expected biometric data, and thus confirm or not the identity of the user.

[0074] These steps related to biometric identification are known to those skilled in the art and are not described in detail in this description. These steps performing biometric identification may be triggered once the device 100 has exited sleep mode after a finger has been detected in proximity to the matrix 104, and a finger is present on the capture surface 102.

[0075] The device 100 can be configured to implement other functions, such as for example a capture of volume images to determine a microvasculature of the finger 101 and implement a detection of the living (via a detection of blood circulation in the microvasculature of the finger 101) and / or a complementary biometric identification from the characteristics of the determined microvasculature.

[0076] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art.

[0077] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. A biometric identification device (100) comprising at least: - an array of ultrasonic transducers (104), each of the ultrasonic transducers comprising first and second electrodes (110, 112) forming, at least in a standby mode of the biometric identification device (100), an electrical capacitance; - a wake-up circuit (126) configured to detect, when the biometric identification device (100) is in the standby mode, a variation in an overall electrical capacitance value formed by at least a portion of the ultrasonic transducers.

2. A biometric identification device (100) according to claim 1, wherein the ultrasonic transducers are of the PMUT or CMUT type.

3. Biometric identification device (100) according to one of the preceding claims, wherein: - the first electrodes (110) of the ultrasonic transducers of the same row of the matrix (104) are electrically coupled to each other, and / or - the second electrodes (112) of the ultrasonic transducers of the same column of the matrix (104) are electrically coupled to each other.

4. A biometric identification device (100) according to one of the preceding claims, further comprising at least one first analog front-end circuit (116) electrically coupled to the first electrodes (110) and at least one second analog front-end circuit (118) electrically coupled to the second electrodes (112), each of the first and second analog front-end circuits (116, 118) being configured to convert an electrode output electrical voltage of the ultrasonic transducers into an output electrical current.

5. The biometric identification device (100) of claim 4, wherein the wake-up circuit (126) is configured to detect the variation in the value of the overall electrical capacitance formed by at least a portion of the ultrasonic transducers by detecting a variation in the output electrical current of at least one of the first and second analog front-end circuits (116, 118).

6. A biometric identification device (100) according to claim 5, wherein the first electrodes (110) of the ultrasonic transducers are arranged between the second electrodes (112) of the ultrasonic transducers and a capture surface (102) of the biometric identification device (100), and wherein the wake-up circuit (126) is configured to detect the variation of the output electrical current of the first analog front-end circuit (116) or of at least one of the first analog front-end circuits (116).

7. Biometric identification device (100) according to one of claims 4 to 6, wherein: - the first electrodes (110) of ultrasonic transducers of the same row of the matrix (104) are electrically coupled to the first analog front-end circuit (116) or to one of the first analog front-end circuits (116), and / or - the second electrodes (112) of ultrasonic transducers of the same column of the matrix (104) are electrically coupled to the second analog front-end circuit (118) or to one of the second analog front-end circuits (118).

8. Biometric identification device (100) according to claim 7, wherein: - the first analog front-end circuit (116) or at least one of the first analog front-end circuits (116) comprises a first multiplexer whose inputs are electrically coupled to first electrodes (110) of ultrasonic transducers belonging to different rows of the matrix (104), and / or - the second analog front-end circuit (118) or at least one of the second analog front-end circuits (118) comprises a second multiplexer whose inputs are electrically coupled to second electrodes (112) of ultrasonic transducers belonging to different columns of the matrix (104).

9. The biometric identification device (100) of any one of claims 4 to 8, wherein each of the first and second analog front-end circuits (116, 118) comprises an analog-to-digital converter. digital such that the output electrical currents delivered by the first and second analog front-end circuits (116, 118) are digital signals, and wherein the wake-up circuit (126) is part of a digital circuit whose inputs are electrically coupled to outputs of the first and second analog front-end circuits (116, 118).

10. Biometric identification device (100) according to one of the preceding claims, further comprising at least one polarization circuit (122) of the ultrasonic transducers, configured to apply the same electrical polarization potential to the first and second electrodes (110, 112) of each of the ultrasonic transducers.

11. The biometric identification device (100) of claim 10, wherein the bias circuit (122) is configured such that the applied bias electrical potential includes a periodically repeated amplitude variation.

12. A biometric identification device (100) according to one of claims 4 to 9 and according to one of claims 10 or 11, comprising several bias circuits (122) each forming part of one of the first and second analog front-end circuits (116, 118).

13. A biometric identification device (100) according to one of the preceding claims, wherein the wake-up circuit (126) is configured to wake the biometric identification device (100) from sleep mode when a variation in the overall electrical capacitance value formed by at least a portion of the ultrasonic transducers is detected.

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