Active matrix capacitive fingerprint sensor with 2-TFT pixel architecture for display integration

Embodiments described herein include an input device including an array of sensing pixels configured to sense an input object in a sensing region. Each of the sensing pixels includes a sense element and a first transistor, wherein the first transistor includes a gate terminal connected to a row select line and a second terminal connected to the sense element. Each of the sensing pixels also includes a second transistor, wherein the second transistor includes a gate terminal connected to the sense element and the second terminal of the first transistor, and wherein the second transistor further includes a second terminal connected to a column output line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 14/788,604, filed Jun. 30, 2015, titled “Active Matrix Capacitive Fingerprint Sensor with 1-TFT Pixel Architecture for Display Integration,” and U.S. patent application Ser. No. 14/788,532, filed Jun. 30, 2015, titled “Active Matrix Capacitive Fingerprint Sensor for Display Integration based on Charge Sensing by a 2-TFT Pixel Architecture,” both filed concurrently herewith.

BACKGROUND

Field of the Disclosure

Embodiments of the present invention generally relate to a method and apparatus for touch sensing, and more specifically, to a fingerprint sensor.

Description of the Related Art

Input devices including proximity sensor devices (also commonly called touchpads or touch sensor devices) are widely used in a variety of electronic systems. Input devices also include fingerprint sensors and other biometric sensor devices. A sensor device typically includes a sensing region, often demarked by a surface, in which the sensor device determines the presence, location, motion, and/or features of one or more input objects. Sensor devices may be used to provide interfaces for the electronic system. For example, sensor devices are often used as input devices for larger computing systems (such as opaque touchpads and fingerprint sensors integrated in, or peripheral to, notebook or desktop computers). Sensor devices are also often used in smaller computing systems (such as touch screens integrated in cellular phones).

SUMMARY

Embodiments described herein include an input device including an array of sensing pixels configured to sense an input object in a sensing region. Each of the sensing pixels includes a sense element and a first transistor, wherein the first transistor includes a gate terminal connected to a row select line and a second terminal connected to the sense element. Each of the sensing pixels also includes a second transistor, wherein the second transistor includes a gate terminal connected to the sense element and the second terminal of the first transistor, and wherein the second transistor further includes a second terminal connected to a column output line.

In another embodiment, a processing system configured to operate an array of sensing pixels to capture an image of an input object includes a readout circuit, wherein the readout circuit includes a current sensing amplifier circuit connected to a column output line and configured to produce a current representing the input object. The processing system also includes a first switch configured to connect and disconnect a positive input terminal of the current sensing amplifier circuit to a first bias voltage, and a second switch configured to connect and disconnect the positive input terminal of the current sensing amplifier circuit to a second bias voltage. A driver module having circuitry is configured to connect a sense element to an enable line through a first transistor; and read the current from a second transistor

In another embodiment, a method for operating device includes asserting a row select line high to set a voltage at a sense element to zero, wherein the row select line is coupled to a gate terminal of a first transistor, and wherein a second terminal of the first transistor is coupled to the sense element. The method also includes asserting the row select line low and biasing an enable line to a negative voltage. The method also includes sensing an output current on a second terminal of a second transistor, wherein a gate terminal of the second transistor is coupled to the second terminal of the first transistor, and wherein the output current is proportional to a feature of the input object.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the embodiments or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

Various embodiments of the present technology provide input devices and methods for improving usability. Particularly, embodiments described herein provide a fingerprint sensor with increased sensor sensitivity and more accurate measurements. Embodiments also provide reduced thicknesses of layers of glass and layers of substrates. Embodiments also provide sensors with a small number of active elements, which may reduce complexity and save space. Embodiments described herein may also substantially nullify parasitic capacitances. Some embodiments integrate a pixel charge over multiple charge and discharge cycles to make an input signal easier to read. Fingerprint sensors described herein provide minimum impact on the optical performance of a display. Embodiments may reduce or cancel the effect of process variations across a pixel array. Some embodiments may provide a calibration process to cancel the effect of transistor performance variation and device mismatch across the pixel array.

InFIG. 1, the input device100is shown as a proximity sensor device (also often referred to as a “touchpad” or a “touch sensor device”) configured to sense input provided by one or more input objects140in a sensing region120. Example input objects include fingers and styli, as shown inFIG. 1.

The input device100may utilize any combination of sensor components and sensing technologies to detect user input in the sensing region120. The input device100comprises one or more sensing elements for detecting user input. As several non-limiting examples, the input device100may use capacitive, elastive, resistive, inductive, magnetic, acoustic, ultrasonic, and/or optical techniques. Some implementations are configured to provide images that span one, two, three, or higher dimensional spaces. Some implementations are configured to provide projections of input along particular axes or planes. In some resistive implementations of the input device100, a flexible and conductive first layer is separated by one or more spacer elements from a conductive second layer. During operation, one or more voltage gradients are created across the layers. Pressing the flexible first layer may deflect it sufficiently to create electrical contact between the layers, resulting in voltage outputs reflective of the point(s) of contact between the layers. These voltage outputs may be used to determine positional information.

Some capacitive implementations utilize “mutual capacitance” (or “transcapacitance”) sensing methods based on changes in the capacitive coupling between sensor electrodes. In various embodiments, an input object near the sensor electrodes alters the electric field between the sensor electrodes, changing the measured capacitive coupling. In one implementation, a transcapacitive sensing method operates by detecting the capacitive coupling between one or more transmitter sensor electrodes (also “transmitter electrodes” or “transmitters”) and one or more receiver sensor electrodes (also “receiver electrodes” or “receivers”). Transmitter sensor electrodes may be modulated relative to a reference voltage (e.g., system ground) to transmit transmitter signals. Receiver sensor electrodes may be held substantially constant relative to the reference voltage to facilitate receipt of resulting signals. A resulting signal may comprise effect(s) corresponding to one or more transmitter signals and/or to one or more sources of environmental interference (e.g., other electromagnetic signals). Sensor electrodes may be dedicated transmitters or receivers, or sensor electrodes may be configured to both transmit and receive. Alternatively, the receiver electrodes may be modulated relative to ground.

InFIG. 1, a processing system110is shown as part of the input device100. The processing system110is configured to operate the hardware of the input device100to detect input in the sensing region120. The processing system110comprises parts of, or all of, one or more integrated circuits (ICs) and/or other circuitry components. For example, a processing system for a mutual capacitance sensor device may comprise transmitter circuitry configured to transmit signals with transmitter sensor electrodes and/or receiver circuitry configured to receive signals with receiver sensor electrodes. In some embodiments, the processing system110also comprises electronically-readable instructions, such as firmware code, software code, and/or the like. In some embodiments, components composing the processing system110are located together, such as near sensing element(s) of the input device100. In other embodiments, components of processing system110are physically separate with one or more components close to sensing element(s) of input device100and one or more components elsewhere. For example, the input device100may be a peripheral coupled to a desktop computer, and the processing system110may comprise software configured to run on a central processing unit of the desktop computer and one or more ICs (perhaps with associated firmware) separate from the central processing unit. As another example, the input device100may be physically integrated in a phone, and the processing system110may comprise circuits and firmware that are part of a main processor of the phone. In some embodiments, the processing system110is dedicated to implementing the input device100. In other embodiments, the processing system110also performs other functions, such as operating display screens, driving haptic actuators, etc.

For example, in some embodiments, the processing system110operates the sensing element(s) of the input device100to produce electrical signals indicative of input (or lack of input) in the sensing region120. The processing system110may perform any appropriate amount of processing on the electrical signals in producing the information provided to the electronic system. For example, the processing system110may digitize analog electrical signals obtained from the sensor electrodes. As another example; the processing system110may perform filtering or other signal conditioning. As yet another example, the processing system110may subtract or otherwise account for a baseline, such that the information reflects a difference between the electrical signals and the baseline. As yet further examples, the processing system110may determine positional information, recognize inputs as commands, recognize handwriting, and the like.

FIG. 2Ashows a portion of an example sensor electrode pattern configured to sense in a sensing region associated with the pattern, according to some embodiments. For clarity of illustration and description,FIG. 2Ashows a pattern of simple rectangles, and does not show various components. This sensor electrode pattern comprises a plurality of transmitter electrodes160(160-1,160-2,160-3, . . .160-n), and a plurality of receiver electrodes170(170-1,170-2,170-3, . . .170-n) disposed over the plurality of transmitter electrodes160.

Transmitter electrodes160and receiver electrodes170are typically ohmically isolated from each other. That is, one or more insulators separate transmitter electrodes160and receiver electrodes170and prevent them from electrically shorting to each other. In some embodiments, transmitter electrodes160and receiver electrodes170are separated by insulative material disposed between them at cross-over areas; in such constructions, the transmitter electrodes160and/or receiver electrodes170may be formed with jumpers connecting different portions of the same electrode. In some embodiments, transmitter electrodes160and receiver electrodes170are separated by one or more layers of insulative material. In some other embodiments, transmitter electrodes160and receiver electrodes170are separated by one or more substrates; for example, they may be disposed on opposite sides of the same substrate, or on different substrates that are laminated together.

The areas of localized capacitive coupling between transmitter electrodes160and receiver electrodes170may be termed “capacitive pixels.” The capacitive coupling between the transmitter electrodes160and receiver electrodes170change with the proximity and motion of input objects in the sensing region associated with the transmitter electrodes160and receiver electrodes170.

In some embodiments, the sensor pattern is “scanned” to determine these capacitive couplings. That is, the transmitter electrodes160are driven to transmit transmitter signals. Transmitters may be operated such that one transmitter electrode transmits at one time, or multiple transmitter electrodes transmit at the same time. Where multiple transmitter electrodes transmit simultaneously, these multiple transmitter electrodes may transmit the same transmitter signal and effectively produce an effectively larger transmitter electrode, or these multiple transmitter electrodes may transmit different transmitter signals. For example, multiple transmitter electrodes may transmit different transmitter signals according to one or more coding schemes that enable their combined effects on the resulting signals of receiver electrodes170to be independently determined.

The receiver sensor electrodes170may be operated singly or multiply to acquire resulting signals. The resulting signals may be used to determine measurements of the capacitive couplings at the capacitive pixels.

A set of measurements from the capacitive pixels form a “capacitive image” (also “capacitive frame”) representative of the capacitive couplings at the pixels. Multiple capacitive images may be acquired over multiple time periods, and differences between them used to derive information about input in the sensing region. For example, successive capacitive images acquired over successive periods of time can be used to track the motion(s) of one or more input objects entering, exiting, and within the sensing region.

The background capacitance of a sensor device is the capacitive image associated with no input object in the sensing region. The background capacitance changes with the environment and operating conditions, and may be estimated in various ways. For example, some embodiments take “baseline images” when no input object is determined to be in the sensing region, and use those baseline images as estimates of their background capacitances.

Capacitive images can be adjusted for the background capacitance of the sensor device for more efficient processing. Some embodiments accomplish this by “baselining” measurements of the capacitive couplings at the capacitive pixels to produce a “baselined capacitive image.” That is, some embodiments compare the measurements forming a capacitance image with appropriate “baseline values” of a “baseline image” associated with those pixels, and determine changes from that baseline image.

In some embodiments, transmitter electrodes160comprise one or more common electrodes (e.g., “V-corn electrode”) used in updating the display of the display screen. These common electrodes may be disposed on an appropriate display screen substrate. For example, the common electrodes may be disposed on the TFT glass in some display screens (e.g., Inplane Switching (IPS) or Plane to Line Switching (PLS)), on the bottom of the color filter glass of some display screens (e.g., Patterned Vertical Alignment (PVA) or Multi-domain Vertical Alignment (MVA)), etc. In such embodiments, the common electrode can also be referred to as a “combination electrode”, since it performs multiple functions. In various embodiments, each transmitter electrode160comprises one or more common electrodes. In other embodiments, at least two transmitter electrodes160may share at least one common electrode.

In various embodiments, the “capacitive frame rate” (the rate at which successive capacitive images are acquired) may be the same or be different from that of the “display frame rate” (the rate at which the display image is updated, including refreshing the screen to redisplay the same image). In some embodiments where the two rates differ, successive capacitive images are acquired at different display updating states, and the different display updating states may affect the capacitive images that are acquired. That is, display updating affects, in particular, the background capacitive image. Thus, if a first capacitive image is acquired when the display updating is at a first state, and a second capacitive image is acquired when the display updating is at a second state, the first and second capacitive images may differ due to differences in the background capacitive image associated with the display updating states, and not due to changes in the sensing region. This is more likely where the capacitive sensing and display updating electrodes are in close proximity to each other, or when they are shared (e.g. combination electrodes).

For convenience of explanation, a capacitive image that is taken during a particular display updating state is considered to be of a particular frame type. That is, a particular frame type is associated with a mapping of a particular capacitive sensing sequence with a particular display sequence. Thus, a first capacitive image taken during a first display updating state is considered to be of a first frame type, a second capacitive image taken during a second display updating state is considered to be of a second frame type, a third capacitive image taken during a third display updating state is considered to be of a third frame type, and so on. Where the relationship of display update state and capacitive image acquisition is periodic, capacitive images acquired cycle through the frame types and then repeats.

FIG. 2Billustrates a system200for sensing an input object according to embodiments of the present disclosure. System200comprises an array210of sensing pixels in a sensing region120, each sensing pixel comprising a sense element220. Sense elements in the embodiments described below could comprise sense plates or any other passive or active elements. Sense elements are operable to determine a feature or effect of an input object. For example, a sense element that is a sense plate could determine a capacitance between the sense plate and an input object, such as a finger. This capacitance can then determine a portion of a fingerprint pattern. The array of sensing elements may also comprise the electrodes described above with respect toFIG. 2A.

Processing system110inFIG. 2Bis operable to transmit and receive signals to and from array210. The processing system110may include a driver module230, a receiver module240, a determination module250, and an optional memory260. The receiver module240is coupled to the array210and configured to receive resulting signals indicative of input (or lack of input) in the sensing region120and/or of environmental interference. The receiver module240may also be configured to pass the resulting signals to the determination module250for determining the presence of an input object (such as a finger) and/or to the optional memory260for storage. In various embodiments, integrated circuits in the processing system110may be coupled to drivers for sending signals to array210. The drivers may be fabricated using thin-film-transistors (TFT) and may comprise switches, combinatorial logic, multiplexers, and other selection and control logic.

The driver module230, which includes driver circuitry, included in the processing system110may be configured for sending signals to array210. The driver module230may send signals that set row select, enable, or supply lines high or low, as described in further detail below. The driver module230may produce signals that turn switches on or off as described in further detail below. Processing system110may be implemented with more circuitry to control the various components described in the example embodiments below.

Embodiments described below comprise fingerprint sensors utilizing thin-film transistors (TFTs). Fingerprint sensors can be incorporated into a display in certain embodiments. For a fingerprint sensor incorporated into a display, the fingerprint sensor elements may be incorporated near a top surface of the display to improve the quality of a signal captured to detect fine features of a fingerprint. The sense elements for fingerprint sensing described below could be incorporated in an entire active area of a display, or in only a part of the active area of the display. The sense elements may have a pixel density that matches the pixel density of the display pixels, in which case a sense element could be incorporated in every pixel of the display, or in every pixel of the relevant portion of the active area configured for fingerprint sensing. The sense elements could also have a pixel density that is greater or less than the pixel density of the display, and if the sensing pixels have a pixel density greater than the display then multiple sense elements of the fingerprint sensor could be incorporated in a single display pixel. Input device100, described above with respect toFIG. 1, comprises a fingerprint sensor in certain embodiments described herein.

Fingerprint sensors detect the valleys and ridges of fingerprints. One technique for detecting a fingerprint comprises detecting changes in sensor capacitances along valleys and ridges of the fingerprint to get an image of the fingerprint, which may be all or a portion of the complete fingerprint pattern of a user's finger. A cover layer may be employed over the fingerprint sensor to protect the sensor. The cover layer can protect display elements and/or proximity sensor elements in addition to fingerprint sensor elements. The cover layer may be made of an opaque material, or a transparent material, such as glass. This cover layer may be 500 microns or less in some embodiments. With fingerprint sensing, capacitances may be measured on the order of 10−18F. A valley depth in a fingerprint may be approximately 60 microns. Ridge-to-ridge spacing may be approximately 400 microns. A thickness of a ridge may be 100-300 microns. Therefore a pixel size for a fingerprint sensor of around 40-70 microns on a side may be sufficient to capture ridge and valley information of a fingerprint. A pixel pitch of around 40-70 microns may also be sufficient to capture ridge and valley information of a fingerprint. Pixel pitch may be 20-100 microns in some embodiments. Smaller pixel sizes and/or pixel pitches can be used to capture smaller features, such as sweat pores, in addition to ridge and valley information of a fingerprint.

With many sensor pixels, it is difficult to place 6 or more TFTs for each pixel to operate a fingerprint sensor due to space constraints. Embodiments described below can work with as few as 1 TFT or 2 TFTs for each sensing pixel. The architectures described below could be discrete or incorporated in a display. In addition, architectures described below can produce waveforms large enough to nullify parasitic capacitances.

Operational amplifiers described in embodiments below can be low voltage integrated circuits or may be embodied on a panel. Switches described below may be in an integrated circuit or embodied on a non-conductive supporting substrate, such as glass or plastic. MEMS (micro-electro-mechanical) switches may be utilized and may be formed on a supporting substrate or in an integrated circuit. Switches and transistors may be formed in semiconductor wafers or may be TFTs. Sense elements in the embodiments described below could comprise sense plates, PN diodes, piezoelectric transducers that sense ultrasonic waves, or any passive or active elements that accumulate a charge or transduce an excitation into a charge in the presence of an input object, such as a finger.

Embodiments described below that sense a capacitance associated with an input object may measure absolute capacitance or trans capacitance. Absolute capacitance measures a capacitance between the input object and a sense element. Trans capacitance measures a change in capacitance between two sense electrodes due to the presence of an input object.

Embodiments described below may integrate a charge over multiple cycles to more easily capture the fingerprint.

Features described in separate embodiments below may be combined, removed, or incorporated into the other embodiments where appropriate.

Active Matrix Capacitive Fingerprint Sensor with 1-TFT Pixel Architecture for Display Integration

FIG. 3illustrates a pixel architecture for an active matrix capacitive fingerprint sensor according to one embodiment. Architecture300may operate with as few as one TFT in each sensing pixel. Architecture300comprises an array310of sense elements302(in this example, the sense elements302comprise sense plates302) each addressed through a select thin-film transistor (TFT)304controlled by a row of addressing lines (row select306). Each column of sense plates302is connected to a common output line308. When a row is selected, sense plate302of each column is connected to the common output line308of that column through the respective TFT304. The TFT304may be in-cell if the sensor is integrated in a display.

The array310of architecture300may use as few as one TFT per pixel, one output line per column, and one address line per row which reduces the impact on the optical performance of the display. An external circuit (described in further detail below) comprising four switches, a feedback capacitance, and a high gain operational amplifier provides cancellation of the parasitic capacitance of the output line. In addition, integration of the pixel charge can be performed over multiple charge and discharge cycles in some embodiments.

FIG. 4Aillustrates a schematic for a drive/readout circuit400of a column j connected to sense plate402at row406iand column408j. Select TFT404is coupled to row select line406iand output line408j. Drive/readout circuit400also comprises four switches: S1j412, S2j414, SF418, and SR420. The feedback network comprises feedback capacitance CF422and reset switch SR420, and the amplifier circuit comprises operational amplifier416. Switch S1j412charges the sense plate402by coupling the plate to Vch410through select TFT404. Switch S2j414is utilized for readout of the stored charge on sense plate402. Feedback switch SF418connects and disconnects the feedback capacitance CF422to an input of the operational amplifier416. Reset switch SR420resets the state of drive/readout circuit400between subsequent readout of the rows i. Feedback capacitance CF422provides feedback to operational amplifier416, which has one input coupled to ground426. In some embodiments a clock signal may be coupled to an input terminal of the operational amplifier416.

FIG. 5Aillustrates timeline500that comprises signal waveforms during the drive/readout sequence in accordance withFIGS. 3 and 4A. A 3-step sequence is used to transfer the charge on the capacitance formed between the sense plate402and a finger to the feedback capacitance CF422. This capacitance contains the information related to the topography of the finger surface. The charge can be integrated during multiple charge/discharge cycles to increase the amplitude of the output signal by repeating the 3-step sequence.FIG. 5Aillustrates the waveforms of the row select406iand control signal of the switches S1j, S2j, SF, and SR. At time T1, sense plate402is connected to the charge voltage Vch410through the select TFT404and switch S1j412, i.e. row select406iand S1jsignals are set to High. Meanwhile S2j414and feedback switch SF418remain open. Reset switch SR420remains closed. As shown, S2j414and SF418are Low and SR420is High. During this time (charge stage), charge is stored on sense plate402with a magnitude proportional to the capacitance to the finger.

At time T2, the TFT404is disconnected from the output line408by turning Row select406ito Low. S1j412is opened (S1j412is turned Low) to disconnect the charge voltage Vch410.

At time T3(output pre-charge stage), S2j414is closed (S2j414is turned High) to pre-charge the output line to virtual ground (in the case of a non-ideal operational amplifier, to the input offset voltage of the operational amplifier Vos).

At time T4, SR420is opened (SR420is turned Low). At Time T5, SF418is closed to configure the circuit for readout of the stored charge. At time T6(Integrate stage), Row select406iis closed to transfer the charge to CF422and consequently change the output voltage424to a value proportional to the stored charge on the sense plate402.

At time T7, SF418is opened (SF418is turned Low) to disconnect the feedback capacitance CF422from the operational amplifier416and retain the charge on CF422. At time T8, the circuit can enter another charging stage by connecting the charge voltage VCh410through the select TFT404and switch S1j412; i.e. row select406iand S1j412signals are set to High. Meanwhile, S2j414is opened, SR420is closed, and SF418remains open (S2j414turns Low and SR420turns High). By completing another charge/precharge/integrate cycle, the pixel charge can be added to (integrated on) the feedback capacitor422. At the end of the Nthcycle, the output voltage424can be sampled and the output can be reset by turning on the SRswitch420. At time TR1, the SF418is opened (SF418turns Low) to initialize the circuit for another readout sequence.

FIGS. 6A-6Dillustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit ofFIG. 4Aduring charge, precharge, integrate, and reset stages. The sense plate402capacitance to the finger is denoted by Cinand the parasitic capacitances of the output line are lumped into the capacitance Cp.FIG. 6Aillustrates the equivalent circuit610during a charge stage (T1<t<T2as illustrated inFIG. 5A).FIG. 6Billustrates the equivalent circuit620during a precharge stage (T3<t<T4as illustrated inFIG. 5A).FIG. 6Cillustrates the equivalent circuit630during an integrate stage (T6<t<T7as illustrated inFIG. 5A).FIG. 6Dillustrates the equivalent circuit640during a reset stage (T7N<t<TR1as illustrated inFIG. 5A). Isolation of the readout circuit from the charge voltage Vchusing switch S2jallows the readout circuit, including the operational amplifier and reset switch SR, to be implemented using lower voltage technology than the drive circuit.

At the end of each charge stage, the sense plate402voltage is Vin=Vchand the negative terminal of the operational amplifier416V−=Vout=0 (or equals Vosclose to 0). A charge of Qin=CinVchis accumulated on sense plate402. This charge is retained on sense plate402by turning off the TFT404at the end of the charge stage. During the pre-charge stage, the output line408is isolated from the power supply and connected to the input of the operational amplifier416. At the end of the pre-charge stage, the voltage of the output line Vlj=V−=Vout=0 (or equals Vosclose to 0), and the charge stored on CF422is zero. At the end of the first read stage, the voltage of the output line Vlj=Vin=V−, Vout=AV−, and −VCF=Vout−V−=(A−1)V−. If the gain of operational amplifier416is large enough, the charge transferred to the parasitic capacitance Cpduring the readout of the sense capacitor is negligible compared to the charge transferred to CF, as the voltage of Cpdoes not change during the readout. Hence, the effect of the parasitic capacitance is cancelled. The SF418is closed during the integration stage to allow charge to be accumulated on the feedback capacitor CF422, while SF418is open during charge and precharge stages. The SF418and SR420are closed in the reset stage to discharge the feedback capacitor CF422and reset the output voltage424.

FIGS. 4B, 5B, and 6E-6Gare schematic diagrams illustrating another embodiment of a drive/readout circuit. The embodiment illustrated inFIG. 4Bis similar toFIG. 4Awith the exception of the removal of the feedback switch inFIG. 4B.FIGS. 5B and 6E-6Gare also associated with the embodiment ofFIG. 4B.FIG. 4Billustrates the schematic of the drive/readout circuit450of the column j connected to the sense plate at row i and column j. The readout circuit includes 3 switches (S1j412, S2j414, and SR420), an operational amplifier416, and a feedback capacitance CF422. Switch S1j412is used for charging the plate, switch S2j414is used for readout of the stored charge on the sense plate, switch SR420is used to reset the state of the circuit between subsequent readout of the rows, and CFprovides the feedback to the operational amplifier.

FIG. 5Billustrates timeline550that comprises signal waveforms during the drive/readout sequence in accordance withFIGS. 3 and 4B. A 3 step sequence is used to measure the capacitance formed between the sense plate and a finger; this capacitance contains the information related to the topography of the finger surface.FIG. 5Bshows the waveforms of the row select (i) and control signal of the switches S1j, S2j, and SR.

At time T1, the sense plate is connected to the charge voltage VChthrough the select TFT and switch S1j; i.e. row select (i) and S1jsignals are set to High. Meanwhile S2jremains open and SRremains closed (S2jis Low and SRis High). This disconnects the sense plate from the readout circuit and resets the output voltage by discharging the charge stored on feedback capacitance CF. During this time (the charge stage), charge is stored on the sense plate with a magnitude proportional to the capacitance to the finger.

At time T2, the TFT is disconnected from the output line by turning Row select (i) to Low and S1jis opened (S1jis turned Low) to disconnect the charge voltage. At time T3(output pre-charge stage), S2jis closed (S2jis turned High) to pre-charge the output line to virtual ground (in the case of a non-ideal op-amp to the input offset voltage of the op-amp Vos). At time T4, SRis opened (SRis turned Low) to configure the circuit for readout of the stored charge. At time T5(Read stage), Row select (i) is closed to transfer the charge to CFand consequently change the output voltage to a value proportional to the stored charge on the sense plate.

FIGS. 6E-6Gillustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit ofFIG. 4Bduring charge, precharge, and read stages. The sense plate402capacitance to the finger is denoted by Cinand the parasitic capacitances of the output line are lumped into the capacitance Cp.FIG. 6Eillustrates the equivalent circuit650during a charge stage (T1<t<T2as illustrated inFIG. 5B).FIG. 6Fillustrates the equivalent circuit660during a precharge stage (T3<t<T4as illustrated inFIG. 5B).FIG. 6Gillustrates the equivalent circuit670during a read stage (T5<t<T6as illustrated inFIG. 5B).

With respect toFIGS. 4B, 5B, and 6E-6G, at the end of the charge stage, the plate voltage is Vin=Vchand the negative terminal of the op-amp V−=Vout=0 (or Vosclose to 0). A charge of Qin=CinVchis accumulated on the sense plate; this charge is retained on the sense plate by turning off the TFT at the end of the charge stage. During the pre-charge stage, the output line is isolated from the power supply and connected to the input of the operational amplifier. At the end of the pre-charge stage, the voltage of the output line Vlj=V−=Vout=0 (or Vosclose to 0) and the charge stored on CFis zero. At the end of the read stage, the Vlj=Vin=V−, Vout=AV−, and −VCF=Vout−V−=(A−1)V−. For the case of large enough gain of the operational amplifier, the charge transferred to the parasitic capacitance Cpduring the readout of the sense capacitor is negligible compared to the charge transferred to CFas the voltage of CPdoes not change during the readout. Hence the effect of the parasitic capacitance is cancelled.

For a first case, (infinite gain (A) and zero VOS): V−=Vlj=0, as the gain is infinite and the offset voltage is zero. Therefore, the charge stored on the sense plate is transferred to CF.
Vout=V−−VCF=−Qin/CF=−Cin/CFVch

For a second case of a non-ideal operational amplifier:

To minimize the effect of Cpon the output voltage:

In many practical cases, Cp>>CF>>Cin, simplifying the condition to:

Under this condition:

From this equation, the effect of the offset voltage can be neglected if Vch>>Vos.

FIG. 7illustrates a schematic for a drive/readout circuit700of a column j connected to sense plate702at row706iand column708j.FIG. 7is similar toFIG. 4A, with the addition of switch S3j728added to each column for precharging the output line to ground726. In the embodiments ofFIGS. 4-6, the pre-charge state is implemented by connecting the output line to virtual ground through the switch S2j414and reset switch SR420. InFIG. 7, switch S3j728allows the pre-charge state to instead be implemented by connecting the output line708directly to system ground726. Switch S3j728can be implemented using a TFT on a display/sensor backplane or using a transistor in a driver circuit. Select TFT704is coupled to row select line706iand output line708j. Drive/readout circuit700comprises four other switches: S1j712, S2j714, feedback switch SF718, and reset switch SR720. Drive/readout circuit700further comprises feedback capacitance CF722and operational amplifier716.

Implementation of the pre-charge switch S3j728on the backplane allows the charge integrator to be isolated from the high voltage built up on the output line708jduring the charging step. This allows implementation of charge integrator using a low-voltage technology for better performance and smaller chip footprint. This also allows for a decrease in the time needed for the pre-charge phase, as the pre-charge switch S3j728can have a higher limit on current than a limit in the operational amplifier circuits.

After the charge stage and isolation of the sense plate702using select TFT704, the output line708is biased to ground726using the pre-charge switch S3j728. Next, the switch S2j714is closed and the output line is connected to the input stage of the integrator. At this stage, select TFT704is opened to transfer the charge to the feedback capacitance722. As the line parasitic capacitance is orders of magnitude larger than the sense plate702capacitance, the charge integrator is only exposed to a very small transient voltage. Hence, charging voltages with magnitudes substantially larger than the operating voltage rating of the charge integrator circuit can be employed.

Alternatively, both charging and pre-charging biases can be applied to the output line through S1j712using a signal with a proper waveform.

FIG. 8illustrates timeline800that comprises signal waveforms during the charge/precharge/integrate sequence in accordance withFIG. 7. The waveforms are similar to timeline500illustrated inFIG. 5Aand described in detail above. Timeline800introduces the waveform S3jfor switch728. Switch S3j728is asserted High during the precharge stage at time T3. Switch S3j728is then asserted Low at time T4.

FIG. 9is a flowchart illustrating a method900for operating an input device, according to one embodiment. The steps of method900may be performed in any suitable order. The method begins at step910, where a driver module applies a charge voltage to a sense element through a first transistor and a first switch. The driver module may also set a row select high at this step. At step920, an electric charge is stored on the sense element. The electric charge comprises a magnitude proportional to a feature of an input object. This feature may be a capacitance associated with the input object. The feature may be a capacitance between the sense element and the input object. If a finger is the input object and a fingerprint is being sensed, the magnitude of the capacitance is measured to determine the depth of a ridge or valley of a fingerprint.

At step930, a gate terminal of the first transistor is driven low and the first switch is opened by the driver module to disconnect the charge voltage. The gate terminal can be driven low by driving the row select line to low. At step940, the charge voltage is transferred to a feedback capacitor. After the charge is transferred to the feedback capacitor, the charge can be read with a readout circuit, or additional cycles may be performed to integrate additional charge on the feedback capacitor before the charge is read out. After the charge is read out, the circuit can be initialized for another drive/readout sequence.

Active Matrix Capacitive Fingerprint Sensor with 2-TFT Pixel Architecture for Display Integration

FIG. 10illustrates a pixel architecture for an active matrix capacitive fingerprint sensor according to one embodiment. Architecture1000may operate with as few as two TFTs in each sensing pixel. Architecture1000comprises an array1020of sense elements1002(in this example sense elements1002comprise sense plates1002) each addressed through a TFT circuit1004controlled by a row of addressing lines (row select1006) and a row of enable lines1012. Each TFT circuit1004is connected to a common output line1008and to a supply line1010.

FIG. 11illustrates a schematic of pixel circuit1100of a column j connected to sense plate1102at row1106iand column1108j. Each sense electrode is connected through a first TFT T1i,j1112to an enable line1110. The first TFT1112T1i,jis controlled by a row select line1106coupled to a gate electrode. Each sense plate1102is connected to the gate of a second TFT T2i,j1116while the drain of the second TFT T2i,j1116is connected to the supply line1104and its source is connected to the output line1108. The reference capacitor CR1114is connected between the gate and source of the second TFT1116. Each row of pixels shares the same enable line1110and row select line1106, and all pixels in the same column share the same supply line1104and output line1108. In a variation of the pixel architecture discussed in further detail below, no supply line1104is included and the drain of the second TFT T2i,j1116is connected to the row select line1106(see, e.g.,FIG. 17). In this sensor, the capacitance formed between the sense plate1102and surface of the finger controls the steady-state output current of the second TFT T2i,j1116. By measuring the output current of the pixel, the capacitance between the sense plate1102and the finger can be determined for each pixel, thereby providing an image of the finger surface.

The architecture illustrated inFIGS. 10 and 11provides a minimum impact on optical performance of the display, as the architecture uses 2 TFTs per pixel with small dimensions. As the steady state current of the pixel represents the value of the capacitance between the finger and the sense plate1102(which is determined by the shape of the finger surface), the measurement time of the output current can be increased to enhance the accuracy of the measurement.

Operation of the second TFT T2i,j1116in the sub-threshold regime is possible to benefit from an exponential current-voltage dependence (i.e., the current has an exponential dependence to the value of the finger-sense plate capacitance). To the first order, the parasitic elements do not impact the response of the sensor output as the sensor operates in steady-state mode.

The circuit can be operated in a three-stage drive/readout sequence to extract the TFT IV characteristics for accurate calculation of the finger capacitance. This method cancels the effect of process variation resulting in characteristic mismatch across the array. It is also possible to calibrate the device by scanning the array when no finger is present to cancel the effect of TFT performance variation and device mismatch across the array.

FIG. 12illustrates a schematic1200of a drive/readout circuit of the column j connected to the pixel at row i and column j for the structure illustrated inFIGS. 10 and 11. The readout circuit includes two switches S1j1232and S2j1234, an operational amplifier1224, and a feedback resistor RF1222. Switch S1j1232is used to connect the output to a first bias voltage −VBias11228, and switch S2j1234is used to connect the output to the second bias voltage −VBias21230. Schematic1200further comprises TFTs1212and1216and capacitances CR1214and Cin i,j1218(the input object is assumed to be coupled to ground1220). Feedback resistor RF1222is coupled to amplifier1224and Vout1226. Row select line1206, enable line1210, supply1204, and output1208are also illustrated inFIG. 12.

FIG. 13illustrates timeline1300that comprises signal waveforms during the drive/readout sequence in accordance withFIGS. 10, 11, and 12. A 3-step sequence is used to measure the capacitance formed between the sense plate1102and a finger. This capacitance contains the information related to the topography of the finger surface. The readout sequence consists of an enable step, readout step, and a disable step. To enable the pixel, at time T1, sense plate1102is connected to the enable line1210through the TFT T1i,j1212; i.e., Row Select1206iis set to High and enable1210iis biased at 0 V. The Supply1204jis also set to Vdd. This will set the potential of sense plate1102to 0 V. During this time switch S1j1228is High (closed or connected) and switch S2j1234is Low (open or disconnected). As a result the output voltage is at −VBias1−RISense1. ISense1is a function of IV characteristics of T2i,j1216and VBias11228. It is important to note that ISense1is independent of the absolute or trans capacitance of the input object. During the readout step, at time T2, sense plate1102is isolated from enable line1210; i.e., Row Select1206iis set to Low (0 or a negative voltage) and enable1210iis biased at −VSS. Next, at time T3, switch S1j1232is set to Low and switch S2j1234is set to High. This connects VBias21230to the positive terminal of the operational amplifier1224and isolates VBias11228from the operational amplifier1224. For an op-amp with large enough gain, the voltage of the negative terminal of the op-amp becomes −VBias2; hence the Output (j)1208is pulled down to −VBias2(from −VBias1). As a result, the output current of the T2i,j1216changes and the Voutwill change to −VBias2−RISense2, where ISense2is a function of the measured capacitance (either absolute or trans capacitance) and characteristics of the TFT1216. At time T4(start of the Disable step), T2i,j1216is turned Off by biasing the gate of T2i,j1216at −VSSby setting the row select line1206ito Vddand the enable line1210ito −VSS. This will set the voltage of Vout1226and output line1208jto −VBias2. At time T5, switch S1j1232is set to High and switch S2j1234to Low, to reset the voltage of output line1208jand Vout1226to −VBias1. Finally, at time T6, the Disable stage is finalized by setting the row select1206ito 0 V. At this point the pixel is ready for the next Enable/Readout/Disable sequence.

FIGS. 14A-14Cillustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit during enable, readout, and disable stages, respectively. The sense plate1102capacitance to the finger (absolute capacitance) is denoted by Cinand it is assumed that the parasitic gate-source capacitance of T2i,j1216is included in CR, and the rest of the parasitic elements are ignored.FIG. 14Aillustrates the equivalent circuit1410during an enable stage (T1<t<T2as illustrated inFIG. 13).FIG. 14Billustrates the equivalent circuit1420during a readout stage (T3<t<T4as illustrated inFIG. 13).FIG. 14Cillustrates the equivalent circuit1430during a disable stage (T5<t<T6as illustrated inFIG. 13).

FIG. 15illustrates signal waveforms1500and drive circuit1510during the drive/readout sequence for the pixel architecture ofFIGS. 10-12implemented without switch S1jand switch S2j. As the state of switches S1jand S2jare opposite (illustrated in waveform1500), it is possible to remove both switches and apply the proper signal directly to the positive terminal of operational amplifier1224as shown inFIG. 15.

FIG. 16illustrates a 2-TFT pixel architecture for an active matrix capacitive fingerprint sensor according to another embodiment. Architecture1600comprises an array1620of sense elements (sense elements1602comprise sense plates1602in this embodiment) each addressed through a TFT circuit1604controlled by a row of addressing lines (row select1610) and a row of enable lines1612. Each TFT circuit1604is connected to a common output line1608. In this architecture, no separate supply line is included and the drain of the second TFT is coupled to the row select line (compare to architecture1000inFIG. 10).

FIG. 17illustrates a schematic for a pixel circuit1700of a column j connected to sense plate1702at row i1704. Each sense electrode is connected through a first TFT T1i,j1712to an Enable line1710. The first TFT T1i,j1712is controlled by a row select/supply line1704coupled to a gate electrode of TFT1712(no separate supply line is included in this embodiment). Each sense plate1702is connected to the gate of a second TFT T2i,j1716while the drain of the second TFT T2i,j1716is connected to the row select/supply line1704and its source is connected to the output line1708. The reference capacitance CR1714is connected between the gate and source of the second TFT T2i,j1716. The drain of the second TFT T2i,j1716is connected to the row select/supply line1704. In this schematic, the capacitance formed between the sense plate1702and surface of the finger controls the steady-state output current of the second TFT T2i,j1716. By measuring the output current of the pixel, the capacitance between the sense plate1702and a finger can be determined, thereby providing an image of the finger surface.

FIG. 18illustrates a schematic1800of a drive/readout circuit of the column j connected to the pixel at row i and column j for the structure illustrated inFIGS. 16 and 17. The readout circuit includes two switches S1j1832and S2j1834, an operational amplifier1824, and a feedback resistor RF1822. Switch S1j1832is used to connect the output to a first bias voltage −VBias11828, and switch S2j1834is used to connect the output to the second bias voltage −VBias21830. Schematic1800further comprises TFTs1812and1816and capacitors CR1814and Cin1818(the input object is assumed to be coupled to ground1820). Feedback resistor RF1822is coupled to amplifier1824and Vout1826. Row select/supply1806, enable1810, and output1808are also illustrated inFIG. 18.

FIG. 19illustrates timeline1900that comprises signal waveforms during the drive/readout sequence in accordance withFIGS. 16, 17, and 18. A 3-step sequence is used to measure the capacitance formed between the sense plate1702and an input object, such as a finger. This capacitance contains the information related to the topography of the finger surface. The readout sequence consists of an enable step, readout step, and a disable step. The waveforms are similar to the waveforms discussed above with respect toFIGS. 11-13, except that there are no supply lines forFIGS. 16-18, and the select/supply lines replace the row select lines. For a detailed discussion of the operations, seeFIG. 13above.

FIGS. 20A-20Cillustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit during enable, readout, and disable stages, respectively. The capacitance between sense plate1702and the finger is denoted by Cinand it is assumed that the parasitic gate-source capacitance of T2i,j1816is included in CR, and the rest of the parasitic elements are ignored.FIG. 20Aillustrates the equivalent circuit2010during an enable stage (T1<t<T2as illustrated inFIG. 19).FIG. 20Billustrates the equivalent circuit2020during a readout stage (T3<t<T4as illustrated inFIG. 19).FIG. 20Cillustrates the equivalent circuit2030during a disable stage (T5<t<T6as illustrated inFIG. 19).

FIG. 21illustrates signal waveforms2100and drive circuit2110during the drive/readout sequence for the pixel architecture ofFIGS. 16-18implemented without switches S1jand S2j. Because the switches S1jand S2jare driven opposite one another (when one is high, the other is low), it is possible to remove both switches and apply the appropriate VBias1or VBias2signal directly to the positive terminal of operational amplifier1824as shown inFIG. 21.

With respect to both 2-TFT architectures illustrated inFIGS. 10-12 and 16-18, during the enable stage, the potential of the gate of the sense transistor (T2i,j) is raised to 0 V. This allows for a current flow in this transistor when a negative voltage is applied to the source of this transistor. During the readout stage, the source voltage of sense transistor changes from −VBias1to −VBias2. Initially (enable stage), the output current of the sense transistor is independent of the capacitance detected by the sense element, but at the later stage the output current will be a function of this capacitance (see equations below). Hence, in the initial stage (enable stage), the sense transistor can be characterized and the sense capacitor can be accurately determined in the second stage. This will eliminate the effect of process variation and device mismatch across the array. During the Disable stage, a −VSSpotential is applied to the gate of the sense transistor to ensure that the TFT remains in the off state when the rest of the pixels in the same column are addressed. The following provides the equations for sense current and output voltage during the readout of a pixel. It is assumed that the circuit has reached steady state condition. The current of the TFT is a function of VDSand VGSexpressed as f(VGS2, VDS2).

Equations for the fingerprint sensor ofFIGS. 10-12:

At T4−(just before changing the state of row select (i)):

Equations for the fingerprint sensor ofFIGS. 16-18:

At T4−(just before changing the state of Row Select (i))

Assuming the TFT operates in subthreshold regime with I∝eKVGSdependence:

where A, B, and D are constants and a function of TFT characteristics, VBias1, and VBias2. The current therefore has an exponential relationship to the input capacitance, and a small change in the input capacitance can produce a large variation in sense current.

Although in the 2-TFT example architectures above, the −VBias1>−VBias2, it is possible to run the embodiments in the condition where −VBias2>−VBias1. Also, the reference capacitance CRmay be implemented via an additional reference capacitor connected to the two terminals of the second TFT T2i,jtransistor, or the gate to source capacitance of the second TFT may be sufficient.

In the 2-TFT example architectures above, it is possible to read the output current only once by applying a voltage pulse to the positive terminal. A calibration step can be applied occasionally to determine the IV characteristics of the sense TFTs across the array. These parameters can be stored and used to avoid measuring the current two times in the same frame. Under this condition, the VBiassignal (seeFIG. 15andFIG. 21) changes from 0 to −VBiasand the current is only measured at T4−.

FIG. 22is a flowchart illustrating a method2200for operating an input device, according to one embodiment. The steps of method2200may be performed in any suitable order. Method2200describes an enable/readout/disable sequence for a fingerprint sensor with a 2-TFT pixel architecture. The method begins at step2210, where a driver module asserts a row select line high to set a voltage at a sense element to zero. The row select line is coupled to a gate terminal of a first transistor, and a second terminal of the first transistor is coupled to the sense element. A third terminal of the first transistor is coupled to an enable line.

At step2220, the driver module asserts the row select line low and the enable line is biased to a negative voltage. This step isolates the sense element from the enable line. At step2230, an output current is sensed on a second terminal of a second transistor. The gate of the second transistor is coupled to the second terminal of the first transistor. A third terminal of the second transistor may be coupled to a supply line, or to a combined select/supply line in some embodiments. The output current is proportional to a feature of the input object. For example, the output current may be proportional to a capacitance between the input object (such as a finger) and the sense element. The output current can therefore be used to determine an image of a fingerprint pattern, which may be all or a portion of a complete fingerprint of a user.

Active Matrix Capacitive Fingerprint Sensor for Display Integration Based on Charge Sensing by a 2-TFT Pixel Architecture

FIG. 23illustrates a pixel architecture for an active matrix capacitive fingerprint sensor for display integration based on charge sensing according to one embodiment. Architecture2300may operate with as few as two TFTs, or one TFT and one diode in each sensing pixel. Architecture2300comprises an array2320of sense elements (sense elements2302comprise sense plates2302in this embodiment) each addressed through a TFT circuit2304controlled by a row of addressing lines (row select2310) and a row of enable lines2312. Each TFT circuit2304is connected to a common output line2308.

FIG. 24illustrates a schematic of a pixel2400of a column j connected to sense plate2402at row2404iand column2408j. Reference capacitor CR2414may employed in some embodiments. Each sense plate2402is connected through the first TFT T1i,j2412to an enable line2410and the first TFT T1i,j2412is controlled by a row select line2404. Each sense plate2402is connected to the gate and drain of a second TFT T2i,j2416while the source of TFT T2i,j2416is connected to the output line2408(the second TFT is diode-connected to create a two terminal device). The reference capacitor CR2414(if used) is connected between the gate and source of the second TFT T2i,j2416. Each row of pixels share the same enable line2410and row select line2404, and all pixels in the same column share the same output line2408. In a variation of the pixel architecture discussed in further detail below, the second TFT T2i,j2416is replaced by a diode or other non-linear circuit element (seeFIGS. 28-30). In this architecture, the charge stored on the sense plate2402is measured to determine the capacitance between the sense plate2402and the finger, hence providing an image of the finger surface.

The architecture illustrated inFIGS. 23 and 24provides a minimum impact on optical performance of the display as the sensor may use as few as two TFTs (or one diode and one TFT) per pixel with smallest possible dimensions. Parasitic capacitance of the output line may be effectively cancelled and produces no artifact on the measured charge as the voltage of the output line remains constant during the enable and readout stages.

The steady state current flowing through the sense transistor (second TFT T2i,j2416) can be used to measure the IV characteristics of the device to cancel the effect of TFT characteristics mismatch across the array. Finally, it is possible to calibrate the device by scanning the array when no finger is present to cancel the effect of TFT performance variation and device mismatch across the array.

FIG. 25illustrates a schematic2500of a drive/readout circuit of the column j connected to the pixel at row i and column j for the pixel structure illustrated inFIG. 23. The readout circuit includes a switch SRj2522, an operational amplifier2524, and a feedback capacitor CF2526. Switch SRj2522is used to reset the charge stored on feedback capacitor CF2526between consecutive readouts. Schematic2500further comprises TFTs T1i,j2512and T2i,j2516and capacitances CR2514and Cin i,j2518(input capacitance, coupled to ground2520). Feedback capacitor CF2526and switch SRj2522are coupled to operational amplifier2524and Vout2530. Row select2506, enable2510, and output2508are also illustrated inFIG. 25.

FIG. 26illustrates timeline2600that comprises signal waveforms during the drive/readout sequence in accordance withFIGS. 23, 24, and 25. A 3-step sequence is used to determine the capacitance formed between the sense plate2402and a finger by measuring the charge stored on sense plate2402due to this capacitance. This capacitance represents the information related to the topography of the finger surface.FIG. 26shows the waveforms of the lines for pixels of24and25. The readout sequence consists of an Enable step, Readout step, and a Disable step. To enable the pixel, at time T1, the sense plate2402is connected to the enable line2510ithrough the TFT T1i,j2512; i.e., row select2506iis set to High and enable2510iis biased at Vdd. During this time, switch SRj2522is closed (switch SRjis High) so Voutis held at ground as the positive terminal of the operational amplifier2524is grounded at2528and the output is connected to the negative terminal. In this step, the current following through the TFT T2i,j2516is only a function of the TFT characteristics, and may be measured for calibration purposes.

At time T2, Row Select2506iand Enable2510ilines are connected to ground and switch SRj2522is opened (switch SRj2522is turned Low). This step isolates the sense plate2402from the Enable line2510iand transfers the charge stored on the sense plate2402(shown as Cin i,j2518) into feedback capacitor CF2526. Consequently, Vin i,jdrops to a value below the threshold voltage of TFT T2i,j2516, and Voutdrops to a negative value depending on the stored charge according to the equations presented below.

At time T3, row select2506iis connected to Vddto turn on TFT T1i,j2516, and switch SRj2522is closed (switch SRj2522turns High). Hence, the pixel is disabled by setting the voltage of Vin i,jto 0 V to eliminate the charge leakage through TFT T2i,j2516during the readout of the pixels in other rows. The Voutis also set to 0 V by discharging the feedback capacitor CF2526.

At time T4, row select line2506iis set to 0 V to prepare the pixel for another Enable/Readout/Disable sequence. To increase the speed of sensing an input capacitance, it is possible to combine the enable step of the row (i+1) with the disable step of the row (i).

FIGS. 27A and 27Billustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit during enable, readout, and disable stages. The capacitance between sense plate2402and a finger is denoted as Cin i,jand it is assumed that the parasitic gate-source capacitance of the TFT T2i,j2516is included in CR, and the rest of the parasitic elements are ignored.FIG. 27Aillustrates the equivalent circuit2710during the enable or disable stage (T1<t<T2and T3<t<T4as illustrated inFIG. 26).FIG. 27Billustrates the equivalent circuit2720during a readout stage (T2<t<T3as illustrated inFIG. 26).

FIG. 28illustrates a schematic for a drive/readout circuit2800of a column j connected to sense plate2802at row2804iand column2808j. Drive/readout circuit2800is identical to circuit2400illustrated inFIG. 24with the exception of the non-linear circuit element (rectifying element) comprising the second TFT T2i,j2416being replaced by a non-linear circuit element (rectifying element) comprising a diode Di,j2816. The structure and operation of the circuits are similar. InFIGS. 24 and 28, like numerals denote like elements (i.e., sense plate2402is equivalent to sense plate2802, etc.). The operation and advantages described above with respect toFIGS. 23-27also apply toFIGS. 28-30.

FIG. 29illustrates a schematic2900of a drive/readout circuit of the column j connected to the pixel at row i and column j for the pixel structure illustrated inFIG. 28. Schematic2900is identical to schematic2500illustrated inFIG. 25with the exception of TFT2516being replaced by diode2916. The structure and operation of the circuits are similar. InFIGS. 25 and 29, like numerals denote like elements (i.e., CF2526is equivalent to CF2926, etc.).

FIGS. 30A and 30Billustrate equivalent circuits of a pixel (i, j) connected to the drive/readout circuit during enable, readout, and disable stages. Equivalent circuit3010is identical to equivalent circuit2710illustrated inFIG. 27with the exception of TFT T2i,jbeing replaced by diode Di,j. The structure and operation of the circuits are similar.

With respect to both the 2-transistor structure ofFIGS. 24-27and the transistor-plus-diode structure ofFIGS. 28-30, during the enable stage, the potential of the gate of the sense transistor (T2i,j) or the anode terminal of the diode (DO (i.e., the non-linear circuit element) is raised to Vdd. This acts to store a charge on the sense plate proportional to a measured capacitance (either absolute capacitance or trans capacitance). A constant current also follows through the transistor or diode (i.e., the non-linear circuit element), which is only a function of the IV characteristics of the device and can be measured to calibrate the sensor to cancel the effect of device mismatch across the pixel array. During the readout, the sense plate is isolated from the enable line and the charge stored on the sense plate is transferred to CF. As the output current remains constant, no charge is transferred to the output line parasitic capacitance. This will eliminate the effect of the parasitic elements of the output line. During the Disable stage, the sense TFT or diode is turned off by setting the voltage of the sense plate to 0 V. Hence, the pixel remains in the off state when the rest of the pixels in the same column are addressed. The following provide the equations for Voutduring the readout of the pixel. It is assumed that the diode or TFT stops conducting at VTor VON.

During Enable and Disable steps:
Vout=0 V

At T3−(just before changing the state of row select (i)):

With respect to both the 2-transistor structure ofFIGS. 24-27and the transistor-plus-diode structure ofFIGS. 28-30, it is possible to connect the positive terminal of the operational amplifier to an arbitrary bias voltage (for example −VBias) to increase the stored charge over the pixel capacitor by increasing the bias voltage from Vddto Vdd+VBiasacross the pixel capacitor. Under this condition, the Enable line should be biased at −VBiasduring the disable stage.

A calibration step can be applied occasionally to determine the IV characteristics of the sense TFT or diode across the array by measuring the current flowing through the device during the Enable step.

FIG. 31is a flowchart illustrating a method3100for operating an input device, according to one embodiment. The steps of method3100may be performed in any suitable order. Method3100describes an enable/readout/disable sequence for a fingerprint sensor with a 2-TFT (or one TFT and one diode) pixel architecture. The method begins at step3110, where a driver module asserts a row select line high to couple a sense element to an enable line through a first transistor. The row select line is coupled to a gate terminal of the first transistor, and a first terminal of the first transistor is coupled to the sense element. The enable line is coupled to a second terminal of the first transistor.

At step3120, a charge is collected at the sense element, where the charge is proportional to a feature of an input object. The charge may be proportional to a capacitance between an input object (such as a finger) and the sense element. At step3130, the driver module asserts the row select line and the enable line low to isolate the sense element from the enable line. At step3140, the charge stored on the sense element is transferred to a feedback capacitor (as a result of step3130). The charge is transferred through a non-linear circuit element. The non-linear circuit element may be a diode or a transistor-connected diode.

At step3150, an output voltage is read. The output voltage is proportional to the feature of the input object, and may be used to determine at least a portion of a fingerprint. After the output voltage has been read, the pixel may be reset to prepare for another enable/readout/disable sequence.

Thus, the embodiments and examples set forth herein were presented in order to best explain the embodiments in accordance with the present technology and its particular application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed.