Active Thin-Film Charge Sensor Element

Abstract

A charge sensor element includes a charge collecting detector configured to generate an intensity signal indicative of an amount of charge at an internal charge sensor element node, an amplifier transistor that is electrically connected to the internal charge sensor element node and configured to amplify the intensity signal, and a reset transistor that is electrically connected to the internal charge sensor element node and configured to reset the intensity signal. The amplifier transistor or the reset transistor includes a front gate and a back gate that are configured to control the amplifier transistor or the reset transistor.

Claims

1. A charge sensor element comprising: a charge collecting detector configured to generate an intensity signal indicative of an amount of charge at an internal charge sensor element node; an amplifier transistor that is electrically connected to the internal charge sensor element node and configured to amplify the intensity signal; and a reset transistor that is electrically connected to the internal charge sensor element node and configured to reset the intensity signal, wherein the amplifier transistor or the reset transistor comprises a front gate and a back gate that are configured to control the amplifier transistor or the reset transistor.

2. The charge sensor element according to claim 1, wherein the amplifier transistor comprises the front gate and the back gate, and wherein the back gate of the amplifier transistor is electrically connected to the front gate or a select signal line.

3. The charge sensor element according to claim 1, wherein the reset transistor comprises the front gate and the back gate, and wherein the back gate of the reset transistor is electrically connected to the front gate of the reset transistor.

4. The charge sensor element according to claim 1, wherein each of the amplifier transistor and the reset transistor comprises the front gate and the back gate.

5. The charge sensor element according to claim 1, wherein the back gate of the amplifier transistor is configured to be controlled by an adjustable voltage.

6. The charge sensor element according to claim 1, further comprising a select transistor electrically connected to the amplifier transistor, wherein the select transistor comprises a front gate and a back gate, and wherein the front gate of the select transistor is electrically connected to the back gate of the select transistor.

7. The charge sensor element according to claim 1, wherein the reset transistor and the amplifier transistor are electrically connected to an anode or cathode of the charge collecting detector.

8. The charge sensor element according to claim 1, wherein the amplifier transistor and the reset transistor are based on an etch-stop layer, back-channel etch, and/or self-aligned transistor architecture.

9. The charge sensor element according claim 1, wherein the charge collecting detector comprises a photodetector.

10. The charge sensor element according claim 1, wherein the charge collecting detector comprises a pyroelectric sensor. an ion-sensitive field-effect transistor or a bio-sensitive field-effect transistor.

11. The charge sensor element according claim 1, wherein the charge collecting detector comprises an ion-sensitive field-effect transistor.

12. The charge sensor element according claim 1, wherein the charge collecting detector comprises a bio-sensitive field-effect transistor.

13. A charge sensor element array, comprising a plurality of the charge sensor element according to claim 1.

14. A method for controlling a charge sensor element, the method comprising: generating, via a charge collecting detector, an intensity signal indicative of an amount of charge at an internal charge sensor element node; amplifying the intensity signal via an amplifier transistor that is electrically connected to the charge collecting detector at the internal charge sensor element node; and resetting the intensity signal via a reset transistor that is electrically connected to the internal charge sensor element node, wherein the amplifying is controlled by a front gate and a back gate of the amplifier transistor or the resetting is controlled by a front gate and a back gate of the reset transistor.

15. The method according to claim 14, further comprising applying a select signal to the back gate of the amplifier transistor or the front gate of the amplifier transistor.

16. The method according to claim 14, wherein a signal applied to the back gate or the front gate of the amplifier transistor is varied over time.

17. The method according to claim 14, wherein a signal is applied to the back gate of the amplifying transistor and the front gate of the amplifier transistor.

18. The method according to claim 14 further comprising applying a reset signal to the front gate and the back gate of the reset transistor electrically connected to the charge collecting detector and the amplification transistor, for resetting the intensity signal.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0048] The above, as well as additional, features will be better understood through the following illustrative and non-limiting detailed description of example embodiments, with reference to the appended drawings.

[0049] FIG. 1 illustrates an active thin-film charge sensor element based on a voltage-mode 2T topology, according to an example.

[0050] FIG. 2 illustrates an active thin-film charge sensor element based on a voltage-mode 3T topology and back gates electrically connected to front gates, according to an example.

[0051] FIG. 3 illustrates an operation of a 2T and 3T topology for amplification of a corresponding intensity signal from a photodetector, according to an example.

[0052] FIG. 4A illustrates an active thin-film charge sensor element based on a current-mode 2T topology, according to an example.

[0053] FIG. 4B illustrates an active thin-film charge sensor element based on a current-mode 3T topology, according to an example.

[0054] FIG. 5A illustrates a 2T topology based on an active thin-film charge sensor element 10, specifically a source follower topology, according to an example.

[0055] FIG. 5B illustrates a 2T topology based on an active thin-film charge sensor element 10, specifically a source follower variation topology, according to an example.

[0056] FIG. 5C illustrates a 2T topology based on an active thin-film charge sensor element 10, specifically a common source topology, according to an example.

[0057] FIG. 5D illustrates a 2T topology based on an active thin-film charge sensor element 10, specifically a common source variation topology, according to an example.

[0058] FIG. 6 illustrates an active pyroelectric fingerprint sensor, based on the detection of temperature changes, according to an example.

[0059] FIG. 7 illustrates an active pyroelectric fingerprint sensor, based on the detection of temperature changes, according to an example.

[0060] FIG. 8 illustrates a layer stack of a pyroelectric sensor element, comprising a pyroelectric front plane and a thin-film transistor (TFT) backplane, according to an example.

[0061] FIG. 9 illustrates an array of biochemical sensor elements, according to an example.

[0062] FIG. 10 illustrates a detection mechanism of a biochemical sensor, based on the use of a marker such as a fluorescent marker or a charged marker, according to an example.

[0063] FIG. 11 shows a cross section of an ion-sensitive field-effect transistor (ISFET) that may be used for detecting a charge, according to an example.

[0064] FIG. 12A shows a cross section of a DNA field-effect transistor (DNAFET) for detecting an intrinsic DNA charge, according to an example.

[0065] FIG. 12B schematically illustrates an array of connected biomechanical sensors, according to an example.

[0066] FIG. 13 is a flowchart of a method for controlling an active thin-film charge sensor element, according to an example.

[0067] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary to elucidate example embodiments, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

[0068] Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings. That which is encompassed by the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example. Furthermore, like numbers refer to the same or similar elements or components throughout.

[0069] FIG. 1 illustrates an active thin-film charge sensor element 10 based on a voltage-mode 2T topology. The active thin-film charge sensor element 10 comprises an electromagnetic detector 1 (charge collecting detector 1). The electromagnetic detector 1 is configured to detect electro-magnetic radiation and generate a corresponding intensity signal at an internal charge sensor element node or internal pixel node IPN. The active thin-film charge sensor element 10 further comprises an amplifier transistor 2. The amplifier transistor 2 is electrically connected to the electromagnetic detector 1 at the internal pixel node IPN and configured to amplify the intensity signal. The active thin-film charge sensor element 10 also comprises a reset transistor 3 that is electrically connected to the electromagnetic detector 1 at the internal pixel node IPN and configured to reset the internal charge sensor element node. At least one of the amplifier transistor 2 or reset transistor 3 comprises a front gate 21, 31 and a back gate 22, 32 wherein the front gates 21, 31 and back gates 22, 32 are configured to control the amplifier transistor 2 or reset transistor 3.

[0070] The 2T topology discussed above based on thin-film technology will reduce the active thin-film charge sensor element area 10 compared to single-gate implementations, lowering the charge sensor element pitch for a given performance and/or gain, and increasing the resolution of a fixed size image sensor (charge-based sensor). The use of dual-gate thin-film transistors will also increase the conversion gain for a given size of the charge sensor element 10 compared to thin-film single-gate transistors of a same size.

[0071] Further, the 2T topology illustrated in FIG. 1 allows for applying a select signal to the back gate 22 of the amplifier transistor 2 which means that—as compared to other topologies—an extra select transistor 4 or capacitance can be omitted, thus reducing the charge sensor element pitch of the charge-based sensor. In this case when the back gate 22 of the amplifier transistor 2 is connected to the select signal, the transconductance of the amplifier transistor 2 is reduced.

[0072] The charge collecting detector 1 may be connected to the front and/or the back gate 21, 22 of the amplifier transistor 2. The charge collecting detector 1 can also be any type of charge generating detector such as a visible light detector, an infrared detector, a pyroelectric detector, an ultraviolet detector, an X-ray detector, a piezoelectric detector or a charge detector.

[0073] The back gate 22 of the amplifier transistor 2 may be connected to the front gate 21 (not shown in FIG. 1) or it may be connected to the select signal line (as illustrated in FIG. 1). In case the back gate 22 is connected to the front gate 21 of the amplifier transistor 2, there may be a physical connection inside the charge sensor element 10, as for example illustrated in FIG. 5A.

[0074] FIG. 1 further illustrates an electrical connection between the back gate 32 of the reset transistor 3 and the front gate 31 of the reset transistor 3.

[0075] FIG. 2 illustrates the active thin-film charge sensor element 10 based on a voltage-mode 3T topology wherein back gates 22, 32 of the amplifier and the reset transistor 2, 3 are electrically connected to the front gates 21, 31 of the amplifier and reset transistor 2, 3. It can also be seen that a select transistor 4 may be electrically connected to the amplifier transistor 2, for example to a source or to a drain of the amplifier transistor 2, and in the illustrated example a front and back gate 41, 42 of the select transistor 4 are electrically connected.

[0076] The operation of the 2T and 3T voltage-mode topology, discussed above, is illustrated in FIG. 3. FIG. 3 illustrates that initially a reset signal is high for a duration of time t.sub.reset. During that reset time t.sub.reset, a select signal is low resulting in a voltage level at the internal charge sensor element node IPN (at the front gate 21 of the amplifier transistor 2) being reset.

[0077] During an integration time t.sub.int, both reset and select signals are low, and the detection of a physical quantity (e.g. electromagnetic radiation) by the charge collecting detector (e.g. an electromagnetic detector) 1 charges the charge sensor element capacitance C.sub.PIX, thereby generating an intensity signal (voltage signal) at the internal charge sensor element node. When the integration time t.sub.int is over, the select signal turns high and the intensity signal resulting from the detected physical quantity (e.g. electromagnetic radiation) by the charge collecting detector (e.g. electromagnetic detector) 1 is amplified by the amplifier transistor 2, thereby generating an output signal on the data line.

[0078] The operation is thus the same as today's voltage mode active charge sensor element sensors using transistors formed on a wafer substrate. It would therefore be possible to replace today's active charge sensor element sensors with a plurality of active thin-film charge sensor elements 10, since the drivers and/or logic do not need to be replaced but can be used with possibly minor adjustments.

[0079] The amplifier transistor 2 may be configured to be controlled by an adjustable voltage over time. By adjusting the voltage at the back gate 22 or at a source or a drain of the amplifier transistor 2, the amplifier transistor 2 can be controlled to amplify when desired. The adjustable voltage may originate from the select line. This allows for great flexibility and triggering of when the amplifier transistor 2 is to be conductive, i.e. when the amplifier transistor 2 is amplifying the intensity signal.

[0080] Additionally, a physical connection of the back gate 32, 42 of the reset transistor 3 or select transistor 4, or the back gate 22 of the amplifier transistor 2 can be connected externally to the individual active charge sensor elements 10, i.e. outside of the active charge sensor element 10, allowing for omitting a via that is normally needed to connect a back gate connection with a source-drain layer in other types of MOSFETs.

[0081] All effects and modes of operation that are discussed above for the 2T and 3T voltage mode topology also apply for the active thin-film charge sensor element 10 based on a current-mode 2T and 3T topology, illustrated in FIGS. 4A and 4B.

[0082] FIGS. 5A-5D further illustrate other 2T topologies based on the active thin-film charge sensor element 10. FIG. 5A illustrates a source follower topology (voltage mode topology), FIG. 5B illustrates a variation of this source follower topology, FIG. 5C illustrates a common source topology (current mode topology) and FIG. 5D illustrates a variation of this common source topology.

[0083] In the topologies illustrated in FIGS. 5B and 5D, an extra capacitance (C.sub.ST) is omitted compared to the topologies illustrated in FIGS. 5A and 5C. This presence of a capacitance (C.sub.ST) increases the size or area of the active charge sensor element 10 and reduces the sensitivity of the active charge sensor element 10. Thus, by omitting this capacitance (C.sub.ST), the performance of the charge sensor element 10 is increased, and higher resolution charge-based sensors can be achieved. Depending on the physical implementation of the charge collecting detector, e.g. electromagnetic detector 1, the amplifier and reset transistors 2, 3 are electrically connected to an anode or cathode side of the electromagnetic detector 1.

[0084] For the topologies illustrated in FIGS. 5A-5D, a mode of operation may be that at a beginning of a readout cycle, the front or back gate 21, 22 of the amplifier transistor 2 is reset to a voltage on the data line.

[0085] During the integration time, a small current from the electromagnetic detector 1 is collected on the internal node capacitance C.sub.PIX. In FIG. 5A, the voltage at the front and back gate 21, 22 will increase, whereas in FIG. 5C, the voltage will decrease. In either case, the voltage should stay below a threshold voltage (conduction limit) of the amplifier transistor 2 such that there is no amplification by the amplifier transistor 2, either as voltage or as current.

[0086] In the readout phase, a voltage and/or current pulse is applied on the select line. This will increase the voltage at the front and back gate 21, 22 of the amplifier transistor 2 due to the capacitive coupling caused by the capacitance C.sub.ST on the select line.

[0087] The voltage at the front and back gate 21, 22 of the amplifier transistor 2 should now be above the threshold voltage of the amplifier transistor 2, and the amplifier transistor 2 is now active and amplifies the intensity signal.

[0088] The active thin-film charge sensor element 10, the amplifier transistor 2, and/or the reset transistor 3 may be based on an etch-stop layer, back-channel etch, and/or self-aligned transistor architecture.

[0089] As discussed above, a charge-based sensor, e.g. an image sensor, may comprise a plurality of active thin-film charge sensor elements, e.g. active thin-film pixels 10. The charge-based sensor may comprise rows and/or columns of the active thin-film charge sensor elements 10.

[0090] The active thin-film charge sensor element 10 may also be used in a fingerprint sensor. An example of such a fingerprint sensor is illustrated in FIG. 6. The fingerprint sensor may be of an active thermal or passive thermal type. In a passive thermal type of fingerprint sensor, the active thin-film charge sensor element 10 is configured to detect changes in temperature caused by a presence of a finger 20 and generate charges. After a while, the temperature of the active thin-film charge sensor element 10 settles to the temperature of the finger, and the generation of charges stops.

[0091] In an active fingerprint sensor type, a small quantum of heat Q_is injected at the same location as the active thin-film charge sensor element 10, and the local thermal mass is observed. When a fingerprint ridge is present, a higher thermal mass is observed, and a lower temperature increase ΔT is observed for a given heat Q, which then generates charges.

[0092] FIG. 6 illustrates a pyroelectric fingerprint sensor of an active type comprising of an array of active thin-film charge sensor elements 10. A pyroelectric layer will generate charges ΔQ depending on the temperature change ΔT:

ΔQ=pAΔT

[0093] Wherein A is the area of the layer and p is a material dependent pyroelectric coefficient. An example of the pyroelectric material is a Poly-VinylDiFluoride-TriFluoroEthylene (PVDF-TrFE), which is sometimes abbreviated as PVDF.

[0094] The pyroelectric material, PVDF, combined with the electrodes can be considered analogous to the OPD in FIGS. 1-5D and thus will generate charges on the internal sensor element node IPN.

[0095] As discussed above and illustrated in FIG. 6, when the finger is placed above the heater, the finger will partially absorb heat from the heater. The remaining heat will spread to the PVDF material and result in a lower generation of charges compared to other areas of the PVDF material where the finger is not present. By dividing an electrode layer on one side of the PVDF material into fixed size isolated areas it is possible to generate a matrix or array of active thin-film charge sensor elements 10 for finger detection over a surface such as a mobile phone screen. Hence, similar to optical large-area optical imagers, the active thin-film charge sensor elements 10 can be arranged in a large array.

[0096] Illustrated in FIG. 7, the array of active thin-film charge sensor elements 10 is arranged in a front plane. The output of the front plane is a quantum of charge Q for each active thin-film charge sensor element 10, typically expressed in number of electrons. The collected charges are processed by a backplane, which consists of active thin-film charge sensor element 10 circuits and peripherals. A purpose of the backplane is to quantize the charges Q as accurate and fast as possible for the full array.

[0097] The charges may be further processed and possibly also similarly processed as discussed above in relation to optical imagers, i.e. the backplane is agnostic to how the charges are generated.

[0098] The front plane and backplane may be arranged as illustrated in FIG. 8. FIG. 8 illustrates a layer stack of the front plane and backplane of one active thin-film charge sensor element 10. This stack layer uses a soluble PVDF-based pyroelectric material as the active layer. In the active layer the temperature increase is translated into charges, which in turn creates a current that charges or discharges a node where the backplane and the bottom electrode are connected, depending on the polarity of the active thin-film charge sensor element 10.

[0099] The active thin-film charge sensor element 10 may also be used in a biochemical detector application. The biochemical detector may comprise a plurality of active thin-film charge sensor elements 10 arranged in an array or matrix configuration also known as an assay, illustrated in FIG. 9.

[0100] The active thin-film charge sensor elements 10 in the assay may be doped or prepared with different detection chemicals, which react to different analytes and then generates charges. This allows for a sample to be analyzed having different types of molecules or analytes, of which the presence is to be detected.

[0101] To ensure proper localization, the local reagent is fixed to the substrate in each sensor element (pixel), e.g. by chemically bonding to the surface there. There are several ways to measure whether a reaction has taken place at a specific site or pixel after the sample is applied to the assay. Most detection methods use some kind of marker which is attached to the molecules in the sample to indicate their presence. This marker can be a fluorescent complex or an electric charge. FIG. 10 illustrates a possible scenario at the surface of the at the active thin-film charge sensor element 10.

[0102] Two pixels are shown, one with reagent A, and the other with reagent B. The analyte bonds to reagent A only, bringing the marker close to the surface of the sensor.

[0103] After the sample is applied to the surface, the sample is cleaned and/or washed again, so that only the attached analytes remain at each pixel.

[0104] Depending on the type of the marker, different readout methodologies can be used. For a fluorescent marker, optical techniques can be used, like laser scanning, or an integrated optical sensor array, i.e. a large-area imager.

[0105] Illustrated in FIG. 11 is an example of a setup for an electrical readout when the marker is charged. A typical example of this concept is the ion-sensitive field-effect transistor (ISFET). An ISFET is typically used for measuring local pH, by checking the local charge caused by the acidity of the fluid. Here, this charge is used to bring a semiconductor in inversion or accumulation, thereby opening a FET channel.

[0106] Other types are e.g. the DNAFET, where DNA is matched with DNA strands fixed to the surface, and the intrinsic charge of the DNA is used to bias a gate of a nanowire transistor.

[0107] Another example based on the active thin-film charge sensor element 10 is a large-area platform comprising an array comprising a semiconductor material being a thin-film semiconductor. This would be another charge-based sensor, where it would be possible to measure charges on the internal node IND, but the internal node IND is a top surface of a DNA-covered surface. An example of this configuration is illustrated in FIGS. 12A and 12B.

[0108] FIG. 13 illustrates a method for controlling the active thin-film charge sensor element 10. The method comprises detecting 100 a predetermined physical quantity with a charge collecting detector 1 of the charge sensor element 10 and generating a corresponding intensity signal at an internal charge sensor element node IND. The method comprises amplifying 200 the intensity signal by an amplifier transistor 2 comprising at least one gate that is electrically connected to the charge collecting detector 1 at the internal charge sensor element node IND.

[0109] The method comprises resetting 300 the intensity signal by a reset transistor 3, wherein the reset transistor is electrically connected to the internal charge sensor element node IND, and wherein the amplifying 200 and/or resetting 300 is controlled by a front gate and a back gate of the amplifier transistor 2 or reset transistor 3.

[0110] A reset signal may be applied to a gate of the amplifier transistor 2 for resetting the intensity signal.

[0111] From the description above follows that, although various examples of the disclosure have been described and shown, the disclosure is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims. While some embodiments have been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative and not restrictive. Other variations to the disclosed embodiments can be understood and effected in practicing the claims, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures or features are recited in mutually different dependent claims does not indicate that a combination of these measures or features cannot be used. Any reference signs in the claims should not be construed as limiting the scope.