Photodetector, image sensor and photodetection method

Abstract

A photodetector which comprises: an electrode (1); a semiconductor layer (2) suitable for a photogeneration of charge carriers, the semiconductor layer (2) being connected to the electrode (1); a charge transfer layer (3) which comprises a two-dimensional semiconductor and is connected to the semiconductor layer (2) for transferring a first type of the charge carriers from the semiconductor layer (2) to the charge transfer layer (3); a substrate (4) supporting the charge transfer layer (3); wherein, the electrode (1), the semiconductor layer (2), the charge transfer layer (3) and the substrate (4) are arranged in the same order along a straight line (A); the charge transfer layer (3) is configured to transfer via the two-dimensional semiconductor the first type of the charge carriers to a charge extraction region (3b) of the charge transfer layer (3). Also, a photodetection method and an image sensor.

Description

[0001] PHOTODETECTOR, IMAGE SENSOR AND PHOTODETECTION METHOD

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a photodetector, an image sensor and a photodetection method, i.e. a method for detecting photons (electromagnetic radiation).

[0004] STATE OF THE ART

[0005] There are known photodetectors which are used for detecting photons i.e. for detecting electromagnetic radiation. Some photodetectors of technological importance are those which can detect visible electromagnetic radiation, as well as radiation of wavelengths on either side of the visible (VIS) spectrum, such as ultraviolet (UV) or infrared (IR) radiation. Also, a particularly important application of photodetectors is imaging. There are known image sensors, i.e. devices or systems which can image objects, wherein said image sensors comprise photodetectors or are considered as being photodetectors. US 2022 / 0102570 A1 describes a photodetector which comprises a measurement layer and at least a first photoactive layer which covers the measurement layer. US 2021 / 0359147 A1 describes an imaging device which comprises a stack of photoactive layers including a p-type photoactive layer and an n-type photoactive layer, the photoactive layers comprising quantum dots.

[0006] A technologically important type of known image sensors are the CMOS image sensors which are based on complementary metal-oxide-semiconductor (CMOS) technology. CMOS image sensors typically integrate, on a single chip, photodetectors and signal processing circuitry. A typical CMOS image sensor comprises a plurality of pixels connected to a circuitry, wherein each pixel comprises a photodiode which acts as a photodetector. CMOS image sensors have numerous applications, and for example can be found in many modern cameras, such as in cameras of mobile phones. Moreover, there are known hybrid CMOS sensors or photodetectors which combine a first part which comprises semiconductive polymers or organic or inorganic semiconducting nanomaterials, with a second part which comprises a CMOS substrate that integrates signal processing circuitry, wherein the first part serves for the photogeneration of an electrical signal, and the second part serves for the readout of the photogenerated signal and for electrically controlling the photodetector or sensor.

[0007] Many known types of conventional photodetectors and related image sensors suffer from an inefficient extraction of the photogenerated charge carriers from the photodetector’s photoactive material, i.e. from the material wherein photogeneration takes place. Said extraction is required so that the extracted charge carriers can be used to generate a detectable (readable) electrical signal. However, said extraction is often inefficient due to suboptimal electronic properties of the photodetector’s components and related materials used for extracting the photogenerated charge carriers. An inefficient and suboptimal extraction of the photogenerated charge carriers may limit considerably the external quantum efficiency (EQE) and, hence, the overall performance of the photodetector (or image sensor), particularly the latter’s sensitivity, responsivity and signal-to-noise ratio (SNR).

[0008] Moreover, known types of conventional photodetectors and related image sensors suffer from considerable leakage currents. The leakage current of a photodetector may contribute to the increase of the photodetector’s noise. If the photodetector is a photodiode, said leakage current is often called dark current. The leakage current may be particularly high in photodiodes which employ low bandgap semiconductors which exhibit a photo-response in the near (NIR), short wave (SWIR), mid wave (MWIR) of far (FIR) infrared.

[0009] Moreover, the dark or leakage current of a photodetector may be particularly high in the case that the photodetector comprises layers forming between them interfaces which exhibit a high number of structural defects. Said defects may act as electronic traps which can contribute to the photodetector having a substantial dark current. Likewise, photodetectors employing nanostructured materials, e.g. nanostructured semiconductors, may exhibit substantial leakage currents due to the possible presence of defects, e.g. dangling bonds or impurities, on the surface the nanostructured materials. Similarly, photodetectors which employ thin films may suffer from substantial leakage current associated with the possible defects at the interface(s) between the thin films. Generally, the leakage current is detrimental to the performance of the photodetector or image sensor.

[0010] Therefore, there is a need to improve the efficiency of the charge extraction, and to reduce the leakage / dark current and, hence, enhance the performance of photodetectors, image sensors and photodetection methods. In addition, there is a particular need for highly performing infrared and / or hybrid CMOS photodetectors and image sensors which may exhibit good charge extraction of photogenerated charge carriers, and a small leakage current.

[0011] DESCRIPTION OF THE INVENTION

[0012] The present invention overcomes many of the drawbacks of conventional photodetectors, image sensors and photodetection methods. In particular, the present invention allows for reducing the leakage current, for improving the extraction of photogenerated charge carrier, and for improving the performance of photodetectors and image sensors. At least some embodiments of the present invention advantageously allow for actively tuning, i.e. modulating, the extraction of the photogenerated carriers. Moreover, at least some embodiments of the present invention advantageously allow for mitigating the effect of the thermal noise of a photodetector or image sensor, for thereby improving the performance of said photodetector or image sensor. Moreover, the present invention advantageously allows for improving photodetectors or image sensors for IR detection or imaging, respectively. The present invention is particularly compatible with CMOS technology and allows for optimizing hybrid CMOS image sensors. Also, the present invention advantageously allows for improving the performance of photodetectors employing layers or thin films comprising nanostructured materials such as nanostructured semiconductors. Advantageously, the present invention is relatively simple and easy to implement.

[0013] A first aspect of the invention concerns a photodetector which comprises: an electrode, a semiconductor layer, a charge transfer layer, and a substrate. The electrode, the semiconductor layer, the charge transfer layer and the substrate are arranged in the same order along a straight line. The semiconductor layer is suitable for a photogeneration of charge carriers in the semiconductor layer. The semiconductor layer is connected to the electrode. The charge transfer layer comprises a two-dimensional semiconductor and is connected to the semiconductor layer for transferring a first type of the charge carriers from the semiconductor layer to the charge transfer layer. The first type of the charge carriers are holes or electrons. The substrate supports the charge transfer layer. Also, the charge transfer layer is configured to transfer via the two-dimensional semiconductor the first type of the charge carriers to a charge extraction region of the charge transfer layer. The charge extraction region is away from the straight line.

[0014] As described above, the charge transfer layer “is connected to the semiconductor layer for transferring a first type of the charge carriers from the semiconductor layer to the charge transfer layer”, and this can be understood as meaning that the charge transfer layer is connected to the semiconductor layer such that a first type of the charge carriers can be transferred (i.e. is transferable) from the semiconductor layer to the charge transfer layer. Hence, it may be understood that the charge transfer layer can receive the first type of the charge carriers.

[0015] Also, from the above it can be understood that the photodetector according to the first aspect of the invention employs a layered structure, i.e. a stack of layers, located on the photodetector’s substrate. One of said layers is the semiconductor layer, i.e. a layer employing one or more semiconductor materials in which photogeneration of charge carriers may take place due to the absorption of photons, i.e. due to the absorption of electromagnetic radiation, by the semiconductor material. Likewise, the photodetector may optionally comprise more than one semiconductor layers which may directly contact each other or may be separated by optional other layers located therebetween. In some non-limiting examples, the photodetector comprises two, three or more semiconductor layers of different n, i or p doping densities, said layers forming junctions such as a p-n junction, an n-p junction, a p-i-n junction or a different type of junction. Preferably, the one or more semiconductor materials used for the photodetector’s one or more semiconductor layers are selected such that their optoelectronic properties, e.g. their bandgap and their electronic doping, are suitable for optimizing the optical absorption of the electromagnetic radiation of interest by the semiconductor(s), and for optimizing the photocurrent that may be created due to the photogeneration in the photodetector’s one or more semiconductors.

[0016] A class of semiconductor materials the optoelectronic properties of which can be tailored and optimized for use in photodetectors, is semiconductor particles or nanoparticles, particularly semiconductor quantum dots. Hence, in a preferred embodiment of the invention, the semiconductor layer comprises semiconductor particles. More preferably said semiconductor particles are nanoparticles. Most preferably said nanoparticles are quantum dots. These materials are particularly suitable for use in a photodetector according to the invention, because their electronic and optical properties can be controlled in different ways, such as for example by controlling the stoichiometry and surface chemistry of the nanoparticles, by controlling the shape and size of the nanoparticles, by mixing different types of semiconductor nanoparticles or by mixing the semiconductor nanoparticles with other types of materials. In addition, these materials, or the layers comprising said materials, can be manufactured using a variety of different solution-based (wet processing) or physical deposition techniques which are suitable for the manufacturing of optoelectronics and photodetectors at an industrial scale. Colloidal semiconductor nanoparticles and quantum dots can be manufactured and processed into forming or being incorporated in thin films, using relatively low-cost and scalable wet chemistry and wet processing methods. In addition, there is a variety of different types of quantum dots (QDs) the optical properties of which can be tailored, e.g. by controlling the size of the QDs, for optimizing the quantum dots’ photo-absorption at specific wavelengths of interest in the UV, Vis and / or IR spectral range(s), for, thereby, optimizing the photocurrents that may result from the photogeneration in the QDs when the latter are used as a photo- responsive material in the photodetector according to the invention.

[0017] In a preferred embodiment of the invention, the semiconductor layer is suitable for absorbing infrared (IR) electromagnetic radiation for the photogeneration of the charge carriers. Some non-limiting examples of semiconductor materials which due to their bandgaps are suitable for absorbing IR radiation and for being used, particularly in the form of quantum dots, in the photodetector according to the present invention are PbS, PbSe, PbTe, InAs, InSb, Ag2Te, AgBiSe2, and others. Moreover, the semiconductor layer(s) of the photodetector may optionally comprise a bulk semiconductor material, or a semiconducting thin film, or one or more organic semiconductor(s).

[0018] The photons, i.e. the electromagnetic radiation, that is detected by the photodetector when the latter is in operation, may enter the photodetector and the latter’s semiconductor layer from different directions. However, in preferred embodiments said radiation impinges and enters the photodetector from the electrode or from the substrate, in which cases preferably said electrode or substrate, respectively, is at least partially transparent to said radiation for allowing the latter to reach the semiconductor layer. Therefore, in a preferred embodiment of the invention, the semiconductor layer is suitable for absorbing electromagnetic radiation for the photogeneration of the charge carriers, and the electrode is at least partially transparent to the electromagnetic radiation. In particularly preferred embodiments the electrode comprises or is made of a transparent conductive oxide. In some non-limiting examples, said transparent conductive oxide is indium tin oxide (ITO), fluorine doped tin oxide (FTO), doped zinc oxide or another oxide. In other embodiments, said electrode is made of other relatively transparent and conductive materials, such as graphene, or metal nanowires or a different type of conductive and at least partially transparent material. However, as mentioned the photodetector may be configured so that the electromagnetic radiation to be detected impinges and enters the structure of the photodetector from the substrate, and for this reason the substrate may be made or comprise a material, such as a semiconductor, glass or polymer, that is at least partially transparent to said electromagnetic radiation.

[0019] As mentioned further above, it can be understood that in preferred embodiments of the invention the photodetector has a layered structure, meaning that it comprises different layers arranged or stacked along the straight line which is a geometrical line that is considered for defining the arrangement and position of several of the photodetector’s components. In a preferred embodiment of the photodetector according to the invention, the electrode, the substrate, the semiconductor layer and the charge extraction layer are substantially planar in shape, with their respective planes being substantially normal to said geometrical straight line. However, there is also contemplated the possibility that the photodetector is flexible and bendable, in which case, although the aforementioned components of the photodetector may be planar, when said components are bended, parts of them, e.g the parts which are away from the geometrical straight line, may not be normal to said geometrical line.

[0020] The photogeneration of charge carriers may occur where the absorption of the electromagnetic radiation takes place in the semiconductor layer. The region of the semiconductor layer where photogeneration may take place may be called “photogeneration region”. Said photogeneration region may be at or around the geometrical straight line. When the photodetection is in operation, the two types of the photogenerated carriers, i.e. the photogenerated electrons and holes, in the semiconductor material may travel (i.e. be transferred), starting from the photogeneration region, towards different directions, with a first type of the carriers travelling via the charge transfer layer towards the latter’s charge extraction region, and the second type of the charge carriers travelling towards the electrode. The transferring of the charge carriers towards the electrode and the charge transfer layer may happen along, or substantially parallel to, the geometrical straight line. Considering the above and that the charge extraction region is away from the straight line, it can be understood that when the first type of the photogenerated charge carriers reach the charge transfer layer, the latter may serve for transferring the first type of the charge carriers away from the geometrical line and further away from the photogeneration region.

[0021] The transfer of the charge carriers from the photogeneration region to the electrode or to the charge transfer layer, may be facilitated and promoted by the optional presence of one or more transfer layers located between the semiconductor layer and the electrode and / or charge transfer layer, respectively. The use of said optional transfer layers may result to increasing the photodetector’s photocurrent and / or photovoltage. For this purpose, each of said transfer layer’s electronic properties may be suitable for optimizing the separation of the photogenerated charge carriers, and / or for allowing the efficient transfer of the photogenerated charge carriers away from the semiconductor layer. Said electronic properties may be the transfer layer’s electronic mobility values, and the position of the electronic levels or bands (e.g. valence and conduction band) of said optional transfer layer with respect to the electronic levels or bands of the one or more materials of the at least one semiconductor layer. Hence, in a preferred embodiment of the photodetector according to the first aspect of the invention, the photodetector further comprises a first transfer layer which along the straight line is arranged between the semiconductor layer and the charge transfer layer and is suitable for transferring the first type of charge carriers from the semiconductor layer towards the charge transfer layer. Preferably the first transfer layer is further suitable to hinder a transfer of a second type of charge carriers from the semiconductor layer to the charge transfer layer. Hence, it may be understood that the aforementioned first transfer layer is configured to transfer the first type of charge carriers from the semiconductor layer towards the charge transfer layer, and preferably to hinder or block a transfer of a second type of charge carriers from the semiconductor layer to the charge transfer layer. Therefore, in the optional case that the second type of charge carriers are electrons, the first transfer layer may act as a, and respectively be called, “hole transport layer” or “electron blocking layer”, and in the other optional case that the second type of charge carriers are holes, the first transfer layer may act as an, and respectively be called, “electron transport layer” or “hole blocking layer”. Also, in a preferred embodiment of the photodetector according to the first aspect of the invention, the photodetector further comprises a second transfer layer which along the straight line is arranged between the electrode and the semiconductor layer and is suitable for transferring a second type of the charge carriers from the semiconductor layer towards the electrode, wherein the second type of the charge carriers is holes when the first type of charge carriers is electrons, or the second type of charge carriers is electrons when the first type of charge carrier is holes. Preferably said second transfer layer is further suitable to hinder a transfer of the first type of charge carriers from the semiconductor layer to the electrode. Hence, it may be understood that the aforementioned second transfer layer is configured to transfer the second type of charge carriers from the semiconductor layer towards the electrode, and preferably to hinder or block a transfer of the first type of charge carriers from the semiconductor layer to the electrode. Therefore, in the optional case that the first type of charge carriers are electrons, the second transfer layer may act as, and respectively be called, “a hole transport layer” or “an electron blocking layer”, and in the other optional case that the first type of charge carriers are holes, the first transfer layer may act as an, and respectively be called, “electron transport layer” or “hole blocking layer”. The hindering of the transfer of the first or second type of charge carriers via the second or first transfer layer, respectively, may advantageously contribute towards efficiently separating the photogenerated holes from the photogenerated electrons.

[0022] The charge carriers reaching the electrode and the charge extraction region may be extracted therefrom, and for this reason preferably the electrode and the charge extraction region may be respectively connected to opposite sides of an external circuit used for the read out of the photocurrent, i.e. the electrical current that corresponds to the photogenerated and extracted charge carriers. Accordingly, in a preferred embodiment of the invention, the photodetector further comprises a read-out circuit electrically connected to the charge extraction region. More preferably the read-out circuit is embedded in the substrate. Having the read-out circuit embedded and, hence, monolithically integrated in the substrate facilitates and simplifies the possible integration and use of the photodetector in a larger system or device, such as in a camera, in a mobile device or in a radiation measurement apparatus. A particular type of technology that may be used for the manufacturing of the photodetector of the first aspect of the invention, and particularly for implementing the optional feature of the photodetector comprising a read-out circuit that is a monolithically integrated circuit, i.e. a circuit integrated in the substrate of the photodetector, is CMOS technology. Hence, in a preferred embodiment the substrate is a CMOS substrate e.g. a silicon substrate compatible and suitable for CMOS fabrication processes. In a preferred embodiment according to the first aspect of the invention, the photodetector further comprises a floating diffusion, and the charge extraction region is on, or is connected to, the floating diffusion. Said floating diffusion may also be called sensing node. The use of the floating diffusion as an optional part of a photodetector or of an image sensor according to the present invention, may advantageously facilitate converting the charge accumulated from the photogeneration in the photodetector or in each pixel of the image sensor, into a voltage signal. Overall, the use of the floating diffusion may advantageously offer conversion of charge to voltage, amplification of weak signals, noise reduction, and pixel reset functionality. These benefits may advantageously further contribute to the accurate and efficient capture of light in a photodetector or image sensor according to the invention.

[0023] In a preferred embodiment herein referred to as “first exemplary embodiment”, the substrate comprises a transfer gate region that is adjacent to the charge transfer layer and is arranged such that when the first type of the charge carriers are transferred via the charge transfer layer towards the charge extraction region, they pass over the transfer gate region of the substrate before they reach the charge extraction region; the substrate comprises embedded at said transfer gate region a first dielectric in contact with the charge transfer layer, and a transfer gate electrode separated from the charge transfer layer via the first dielectric; and the transfer gate region and the transfer gate electrode are away from the straight line. The combination of the transfer gate electrode and the first dielectric can be referred to (i.e. be called), or can be considered as being, a “transfer gate” which may advantageously serve for controlling the flow of the charge carriers through the charge transfer layer. Hence, it may be understood that the “transfer gate region” refers to a region, i.e. an area or part of the substrate, wherein at said area or part (i.e. region) of the substrate there are embedded the transfer gate electrode and the first dielectric. Due to the fact that in the photodetector of the first aspect of the invention, the first type of the photogenerated charge carriers are transferable via the charge transfer layer, advantageously, in said first exemplary embodiment that comprises the transfer gate (i.e. the transfer gate electrode and the first dielectric), said transfer gate may advantageously enable controlling the flow of the charge carriers of the first type via the charge transfer layer. Hence, the photodetector’s transfer gate can be used to control or block the charge flow to the floating diffusion. Said control can be achieved by controllably applying a bias voltage to the transfer gate electrode (i.e. the electrode of the transfer gate) for, thereby, creating and adjusting a potential barrier to the charge carriers at the part of the charge extraction layer over the transfer gate region. For this reason, in the optional case that the photodetector comprises said transfer gate electrode, the latter may preferably be connected to an optional biasing circuit or system which is configured to controllably apply a bias voltage to the transfer gate electrode, and said biasing circuit may be an external circuit or a part of the photodetector. In a very preferred embodiment said biasing circuit is integrated in the substrate. Said biasing circuit (biasing circuitry) may also be called transfer gate control circuit or circuitry.

[0024] From the above it can be understood that the transfer gate electrode and the first dielectric are optional components which may be used for enabling controlling the photocurrent flow. However, said control may be advantageously further promoted and optimized by the optional presence of a second dielectric. Hence, in a preferred embodiment which is according to the first exemplary embodiment, the photodetector further comprises a second dielectric which is over the transfer gate region of the substrate and in contact with the charge transfer layer opposite the first dielectric, said second dielectric extending over the charge extraction region. More preferably said second dielectric is configured to prevent a direct contact between the semiconductor layer and the charge transfer layer over the charge extraction region. In the latter embodiment, the use of the second dielectric over the substrate’s transfer gate region and in contact with the charge transfer layer opposite the first dielectric, can advantageously prevent the semiconductor layer from directly contacting, over the substrate’s transfer gate region, the charge transfer layer. Likewise, at least in some embodiments wherein said second dielectric is part of the photodetector and extends over the charge extraction region, the second dielectric may advantageously also prevent the semiconductor layer from directly contacting, over the charge extraction region, the charge transfer layer. Hence, in a preferred embodiment of the invention, the photodetector comprises said second dielectric, and the latter is configured to prevent a direct contact between the semiconductor layer and the charge extraction region. In the latter case, the second dielectric by preventing the semiconducting layer from directly contacting the charge extracting region, may advantageously allow for reducing the photodetector’s leakage current, and for optimizing the functionality of the photodetector’s transfer gate. From the above, it may be understood that in in a preferred embodiment which is according to the aforementioned first exemplary embodiment and also comprises the aforementioned second dielectric layer, the semiconductor layer extends over the transfer gate region, the second dielectric layer and the charge extraction region. Hence, in the latter case advantageously for the fabrication of said preferred embodiment it is not necessary to pattern the semiconductor layer for avoiding the latter’s deposition or presence over said transfer gate region, second dielectric and charge extraction region. This advantageously simplifies the design and fabrication of the photodetector. It may also be understood that in the photodetector of the first aspect of the invention, optionally and preferably the semiconductor layer extends over the charge extraction region. Nevertheless, it is noted that optionally the semiconductor layer may be patterned. Advantageously, in the optional case that photodetector comprises said transfer gate and the second dielectric, the combination of the transfer gate and second dielectric can allow for significantly reducing the readout noise, for the following reason. The charge transfer layer’s part between the first and the second dielectric can be considered as being a channel through which the photogenerated carriers of the first type can flow. Therefore, an electrical voltage can be applied to the transfer gate electrode for closing said channel. The presence of the second dielectric allows for closing said channel more effectively. When the channel is closed, it can be considered that the photoactive region of the photodetector that is one side of said channel is effectively electrically isolated / disconnected from the charge extraction region that is on the other side of said channel. Hence, when the channel is closed, the capacitance of the photodetector’s photoactive region may not contribute to the capacitance that can be felt by a readout circuit that preferably is connected directly or indirectly to said charge extraction region. However, the capacitance felt by a readout circuit connected at the charge extraction region affects the readout noise which typically is reduced when said capacitance is also reduced. Hence, the transfer gate electrode, the first dielectric and the second dielectric can enable, i.e. can be used for, reducing by a significance amount the capacitance felt at the charge extraction region, and therefore, can also enable reducing the readout noise.

[0025] Considering the above, in a preferred embodiment which is according to the first exemplary embodiment, the photodetector further comprises a circuitry that is connected to the charge extraction region and comprises a transfer gate control circuit which is configured to set and / or maintain the transfer gate electrode to a first state that permits the flow of the charge carriers of the first type via the charge transfer layer, and to a second state that impedes, and preferably blocks, the flow of the charge carriers of the first type via the charge transfer layer. Said circuitry may also be called “control circuitry” or “control circuit”. More preferably, said circuitry (control circuitry) comprises a reset circuit that is connected to the charge extraction region and is configured to set or reset and / or maintain said charge extraction region to a specific state which may also be called “reset state” or “baseline state”, and is also configured to stop maintaining the charge extraction region to said reset state. Further preferably, said circuitry comprises or is connected to the readout circuit that is also connected to the charge extraction region and is configured to measure a first signal, which may also be called “reference signal”, and a second signal which may called “photo-signal” and is produced by the photogenerated carriers. Even more preferably, the transfer gate control circuit, the reset circuit and the readout circuit are configured to operate in synchronization to each other, and for this reason the circuitry may further comprise additional control electronics which are configured to control or synchronize the operation of said transfer gate control circuit, reset circuit and readout circuit. The aforementioned circuitry may enable mitigating the effect of the thermal noise component of the overall measured noise as follows. In a preferred embodiment which is according to the first exemplary embodiment and further comprises a control circuitry, the latter is configured to perform, preferably in the same order, the following operations: i) Set and / or maintain the charge extraction region to a reset (baseline) state; ii) set and maintain the transfer gate electrode to a first state for permitting the flow of the charge carriers of the first type via the charge transfer layer. iii) set and maintain, for a first period of time, the transfer gate electrode to a second state for impeding or blocking the flow of the charge carriers of the first type via the charge transfer layer; iv) measure a first signal (reference signal) at the charge extraction region; v) stop maintaining the charge extraction region to the reset state; vi) set and maintain the transfer gate electrode to the first state for permitting the flow of the charge carriers of the first type via the charge transfer layer (i.e. via the channel between the first and second dielectric); vii) set and maintain for a second period of time the transfer gate electrode to the second state; viii) within said second period of time, measure a second signal (photo-signal) at the charge extraction region;

[0026] Preferably, the first signal is measured within the first period of time. More preferably, the first signal is measured within, and towards the end of, the first period of time. Also, preferably the second period of time is much smaller compared to the first period of time. Also, very preferably, the first and second signal measurements last the same with respect to each other.

[0027] The first period of time in step iii) may also be called “photon integration time” because when the photodetector is in operation and receives electromagnetic radiation that causes the photogeneration of charge carriers, the carriers being photogenerated during said photon integration time can be accumulated and contribute to the second signal that is measured in step viii). Advantageously, subtracting the first signal (the baseline signal) from the second signal (the photo-signal) enables approximately subtracting the thermal noise from the measured photo-signal, because the first and the second signal are closely corelated in time, and because, in between measuring the first and the second signal, there is no reset operation being applied to reset the charge extraction region to the reset state. Hence, the generation of Johnson noise associated with a possible reset operation between measuring the first and the second signal is avoided. It should be mentioned, that steps i), iv), v) and viii) which are described above with respect to the charge extraction region, may instead be performed with respect to a different optional part that may be electrically connected to said charge extraction, as happens in some embodiments of the present invention. For example, said steps i), iv), v) and viii) may be performed with respect to an floating diffusion that is an optional component found in some preferred embodiments of a photodetector or of an image sensor according to the invention, in which case the term “charge extraction region” mentioned above in steps i), iv), v) and viii) can be replaced by the term “floating diffusion”.

[0028] In the photodetector according to the first aspect of the invention the charge transfer layer comprises a two-dimensional semiconductor. In a preferred embodiment the charge transfer layer is the two-dimensional semiconductor. Preferably said two-dimensional (2D) semiconductor is a two-dimensional transition metal dichalcogenide. The use of a two- dimensional semiconductor material as at least a part of the charge transfer layer can advantageously allow for fabricating a charge transfer layer which is substantially thin and at the same time has good electronic properties, e.g. has good charge carrier mobilities, for allowing the efficient and fast transfer of the first type of charge carriers to the charge collection region. Hence, the use of a two-dimensional semiconductor may advantageously allow for optimizing the photodetector’s signal e.g. the photodetector’s photocurrent. Furthermore, a charge transfer layer which comprises a 2D semiconductor layer may reduce the leakage / dark current due to its low volume and hence low amount of (bulk) trap states. Moreover, in some embodiments where the photodetector comprises a transfer gate as mentioned further above, the charge transfer layer which comprises a 2D semiconductor may advantageously be substantially thin so that the flow of the charge carriers of the first type via the charge transfer layer can be more efficiently and accurately controlled with the use said transfer gate. Also, Moreover, in some embodiments wherein the photodetector comprises a modulation gate as described further below, the charge transfer layer which comprises a 2D semiconductor may advantageously be substantially thin so that the Fermi level of the 2D semiconductor can be more efficiently and accurately controlled with the use said modulation gate. For this reason, in a preferred embodiment the charge transfer layer is made of a two-dimensional transition metal dichalcogenide, because there are two-dimensional transition metal dichalcogenide materials from which it is possible to make thin layers which have a thickness of few atoms or one or more monolayers, and which exhibit good electronic mobilities and can be gated such that their Fermi level is controlled and / or their conductivities can be switched on and off. Also, in a preferred embodiment the two-dimensional semiconductor is a two-dimensional transition metal dichalcogenide. Some non-limiting examples of said dichalcogenide materials are M0S2, MoSe2, MoTe2. WS2, WSe2 or other. In a preferred embodiment according to the first aspect of the invention, the photodetector is an image sensor comprising a plurality of pixels on the substrate, preferably the image sensor being an active pixel image sensor. When the photodetector according to the invention is an active pixel image sensor (APS) it may advantageously offer low noise, power efficiency, high sensitivity, enhanced low-light performance, fast read-out and high dynamic range.

[0029] In a preferred embodiment according to the first aspect of the invention, the photodetector further comprises a modulation gate arranged, along the straight line, in between the substrate and the charge transfer layer or at least partially within the substrate, wherein, the modulation gate comprises a modulation gate dielectric and a modulation gate electrode which is separated from the charge transfer layer by the modulate gate dielectric which is adjacent to the charge transfer layer, and wherein the modulation gate is configured to modulate a Fermi level of the charge transfer layer when an electrical voltage is applied to the modulation gate electrode. Advantageously, said modulation gate may allow for actively tuning, i.e. modulating, the extraction of the photogenerated carriers and, hence, for tuning the photo-response and the sensitivity of the photodetector.

[0030] A second aspect of the invention concerns an image sensor comprising a photodetector which is according to the first aspect of the invention. Advantageously, an image sensor according to the invention can be used as part of a variety of different imaging devices of technological and commercial importance, including digital cameras, smartphones, and systems or devices for scientific imaging, surveillance, medical imaging, or another purpose.

[0031] A third aspect of the invention concerns a photodetection method using a photodetector which is according to the first aspect of the invention, the method comprising: photo-generating charge carriers in the semiconductor layer of the photodetector; transferring a first type of the charge carriers from the semiconductor layer to the charge transfer layer of the photodetector, the first type of the charge carriers being holes or electrons; transferring, via the two- dimensional semiconductor, the first type of the charge carriers to a charge extraction region of the charge transfer layer.

[0032] Another aspect of the invention concerns an imaging method using an image sensor which is according to the third aspect of the invention, said imaging method being similar to the aforementioned photodetection method. Hence, a preferred embodiment of an imaging method according to the invention, using an image sensor which is according to the second aspect of the invention, comprises the following steps: photo-generating charge carriers in the semiconductor layer of the image sensor; transferring a first type of the charge carriers from the semiconductor layer to the charge transfer layer of the image sensor, the first type of the charge carriers being holes or electrons; transferring, via the two-dimensional semiconductor, the first type of the charge carriers to a charge extraction region of the charge transfer layer.

[0033] Additional advantages and features of the invention will become apparent from the detailed description that follows and will be particularly pointed out in the appended claims.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as examples of how the invention can be carried out. The drawings comprise the following figures:

[0036] Fig. 1 A illustrates a preferred embodiment of a photodetector according to the invention.

[0037] Fig. 1 B illustrates a preferred embodiment of a photodetector according to the invention.

[0038] Fig. 1C illustrates a preferred embodiment of a photodetector according to the invention.

[0039] Fig.2 illustrates a preferred embodiment of a photodetector according to the invention.

[0040] Fig. 3 illustrates a preferred embodiment of an image sensor according to the invention.

[0041] Fig. 4 illustrates a preferred embodiment of an image sensor according to the invention.

[0042] Fig. 5 illustrates a flow diagram of a preferred embodiment of a photodetection method according to the invention.

[0043] Fig. 6 illustrates a flow diagram of a preferred embodiment of a photodetection method according to the invention.

[0044] Fig. 7 illustrates a diagram of a control circuit of a preferred embodiment of an image sensor according to the invention.

[0045] Fig. 8 illustrates an operation performed with the control circuit of Fig. 7.

[0046] DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Next embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings, showing apparatuses and methods according to the invention.

[0047] Embodiments of a photodetector according to the invention, are explained next with reference to Fig. 1A, 1 B, 1C and 2. Fig. 1A illustrates a cross section of a preferred embodiment of a photodetector according to the first aspect of the invention. As shown in Fig. 1A, the photodetector along a straight line A, which is the dashed geometric line in Fig. 1A, comprises arranged in the same order: an electrode 1 ; a semiconductor layer 2 suitable for a photogeneration of charge carriers in the semiconductor layer 2, the semiconductor layer 2 being connected to the electrode 1 ; a charge transfer layer 3 which comprises a two- dimensional semiconductor and is connected to the semiconductor layer 2 for transferring a first type of the charge carriers from the semiconductor layer to the charge transfer layer 3, the first type of the charge carriers being holes or electrons; a substrate 4 supporting the charge transfer layer 3; wherein, the charge transfer layer 3 is configured to transfer via the two- dimensional semiconductor the first type of the charge carriers to a charge extraction region 3b of the charge transfer layer 3, the charge extraction region 3b being away from the straight line A.

[0048] The embodiment of Fig. 1 B is similar to the embodiment of Fig. 1A and further comprises the following features which are described with reference to Fig. 1 B. In the embodiment of Fig. 1 B the substrate 4 comprises a transfer gate region 4a that is adjacent to the charge transfer layer 3. Said transfer gate region 4a is arranged such that when the first type of the charge carriers are transferred via the charge transfer layer 3 towards the charge extraction region 3b, they pass over the transfer gate region 4a before they reach the charge extraction region 3b. Hence, the charge transfer layer 3 is configured to transfer, via the same and over the transfer gate region 4a of the substrate 4, the first type of the charge carriers to the charge extraction region 3b. The substrate 4 comprises embedded at said transfer gate region 4a a first dielectric 5 in contact with the charge transfer layer 3, and a transfer gate electrode 6 separated from the charge transfer layer 3 by (via) the first dielectric 5. As illustrated in Fig. 1 B, the charge extraction region 3b, the transfer gate region 4a and the transfer gate electrode 6 are away from the straight line A.

[0049] The embodiment shown in Fig. 1C is similar to the embodiment of Fig. 1 B and further comprises the following features which are described with reference to Fig. 1C. The photodetector of Fig. 1C further comprises a second dielectric 7 over the transfer gate region 4a of the substrate and in contact with the charge transfer layer 3 opposite the first dielectric 5, the second dielectric 7 extending over the charge extraction region 3b. Also, the photodetector of Fig. 1C further comprises a modulation gate 40 arranged along the straight line A in between the substrate 4 and the charge transfer layer. The modulation gate 40 shown in Fig. 1C is (i.e. is embedded) within the substrate, and comprises a modulation gate dielectric 41 and a modulation gate electrode 42 which is separated from charge transfer layer 3 by (i.e. via) the modulation gate dielectric 41 which is adjacent to the charge transfer layer 3. Said modulation gate 40 is configured to modulate a Fermi level of the charge transfer layer 3 when an electrical voltage is applied to the modulation gate electrode 42. Hence, in a preferred embodiment which is similar to the embodiment of Fig. 1 C and comprises said modulation gate 40, the photodetector further comprises a modulation gate control circuitry which is connected to the modulation gate electrode 42 and is configured to controllably apply an electrical voltage to said modulation gate electrode 42.

[0050] From Fig. 1A-1C it can be understood that each of the electrode 1 , the semiconductor layer 2, the charge transfer layer 3 and the substrate 4, is substantially planar and normal to the straight line A which crosses the photodetector and extends from a first side 21 of the photodetector to a second side 22 of the photodetector, wherein the first and the second sides are opposite to each other, and wherein the substrate 4 is (i.e. is adjacent to) at the first side 21 , and the electrode 1 is at the second side 22, as shown in Fig. 1C. Said first and second sides, and the corresponding surfaces of each one of the photodetectors of Fig. 1A-1C, may respectively be called back and front sides / surfaces respectively, i.e. the front side 22 is at the electrode 1 and the back side 21 is at the substrate 4. In the embodiment of Fig. 1C the electromagnetic radiation (hv) impinges and enters the photodetector from the front side, i.e. via the electrode 1 , as shown by the thick arrows in Fig. 1 C. For this reason, the electrode 1 of the embodiment of Fig. 1C is transparent to said radiation. Alternatively, the electrode 1 may be partially transparent to said radiation. In the embodiments of Fig. 1A-1C, the semiconductor layer’s region about the line A is a photo-absorption region i.e. a region where absorption of photons and, hence, generation of charge carriers can take place.

[0051] In an exemplary embodiment which is similar to the embodiment shown in Fig. 1C, the semiconductor layer 2 is made of semiconductor nanoparticles, particularly of semiconductor quantum dots (QDs) which are suitable for absorbing infrared electromagnetic radiation for the photogeneration of the charge carriers. Also, in said exemplary embodiment, the substrate 4 is a silicon substrate that is suitable for CMOS processing, and the top electrode 1 is made of a transparent conductive oxide (TCO) that allows the electromagnetic radiation, which is to be detected, to enter the photodetector’s layered structure from the electrode. For this reason, said TCO electrode is at least partially transparent to the IR radiation which can be absorbed by the QDs of the photodetector. Moreover, in said exemplary embodiment the charge transfer layer 3 is made of a 2D transition metal dichalcogenide. In some non-limiting examples, said charge transfer layer is made of one, two or more stacked monolayers of any of M0S2, MoSe2, MoTe2, WS2, WSe2, WTe2. It is noted that optionally said 2D semiconductor or said 2D transition metal dichalcogenide may not be stoichiometric, and / or may be doped.

[0052] Each one of the aforementioned transfer gate electrode and modulation gate electrode may be made of a suitable material, and for example be made of any of TiN, aluminium, copper, tungsten or other suitable material or a combination thereof. Also, each one of the aforementioned first dielectric, second dielectric and modulation gate dielectric may be made of a suitable material, and for example be made AIOx, SiO2, HfO2, hBN, CaF2 or other suitable material or a combination thereof.

[0053] Fig. 2 illustrates a cross section of a preferred embodiment of a photodetector according to the first aspect of the invention. The embodiment of Fig. 2 is similar to the embodiment of Fig. 1 B, but also comprises additional features. Notably, the photodetector of Fig. 2 further comprises a first transfer layer 9 which along the line A is arranged between the semiconductor layer 2 and the charge transfer layer 3 and is suitable for transferring the first type of charge carriers from the semiconductor layer 2 towards the charge transfer layer 3. Said first transfer layer 9 shown in Fig. 2 is further suitable to hinder a transfer of a second type of charge carriers from the semiconductor layer 2 to the charge transfer layer 3. In some non-limiting examples, the first transfer layer is made of or comprises any of SnC>2, ZnO, AZO (aluminium doped zinc oxide), PbS, InAs, InSb, PbSe, Ag2Te, AgBiSe2, AgBiS2 or other material a combination thereof. Also, the photodetector of Fig. 2 further comprises a read-out circuit 10 electrically connected to the charge extraction region 3b. Said read-out circuit may preferably be embedded in the substrate, and further preferably may comprise a floating diffusion region in, i.e. embedded in, the substrate and in contact with the charge extraction region 3b of the charge transfer layer.

[0054] In addition, the photodetector of Fig. 2 further comprises a second transfer layer 8 which along the straight line A is arranged between the electrode 1 and the semiconductor layer 2 and is suitable for transferring a second type of the charge carriers from the semiconductor layer 2 towards the electrode 1. The second type of the charge carriers is holes when the first type of charge carriers is electrons, and the second type of charge carriers is electrons when the first type of charge carrier is holes. The second transfer layer 8 of the embodiment illustrated in Fig. 2 is further suitable to hinder a transfer of the first type of charge carriers from the semiconductor layer 2 to the electrode 1. In some non-limiting examples, the second transfer layer is made of or comprises any of SnC>2, ZnO, AZO, PbS, InAs, InSb, PbSe, AgTe, AgBiS2 or other material or a combination thereof. Fig. 1C may also be considered as illustrating a cross section of a respective pixel of a preferred embodiment of an image sensor according to the invention. Said preferred embodiment of an image sensor is further illustrated by Fig. 3 and 4, and its operation is explained below with reference to Fig. 3 and Fig. 4. The image sensor comprises an array of pixels 31 on the substrate 4, and each pixel’s region at and around its corresponding straight line A is a photogeneration region 32 where photogeneration and separation of charge carriers takes place. It is noted that as shown in Fig. 3 and 4, the straight lines A are substantially normal to the planar substrate 4 and to the back and front surfaces / sides 21 , 22 of the image sensor. The photogeneration happens due to absorption of electromagnetic radiation (photons) by the semiconductor layer 2, said radiation being symbolized as “hv” in Fig. 1C and Fig. 4. The electromagnetic radiation enters the image sensor via the front side 22 and the electrode 1 which is located and forms said first side 22. In other embodiments other optional components, e.g. encapsulation layers, color filters, or others, may be located and form the front side / surface 22. In the embodiment of Fig. 3 and 4, the electrode 1 is electrically connected to a respective metallic contact 11. The straight line A shown for each pixel in Fig. 4, is at approximately the center of the photogeneration region 32 of each pixel. The second type of the photogenerated carriers are transferred from the semiconductor layer 2 to the electrode 1 , and the first type of the photogenerated layers are transferred to the charge extraction layer 3. Then, the charge transfer layer can transfer the carriers laterally towards the charge extraction region, which is in contact to and over, i.e. is adjacent to, a floating diffusion 12 of the substrate. The floating diffusion 12, which may also called “sense node”, is electrically connected to a read-out circuit 10. The first type of charge carriers, as they travel through the charge transfer layer 3, and before they reach the flowing diffusion 12, they pass over the transfer gate, and hence between the first dielectric 5 of the transfer gate, and the second dielectric 7. The first and the second dielectrics 5, 7 are at opposite sides / surfaces of the charge transfer layer 3. The transfer gate allows for controlling the flow of the charge carriers through the charge transfer layer, and hence, allows for controlling the photodetector’s measured electrical photo-response (signal). This can be done by controllably applying a bias voltage to the transfer gate’s electrode 6 which for this reason is electrically accessible e.g. is connected to a respective circuit (e.g. a transfer gate control circuitry) which is configured for controllably applying bias voltages to the transfer gates of the pixels. The second dielectric 7 shown in Fig. 3, allows for preventing the formation of a direct electrical contact and related leakage current between the semiconductor layer and the charge extraction region, and more generally, the second dielectric 7 allows for optimizing the function of the transfer gate for controlling the electrical response of the photodetector.

[0055] Embodiments of a method according to the invention, are explained next with reference to Fig. 5 and 6. The embodiment of the method shown in Fig. 5 is realized using the photodetector of Fig. 1A, and comprises the following steps: in step 101 , charge carriers are photogenerated in the semiconductor layer 2 of the photodetector due to the absorption of electromagnetic radiation (photons) by the semiconductor layer; in step 102, a first type of the charge carriers are transferred from the semiconductor layer 2 to the charge transfer layer 3 of the photodetector, the first type of the charge carriers being holes or electrons; in step 103, the first type of the charge carriers are transferred, via the two-dimensional semiconductor of the charge transfer layer 3, to a charge extraction region 3b of the charge transfer layer 3.

[0056] The embodiment of the method shown in Fig. 6 may be implemented using the photodetector of Fig. 1C, and comprises the steps shown in Fig. 5 but additionally comprises step 101a in which the modulation gate is used to control the Fermi level of the charge transfer layer. For this purpose, in step 101a a bias voltage is applied to the modulation gate electrode 42 for thereby controlling the Fermi level of the charge transfer layer 3. Also, the embodiment of Fig.

[0057] 6 further comprises step 103a in which the transfer gate electrode 6 of the photodetector is used to control, over the gate transfer region, the transferring of the first type of the charge carriers to the charge extraction region 3b. For this purpose, a bias voltage is applied to the transfer gate electrode 6, to induce the creation of a potential barrier in the part of charge transfer layer which is adjacent to the first dielectric and, hence, over the transfer gate region. This potential barrier can affect and control the flow of the charge carriers via the charge transfer layer.

[0058] A preferred embodiment of an image sensor according to the second aspect of the invention, comprises a transfer gate as the one shown in Fig. 1 C, and also comprises a control circuitry connected to each pixel of the sensor, as illustrated in Fig. 7. Each pixel, which may also be called “sensor node” (SN), of the embodiment of Fig. 7 comprises a photodiode PD, a transfer gate TG that is configured to control the flow of charge between the photodiode and a floating diffusion of the pixel, and a reset transistor RST via which a reset signal VRST can be applied to (re)set the pixel’s floating diffusion to a reset state which can also be called baseline or ground state. The photodiode PD shown in Fig. 7 comprises the pixel’s photogeneration region. As shown in Fig. 7, the floating diffusion is further connected to a readout (“READ”) circuitry, and the pixel. The capacitance of the sensor node (pixel) that is felt by the readout circuitry is symbolized in Fig. 7 as CSN. When the transfer gate blocks the flow of the charge carriers between the photodiode and the floating diffusion, i.e. when the transfer gate TG electrically disconnects the pixel’s photodiode from the floating diffusion, the read-out circuit only sees the CSN and not the capacitance of the photodiode, thereby reducing the read noise.

[0059] An operation performed using the control circuitry of Fig. 7 is illustrated by the diagram of the FIG. 8 and is as follows: i) using the reset transistor RST, set and maintain the floating diffusion to a reset (baseline, ground) state; ii) using the transfer gate TG, permit the flow of the photogenerated charge carriers of the first type from the photodiode to the floating diffusion; iii) using the transfer gate TG, impede and block over a photon integration time tint, the flow of the photogenerated charge carriers of the first type from the photodiode to (towards) the floating diffusion; iv) using the read circuitry READ, towards the end of the photon integration time tint, measure a first signal S1 (reference signal) at the floating diffusion; v) using the reset transistor RST, stop maintaining the floating diffusion at the reset state; vi) using the transfer gate TG, permit the flow of the photogenerated charge carriers of the first type from the photodiode to the floating diffusion; vii) using the transfer gate TG, block for (over) a second period of time the flow of the photogenerated charge carriers of the first type from the photodiode to the floating diffusion; viii) using the read circuitry READ, within said second period of time, measure a second signal S2 (photo-signal) at the floating diffusion;

[0060] For achieving the above steps (i)-(viii) the reset transistor RST, the transfer gate TG and read circuitry READ may be set to respective high H or low L states, as shown in Fig. 8. Hence, for step (i) the reset transistor RST may be set to a respective high H state. For step (ii) the transfer gate TG may be set or maintained to a respective high H. For step (iii) the transfer gate TG may be reset from said high H state to a low L state for a time duration equal to tint. For step (iv) the read circuitry READ may be reset from a respective low state L to a high state H and then be reset back to the low state L. For step (v) the reset transistor RST may be set to a respective low state L. For step (vi) the transfer gate TG may be reset from the low state L to the high state H. For step (vii) the transfer gate TG may be reset from the high state H to the low state L and maintained at set low state L for a time duration equal to the second period of time. For step (viii) the read circuitry READ may be set from the respective low state L to the high state H and then be reset back to the low state L.

[0061] In this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

[0062] The invention is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

[0063] Research leading to these results has received funding from the European Innovation Council under grant agreement n° 101113088.

Claims

CLAIMS1 . A photodetector which comprises: an electrode (1); a semiconductor layer (2) suitable for a photogeneration of charge carriers in the semiconductor layer (2), the semiconductor layer (2) being connected to the electrode (1); a charge transfer layer (3) which comprises a two-dimensional semiconductor and is connected to the semiconductor layer (2) for transferring a first type of the charge carriers from the semiconductor layer (2) to the charge transfer layer (3), the first type of the charge carriers being holes or electrons; a substrate (4) supporting the charge transfer layer (3); wherein, the electrode (1), the semiconductor layer (2), the charge transfer layer (3) and the substrate (4) are arranged in the same order along a straight line (A); the charge transfer layer (3) is configured to transfer, via the two-dimensional semiconductor and over a transfer gate region (4a) of the substrate (4), the first type of the charge carriers to a charge extraction region (3b) of the charge transfer layer (3); the substrate (4) comprises embedded at said transfer gate region (4a) a first dielectric (5) in contact with the charge transfer layer (3), and a transfer gate electrode (6) separated from the charge transfer layer (3) via the first dielectric (5); the charge extraction region (3b), the transfer gate region (4a) and the transfer gate electrode (6) are away from the straight line (A).

2. A photodetector according to claim 1 , wherein the semiconductor layer (2) comprises semiconductor particles, preferably quantum dots.

3. A photodetector according to any of the preceding claims, further comprising a modulation gate (40) arranged along the straight line (A) in between the substrate (4) and the charge transfer layer or at least partially within the substrate, wherein the modulation gate (40) comprises a modulation gate dielectric (41) and a modulation gate electrode (42) which is separated from charge transfer layer (3) by the modulation gate dielectric (41) which is adjacent to the charge transfer layer (3), and wherein the modulation gate (40) is configured to modulate a Fermi level of the charge transfer layer (3) when an electrical voltage is applied to the modulation gate electrode (42).

4. A photodetector according to any of the preceding claims, wherein the two-dimensional semiconductor is a two-dimensional transition metal dichalcogenide.

5. A photodetector according to any of the preceding claims, wherein the photodetector further comprises a second dielectric (7) over the transfer gate region (4a) of the substrate and in contact with the charge transfer layer (3) opposite the first dielectric (5), the second dielectric (7) extending over the charge extraction region (3b), preferably the second dielectric (7) being configured to prevent a direct contact between the semiconductor layer (2) and the charge transfer layer (2) over the charge extraction region (3b).

6. A photodetector according to claim 5, wherein the semiconductor layer (2) extends over the transfer gate region (4a), the second dielectric (7) and the charge extraction region (3b).

7. A photodetector according to any of the preceding claims, wherein the photodetector further comprises a floating diffusion (12), and the charge extraction region (3b) is on or connected to the floating diffusion (12).

8. A photodetector according to any of the preceding claims, further comprising a first transfer layer (9) which along the straight line (A) is arranged between the semiconductor layer (2) and the charge transfer layer (3) and is suitable for transferring the first type of charge carriers from the semiconductor layer (3) towards the charge transfer layer (3), preferably the first transfer layer (9) being further suitable to hinder a transfer of a second type of charge carriers from the semiconductor layer (2) to the charge transfer layer (3).

9. A photodetector according to any of the preceding claims, further comprising a second transfer layer (8) which along the straight line (A) is arranged between the electrode (1) and the semiconductor layer (2) and is suitable for transferring a second type of the charge carriers from the semiconductor layer (3) towards the electrode (1), wherein the second type of the charge carriers is holes when the first type of charge carriers is electrons, or the second type of charge carriers is electrons when the first type of charge carrier is holes, preferably the second transfer layer (8) being further suitable to hinder a transfer of the first type of charge carriers from the semiconductor layer (2) to the electrode (1).

10. A photodetector according to any of the preceding claims, wherein the semiconductor layer (3) is suitable for absorbing electromagnetic radiation for the photogeneration of thecharge carriers, and the electrode (1) is at least partially transparent to the electromagnetic radiation, preferably the electrode (1) comprising or being made of a transparent conductive oxide.

11. A photodetector according to any of the preceding claims, further comprising a readout circuit (10) electrically connected to the charge extraction region, preferably the read-out circuit being embedded in the substrate.

12. A photodetector according to any of the preceding claims, wherein the photodetector is an image sensor comprising a plurality of pixels (31) on the substrate (4), preferably the image sensor being an active pixel image sensor.

13. A photodetector according to any of preceding claims, wherein the substrate is a CMOS substrate.

14. An image sensor comprising a photodetector which is according to any of the preceding claims.

15. A photodetection method using a photodetector which is according to any of claims 1 to 13, the method comprising: photo-generating charge carriers in the semiconductor layer (2) of the photodetector; transferring a first type of the charge carriers from the semiconductor layer (2) to the charge transfer layer (3) of the photodetector, the first type of the charge carriers being holes or electrons; transferring, via the two-dimensional semiconductor, the first type of the charge carriers to a charge extraction region (3b) of the charge transfer layer (3).

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