Patent Publication Number: US-2022223643-A1

Title: Image sensor comprising stacked photo-sensitive devices

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Application No. 21151621.6, filed on Jan. 14, 2021 incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention and concept relate to photo-sensitive devices and their methods of fabrication. In particular, the present inventive concept relates to photo-sensitive devices, preferably formed by thin-film technology. And more particularly, to stacked photo-sensitive devices comprising a fully depleted pixel structure and a floating electrical connection. 
     BACKGROUND 
     Photo-sensitive devices fabricated on a silicon substrate limit the specific detectable wavelengths and include other shortcomings inherent to the optical characteristics and dimensional requirements of silicon. Accordingly, there is a need for a more compact and more optically efficient solution for a photo-sensitive device. Particularly, there is a growing demand and strong need for thin-film, depleted photo-diode technology photo-sensitive devices that are monolithically integrated on semiconductor substrates. The semiconductor substrates enable circuitry for read-out and processing of signals from the photo-sensitive devices. 
     Traditionally, a color image sensor consists of a color filter array (CFA) stacked on top of a pixel (i.e. read-out-circuitry) to selectively absorb each color signal. One typical example includes a Bayer colour filter array, wherein three different colour signals are combined to generate a colour image. This traditional approach, however, results in one-third loss of the light signal and three-quarters loss of resolution. 
     More specifically, known approaches to color signals have inferior kTC noise performance, high dark current, and low conversion gains. One example, United States Published Patent Application number US 2016/0181325 discloses a CMOS image sensor combining CMOS read-out integrated circuits (ROICs) and photodiode on active pixel (POAP) technology with quantum dot (PbS-CQD) detector material. This three-transistor pixel circuit results in an unacceptably high reset noise due to un-correlated read-out (i.e. a pixel signal is read first, then a reset scheme is applied). Moreover, the photodiode area is directly connected to a pixel circuit below the photodiode area. Thus, a defective contact region will generate high dark current. In addition, conversion gain may be low because a photodiode capacitance is added to a floating diffusion node where charge-to-voltage conversion takes place. This results in poor noise characteristics overall and poor image quality. 
     Other examples of three-transistor, layered photodiodes include U.S. Pat. No. 5,965,875 issued on 1999 Oct. 12 to Merrill; Published U.S. Pat. App. No. 20120298841 published on 2012 Nov. 29 by Yamashita et al.; and Published U.S. Pat. App. No. 20130009263 published on 2013 Jan. 10 by Hatano et al.: Each of these aforementioned examples similarly result in undesirable noise characteristics and loss of resolution, as discussed above. 
     Other examples in the art included layered organic photodiodes. Representative examples include the non-patent literature (NPL) reference titled “Image Sensor with Organic Photoconductive Films by Stacking Red/Green and Blue Components,” by Takagi et al. published in 2016 in the Society for Imaging Science and Technology DOI: 10.2352/ISSN.2470-1173.2016.12.IMSE-264: However, this reference suffers similar problems previously discussed as it too presents a three-transistor layered photodiodes with unacceptable noise and resolution characteristics. 
     Yet another example of a stacked photodiode, with a combination of organic and silicon layers, includes U.S. Pat. No. 9,748,295 to Lee et al. issued on 2017 Aug. 29. However, this approach suffers similar limitations of results in one-third loss of the light signal and three-quarters loss of resolution. 
     Therefore, those skilled in the art will appreciate that remains yet a need for an improved color imager that maximized the light absorbed, improves resolution, and reduces noise. 
     SUMMARY 
     Objects of the present invention and inventive concept enable a device and method of fabricating such a device comprising a photo-sensitive, thin-film, stacked, fully depleted pixel structure with a common floating electrical connection and such a device enabled for integration with a read-out integrated circuit (ROIL). 
     The present invention, as defined in the appended claims, meets these objectives and overcomes limitations in the known art. Exemplary embodiments, including one or more preferred embodiments are discussed herein and further set out in the dependent claims. 
     The present invention contemplates a device comprising at least two stacked layer devices wherein each stacked layer structure configures to respond to a respective predefined range of visible light, near infrared light, ultraviolet light, IR light, or any desired spectrum of electro-optical wavelengths. 
     A first embodiment discloses an image sensor ( 100 ) comprising a second photo-sensitive device ( 20 ) arranged to vertically stack over a first photo-sensitive device ( 10 ) in electrical contact with the read-out-circuitry ( 1 ), wherein the first photo-sensitive device ( 10 ) comprises a top electrode ( 111 ) configured to control a first electrical potential, a first charge transport layer ( 112 ) arranged beneath the top electrode ( 111 ), responsive to the first electrical potential, and vertically stacked on top of an active layer ( 113 ), the active layer ( 113 ) configured to generate electrical charges in response to a first predefined range of wavelengths of light incident on the device ( 100 ), a second charge transport layer ( 114 ) arranged under the active layer ( 113 ), the second charge transport layer ( 114 ) comprising a semiconductor material comprising a first portion ( 114   a ) of the charge transport layer ( 114 ), the first portion ( 114   a ) being vertically aligned underneath the active layer ( 113 ), a second portion, i.e. a transfer region ( 114   b ), protruding laterally to extend beyond the active layer ( 113 ) and a third portion ( 114   c ), a bottom electrode ( 116 ) separated by a dielectric material ( 115 ) from the second charge transport layer ( 114 ) wherein the bottom electrode ( 116 ) is configured to provide a first electrical voltage for at least partially depleting the first portion ( 114   a ) of the corresponding second charge transport layer ( 114 ), a transfer gate electrode ( 117 ) separated by a dielectric material ( 115 ) from the second charge transport layer ( 114 ), wherein the transfer gate electrode ( 117 ) is configured to control transfer of electrical charges accumulated in the first portion ( 114   a ) via the transfer region ( 114   b ) to the third region ( 114   c ) for read-out of light detected by the first stacked layer structure ( 10 ), and a first floating electrical connection ( 118 ) extending from the third portion ( 114   c ) of the charge transport layer ( 114 ) and protruding downward therefrom onto the read-out-circuitry ( 1 ). 
     The second photo-sensitive device ( 20 ) comprises a top electrode ( 211 ) configured to control a corresponding second electrical potential, a first charge transport layer ( 212 ) arranged beneath the top electrode ( 211 ), responsive to the second electrical potential, and vertically stacked on top of an active layer ( 213 ), the active layer ( 213 ) configured to generate electrical charges in response to a second predefined range of wavelengths of light incident on the device ( 100 ), a second charge transport layer ( 214 ) arranged under the active layer ( 213 ), the second charge transport layer ( 214 ) comprising a semiconductor material comprising a first portion ( 214   a ) of the charge transport layer, the first portion ( 214   a ) being vertically aligned underneath the active layer ( 213 ), a second portion ( 214   b ), i.e. a transfer region, protruding laterally to extend beyond the active layer ( 18   b ) and a third portion ( 214   c ), a bottom electrode ( 216 ) separated by a dielectric material ( 215 ) from the second charge transport layer ( 214 ), wherein the bottom electrode ( 216 ) configures to provide a second electrical voltage for at least partially depleting the first portion ( 214   a ) of the corresponding second charge transport layer ( 214 ), a transfer gate electrode ( 217 ) separated by a dielectric material ( 215 ) from the second charge transport layer ( 214 ), wherein the transfer gate electrode ( 217 ) is configured to control transfer of electrical charges accumulated in the first portion ( 214   a ) via the transfer region ( 214   b ) to the third region ( 214   c ) for read-out of light detected by the second stacked layer structure ( 20 ), and, a second floating electrical connection ( 218 ) extending from the third portion ( 214   bc ) of the charge transport layer ( 121 ) and protruding downward therefrom onto the third portion ( 114   c ) of the first photo-sensitive device ( 10 ). 
     A second embodiment discloses an image sensor ( 100 ) according to the first embodiment, but further comprising: a third photo-sensitive device ( 30 ) arranged to vertically stack on the second photo-sensitive device ( 20 ), the third photo-sensitive device ( 30 ) comprising a top electrode ( 311 ) configured to control a corresponding third electrical potential, a first charge transport layer ( 312 ) arranged beneath the top electrode ( 311 ), responsive to the first electrical potential, and vertically stacked on top of an active layer ( 313 ), the active layer ( 313 ) configured to generate electrical charges in response to a third predefined range of wavelengths of light incident on the device ( 100 ), a second charge transport layer ( 314 ) arranged under the active layer ( 313 ), the second charge transport layer ( 314 ) comprising a semiconductor material comprising a first portion ( 314   a ) of the charge transport layer, the first portion ( 314   a ) being vertically aligned underneath the active layer ( 313 ), a second portion ( 314   b ), i.e. a transfer region, protruding laterally to extend beyond the active layer ( 313 ) and a third portion ( 314   c ), a bottom electrode ( 316 ) separated by a dielectric material ( 315 ) from the second charge transport layer ( 314 ), wherein the bottom electrode ( 316 ) configures to provide a second electrical voltage for at least partially depleting the first portion ( 314   a ) of the corresponding second charge transport layer ( 314 ), a transfer gate electrode ( 317 ) separated by a dielectric material ( 215 ) from the second charge transport layer ( 314 ), wherein the transfer gate electrode ( 317 ) is configured to control transfer of electrical charges accumulated in the first portion ( 314   a ) via the transfer region ( 314   b ) to the third region ( 314   c ) for read-out of light detected by the third stacked layer structure ( 30 ), a third floating electrical connection ( 318 ) extending from the third portion ( 314   c ) of the second charge transport layer ( 314 ) and protruding downward therefrom onto the third portion ( 214   c ) of the second stacked layer structure ( 10 ). 
     In a third embodiment, the image sensor of the foregoing embodiments is configured to have the floating electrical connections ( 118 ,  218 ,  318 ) vertically stack and are configured to transfer an electrical signal in response to any at least one range of first, second, or third predefined wavelengths of incident light from the photosensitive device to the read-out-circuitry ( 1 ). The read-out-circuitry ( 1 ) is configured to receive a signal from any of the at least one floating electrical connections ( 118 ,  218 ,  318 ) in response to light incident on any of the at least one active layers ( 113 , 213 , 313 ). 
     The following illustrative and non-limiting detailed description and accompanying drawings set forth illustrative embodiments of the present invention to enable a better understanding of the feature and advantages of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings like reference numerals will be used for like elements unless stated otherwise. Some features may not be illustrated in some of the views for clarity or illustrative purposes. The drawings are not to scale, unless stated otherwise. 
         FIG. 1  is a schematic cross-sectional view of an image sensor device according to one embodiment of the present invention. 
         FIG. 2  is a schematic cross-sectional view of an image sensor device according to another embodiment of the present invention. 
         FIG. 3  is a schematic cross-sectional view of an image sensor device according to yet another embodiment of the present invention. 
         FIG. 4  is a table describing some suitable layer material based on electro-optic frequency response. 
         FIG. 5  is a schematic layout of read-out-circuitry suitable for use in the embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention discussed herein are exemplary and non-limiting. Those skilled in the art will appreciate that additional features may be added, or some features may be removed without impacting the spirit of the present invention. The scope of the present invention is particularly defined by the appended claims. 
     One contemplated and representative first embodiment of the present invention includes a light, or more generally, an image sensor comprising three stacked layer devices wherein the first stacked layer device  10  configures to respond to blue light, a second stacked layer device  20  configures to respond to red light, and a third stacked layer device  30  configures to respond to green light. The operation of each respective layer structure ( 10 ,  20 ,  30 ) is similar, so for brevity, the operating principle is summarized in the following paragraphs and further explanation is provided in the detailed description of the invention. Accordingly, each respective stacked layer device enables a photo-sensitive device with good noise characteristics and high conversion gain. 
       FIG. 1  illustrates a first embodiment of the present invention, an image sensor device  100  comprising a layer structure having at least two photo sensitive-devices, a first photo sensitive-device  10 , and a photo sensitive-device  20 . 
     The first photo sensitive-device  10  is a vertically arranged layer structure comprising multiple layers. Such a vertically stacked first layer structure comprises a top electrode  111  configured to control a first electrical potential. Disposed below the top electrode  111  is a first charge transport layer  112 , also called a hole transport layer, which responds to the first potential. As a non-limiting example, the first potential corresponds to blue visible light range. 
     An active layer  113  arranges, and vertically aligns, beneath the first charge transport layer  112 . The active layer  113  configures to generate charges in response to incident light on the active layer: In this exemplary embodiment—blue light. The active layer configures to absorb incident light to generate charges. Thus, an amount of charges generated is indicative of an amount of incident light on the active layer. The generated charges transport through the active layer and accumulate in the charge transport layer. In this embodiment, the active layer  113  configures to absorb light in a first pre-defined range of the electromagnetic spectrum, specifically, in the range of corresponding to blue light. The active layer arranges in a vertical stack between charge transport layers for collecting holes and electrons, respectively. Accordingly, the charge transport layer arranged below the active layer consists of either a hole transport layer or an electron transport layer. 
     A second charge transport layer  114  arranges, and vertically aligns, beneath the active layer  113 . The second charge transport layer  114  comprises a semiconductor and includes a first portion  114   a  and a second portion, which is a transfer  114   b  that protrudes horizontally out of the vertical stack of aligned layers, thus extending beyond the active layer  113  to provide connectivity to the floating diffusion  114   c . The transfer region  114   b  laterally displaces in a plane of the charge transport layer in relation to the first portion  114   a . The transfer gate controls the movement of charges from the first portion to the second portion after accumulation. Thus, generating and accurately controlling charges in the charge transport layer. This results in good noise characteristics of the photo-sensitive device. Accordingly, once the charges accumulate in the first portion  114   a , the charges may be transferred to the second portion  114   b , under control of a transfer gate electrode  117 , and from there to the floating diffusion  114   c , ultimately enabling reading out a signal corresponding to, for example, blue light. 
     A bottom electrode  116  arranges, and vertically aligns, beneath a dielectric material  115 , which, in turn arranges, and vertically aligns, beneath the second charge transport layer  114 . Bottom electrode  116  and the second charge transport layer  114  arrange such that the first portion  114   a  of the second charge transport layer  114  is between bottom electrode  116  and the active layer  113  and wherein bottom electrode  116  configures to provide a first voltage, which configures to deplete the first portion  114   a  of the corresponding first charge transport layer  114 . 
     A floating diffusion  114   c  extends downward from the second portion  114   b  of the second charge transport layer  114  configures to control transfer of accumulated charges from this first stacked layer structure  10  for read-out of detected light by the photo-sensitive device  100 . 
     In this embodiment, the device  100  is describes a vertically stacked layer structure wherein the charge transport layer and the active layer arrange in separate, parallel planes, stacked vertically with respect to each other (as reference in this document based on the orientation of the associated drawings). The photo-sensitive device orients such that the charge transport layer is above the gate and, in such case, the active layer is above the charge transport layer. However, it should be realized that a layer defined as being arranged “above”, “on” or “below” another layer, need not be arranged directly on top of or directly below the other layer. Rather, there may be other intermediate layers in-between. 
     The photo-sensitive device is configured such that it is possible to achieve full depletion of the first portion of the charge transport layer wherein charges are accumulated. It is also advantageous to configure the photo-sensitive device such that the gate will provide a voltage so that the first portion will be fully depleted. Full depletion is beneficial in providing good noise characteristics of the photo-sensitive device, since reset noise may be limited, a dark current may be limited and a high conversion gain may be provided. 
     Still referring to  FIG. 1 , the device  100  further contemplates a second stacked layer structure  20  arranged below the first structure  10 . In this embodiment, the second stacked layer structure  20  configures to respond and absorb a second predefined range of visible light on the electromagnetic wave spectrum, corresponding to a light range other than of the first structure  10  for example, as would be understood commonly as red light. Some materials for portions of the second layer structure  20  may differ from those similarly corresponding stacked layers of the first layer structure  10 : However, the operation principles are significantly similar and, accordingly, some features are omitted here for brevity, but will be well understood by those skilled in this art. 
     The second layer structure  20  comprises a top electrode  211  configured to control a corresponding second potential corresponding to absorbed light in the second predefined range of the electromagnetic spectrum and corresponding to red light. 
     A first charge transport layer  212  arranges beneath the top electrode  211  and vertically stacks on top of an active layer  213 . The active layer  213  configures to generate charges in response to the second predefined range of wavelengths of incident light. 
     A second charge transport layer  214  arranges under the active layer  213 . The second charge transport layer  214  comprises a semiconductor material comprising a first portion  214   a  that vertically aligns underneath the active layer  213 . The second charge transport layer  214  further comprises a second portion, which is a transfer region  214   b . This transfer region  214   b  protrudes laterally to extend beyond the active layer  213  and otherwise enables connectivity to a floating diffusion  214   c.    
     A bottom electrode  216  arranged below and vertically aligns with the second charge transport layer  214 . A dielectric material layer  215  arranges between the bottom electrode  216  and the charge transport layer  214 . The bottom electrode  216  configures to provide a second voltage for fully depleting the first portion  214   a  of the corresponding second charge transport layer  214 . 
     A second floating diffusion  214   c  extends from the transfer gate  217  of the second charge transport layer  214  and protruding therefrom. 
       FIG. 2  illustrates a second aspect of the present invention. According to this embodiment, a photo-sensitive device  100  comprises three stacked structures ( 10 ,  20 ,  30 ) combined with a read-out circuit  250 . The first layer structure  10  and the second layer structure  20  are described above in relation to the first embodiment. The third layer structure  30  is similar in construction to the first or second layer structure; however, in the third layer structure is configured to respond to a light range, other than the first  10  and the second structure  20 , e.g. green light, and the materials and structure of the multiple-layered third layer structure  30  are selected to enable the third layer structure  30  to respond to a third predetermined wavelength range. 
     The third stacked layer structure  30  configures and arranges to vertically stack underneath the second stacked layer structure  20 . The third stacked layer structure  30  comprises a corresponding a top electrode  311  configured to control a corresponding third potential. One exemplary third potential corresponds to green light in a third predefined wavelength range. The third stacked layer structure further comprises a first charge transport layer  312  arranged beneath the top electrode  311  and vertically stacked on top of an active layer  313 . The active layer  313  configures to generate charges in response to the third predefined range of wavelengths of incident light. A second charge transport layer  314  arranges under the active layer  313 . The second charge transport layer  314  comprises a semiconductor material having a first portion  314   a  that vertically aligns underneath the active layer  313  and a second portion  314   b  (also called a transfer region) protruding laterally to extend beyond the active layer  313 . A gate  317  separated by a dielectric material  315  from the second charge transport layer  314 , and a third floating diffusion  314   c  extending from the transfer region  314   c  of the charge transport layer  314  and protruding therefrom completes the third stacked layer structure  30 . 
     Additionally, the device  200  arranges the three stacked layer structures ( 10 ,  20 ,  30 ) to vertically align their respective floating diffusions ( 114   c ,  214   c ,  314   c ) to enable the Read Out Integrated Circuit (ROIC)  1  to receive the receive the electrical signal in response of light impinging on the photo-sensitive device  100 . 
     The photo-sensitive device  100  thus integrates with a substrate on which read-out integrated circuit  1  is provided. This ensures that the photo-sensitive device with read-out circuitry may be very compact and allows processing of detected signals in a small-scale device. The read-out integrated circuit may be used for reading out signals of detected amount of light, but may also be used for more advanced processing of the signals. 
     The read-out integrated circuit  1  may be formed by integrated semiconductor technology, such as using complementary metal-oxide-semiconductor (CMOS) circuitry. Thus, the photo-sensitive device may make use of existing technology for manufacturing of small-scale circuitry. 
     The active layer and charge transport layer may be formed on a silicon CMOS wafer, on which a read-out integrated circuit is formed. However, it should be realized that the active layer and charge transport layer may alternatively be formed on other substrates, such as a thin-film technology wafer, which may for example use organic materials. 
     It should be realized that the active layer and the charge transport layer may be arranged in different relations to a read-out integrated circuit. For instance, the active layer and the charge transport layer may be arranged on top of the read-out integrated circuit on the substrate. However, in another embodiment, the active layer and the charge transport layer may be arranged on a common substrate with the read-out integrated circuit, but the active layer and the charge transport layer of the photo-sensitive device may be arranged next to the read-out integrated circuit on the substrate. For instance, the active layer and the charge transport layer of the photo-sensitive device and the read-out integrated circuit may be arranged on the same polyimide substrate. In such case, the read-out integrated circuit may be designed by using thin-film technology. 
       FIG. 3  illustrates a third contemplated aspect in another embodiment according to the present invention. A photo-sensitive device  100  comprises two sets of a triple-stacked multi-layer structure. Thus, (as illustrated in  FIG. 3 ) the left side of the device  100  comprises a first stacked layer structure  10 ′ adapted to respond to blue light as defined in the first predetermined range of light, a second stacked layer structure  20 ′ adapted to respond to red light as defined in the second predetermined range of light, and a third stacked layer structure  30 ′ adapted to respond to green light as defined in the third predetermined range of light. Further, the device  100  additionally comprises, on the right-hand side (of  FIG. 3 ), a first stacked layer structure  10  adapted to respond to blue light as defined in the first predetermined range of light, a second stacked layer structure  20  adapted to respond to red light as defined in the second predetermined range of light, and a third stacked layer structure  30  adapted to respond to green light as defined in the third predetermined range of light. The configuration, operation, and performance of each layer structure ( 10 ,  10 ′,  20 ,  20 ′,  30 ,  30 ′) is identical to the corresponding structures defined in the first and second embodiments/aspects, above. 
     Additionally, the device  100  includes a read-out integrated circuit  1 , as previously described in relation to the second embodiment. 
     Notably, the device  300  uses a common stack of floating diffusions ( 314   c ,  214   c ,  114   c ) whereby left and right pairs of stacked devices share a single, common floating diffusion. Thus, stacked device  10  and  10 ′ use the common floating diffusion  114   c . Likewise, the pairs of stacked devices  20  and  20 ′ a use the common floating diffusion  214   c , and pairs of stacked devices  30  and  30 ′ use the common floating diffusion  214   c . Further the floating diffusions  114   c ,  214   c ,  314   c  align in a vertical stack to electrically couple to the ROIC  1 . The read-out-circuitry  1  is configured to receive a signal from any of the at least one floating electrical connections  118 ,  218 ,  318  in response to light incident on any of the at least one active layers  113 ,  213 ,  313 .  FIG. 5  illustrates a read-out circuitry  1  connected to the stack of electrical connections  118 ,  218 ,  318 . The  FIG. 5  illustrates a state-of-the art ROIC comprising a Reset Transistor RST, a select transistor SEL and a sense transistor SF. 
     According to an embodiment, the active layer comprises a quantum dot, an organic photodetector material or a perovskite material. These materials allow detection of light in different wavelength intervals or predetermined ranges. Thus, the photo-sensitive device may for instance be used for detecting infrared light, which may not be possible if silicon is used for light detection. Use of quantum dot, organic photodetector material and perovskite materials may be well suited to the configuration of the photo-sensitive device. However, it should be realized that it may also be possible to use other materials. 
     According to an embodiment, wherein the active layer and charge transport layer are formed using thin-film layer deposition. Thin-film layer deposition may be suitable for monolithic integration of structures using vast amount of materials, e.g. organic materials. This may be advantageously used in the photo-sensitive device, which may be suited for use of e.g. organic materials in light detection. 
     According to an embodiment, the charge transport layer is an electron transport layer and the photo-sensitive device further comprises a hole transport layer arranged such that the active layer is between the hole transport layer and the electron transport layer. It may be particularly suitable to use an electron transport layer as the charge transport layer. Suitable materials may be readily available. 
     According to an embodiment, the photo-sensitive device further comprises a top electrode layer arranged above the hole transport layer, wherein the top electrode layer is configured to control a potential of the hole transport layer. Thus, the active layer and the charge transport layers are arranged between two electrodes controlling potential of the layers to control function of the photo-sensitive device. 
     The top electrode layer may be configured to be transparent to relevant wavelengths (to be detected by the photo-sensitive device) to ensure that the light is passed to the active layer for detection with high sensitivity. 
     According to another embodiment, the charge transport layer is a hole transport layer and wherein the photo-sensitive device further comprises an electron transport layer arranged such that the active layer is between the electron transport layer and the hole transport layer. Thus, it should be realized that the photo-sensitive device need not necessarily be formed using an electron transport layer as the charge transport layer for detecting the amount of light incident on the photo-sensitive device, but rather a hole transport layer may alternatively be used. 
     According to an embodiment, the charge transport layer is formed by a metal-oxide semiconductor. This may ensure that the charge transport layer may be formed by a suitable material. Various alternatives exist, such as indium-gallium-zinc oxide (IGZO), indium-tin-zinc-oxide or hafnium-indium-zinc-oxide using an n-type semiconductor. Other alternatives are tin-oxide, copper-oxide, selenides and sulfides using a p-type semiconductor. 
       FIG. 4  lists suitable layer material based on their electro-optic frequency response. 
     According to an embodiment, the gate and the transfer gate are arranged on different sides of the charge transport layer or the gate and the transfer gate are arranged on a same side of the charge transport layer or the transfer gate comprises a first portion and a second portion, wherein the first portion of the transfer gate and the second portion of the transfer gate are arranged on opposite sides of the charge transport layer. Thus, the transfer gate may be arranged in different configurations in relation to the charge transport layer. The transfer gate may be arranged to provide a well-controlled transfer of charges from the first portion to the second portion of the charge transport layer. 
     According to a second aspect, there is provided an image sensor comprising an array of photo-sensitive devices  100  according to the first aspect. Effects and features of this second aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect. Thus, the photo-sensitive devices may be used for forming in imaging sensor to enable imaging. For instance, this may be very useful for infrared imaging. 
     In an image sensor, components of the photo-sensitive devices  100  may be shared. This implies that a number of transistors may be used efficiently in the array in relation to the number of photo-sensitive devices. For instance, a read-out integrated circuit  1  associated with the array of photo-sensitive devices  100  may be arranged such that one or more transistors of the read-out integrated circuit are shared per photo-sensitive device. 
     According to an embodiment, at least a second portion of a first photo-sensitive device  100  in the array and a second portion of a second photo-sensitive device  100  in the array are shared in the charge transport layer. This implies that a floating diffusion node of the first and the second photo-sensitive device may be shared. Hence, a number of transistors per photo-sensitive device may be reduced, which may ensure that size of each photo-sensitive device may be reduced for enabling high resolution imaging. 
     By setting a level of the gate voltage properly, the gate voltage provided to the electrode arranged below the first portion of the charge transport layer need not be varied. Rather, a constant gate voltage may be provided, for example a DC signal may be provided to the gate. This implies that control of the gate is very simple and need not be accurately timed. 
     However, according to another embodiment, a high gate voltage is provided to the electrode for biasing the photo-sensitive device to accumulate charges in the first portion of the charge transport layer and a low gate voltage is provided when transferring charges. 
     The low gate voltage is defined as “low” in that it is lower than the high gate voltage. 
     The low gate voltage may bias the photo-sensitive device such that the charge transport layer in the first portion is fully depleted when charges have been transferred from the first portion to the second portion. 
     In the above description the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims. 
     For ease of reference, and as illustrated in the drawings, embodiments of the present invention are described from a cross-sectional viewpoint and includes the arbitrary designation of top and bottom. Herein, “top” refers to the orientation of a surface of an image sensor that receives incoming light. 
     Various alternatives to the embodiments of the invention described herein may be employed in practicing the invention, and although the following claims define the scope of the invention, the methods, structures, and their equivalents, are covered by the claims.