Bio-Inspired Imaging Device with In-Sensor Visual Adaptation

A bio-inspired imaging device mimicking visual adaptation of human vision provides a large dynamic range in imaging an image. The device employs a neuromorphic vision sensor realized with phototransistors each being a field-effect transistor, a channel layer of which is an atomically-thin layer of two-dimensional semiconductor material. The channel layer is intentionally formed with defects trap states for trapping a portion of charge carriers generated by a light beam incident on the phototransistor such that intensity information of the light beam is memorized. A gate-source voltage directs the defects trap states to de-trap the trapped portion of charge carriers or to further trap an additional portion of charge carriers, allowing the phototransistor to exhibit a time-dependent excitation or inhibition effect on drain current to thereby enable the imaging sensor to mimic scotopic or photopic adaptation in imaging the image.

ABBREVIATIONS

2D Two dimensional

BP Boron phosphide

CCR Current change ratio

CMOS Complementary metal oxide semiconductor

DOS Density of states

FET Field effect transistor

IR Infrared

ITO Indium tin oxide

TMD Transition metal dichalcogenide

TMO Transition metal oxide

FIELD OF THE INVENTION

The present invention generally relates to an imaging device. In particular, the present invention relates to a bio-inspired imaging device for mimicking visual adaptation of human vision in imaging an image.

BACKGROUND

The development of machine vision (e.g., in intelligent vehicles, mobile medical devices, real-time video analysis and cooperative autonomous driving) demands imaging sensors with ultrahigh resolution, high image-capturing speed, stable imaging performance, and capability of imaging under a wide range of illumination conditions. The last requirement is translated into a need for imaging sensors to image objects under dim-light and bright-light conditions. A large dynamic range in imaging the objects is required. Accurate imaging under a wide range of illumination conditions is critical for correct perception of the environment by humans, because natural light normally used in illuminating objects for human viewing spans a very large range of 280 dB in light intensity. It requires imaging sensors to accurately capture and detect more shadow and highlighted details. Currently, imaging sensors realized by silicon-based CMOS technologies usually have a dynamic range of 70 dB, much narrower than the range of illumination conditions of interest in illuminating natural scenes. To develop an imaging sensor adaptive to a wide range of lighting conditions, one usually employs one or more of techniques that include controlling an optical aperture, adopting a liquid lens, adjusting an exposure time, and applying denoising algorithms in post processing. These techniques usually require complex hardware and software resources.

There is a need in the art for an improved imaging sensor that enables imaging under a wide range of illumination conditions while being simple to implement or operate.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a bio-inspired imaging device with in-sensor visual adaptation for imaging an image.

The imaging device comprises an imaging sensor. The imaging sensor comprises a plurality of phototransistors. An individual phototransistor for converting a light beam of the image to an electrical signal is a FET comprises a source, a drain, a gate and a channel layer. The channel layer connects the source and the drain. The channel layer is an atomically-thin layer composed of a 2D semiconductor material for generating charge carriers upon irradiation by the light beam in forming the electrical signal. In particular, the channel layer is formed with defects trap states for trapping a portion of the charge carriers such that intensity information of the light beam is memorized by the defects trap states. Furthermore, the channel layer is positioned in proximity to the gate. Hence, a gate-source voltage between the gate and the source directs the defects trap states to de-trap the trapped portion of the charge carriers over time or to further trap an additional portion of the charge carriers over time. It allows the individual phototransistor to exhibit a time-dependent excitation or inhibition effect on the electrical signal to thereby enable the imaging sensor to mimic visual adaptation of human vision in imaging the image. By producing effects similar to visual adaptation, the disclosed imaging device advantageously achieves a large dynamic range in imaging the image.

Preferably, the imaging device further comprises one or more gate drivers and one or more computing processors. The one or more gate drivers is used for providing respective gate-source voltages to the plurality of phototransistors. The one or more computing processors are configured to determine the respective gate-source voltages and control the one or more gate drivers to provide the determined respective gate-source voltages to the plurality of phototransistors for causing the imaging sensor to mimic visual adaptation in imaging the image.

Preferably, the one or more computing processors are further configured to control the one or more gate drivers to provide: a positive gate-source voltage to the individual phototransistor if photopic adaptation is mimicked in imaging the image; and a negative gate-source voltage to the individual phototransistor if scotopic adaptation is mimicked in imaging the image.

In certain embodiments, the respective gate-source voltages applied to the plurality of phototransistors at each time instant are same, allowing reduced complexity implementation of the one or more gate drivers.

For the imaging sensor, it is preferable that the atomically-thin layer used to form the channel layer consists of one or two monolayers of the 2D semiconductor material.

The 2D semiconductor material used to form the channel layer may be selected from TMDs, TMOs, MXenes, BP, nanoribbon graphene and carbon nanotubes.

In certain embodiments, the 2D semiconductor material is selected to be MoS2.

Preferably, the imaging device further comprises a dielectric layer sandwiched between the gate and the channel layer. In certain embodiments, the dielectric layer is substantially composed of a material selected from Al2O3, HfO2, ZrO2and h-BN.

In certain embodiments, the plurality of phototransistors is formed on a substrate. The gate of the individual phototransistor is located on the substrate.

In certain embodiments, the source and drain are made of metal, indium tin oxide or graphene.

In certain embodiments, the gate is made of metal, indium tin oxide or graphene.

In certain embodiments, the plurality of phototransistors is arranged as a rectangular array in the imaging sensor for capturing the image.

It is preferable that the imaging sensor further comprises a plurality of optical components for optically processing respective light beams of the image before the respective light beams reach the plurality of phototransistors. Preferably, the optical components include a microlens located in proximity to the individual phototransistor for focusing the light beam onto the individual phototransistor. Optionally, the plurality of optical components further includes a color filter located in proximity to the individual phototransistor for enabling the individual phototransistor to extract color information of the light beam. It is also optional that the plurality of optical components further includes an IR-stop filter overlying the plurality of phototransistors for filtering off IR components of the respective light beams before the image is imaged by the plurality of phototransistors.

Optionally, the imaging device further comprises one or more ADCs for digitizing respective electrical signals received from the plurality of phototransistors to form a plurality of digitized electrical signals. The one or more computing processors are further configured to: receive the plurality of digitized electrical signals obtained at a time instant to form an image signal of the image at the time instant; and collect respective image signals obtained at a plurality of successive time instants to form a sequence of image signals.

In certain embodiments, the one or more computing processors are further configured to adjust the respective gate-source voltages applied to the plurality of phototransistors according to an incident power density of the light beam of an individual image in an optical-image sequence for realizing Weber's law in generating the sequence of image signals from the optical-image sequence.

Other aspects of the present invention are disclosed as illustrated by the embodiments hereinafter.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the present invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

As used herein, “visual adaptation” means a brief and temporary change in visual sensitivity or perception when a subject is exposed to a new optical stimulus, and by the lingering aftereffects when the stimulus is removed.

As used herein, “scotopic adaptation” means visual adaptation of the eye to vision (viz., to see) in the dark or in an environment with dim-light illumination.

As used herein, “photopic adaptation” means visual adaptation of the eye to vision (viz., to see) in the sunlight or in bright illumination.

As used herein, “two-dimensional material” or “2D material” means a material usable to form a solid consisting of one or few layers of constituents, where an individual constituent is an atom or compound, and each layer is a monolayer having a thickness of one count of the aforesaid atom or compound.

As used herein, “atomically-thin layer” of a 2D material means a solid layer formed by at most nine monolayers of constituents of the 2D material. In the art, there is no consensus on the number of monolayers that collectively form an atomically-thin layer. For example, K. F. MAK et al., in “Atomically thin MoS2: A new direct-gap semiconductor,”Physical Review Letters,105, 136805, 24 Sep. 2010, consider that an atomically-thin layer has at most six monolayers, whereas L. WANG et al., in “Tuning magnetotransport in a compensated semimetal at the atomic scale,” arXiv:1510.04827v2, 19 Oct. 2015, consider that an atomically-thin layer may consist of at most nine monolayers. Herein, an atomically-thin layer has at most nine monolayers of the compound.

Disclosed herein is a bio-inspired imaging device for imaging an image with an advantage of mimicking visual adaptation of human vision in imaging the image. The types of visual adaptation that is mimicked include both scotopic adaptation and photopic adaptation. Furthermore, the imaging device employs a plurality of phototransistors self-adaptive in mimicking the visual adaptation such that in-sensor visual adaptation is achieved. By producing effects similar to visual adaptation, the disclosed imaging device advantageously achieves a large dynamic range in imaging the image.

The disclosed bio-inspired imaging device is developed based on research results on electro-optical properties of an adaptive vision sensor, which is a phototransistor realized as a FET with an atomically-thin layer of a 2D semiconductor material. Before the disclosed imaging device is elaborated, details of the adaptive vision sensor are first provided. Electrical and optical properties thereof are also detailed.

An overview of the adaptive vision sensor is first given. The adaptive vision sensor was inspired by the visual adaptation mechanism of the human retina. The sensor is a semiconductor optoelectronic device, and can adapt to different illumination scenes at photo-sensory terminals. The light-intensity-dependent characteristics of the adaptive vision sensor match well with Weber's law, in which the perceived change in stimuli is proportional to the light stimuli. The gate terminal of the phototransistor facilitates vision adaptation with highly localized and dynamic modulation of photo-sensitivity under different illumination conditions at a pixel level, exhibiting a large effective perception range. Through this in-sensor adaptation process in the receptive field, the phototransistor array shows image memorization and image contrast enhancement for both scotopic and photopic adaptation, exhibiting a wide perception range and improvement of image recognition rate.

FIG.1depicts an exemplary structure of an adaptive vision sensor100. A reference upward direction10is defined as shown inFIG.1. Herein in the specification and appended claims, positional and directional words such as “above,” “below,” “higher,” “upper,” “lower,” “top,” “bottom” and “horizontal” for describing the adaptive vision sensor100are interpreted with reference to the reference upward direction10.

The sensor100is a phototransistor (also referenced as100for convenience) based on a FET structure, which includes a local bottom gate120on a substrate140, a dielectric layer125on the bottom gate120, a channel layer110based on a semiconductor material that is light-sensitive, and a source130and a drain135. The channel layer110is an atomically-thin layer of a 2D semiconductor material. Defects trap states115are intentionally introduced to the channel layer110, e.g., by UVO treatment or oxygen plasma treatment to the channel layer110. Normally, both the UVO treatment and oxygen plasma treatment generate the defects trap states115on a surface of the channel layer110. Typically in operating the adaptive vision sensor100, the source130is grounded, a fixed voltage is applied to the drain135, and the bottom gate120has a voltage that is adjustable. The light is absorbed by the channel layer110, leading to a reduction of channel resistance and an increase of drain current (ID). Note that the bottom gate120, which is simply a gate of the FET, is located below the channel layer110, and a light beam191is irradiated on a top side of the channel layer110so that the bottom gate120does not block the light beam191in arriving the channel layer110. Also note that IDforms an electrical signal192in photoconverting the light beam191.

In the theoretical explanation and experimental results to be given below, the channel layer110is selected to be a bilayer MoS2film, which consists of two monolayers of MoS2, as an example for demonstrating photoconversion properties of the sensor100.

FIG.2Ashows the transfer characteristic curves of the phototransistor100at a drain-source voltage (VD), or a drain voltage in simplicity, of VD=1V under different illumination conditions ranging from 6 nW/cm2to 60 mW/cm2(at 660 nm wavelength) in incident power density (Pin). The threshold voltage (VTH) of the phototransistor100shifts towards a more negative value with an increase of Pin. This result indicates that the carrier density increases under brighter illumination. In analogy to the visual threshold of a retina, one may define a threshold ID=21darkas a just noticeable photocurrent, where Idarkrepresents the drain current at VD=1V under a dark condition (i.e. without any light irradiation on the sensor100). The value of Idarkincreases with VG, the gate-source voltage, or the gate voltage in simplicity. Thus, the threshold IDalso increases with VG. This result can be utilized to emulate Weber's law. The photosensitivity (Sph) is defined as

where Iilluminationis a value of IDunder illumination, and Iphis the photocurrent defined as Iph=Iillumination−Idark.

FIG.2Bplots Sphvalues as a function of Pinunder different values of VG(from −5V to +6V with a step of 1V). When a negative VGis applied, the value of Sphis much higher than that obtained at a positive VG, and the Sphvalue increases almost linearly with Pinbecause of the photoconductive mechanism. In contrast, the value of Sphis relatively low at positive VGand increases sublinearly with Pin, which results from the photogating effect. These characteristics unambiguously show that the adaptive vision sensor100has a high Sphvalue under a negative VG(similar to the characteristics of rod cells), while a low Sphvalue is obtained under a positive VG(similar to the characteristics of cone cells).

FIG.2Cdepicts a plot showing a relationship between VGand Pinat Sph=1. The plot shows a slope of 1 on a linear-log scale. According to different Pinvalues, one can apply different VGvalues to modulate the characteristics of the phototransistor100. A positive VGvalue tunes the phototransistor100into an operating region such that the phototransistor100operates with characteristics similar to the characteristics of cone cells of the photoreceptor, while a negative VGvalue corresponds to rod cells of the photoreceptor.FIG.2Dplots the threshold IDagainst Pinat Sph=1. The plot shows that the threshold IDincreases with an increase of Pinat a slope of 3.5×10−5on a log-log scale, matching well with the trend of Weber's law. The phototransistor100at a positive VG(cone) works well for an illumination level at photopic level (about 10−3to 102mW/cm2), and the phototransistor100at a negative VG(rod) is adapted well for an illumination level at scotopic level (about 10−8to 10−3mW/cm2). In this way, the adaptive vision sensor100can fit well with Weber's law by applying locally different VGvalues to the sensor100according to different Pinvalues. This phenomenon is similar to the switchover between rod and cone cells in photoreceptors by the negative feedback of horizontal cells of retina according to different light illumination conditions.

FIG.3Adepicts a plot of IDof the phototransistor100over time under a negative VG. The result indicates that IDincreases gradually over time at a negative VG, showing a current excitation characteristic due to the de-trapping mechanism of the defects trap states115at the surface of channel layer110. The de-trapped electrons lead to the increase of ID. This phenomenon is similar to the regeneration of photopigment in photoreceptors of an eye, leading to an increase of visual sensitivity of photoreceptors. In this case, a fixed negative VG(horizontal cell) gives rise to electron de-trapping (photopigment regeneration).

FIG.3Bdepicts a plot of IDof the phototransistor100over time under a positive VG. In contrast to the case of a negative VG, the result ofFIG.3Bindicates that IDdecreases gradually over time when a positive VGis applied to the phototransistor100, showing a current inhibition behavior of the phototransistor100. The phenomenon of the decrease of IDover time under continuous illumination is analogous to the decrease of visual sensitivity of photoreceptor cells over time under a bright light condition. The mechanism of trap states trapping electrons is similar to the bleaching of photopigment in the retina, leading to the decrease of visual sensitivity of photoreceptors. A positive VG(horizontal cell) leads to electron trapping (photopigment bleaching).

To quantitatively compare the degree of current excitation and inhibition effect, one may compute a CCR of the sensor100by

where ID-0 sis the value of IDat an initial state, and ID-120 sis the value of IDmeasured at a time of 120 s.FIG.3Cplots CCR against VG. When VGis more negative, the CCR value is greater than 1, indicating a more obvious current excitation effect. When VGis more positive, the CCR value is less than 1, suggesting a more obvious current inhibition effect.

FIG.4depicts a schematic band structure of the phototransistor100against DOS under VGvalues with different polarities. The band diagrams of the phototransistor100, as shown inFIG.4, may be used to explain the above-mentioned phenomena of time-dependent current excitation (inhibition) effect under a negative (positive) VG. The channel layer110, which is an atomically-thin MoS2layer that is UVO treated, possesses a lot of localized trap states in the band gap. These trap states are mainly resulted from S vacancies defect configurations. These trap states are distributed over a broad energy range in the band gap, exhibiting ambipolar trap states. At the initial state (VG=0V), the net charge of these trap states is close to zero, as shown in subplot (I) ofFIG.4(all donor-type traps are occupied with electrons, and all acceptor-type traps are vacant). When a negative VG(VG=−2V) is applied, the Fermi level (EF) is lowered. The donor-type traps above EFde-trap electrons and become positively charged after accomplishing the de-trapping process of electrons, inducing more electrons in the conductive band (Er) and increasing ID. The shallow donor-type traps firstly de-trap electrons after a negative VGis applied. As the time prolongs, the deeper traps de-trap electrons, as shown in subplot (II) ofFIG.4. The de-trapped electrons lead to the increase of ID. This phenomenon is similar to the regeneration of photopigment and results in the increase of visual sensitivity of photoreceptor. In this case, a fixed negative VG(horizontal cell) gives rise to electron de-trapping (photopigment regeneration).

In contrast, when a positive VG(VG=+4V) is applied, EFis elevated. The acceptor-type states below EFtrap the electrons from EC, leading to the decrease of electrons in ECas shown in subplot (III) ofFIG.4. The shallow acceptor-type traps firstly trap electrons after a positive VGis applied. As the time goes on, the deep trap states start to trap electrons. The decrease of IDover time under continuous illumination is analogous to the decrease of visual sensitivity of photoreceptor cells over time under a bright light condition. The mechanism of trapping electrons is similar to the bleaching of photopigment in the retina, leading to the decrease of visual sensitivity of photoreceptors. A positive VG(horizontal cell) gives rise to electron trapping (photopigment bleaching).

Based on the time-varying excitation or inhibition characteristics depending on VG, one can realize the visual adaptation function (both scotopic and photopic adaptations) by the phototransistor100. One can apply a more negative VGunder a dim-background condition for invoking excitation characteristics, and a more positive VGunder a bright-light condition for invoking inhibition behavior of the phototransistor100. For illustration,FIG.5depicts time-dependent profiles of IDof the sensor100under different VGvalues, where individual VGvalues are selected according to Pin. For example, the CCRs are 214, 6.67, and 2.22 under dim-light conditions of 60 nW/cm2, 600 nW/cm2and 6 μW/cm2by applying VGof −3V, −2V and −1V (scotopic adaptation), respectively. In contrast, the CCRs are 0.89, 0.78 and 0.77 under bright-light conditions of 600 μW/cm2, 6 mW/cm2and 60 mW/cm2by applying VGof +2V, +4V and +6V (photopic adaptation), respectively.

The trapping and de-trapping processes can change the conductance of the phototransistor100, and can be electrically reset by the applied VG. Therefore, the phototransistor100can work as an optoelectronic memory by recording the perceived light information over a long time even after turning off the light stimulus due to the persistent photoconductivity effect, as shown inFIG.6.FIG.6depicts an operating sequence of IDreadouts obtained from the phototransistor100.

Based on the light-intensity-dependent and time-dependent characteristics of the phototransistor100, the sensing and adaptation functions (both scotopic and photopic adaptations) in the human retina can be emulated with the MoS2phototransistor array. An 8×8 MoS2phototransistor array was implemented for experimentally demonstrating perception of the pattern of “8”.

For the scotopic adaptation test of the MoS2phototransistor array, a gate-source voltage of VG=−2V was applied to all phototransistors in the array. In the array, 20 phototransistors corresponding to the image “8” pattern were illuminated with a weak light of 6 μW/cm2, and the remaining 44 phototransistors were under a dim-background illumination of 600 nW/cm2.FIG.7shows the perceived pattern of “8” obtained at different time instants under dim-light illumination, where the perceived pattern was extracted from IDvalues at these time instants. The pattern of “8” could not be recognized at the beginning with zero contrast because of low photocurrent under relatively low light illumination. With the visual adaptation effect, the image contrast increased over time. This image contrast enhancement over time is similar to the scotopic adaptation of the retina.

For the photopic adaptation test, all phototransistors in the array were applied with VG=+4V. The 20 phototransistors corresponding to the pattern of “8” were illuminated with a strong light beam of 60 mW/cm2, and the other 44 phototransistors were subject to a bright background illumination of 6 mW/cm2.FIG.8shows the perceived pattern of “8” at different time instants under bright light. The perceived pattern was extracted from IDvalues at these time instants. The image contrast of the pattern of “8” was zero at the time instant of 0 s, and increased over time. The “dazzling” pattern of “8” at the initial stage was gradually changed to a comfortable image for the human eye, similar to the photopic adaptation of the retina. The phototransistor array can realize image brightness modulation and contrast enhancement when a positive VGis applied to the phototransistors in the array.

In the disclosed bio-inspired imaging device for imaging an image, the adaptive vision sensor100is used as a basic unit for imaging a pixel of the image.FIG.9depicts an exemplary bio-inspired imaging device900.

The imaging device900comprises an imaging sensor910. The imaging sensor910comprises a plurality of phototransistors912collectively used for imaging the image. Usually, the plurality of phototransistors912is arranged as a rectangular array in the imaging sensor910for capturing the image. Although the rectangular-array arrangement is most often used in practice, the present invention is not limited to this arrangement. Other arrangements, such as a circular arrangement, may be used to organize the plurality of phototransistors912.

Advantageously, each of the phototransistors912is realized as an embodiment of the phototransistor100. Practically, most often all the phototransistors912are realized with the same embodiment of the phototransistor100. For illustration of the imaging device900, without loss of generality the phototransistor100is used to illustrate an individual phototransistor in the plurality of phototransistors912.

Refer toFIG.1. The phototransistor100used for converting a light beam191of the image to an electrical signal192is a FET comprising a source130, a drain135, a gate120and a channel layer110. The channel layer110connects the source130and the drain135. Furthermore, the channel layer110is an atomically-thin layer composed of a 2D semiconductor material for generating charge carriers (electrons and/or holes) upon irradiation by the light beam191in forming the electrical signal192(which is the drain current ID). Examples of appropriate 2D semiconductor materials include: TMDs such as MoS2, WS2, MoSe2, WSe2, MoTe2; TMOs such as MoO3and V2O5; MXenes such as Ti2C; BP; nanoribbon graphene; and carbon nanotubes. As an advantageous feature, the channel layer110is formed with defects trap states115for trapping a portion of the charge carriers such that intensity information of the light beam191is memorized by the defects trap states115. Specifically, the defects trap states115are intentionally introduced to the channel layer110. As mentioned earlier, UVO treatment or oxygen plasma treatment may be used to create the defects trap states115on the surface of channel layer110. Note that the channel layer110is positioned in proximity to the gate120. As a result and based on the explanation regardingFIG.4, a gate-source voltage between the gate120and the source130directs the defects trap states115to de-trap the trapped portion of the charge carriers over time or to further trap an additional portion of the charge carriers over time. It allows the phototransistor100to exhibit a time-dependent excitation or inhibition effect on the electrical signal192. Thereby, it enables the plurality of phototransistors912to mimic visual adaptation of human vision in imaging the image to thereby achieve in-sensor visual adaptation.

There are several advantages resulted from the imaging sensor910, which produces an effect similar to visual adaptation of human vision. First, as mentioned above, the produced effect enables the imaging device900to provide a large dynamic range in imaging the image, thus facilitating the imaging device900to perform imaging under a wide range of illumination conditions from dim-light conditions to bright-light conditions. Second, the imaging sensor910is self-autonomous in achieving the visual-adaptation effect. It allows in-sensor visual adaptation to be gradually achieved over time automatically even if the gate-source voltage is maintained at a certain constant level. It simplifies the design of an algorithm in controlling the gate-source voltage. Third, the imaging sensor900integrates light sensing, image memorization and visual adaptation in one array of phototransistors. It can largely simplify design of artificial vision circuitry and exhibit a wide perception range and improvement in image recognition rate. Note that the imaging sensor910is a neuromorphic vision sensor in that visual adaptation functions are built-in functions in the imaging sensor910. External processing of the analog electrical signals generated by the plurality of phototransistors912, which often involves complicated circuitry for analog signal processing, is avoided. The analog outputs of the imaging sensor900can be directly used as a human visual perception in neuromorphic processing.

Other implementation details of the imaging sensor910are elaborated as follows.

One important parameter in designing the channel layer110is the number of monolayers of the selected semiconductor compound. Although the channel layer110is allowed to have at most nine monolayers, it is believed that one or two monolayers are preferable in forming the channel layer110. The rationale is that the defects trap states115are mostly located on the top surface of the channel layer110. Keeping the number of monolayers to one or two may enable a substantial portion of the charge carriers to be trapped in the defects trap states115. It is believed that the resultant in-sensor scotopic or photopic adaptation process (i.e. time-dependent excitation or inhibition effect on the electrical signal192) may be more effective in adapting the photosensitivity of the phototransistor100to the intensity level of the light beam191.

Since the channel layer110is held in proximity to the gate120, preferably the phototransistor100further comprises a dielectric layer125sandwiched between the gate120and the channel layer110. The dielectric layer125is sufficiently thin to allow an electric field generated from the gate120to substantially penetrate into the channel layer110. It is also preferable that the dielectric layer125is a high-κ dielectric having a high dielectric constant as compared to silicon dioxide. In certain embodiments, the dielectric layer125is a high-κ dielectric substantially composed of a material selected from aluminum oxide (Al2O3), hafnium dioxide (HfO2), zirconium dioxide (ZrO2) and hexagonal boron nitride (h-BN).

In certain embodiments, the plurality of phototransistors912is formed on a substrate140, e.g., silicon. In particular, the gate120of the phototransistor100is located on the substrate140.

The gate120, the source130and the drain135are electrodes made of appropriate conductive materials.

Preferably, the source130and the drain135are made of metal for an advantage of high conductivity. Generally, an appropriate metal is required to be selected because different 2D semiconductor materials selected in forming the channel layer110need to match to different metals with different work functions. Gold (Au) is a high-work-function metal, which is suitable for contacting with p-type semiconductors. For n-type semiconductors, a low-work-function metal, e.g., titanium (Ti), nickel (Ni), indium (In) and bismuth (Bi), is suitable. Gold may be deposited on the low-work-function metal as a protective layer against oxidation by air.

Apart from metals, other conductive materials may also be used for forming the source130and the drain135. These other conductive materials may be ITO, graphene, etc.

The gate120may be made of metal, ITO, graphene, etc.

Although the plurality of phototransistors912may directly receive the image for imaging, it is advantageous to optically process the image before it is imaged as in other commonly-used imaging sensors. Preferably, the imaging sensor further comprises a plurality of optical components913for optically processing respective light beams of the image before the respective light beams reach the plurality of phototransistors912.FIG.10depicts some examples of optical components that are found in existing imaging sensors in the art and that are useful for realizing the plurality of optical components913. The plurality of optical components913may include a microlens1010located in proximity to the phototransistor100for focusing the light beam191onto the phototransistor100. The plurality of optical components913may further include a color filter1020located in proximity to the phototransistor100for enabling the phototransistor100to extract color information of the light beam191. It is also possible that the plurality of optical components913further includes an IR-stop filter1030overlying the plurality of phototransistors912such that the IR-stop filter1030filters off IR components of the light beam191before the phototransistor100converts the light beam191into the electrical signal192. The use of the IR-stop filter1030may enhance the contrast of the image in case the semiconductor compound selected to form the channel layer110is undesirably responsive to the IR light.

Refer toFIG.9. Generally, the imaging device900is incorporated with electronic circuits working with the imaging sensor910in imaging the image.

Preferably, the imaging device900further comprises one or more gate drivers920and one or more computing processors930. The one or more gate drivers920are used for providing respective gate-source voltages to the plurality of phototransistors912. The one or more gate drivers920are controllable by the one or more computing processors930. The one or more computing processors930are used for determining the respective gate-source voltages and controlling the one or more gate drivers920to provide the determined respective gate-source voltages to the plurality of phototransistors912for causing the imaging sensor910to mimic visual adaptation in imaging the image.

Usually, the one or more computing processors930are also used for controlling other electronic components and subsystems in the imaging device900.

As mentioned above, a negative VGcan be applied to the phototransistor100to invoke an in-sensor scotopic adaptation process, which is useful for imaging under a dim-background condition. Similarly, a positive VGcan be applied to the phototransistor100to invoke an in-sensor photopic adaptation process, which is useful for imaging under a bright-light condition. Preferably, the one or more computing processors930are further configured to control the one or more gate drivers920to provide: (1) a positive gate-source voltage to an individual phototransistor in the plurality of phototransistors912if photopic adaptation is mimicked in imaging the image; and (2) a negative gate-source voltage to the individual phototransistor if scotopic adaptation is mimicked in imaging the image.

In certain embodiments, the respective gate-source voltages applied to the plurality of phototransistors912at each time instant are same. It allows reduced complexity implementation of the one or more gate drivers920and possibly the one or more computing processors930in comparison to an opposite case that the respective gate-source voltages are not required to have the same value.

If neuromorphic processing, which directly uses analog electrical signals generated from imaging sensor910, is not targeted to, most often the one or more computing processors930are further used to read respective electrical signals generated from the plurality of phototransistors912. Since the electrical signal192generated from the phototransistor100is an analog signal, it is required to digitize the electrical signal192before the electrical signal192is recognizable by the one or more computing processors930. Optionally, the imaging device900further comprises one or more ADCs for digitizing the respective electrical signals received from the plurality of phototransistors912to form a plurality of digitized electrical signals. The plurality of digitized electrical signals is readable by the one or more computing processors930.

It is desirable that the one or more computing processors930can automatically determine the polarity of the respective gate-source voltages supplied to the plurality of phototransistors912for invoking scotopic or photopic adaptation as desired in imaging the image. This determination involves determining whether the image is under bright-light illumination or under dim-light illumination.FIG.11depicts an exemplary procedure executable by the one or more computing processors930for determining the polarity of the respective gate-source voltages.

In step1110, the respective gate-source voltages are initialized to zero volt (VG=0V). The one or more computing processors930control the one or more gate drivers920to feed the plurality of phototransistors912with zero gate-source voltages.

In step1120, the one or more computing processors930controls the imaging sensor910to employ the plurality of phototransistors912to photoconvert the image into the respective electrical signals. The respective electrical signals are subsequently digitized by the one or more ADCs940to form the plurality of digitized electrical signals.

In step1130, the one or more computing processors930determine, from the plurality of digitized electrical signals, if the image is under-illuminated, over-illuminated, or adequately illuminated. A typical rule for determining if the image is under-illuminated is to check whether an average digitized electrical signal is less than a certain small value. To determine if the image is over-illuminated, one typical rule is to determine if the proportion of image pixels reaching an upper limit of possible electrical-signal value exceeds a certain large percentage (e.g., 50%). If it is determined that the image is neither under-illuminated nor over-illuminated, the image is considered to be adequately illuminated. Those skilled in the art will appreciate that other rules may also be used for determining whether the image is under-illuminated, over-illuminated, or adequately illuminated.

If the step1130determines that the image is under-illuminated, scotopic adaptation is required. A negative gate-source voltage (VG<0V) is applied to each of the phototransistors912to invoke an in-sensor scotopic adaptation process in subsequent imaging of the image (step1140). If the image is determined to be over-illuminated, a positive gate-source voltage (VG>0V) is applied to each of the phototransistors912to invoke an in-sensor photopic adaptation process in subsequent imaging of the image (step1160). If the step1130finds that the image is adequately illuminated, VG=0V is maintained in subsequent imaging of the image (step1150).

In practical situations, the imaging device900is often required to capture a sequence of images to form a video signal. In certain embodiments, the one or more computing processors930are further configured to: receive the plurality of digitized electrical signals obtained at a time instant to form an image signal of the image at the time instant; and collect respective image signals obtained at a plurality of successive time instants to form a sequence of image signals.

As one advantageous feature of the imaging device900, the sequence of image signals may be encoded such that Weber's law is satisfied. As explained above, Weber's law can be satisfied via applying locally different VGvalues to the plurality of phototransistors912according to different Pinvalues. In certain embodiments, the one or more computing processors930are further configured to adjust the gate-source voltage applied to the phototransistor100(namely, the individual phototransistor in the plurality of phototransistors912) according to an incident power density of the light beam191of each image in an optical-image sequence for realizing Weber's law in generating the sequence of image signals from the optical-image sequence.

In one implementation of the imaging device900, the one or more gate drivers920, the one or more computing processors930and the one or more ADCs940are integrated in the imaging sensor910for achieving miniaturization of the imaging device900. In another implementation option, the one or more gate drivers920and the one or more ADCs940are housed in the imaging sensor910, allowing the imaging sensor910to be conveniently implemented with an entirely digital interface for communicating with the one or more computing processors930. It is also possible that the one or more gate drivers920, the one or more computing processors930and the one or more ADCs940are outside the imaging sensor910in the imaging device900. Other implementation options are possible.