US20220139972A1

US20220139972A1 - Tft photodetector integrated on display panel

Info

Publication number: US20220139972A1
Application number: US17/574,743
Authority: US
Inventors: Hoon Kim
Original assignee: Individual
Current assignee: Tmrw Electronics Sarl
Priority date: 2019-08-20
Filing date: 2022-01-13
Publication date: 2022-05-05
Also published as: US20240266370A1; US20210335858A1; US11978747B2; US12446335B2; US20220005847A1; US11094724B2; US11152409B2; US20260020360A1; US20210057599A1; US20210056279A1; US12453196B2; US12068341B2; US20240379698A1; US11515346B2; US20260026114A1; US11239270B2; US20230275105A1; US11646330B2; US20210055860A1; US20230088505A1

Abstract

A method of fabricating a thin-film transistor (TFT) photodetector includes forming diffusion layers on a glass substrate or a transparent flexible substrate, wherein the diffusion layers include a P-type diffusion layer of P-type polycrystalline or amorphous silicon, for use as an active layer and P+-type diffusion layers of amorphous or polycrystalline silicon at both sides of the P-type diffusion layer, forming an insulating oxide layer on the formed diffusion layers, forming an N-type diffusion layer of polycrystalline or amorphous silicon on the insulating oxide film, forming a gate to be used as a light receiving part by photo-patterning the N-type diffusion layer, etching the generated insulating oxide layer except for only a necessary part in a photoresist (PR) patterning process, removing a remaining area of the P+-type diffusion layer except for areas to be used as a source and a drain by etching, and generating electrodes by depositing a metal in etched parts of the insulating oxide film in the source and the drain.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 16/729,444 filed on Dec. 29, 2019, which claims priority from Provisional Application 62/889,560 filed on Aug. 20, 2019. The aforementioned applications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to a thin-film transistor (TFT) photodetector, and more particularly, to a TFT photodetector which is implemented in each pixel on a display panel and executes both a display function and an image sensing function.

BACKGROUND

Technologies such as liquid crystals, organic light emitting diode (OLED) cells, touch screens, backlights, and thin film transistors (TFTs) on glass are integrated on a display panel. Particularly, the trend of recent mobile devices is toward a display panel which tends to be as large as or larger than an overall device size, and a display itself is becoming more flexible.
However, the current display system performs only a one-way function of outputting an image or the like to the outside, without a function of efficiently, directly acquiring an input signal. At present, the display system executes only a touch screen function, while a separate image sensor performs a process such as image sensing.
Particularly in a mobile device or a laptop computer to which a biometric recognition and authentication system such as fingerprint or face recognition and authentication is essential, there are technological limitations in acquiring a signal from an image sensor confined to any specific position on a display. Although it is most desirable to incorporate an input signal device into the display system, an image sensor cannot be implemented on a display panel with the current technology, thus making it impossible to integrate the display panel with the image sensor in real implementation.
To solve this problem, the present disclosure provides a technique of making a display module function as an image sensor without the need for using a separate image sensor, by realizing a high-sensitivity image sensor on a glass substrate or a flexible substrate such as a polyimide (PI) film, which is used as a display panel, through application of a photo assisted tunneling-photodetector (PAT-PD) disclosed in a patent application being filed, “Unit Pixel of Image Sensor and Light-Receiving Element Thereof” (U.S. Ser. No. 15/885,757).
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.

SUMMARY

An aspect of the present disclosure is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
Accordingly, an aspect of the present disclosure is to provide a display module which functions as an image sensor by forming a thing film transistor (TFT) photodetector on a glass substrate or a flexible substrate such as a polyimide (PI) film, which is used as the display panel.
Another aspect of the present disclosure is to perform an image sensing process without the need for separately using a light emitting part required for an image sensor, by using a light emitting element or backlight unit (BLU) of a display as a light source for the image sensor.
In accordance with an aspect of the present disclosure, there is provided a thin-film transistor (TFT) photodetector for a display panel. The TFT photodetector includes an amorphous transparent substrate used as the display panel, a source formed of amorphous silicon or polycrystalline silicon on the transparent substrate, a drain formed of amorphous silicon or polycrystalline silicon, opposite to the source on the transparent substrate, an active layer formed between the source and the drain and having a current channel formed between the source and the drain, an insulating oxide film formed on the source, the drain, and the active layer, and a light receiving part formed on the insulating oxide film and configured to absorb light. The light receiving part is isolated from the active layer by the insulating oxide film. When light is incident on the light receiving part, electrons migrate by tunneling through the insulating oxide film between the light receiving part and the active layer which have been excited with the insulating oxide film in between, the amount of charge in the light receiving part is changed by the migration of the electrons, a threshold voltage of the current channel is changed due to the change of the amount of charge, and photocurrent flows through the current channel due to the change of the threshold voltage.
The active layer and the light receiving part of the TFT photodetector may be formed of amorphous silicon or polycrystalline silicon.
A depletion layer may be formed in the light receiving part near to the insulating oxide film by the tunneling of the electrons, and the threshold voltage may be dropped in the active layer near to the insulating oxide film due to the depletion layer.
The transparent substrate may be a glass substrate or a flexible substrate like a polyimide substrate.
The TFT photodetector may further include a metal protection layer formed between the transparent substrate and the source and drain, and configured to block light from being introduced into the source and drain through the transparent substrate.
When the intensity of light incident on the light receiving part decreases, the electrons may be returned to an original state by re-tunneling through the insulating oxide film, thereby reducing the depletion layer and photocurrent formed in the current channel.
The TFT photodetector may further include a shielding layer exposing the light receiving part and configured to shield tops of the drain and the active layer from light.
The TFT photodetector may further include a wavelength extension layer formed of amorphous silicon or polycrystalline silicon on at least one of the light receiving part or the active layer, and configured to generate a plurality of local energy levels.
The wavelength extension layer may form a plurality of localized states between a conduction band and a valence band of the light receiving part or the active layer, and have a plurality of band gap energies different from a band gap energy of single crystalline silicon.
The light receiving part may be doped with a first impurity, and the active layer may be doped with a second impurity different from the first impurity.
Other aspects, advantages, and salient features of the disclosure will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of certain exemplary embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a display module with thin film transistor (TFT) photodetectors implemented thereon, which is used as an image sensor, in an electronic device according to an embodiment of the present disclosure;

FIG. 2 is a diagram illustrating an exemplary TFT photodetector implemented on a pixel basis according to an embodiment of the present disclosure;

FIG. 3 is a sectional view illustrating exemplary implementations of a TFT photodetector on a pixel basis on a display according to an embodiment of the present disclosure;

FIG. 4 is a sectional view illustrating a TFT photodetector according to an embodiment of the present disclosure;

FIG. 5 is sectional views illustrating a process of fabricating a TFT photodetector according to an embodiment of the present disclosure;

FIG. 6 is an energy band diagram illustrating a photo-electric conversion mechanism of a TFT photodetector according to an embodiment of the present disclosure;

FIG. 7 is an energy band diagram illustrating a tunneling mechanism of a TFT photodetector according to an embodiment of the present disclosure; and

FIG. 8 is a diagram illustrating a photo-electric conversion mechanism caused by a plurality of localized states in a TFT photodetector formed of amorphous silicon (a-Si) or polycrystalline silicon (poly-Si).

Throughout the drawings, like reference numerals will be understood to refer to like parts, components, and structures.

DETAILED DESCRIPTION

The present disclosure will be described in detail with reference to the attached drawings. Lest it should obscure the subject matter of the present disclosure, a known technology will not be described in detail. An ordinal number (e.g., first, second, and so on) used in the description of the present disclosure is used simply to distinguish one component from another component.
When it is said that one component is “coupled to or with” or “connected to” another component, it is to be understood that the one component may be coupled to or connected to the other component directly or with a third party in between.
FIG. 1 is a schematic diagram illustrating a display module with thin film transistor (TFT) photodetectors implemented thereon, which is used as an image sensor, in an electronic device according to an embodiment of the present disclosure.
A TFT photodetector 100 according to the present disclosure is formed on a display panel 200 of an electronic device 10. The electronic device 10 may be any of devices equipped with a display, such as a smartphone, a laptop computer, a monitor, or a TV.
Specifically, TFT photodetectors 100 may be formed across the whole or part of the display panel 200, and a TFT photodetector 100 may be formed in each individual pixel, thus operating as a part of the pixel. When TFT photodetectors 100 are formed across the whole display panel 200, the number of the TFT photodetectors 100 may be equal to the number of pixels corresponding to the resolution of the display panel 200. The display panel 200 may be any of a light receiving display requiring a backlight unit (BCU), such as a liquid crystal display (LCD) or a light emitting display which emits light on its own, such as a light emitting diode (LED) (e.g., organic LED (OLED) or active matrix OLED (AMOLED)) display or a plasma display panel (PDP).
The display panel 200 displays a video or an image or operates as an image sensor, according to an operation of the electronic device 10. When the display panel 200 operates as an image sensor, an optical image of an external object may be acquired by means of a plurality of TFT photodetectors 100 implemented on the display panel 200. A light source required for image sensing may be an external light source such as natural light or an external lighting, or an internal light source such as a BCU or OLED elements of the display panel 200.
As such, formation of TFT photodetectors 100 of the present disclosure on the display panel 200 advantageously enables use of the display panel 200 as an image sensor without the need for providing a separate image sensor in the electronic device 10. Further, because the display panel 200 is used as an image sensor, a light source for display may also be used as a light source for image sensing without the need for adding a light source for image sensing in the electronic device 10. Therefore, the effects of device simplification and reduced fabrication cost may be expected.
Further, because the pixels of the image sensor are formed in the same size as the pixels of the display, as many image sensor pixels as the number of pixels corresponding to the resolution of the display may be arranged in the electronic device 10. In this case, the whole display may serve as an image sensor. The electronic device 10 may acquire an image of an external object by controlling image sensor pixels in the whole or part of the display. Hereinbelow, an image sensor pixel is interchangeably used with a unit pixel of the image sensor. Obviously, the TFT photodetector 100 of the present disclosure is formed in a unit pixel of the image sensor. Further, a unit pixel of the display panel is interchangeably used with a display pixel.
Further, the electronic device 10 may acquire biometric information about an external object, such as information about the fingerprint of a finger, a finger vein, a face, or an iris by the display panel 200 with TFT photodetectors 100 formed thereon. For example, a user may touch his or her finger on any area of the display or place the finger within a predetermined distance from the area of display in the electronic device 10, so that a fingerprint image may be acquired from a plurality of image sensor pixels formed in the area of the display. Throughout the specification, the display panel 200 may be referred to as a display or a screen panel of a display.
Now, a description will be given of formation of TFT photodetectors on a display panel.
FIG. 2 is a diagram illustrating exemplary formation of a TFT photodetector in each pixel of a display according to an embodiment of the present disclosure.
Although the TFT photodetector 100 of the present disclosure operates in a similar principle to that of a photo assisted tunneling-photodetector (PAT-PD) disclosed in U.S. patent application Ser. No. 15/885,757, the TFT photodetector 100 and the PAT-PD are different in that the PAT-PD is formed on a single crystalline silicon substrate, and an active layer, a source, a drain, and a gate serving as a light receiving part are formed of single crystalline silicon, whereas the TFT photodetector 100 is formed on the display panel 200 which is a glass substrate or a transparent flexible substrate using a transparent film formed of, for example, polyimide (PI), polyethylene terephthalate (PET), polypropylene (PP), polycarbonate (PC), polymethylmethacrylate (PMMA), polyethylenenaphthalate (PEN), polyetheretherketone (PEEK), polyethersulfone (PES), or polyarylate, and an active layer, a source, a drain, and a light receiving part are formed of amorphous silicon (a-Si) or polycrystalline (poly-Si) silicon. Glass or a PI film is amorphous, which makes it impossible to stack single crystalline silicon thereon. Therefore, when TFT photodetectors are formed on a glass substrate or a flexible substrate, the TFT photodetectors should be implemented in an amorphous or polycrystalline fashion. Under circumstances, the amorphous silicon or the polycrystalline silicon may be replaced with a material with a conductive property controllable by an electric field or tunneling. Throughout the specification, the term “PAT-PD” or “TFT PAT-PD” is interchangeably used with “TFT photodetector”.
Preferably, display pixels and image sensor pixels are matched to each other in a one-to-one correspondence. FIG. 2 illustrates an exemplary pixel structure of the display panel 200 with TFT photodetectors 100 formed thereon. A unit pixel 300 of the display panel 200 includes a light emitting area 310 for display, a switching TFT 320, and a TFT photodetector 100 for image sensing. The display panel 200 may be designed such that each unit pixel of the display panel and each unit pixel of the image sensor occupy areas of similar sizes and thus the display pixels and the image sensor pixels may be matched in a one-to-one correspondence per position. In this case, as the TFT photodetector 100 may operate, using the light emitting area 310 of the display pixel as a light source, a signal may be processed by matching the light emitting area 310 to the TFT photodetector 100, and data may be processed by matching data included in the light source to data collected by the TFT photodetector 100.
Although it is preferable to form the TFT photodetector 100 without any overlap with the light emitting area 310, the TFT photodetector 100 may be formed overlapping with the light emitting area 310 over a predetermined area because the TFT photodetector 100 occupies a small area relative to the light emitting area 310. However, to maximize photoelectric conversion, the introduction of unnecessary light is blocked by shielding an area except for the light receiving part of the TFT photodetector 100 with a metal or the like. The resulting shielding of a part of the light emitting area 310 with the light shielding area except for the light receiving part of the TFT photodetector 100 may decrease the light emission efficiency of the display.
In some cases, the display pixels and the image sensor pixels may be configured in different sizes. For example, when the unit pixels of the image sensor are designed such that one display pixel area corresponds to n image sensor pixels, n TFT photodetectors 100 share the light emitting area of one display pixel as a light source, making it difficult to control the TFT photodetectors 100 individually by light source control. However, the light source control may be simplified, which in turn simplifies an image sensing process. On the contrary, the unit pixels of the image sensor may be designed such that the area of one unit pixel of the image sensor corresponds to m display pixels. In this case, although fewer image sensor pixels than the number of pixels corresponding to the resolution of the display may be arranged, one TFT photodetector 100 uses the light emitting areas of m display pixels as light sources, and thus fine light source control required for image sensing and data processing may become difficult.
The light emitting area 310 may be formed in a different structure according to the type of a used display For example, when the display panel 200 of the electronic device 10 is a light emitting display such as an organic light emitting diode (OLED) display, the light emitting area 310 may be a light emitting pixel with red, green, blue (RGB) sub-pixels arranged therein. When the display panel 200 of the electronic device 10 is a light receiving display such as a liquid crystal display (LCD), RGB sub-filters may be arranged in the light emitting area 310. Obviously, the TFT photodetector 100 may use an external light source such as natural light as a light source for image sensing, instead of the light emitting area 310.
With reference made to FIG. 2 again, a plurality of unit pixels 300 are arranged in a lattice structure. Each unit pixel 300 may be formed by vertically stacking or arranging side by side a display sub-panel formed on a glass substrate or a transparent flexible substrate and an image sensor sub-panel formed on a glass substrate or a transparent flexible substrate. In this regard, FIG. 3 illustrates a cross section of a unit pixel 300 of the display.
Referring to FIG. 3, the unit pixel 300 of the display panel includes a display sub-panel 330 and an image sensor sub-panel 340. The display sub-panel 330 may include a light emitting area 310 for display and its switching TFT 320, and the image sensor sub-panel 340 may include a TFT photodetector 100 for image sensing and a driving TFT 344 for driving the TFT photodetector 100. The display sub-panel 330 or the image sensor sub-panel 340 is formed on a transparent glass substrate or a transparent flexible substrate such as a PI film (hereinafter, a glass substrate and a transparent substrate are interchangeably used with each other). The transparent display panel 200 may be formed by vertically stacking and attaching the two panels as illustrated in FIG. 3(a) or arranging the two panels side by side on the same glass substrate 334 as illustrated in FIG. 3(b).
Alternatively, the light emitting area 310 and the switching TFT 320 of an OLED device for display, and the TFT photodetector 100 for image sensing and the driving TFT 344 may be arranged together on the same glass substrate 332 or 342, as illustrated in FIG. 3(b). In this case, a driving switch 322 may be formed by incorporating a switching TFT for controlling the light emitting area 310 with a switching TFT for controlling the TFT photodetector 100, or the switching TFTs may be formed separately.
Throughout the specification, the display sub-panel 330 and the image sensor sub-panel 340 may also be referred to as a display pixel and an image sensor pixel, respectively.
As described before, the image sensor pixel 340 of a similar size to that of the display pixel 330 senses light and acquires an image by signal processing and detector driving, and includes the TFT photodetector 100 and the driving TFT 344 for driving the TFT photodetector 100. The switching TFT 320 for an output to be used for display, and the driving TFT 344 for driving the TFT photodetector 100 formed on an image sensor pixel basis may be integrated or configured separately. In this manner, the TFT photodetector 100 of the present disclosure is formed on a pixel basis.
Because the TFT photodetector 100 should be formed on an amorphous substrate such as a glass substrate or a PI film, not a single crystalline silicon substrate, the TFT photodetector 100 should be implemented in a different manner from an existing photodetector using single crystalline silicon. Now, a description will be given of a detailed structure, operation mechanism, fabrication method of a TFT photodetector according to the present disclosure.
FIG. 4 is a sectional view illustrating a TFT photodetector according to an embodiment of the present disclosure.
Referring to FIG. 4, the TFT photodetector 100 of the present disclosure is formed on the transparent substrate 342 such as an amorphous glass substrate or a flexible substrate, and includes a gate 150 formed of a-Si or poly-Si on the transparent substrate 342, an insulating oxide film 140 capable of controlling tunneling of optically excited charges, a drain 110, a source 120, and an active layer 130 in which a current channel is formed between the source 120 and the drain 110. While the drain 110, the source 120, the active layer 130, and the gate 150 are formed of a-Si or poly-Si, they may be formed of any other material as far as the material has a conductive property controllable by tunneling or an electric field.
The gate 150 is formed of N-type poly-Si or a-SI by implanting an N-type impurity and operates as a light receiving part that absorbs incident light. The active layer 130 is formed of P-type poly-Si or a-Si, with the insulating oxide film 140 between the active layer 130 and the gate 150. The active layer 130 forms a current channel according to optical excitation between the drain 110 and the source 120 which are P+-type diffusion layers.
An area on which light is incident is confined to the gate 150 serving as the light receiving part and the active layer 130 between which and the gate 150 the insulating oxide film 140 is interposed. For this purpose, a metal protection layer 160 may be formed on a boundary surface of the transparent substrate 342, except for the area between the transparent substrate 342 and the active layer 130, to shield unnecessary light introduced into the TFT photodetector 100. A metal shielding layer 170 may be formed in the remaining area except for the gate 150 in an upper part of the TFT photodetector 100. The shielding layer 170 may be formed by a silicide and metal process. The TFT photodetector 100 limits an area on which light is incident to the gate 150 serving as the light receiving part by means of the shielding layer 170, thereby maximizing the photoelectric change of the gate 150. Hereinbelow, the gate 150 and the light receiving part are interchangeably used throughout the specification.
In a state where no light is introduced, the TFT photodetector 100 controls biases of the gate 150, the drain 110, the source 120, and the active layer 130 to maintain a stable equilibrium state in which electrons are trapped. For this purpose, the metal protection layer 160 is provided on the boundary surface between the overlying shielding layer 170 and the transparent substrate 342 to shield unintended unnecessary light through the transparent substrate 342 of, for example, glass. Specifically, the active layer 130 between the source 120 and the drain 110 is bias-controlled to have a threshold voltage at which the potential state of a silicon surface on which a current channel may be formed in an initial fabrication process is shortly before a sub-threshold state. In this state, when light is not incident on the gate 150 as the light receiving part, photocurrent does not flow in the current channel.
When light is incident on the light receiving part, electrons are introduced into the N-type gate 150 by tunneling from the P-type active layer 130 to the insulating oxide film 140, among charges of the two PN areas excited with the insulating oxide layer 140 in between, the electron migration changes the threshold voltage of the current channel between the source 120 and the drain 110 in correspondence with a change in the total amount of charge in the gate 150, the threshold voltage modulation effect caused by the change in the amount of charge in the light receiving part causes a change in the conductance of the current channel, and thus photocurrent corresponding to the changed conductance flows.
Since the gate 150 is doped with holes, the electrons passed through the insulating oxide film 140 by tunneling are combined with holes in an area of the gate 150 near to the insulating oxide film 140, thereby generating a depletion layer at the top end of the insulating oxide film 140. Therefore, the threshold voltage drops due to a change in the charge of the active layer 130 near to the insulating oxide film 140, thereby exciting the current channel between the source 120 and the drain 110.
In other words, current that flows in the current channel excited between the source 120 and the drain 110 by light reception at the light receiving part is not a direct flow of charges of electron-hole pairs (EHPs) caused by the light reception but an indirect current flow in the current channel excited by tunneling of directly generated charges. Therefore, a very high-efficiency light detection capability may be achieved.
FIG. 5 is a sectional view illustrating a process of fabricating a TFT photodetector according to an embodiment of the present disclosure.
In FIG. 5(a), the P-type poly-Si or a-Si diffusion layer 130 to be used as an active layer is formed on the glass substrate 342 or a flexible substrate of, for example, a PI film, and two P+-type diffusion layers 111 and 121 are formed of a-SI or poly-Si at both sides of the diffusion layer 130.
The diffusion layers 130, 111, and 121 may be formed of a-SI or poly-Si. To increase mobility, the diffusion layers 130, 111, and 121 may be formed by depositing a-SI and then crystalizing the deposited a-Si into poly crystals by thermal treatment such as laser annealing, or directly depositing poly-Si on a transparent substrate.
Subsequently, a thin SiO₂or SiNx insulating oxide layer 141 is formed on the diffusion layers 130, 111, and 121. The insulating oxide layer 141 may be formed by sputtering or plasma enhanced chemical vapor deposition (PECVD).
Subsequently, an N-type diffusion layer 151 is formed of poly-SI or a-Si on the insulating oxide film 141 in the same manner.
Referring to FIG. 5(b), the gate 150 is then formed for use as a light receiving part by photo-patterning the generated diffusion layer 151. Referring to FIG. 5(c), only the generated insulating oxide layer 141 is etched away, remaining only a necessary part by a photoresist (PR) patterning process. Partial insulating oxide films 142 and 143 are removed together on areas of the diffusion layer 121, which are to be used as the source 120 and the drain 110, so that a source electrode and a drain electrode may be connected.
Referring to FIG. 5(d), the remaining area except for the areas to be used as the source 120 and the drain 110 is then removed from the P+-type diffusion layers 111 and 121 by etching. Electrodes are formed by depositing a metal or the like in the areas of the insulating oxide films 142 and 143 which have been removed in the source 120 and the drain 110.
In the TFT photodetector 100 fabricated in the above manner, current flows through a current channel excited between the source 120 and the drain 110 by tunneling, as described before. If the thickness of the active layer 130 is equal to or larger than a predetermined thickness, for example, 100 nm, a neutral area is produced separately in an area under the gate 150, which has not been depleted perfectly, except for the current channel generated by light. Unnecessary extra charges generated by light may be accumulated in the neutral area, and are likely to act as a changing factor to the threshold voltage which is linearly changed by light. Therefore, the extra charges need to be processed separately.
FIG. 6 is an energy band diagram referred to for describing a photoelectric conversion mechanism of a TFT photodetector according to an embodiment of the present disclosure.
When light is incident on the gate 150 as the light receiving part, EHPs are generated in the gate 150 and the active layer 130. Excited electrons of the active layer 130 tunnels through the insulating oxide film 140 by an electric field, thereby depleting a bottom end portion of the gate 150. As a result, the total charge amount of the gate 150 is changed, which leads to a threshold voltage modulation effect equivalent to application of a negative power source to the gate 150. Accordingly, a current channel is formed in the active layer 130 of poly-Si, and thus current flows between the source 120 and the drain 110. The TFT photodetector 100 implemented based on this structure and principle has a high-sensitivity detection capability of sensing even a single photon and enables very intense photocurrent to flow, even with a small amount of light.
FIG. 7 is an energy band diagram referred to for describing a tunneling mechanism of a TFT photodetector according to an embodiment of the present disclosure.
In the TFT photodetector 100, the shielding layer 170 is formed such that only the active layer 130 facing the shielding layer 170 with the gate 150 serving as the light receiving part and the insulating oxide film 140 in between is affected by light, with no effect of light on the remaining area. The shielding layer 170 may be formed by a silicide and metal process, and may not be formed on the gate 150 through a mask.
Light of multiple wavelengths incident on the TFT photodetector 100 is mostly transmitted through or absorbed to the gate 150 formed of poly-Si or a-Si.
If the thickness of the gate 150 is equal to or larger than a predetermined value, for example, 300 nm, short-wavelength light of the blue family in light incident on the TFT photodetector 100 is mostly absorbed to the gate 150, while only very partial short-wavelength light reaches the active layer 130 under the gate 150.
As described above, since the TFT photodetector 100 has an excellent high-sensitivity detection capability compared to a conventional photodetector, even though only a very small part of light of a short wavelength incident on the gate 150 is transmitted through the gate 150 and reaches the active layer 130, the threshold voltage of the current channel is changed and thus even a slight change in light may be sensed.
Light of the other wavelengths is also transmitted through the gate 150 and reaches the active layer 130 in the same principle. Accordingly, the same phenomenon as observed from reception of light of a short wavelength occurs to the gate 150, thereby causing a change in the threshold voltage of the current channel. However, because light of a relatively long wavelength is easily transmitted through the gate 150 and reaches the active layer 130, compared to light of a short wavelength, the light of a long wavelength generates more EHPs in the active layer 130. Therefore, more electrons migrate to the gate 150 through the insulating oxide film 140 by tunneling, causing a change in the threshold voltage of the current channel between the source 120 and the drain 110.
The metal protection layer 160 formed between the transparent substrate 342 and the active layer 130 blocks light introduced through the transparent substrate 342 from reaching an area other than the active layer 130. Therefore, the light is absorbed only to or transmitted only through the active layer 130 contacting the gate 150, and thus efficient tunneling through the insulating oxide film 140 occurs.
For more efficient tunneling, a predetermined voltage may be applied between the gate 150 of poly-Si and the active layer 130 of poly-Si, or a property such as dark current may be adjusted by adjusting a tunneling probability and controlling an initial threshold voltage of the TFT photodetector 100.
Then, when the intensity of light is decreased or light is blocked, tunneled electrons are re-tunneled to the active layer 130, and thus the amount of charge in the gate 150 returns to an original level. Accordingly, the formed depletion layer is reduced and, at the same time, photocurrent generated in the current channel is also reduced.
However, it may occur that charges have not completely disappeared and thus have remained in the active layer 130 even after the light blocking, causing an error such as a signal delay in the next light irradiation. To avert this problem, the thickness of the active layer 130 may be controlled such that an area remaining as an intermediate area, in which no channel is generated, may be reduced, or a reset device may be added to remove the charges remaining in the active layer 130.
FIG. 8 illustrates a mechanism for photoelectric conversion caused by a plurality of localized states in a TFT photodetector formed of a-Si or poly-Si.
FIG. 8(a) illustrates the energy band of general single crystalline silicon, and FIG. 8(b) illustrates the energy bands of the gate and the active layer of a TFT photodetector of a-Si or poly-Si.
In the TFT photodetector 100, electrons are introduced into the N-type gate 150 by tunneling from the P-type active layer 130 to the insulating oxide film 140, among charges of the two PN areas excited with the insulating oxide layer 140 in between, the electron migration changes the threshold voltage of the current channel between the source 120 and the drain 110 in correspondence with a change in the total amount of charge in the gate 150, the threshold voltage modulation effect caused by the change in the amount of charge in the light receiving part causes a change in the conductance of the current channel, and thus photocurrent corresponding to the changed conductance flows.
As the gate 150 as the light receiving part and the active layer 130 are formed of a-SI or poly-Si, instead of single crystalline silicon, according to an embodiment of the present disclosure, a plurality of localized energy levels are formed in the gate 150 and the active layer 130, thereby forming a wavelength extension layer 180 that extends the wavelength range of light absorbed by the TFT photodetector 100.
The wavelength extension layer 180 is formed of a-Si/poly-Si. As illustrated in FIG. 8(b), a plurality of local energy levels are generated through multiple localized states formed in a forbidden band between the conduction band and valence band of the gate 150 and the active layer 130. The localized states are naturally generated in the forbidden band in view of the nature of the a-Si/poly-Si structure, which obviates the need for applying stress or implanting ion to artificially form the localized states. Therefore, processes are simplified.
Accordingly, the TFT photodetector 100 may generate EHPs by absorbing light even at an energy level lower than 1.12 eV which is the band gap energy of the general single crystalline silicon, thereby enabling detection of the wavelength range of the near-infrared area, which is longer than a maximum detectable wavelength of silicon, 1150 nm, and detection of light of a wavelength that a general silicon photodiode is not capable of detecting.
As described above, because the TFT photodetector 100 is formed of a-Si or poly-Si, compared to a conventional photodetector formed of single crystalline silicon, the wavelength extension layer 180 including multiple localized states in the forbidden band exists, and there is no need for artificially forming localized states by applying uniaxial tensile stress on single crystalline silicon, combining hetero elements (e.g., Ge or the like), implanting ions (e.g., P, B, N, Ga, or the like), or increasing the doping density of an oxide film, poly-Si, and/or a substrate to control a thermal process strength. Therefore, a fabrication process is simplified.
As described before, the TFT photodetector 100 according to the embodiment of the present disclosure may generate a flow of photocurrent with an intensity higher than the conventional photodetector by hundreds of times to a few thousands of times, for the same light intensity.
Further, because the TFT photodetector 100 according to the embodiment of the present disclosure includes a plurality of localized states, the wavelength range in which a valid signal is detectable is extended. Thus, the TFT photodetector 100 is applicable to a sensor for biometric recognition, motion recognition, or the like.
While the TFT photodetector 100 has been described as implemented in a similar structure to a P-channel metal-oxide semiconductor (PMOS), this should not be construed as limiting. The TFT photodetector 100 may be implemented in a similar structure to an N-channel metal-oxide semiconductor (NMOS) by exchanging the doping impurities of the gate and the active layer.
As is apparent from the foregoing description, according to various embodiments, image sensing may be performed by using a display system used in a device without the need for providing a separate image sensor to the device.
Further, a light emitting light source for a display in the device may be used as a light source for an image sensor, which obviates the need for a separate light source for an image sensing process.
Further, because of the use of a-Si or poly-Si, not single crystalline silicon, light in the wavelength rage of the near-infrared area, which is longer than a maximum wavelength detectable in general silicon, 1150 nm may be detected.
While the disclosure has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims and their equivalents.

Claims

What is claimed is:

1. A method of fabricating a thin-film transistor (TFT) photodetector, the method comprising:

forming diffusion layers on a glass substrate or a transparent flexible substrate, wherein the diffusion layers include a P-type diffusion layer of P-type polycrystalline or amorphous silicon, for use as an active layer and P+-type diffusion layers of amorphous or polycrystalline silicon at both sides of the P-type diffusion layer;

forming an insulating oxide layer on the formed diffusion layers;

forming an N-type diffusion layer of polycrystalline or amorphous silicon on the insulating oxide film;

forming a gate to be used as a light receiving part by photo-patterning the N-type diffusion layer;

etching the generated insulating oxide layer except for only a necessary part in a photoresist (PR) patterning process;

removing a remaining area of the P+-type diffusion layer except for areas to be used as a source and a drain by etching; and

generating electrodes by depositing a metal in etched parts of the insulating oxide film in the source and the drain.

2. The method according to claim 1, wherein the insulating oxide film is formed by sputtering or plasma enhanced chemical vapor deposition (PECVD).

3. The method according to claim 1, wherein the N-type diffusion layer is formed by depositing amorphous silicon and then crystallizing the deposited amorphous silicon by thermal treatment, or directly depositing polycrystalline silicon on the transparent substrate.