Pixel unit for blood oxygen detection and non-invasive blood oxygen detector comprising same

Abstract

The present application relates to a pixel unit for blood oxygen detection and a non-invasive blood oxygen detector comprising the same. A pixel unit for blood oxygen detection can include a semiconductor substrate; a light gate covering a first region of the semiconductor substrate, the first region serving as a light sensing area of the pixel unit; a transfer gate spaced apart from the light gate by a distance and covering a second region of the semiconductor substrate, the light gate and the transfer gate and the semiconductor substrate being formed with a gate insulating layer therebetween; and a floating diffusion region formed in a third region of the semiconductor substrate adjacent to the transfer gate. The pixel unit of the present application simplifies the pixel structure and the manufacturing process, reduces the manufacturing cost, and reduces noise.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of non-invasive blood oxygen detection, and more particularly, to an improved pixel unit for blood oxygen detection, a non-invasive blood oxygen detector comprising an array of such pixel units, and an electronic device integrated with such non-invasive blood oxygen detector. BACKGROUND

[0002] The oxygen content in human blood (referred to as "blood oxygen" for short) is one of the important vital sign parameters, and it is necessary to monitor blood oxygen.

[0003] Currently, the measurement of blood oxygen saturation mainly includes two ways: invasive and non-invasive. The invasive measurement needs to extract blood from the human body, and the blood gas analysis is performed on the blood sample to determine the blood oxygen saturation, which is generally expressed by the percentage of oxygenated hemoglobin in total hemoglobin. The non-invasive measurement technology includes photoplethysmography (PPG), which uses spectrophotometric principle. Since the absorption rates of different components in blood to the same light are different, the content of different components in blood is calculated by measuring the attenuated spectrum of light passing through the blood. The PPG blood oxygen detection scheme is popular with users because it is convenient and quick, does not damage the human body, and is suitable for continuous monitoring of blood oxygen content changes. SUMMARY

[0004] One aspect of the present application provides a pixel unit for blood oxygen detection, comprising: a semiconductor substrate; a grating covering a first region of the semiconductor substrate, the first region serving as a light sensing area of the pixel unit; a transfer gate spaced apart from the grating by a distance and covering a second region of the semiconductor substrate, wherein the grating and the transfer gate have a gate insulating layer, a semiconductor layer and a metal layer formed between the grating and the semiconductor substrate respectively, and the contact potential difference between the semiconductor layer and the metal layer corresponding to the grating has the opposite sign of the contact potential difference between the semiconductor layer and the metal layer corresponding to the transfer gate; a floating diffusion region formed in a third region of the semiconductor substrate adjacent to the second region covered by the transfer gate.

[0005] According to an embodiment, the pixel unit further comprises: a charge storage region formed in a fourth region of the semiconductor substrate between the grating and the transfer gate.

[0006] According to an embodiment, no doped implantation region is formed in the fourth region of the semiconductor substrate between the grating and the transfer gate.

[0007] According to an embodiment, the semiconductor substrate has a first conductivity type, and the floating diffusion region and the charge storage region have a second conductivity type.

[0008] According to an embodiment, the transfer gate includes a first semiconductor layer formed on the gate insulating layer, formed of a semiconductor material having the first conductivity type, and a second metal layer formed on the first semiconductor layer, formed of a conductive metal material, thereby forming a first contact potential difference between the second metal layer and the first semiconductor layer.

[0009] According to an embodiment, the photo gate includes a third semiconductor layer formed on the gate insulating layer, formed of a semiconductor material having the second conductivity type, and a fourth metal layer formed on the third semiconductor layer, formed of a conductive metal material, thereby forming a second contact potential difference between the fourth metal layer and the third semiconductor layer.

[0010] According to an embodiment, during a photo sensing operation, the photo gate receives a first potential to form a depletion region in a first region of the semiconductor substrate, and the transfer gate receives a second potential to turn off a second region of the semiconductor substrate. At a read operation, the transfer gate receives the first potential to turn on the second region of the semiconductor substrate, and the photo gate receives the second potential, with an electric field formed by the first potential and the second potential, to move photo-generated charges formed in the first region of the semiconductor substrate into the floating diffusion region.

[0011] Another aspect of the present application provides a non-invasive blood oxygen detector, comprising: a light source configured to emit a probe light; a photoelectric sensor comprising an array of pixel units as described above, configured to detect a transmitted light or a reflected light formed after the probe light irradiates a detection object; and a control unit configured to determine a blood oxygen content of the detection object based on the detected transmitted light or reflected light.

[0012] Another aspect of the present application provides a portable electronic device comprising the non-invasive blood oxygen detector as described above. The electronic device can be a mobile phone, a tablet computer, or a smart wearable device.

[0013] The above and other features and advantages of the present application will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a schematic diagram of a pixel unit for PPG blood oxygen detection.

[0015] Figure 2 is a schematic diagram of another pixel unit for PPG blood oxygen detection.

[0016] Figure 3 is a schematic diagram of a pixel unit for PPG blood oxygen detection according to an embodiment of the present application.

[0017] Figure 4is a schematic diagram of a pixel cell for PPG blood oxygen detection according to another embodiment of the application.

[0018] Figure 5 is a schematic diagram of a gate structure in a pixel cell for PPG blood oxygen detection according to an embodiment of the application.

[0019] Figure 6 is a schematic diagram of a non-invasive blood oxygen detector according to an embodiment of the application.

[0020] Figure 7 is a schematic diagram of an electronic device comprising a non-invasive blood oxygen detector according to an embodiment of the application. DETAILED DESCRIPTION

[0021] In the following, example embodiments according to the present application will be described in detail with reference to the accompanying drawings. Note that the accompanying drawings can not be drawn to scale. Obviously, the described embodiments are only a part of the embodiments of the present application, and the present application is not limited to the example embodiments described herein.

[0022] Figure 1 is a schematic diagram of a pixel cell for PPG blood oxygen detection employing a single p-n junction and three transistors, also referred to as an active pixel structure. Referring to Figure 1 , the pixel cell comprises an n+ doped region 11 formed in a p-type substrate 10, which forms a p-n junction with the p-type substrate 10 and serves as a photosensitive region. A readout circuit associated with the p-n junction comprises a reset transistor RST, a source follower transistor SF serving as a signal amplifier, and a row selector transistor RS. The reset transistor RST can be a PMOS transistor, whose source terminal can be connected to a common drive voltage V DD , and whose drain terminal can be connected to the photosensitive region 11 for performing a reset operation on the photosensitive region 11 under control of a reset signal. The source follower transistor SF and the row selector transistor RS can be NMOS transistors, wherein the gate of the source follower transistor SF is connected to the photosensitive region 11, the drain is connected to the common drive voltage V DD , and the source is connected to the drain of the row selector transistor RS. The gate of the row selector transistor RS receives a row selection signal, and the source provides an output signal OUT. In operation, the reset transistor RST is first turned on to perform a reset operation on the photosensitive region 11 to remove residual charges therein. After the reset operation is completed, the reset transistor RST is turned off, and an exposure process is then performed, in which transmitted or reflected light after passing through a human body is incident on the photosensitive region 11 to generate photo-generated charges. After the exposure process is completed, the row selector transistor RS is turned on, and the photo-generated charges in the photosensitive region 11 are amplified by the source follower transistor SF and read out, thereby generating the output signal OUT. The above reset, exposure and readout steps can be repeated to obtain a plurality of output signals OUT.

[0023] However, Figure 1 The pixel unit of the PPG pixel structure introduces KTC noise when performing the reset operation, which is essentially a kind of thermal noise generated by the thermal noise conduction to the PN junction capacitor when the channel resistance of the reset transistor RST is turned on. The higher the temperature, the greater the KTC noise.

[0024] In order to solve the reset KTC noise and reduce the dark current, a clamping photodiode (PPD) and a four-transistor (4T) structure are proposed, as shown in Figure 2 Referring to Figure 2 , a p+ doped region 12 is further formed in the surface portion of the n+ doped region 11, so as to obtain a p-n-p structure of the light sensing region, which can reduce the dark current. In addition, the photo-generated charges generated in the light sensing region are read into the floating diffusion region 13 through the transfer transistor TX, and then read out by the readout circuit, which can be an n+ doped region in the example of Figure 2 . In operation, the exposure operation can be performed, and the electron-hole pairs generated in the p-n-p light sensing region are separated due to the existence of the PPD electric field, and the electrons move towards the n region and the holes move towards the p region. At the end of the exposure, the reset transistor RST is turned on to perform the reset operation on the floating diffusion region 13 to remove the residual charges therein. After the reset is completed, the first sampling is performed, i.e. the reset transistor RST is turned off, and the reset level in the floating diffusion region 13 is read out by using the source follower transistor SF and the row selector transistor RS to obtain the reset level output OUT1, which contains the KTC noise introduced by the reset operation and the flicker noise (1 / f noise) and offset noise of the source follower transistor SF serving as a signal amplifier. Then, the transfer transistor TX is activated to transfer the photo-generated charges to the floating diffusion region 13, and the second sampling, i.e. the signal level readout operation, is performed to obtain the signal level output OUT2. The signal level OUT2 is subtracted from the reset level OUT1 to obtain the final output signal. Through the above correlated double sampling (CDS) operation, the KTC noise, the flicker noise and the offset noise can be removed, and the detection accuracy is greatly improved.

[0025] However, in the PPG pixel structure of Figure 2 , in order to facilitate the transfer of the photo-generated charges (electrons in this example) to the floating diffusion region 13 through the channel region below the transfer gate TX, it is required that the edge of the p+ doped region 12 on the side of the transfer transistor TX cannot be aligned with the edge of the n+ doped region 11 (otherwise it will affect the transfer of the electrons), but slightly retreats, which leads to the complication of the manufacturing process and high precision requirements. Generally, the standard CMOS image sensor manufacturing process needs to be used to manufacture Figure 2The PPG non-invasive blood oxygen detection pixel structure of the prior art involves using more mask plates to form a doping mask layer, and different doping angles, doping concentrations, etc. may need to be adopted, greatly increasing the manufacturing cost of the PPG pixel structure.

[0026] Figure 3 A schematic diagram of a pixel unit for PPG blood oxygen detection according to an embodiment of the present application is shown, which has a simple structure while ensuring a good signal-to-noise ratio, can simplify the manufacturing process, and thus reduce the cost.

[0027] Referring to Figure 3 , the PPG pixel unit includes a semiconductor substrate 100, which can be, for example, a p-type substrate. A photogate PG is formed on the substrate 100 and covers a first region of the substrate 100, which serves as a light-sensing region of the PPG pixel unit. In Figure 3 the embodiment shown, no doping implantation region can be formed in the first region of the semiconductor substrate 100 covered by the photogate PG, or in other words, no p-n junction can be formed in the light-sensing region. The photogate PG is spaced apart from the semiconductor substrate 100 by a gate insulating layer. For example, the semiconductor substrate 100 can be a silicon substrate, and the gate insulating layer can be a silicon dioxide layer. A transfer gate TX is formed on the substrate 100 and covers a second region of the substrate 100, and the transfer gate TX is spaced apart from the photogate PG by a distance, which can be appropriately set to facilitate the transfer operation of photo-generated charges, as described in detail below. The transfer gate TX is also spaced apart from the semiconductor substrate 100 by a gate insulating layer.

[0028] A floating diffusion region 130 can be formed in a third region of the substrate 100 adjacent to the second region covered by the transfer gate TX. In some embodiments, the third region can be located on the side of the second region covered by the transfer gate TX opposite to the first region covered by the photogate PG. In Figure 3 the embodiment shown, the floating diffusion region can be, for example, an n+ doped region.

[0029] The readout circuit can be the same as the readout circuit shown in Figure 1 and Figure 2 , including a reset transistor RST, a source follower transistor SF serving as a signal amplifier, and a row selector transistor RS. The reset transistor RST can be a PMOS transistor, the source terminal of which can be connected to a common drive voltage V DD , and the drain terminal of which can be connected to the floating diffusion region 130. The source follower transistor SF and the row selector transistor RS can be NMOS transistors, in which the gate of the source follower transistor SF is connected to the floating diffusion region 130, and the drain is connected to the common drive voltage V DDThe source is connected to the drain of the row selector transistor RS. The gate of the row selector transistor RS receives the row select signal, and the source provides the output signal OUT.

[0030] for Figure 3 The pixel unit shown can employ correlated double sampling to improve the signal-to-noise ratio of the output signal, thereby improving the accuracy of blood oxygen detection. Specifically, during operation, a high voltage can be applied to the grating PG, which forms a depletion region in the first region (i.e., the photosensitive region) of the semiconductor substrate 100 covered by the grating PG. At least a portion of the transmitted or reflected light generated after irradiation of the human body can pass through the grating PG and irradiate the depletion region, generating photogenerated charges, where electrons and holes are separated by the electric field. At this time, a low voltage can be applied to the transfer gate TX, causing the second region of the semiconductor substrate 100 it covers to be cut off / turned off, so that the photogenerated charges will not be transferred to the floating diffusion region 130 through the second region.

[0031] After exposure, the reset transistor RST can be turned on to reset the floating diffusion region 13, removing any residual charge. In some embodiments, the reset operation can be performed first, followed by the exposure operation, or the exposure and reset operations can be performed simultaneously. This is because the low voltage applied to the transfer gate TX ensures that the second region (channel region) it covers is in a cutoff state, thus preventing charge leakage between the photosensitive region (first region) and the floating diffusion region (third region) 130. Embodiments of the present invention are not limited to any specific execution order of the reset and exposure steps.

[0032] After the reset operation and the exposure operation are completed, a correlated double sampling operation can be performed. Specifically, the reset level in the floating diffusion region 130 can be first sampled, at which time the reset transistor RST can be rendered non-conductive, the row selector transistor RS is rendered conductive by controlling the row selection signal, and the reset level in the floating diffusion region 130 is amplified by the source follower transistor SF and then read out, thereby generating a reset level output signal OUT1, which can be stored in, for example, a first capacitor (not shown). Then, a low voltage can be applied to the gate PG, and a high voltage can be applied to the transfer gate TX, under the action of the electric field formed by the two, the photo-generated charges, in this embodiment, electrons, in the photosensitive region are transferred to the floating diffusion region 130. The signal level in the floating diffusion region 130 can then be sampled, at which time the reset transistor RST remains in a cut-off state, the row selector transistor RS is rendered conductive by controlling the row selection signal, and the signal level in the floating diffusion region 130 is amplified by the source follower transistor SF and then read out, thereby generating a signal level output signal OUT2, which can be stored in, for example, a second capacitor (not shown). The final output signal is obtained by subtracting the reset level OUT1 from the signal level OUT2. Through the above-mentioned correlated double sampling (CDS) operation, KTC noise, flicker noise, and offset noise, etc. can be removed, and the detection accuracy can be greatly improved.

[0033] In Figure 3 the embodiment, by using the gate PG without forming a p-n junction in the photosensitive region, the process of doping injection related to the formation of the p-n junction can be avoided, and the process precision requirement comparable to the standard process of the image sensor can also be avoided, so that the manufacturing process can be greatly simplified, and the manufacturing cost can be reduced. At the same time, Figure 3 the pixel unit of the embodiment can adopt a correlated double sampling operation, so that a low noise level can be maintained, and the signal-to-noise ratio of the sampled signal can be ensured.

[0034] Figure 4 is a schematic diagram of a pixel unit for PPG blood oxygen detection according to another embodiment of the present application. Figure 4 The pixel unit shown in Figure 3 includes some same features, which are indicated by the same reference numerals, and repeated description thereof will be omitted here, and the differences between the pixel unit of Figure 4 and the pixel unit of Figure 3 will be mainly described.

[0035] As shown in Figure 4 , in the fourth region of the semiconductor substrate 100 between the gate PG and the transfer gate TX, the charge storage region 110 can be formed. In Figure 4In some embodiments, when the substrate 100 is a p-type substrate, the charge storage region 110 can be an n+ doped region. The charge storage region 110 can be used to temporarily store photogenerated charges generated during exposure. Figure 4 In this embodiment, the charge is electrons, which allows for the accumulation of more photogenerated charge during exposure, preventing charge saturation in the photosensitive area. The photogenerated charge in the charge storage region 110 can then be transferred to the floating diffusion region 130 and read. Signal reading can be performed using the related double sampling operation described above, which will not be repeated here. Figure 4 Other aspects of the pixel unit shown can be compared with Figure 3 The embodiments shown are the same and will not be described again here.

[0036] From the above about Figure 3 and Figure 4 As can be understood from the described embodiments, ensuring effective isolation between the floating diffusion region 130 and the photosensitive region or charge storage region 110 is very effective in reducing noise. To ensure effective isolation between the floating diffusion region 130 and the photosensitive region or charge storage region 110, in some embodiments of the invention, the structure of the transfer gate TX is also improved, as shown in… Figure 5 In the schematic diagram. Figure 5 Other aspects of the pixel structure shown can be compared with Figure 4 Same, and Figure 5 The read circuitry, namely the reset transistor RST, the source follower transistor SF, and the row selector transistor RS, is omitted.

[0037] Reference Figure 5 The transfer gate TX may include a semiconductor layer 122 formed on a gate insulating layer and a metal layer 124 formed on the semiconductor layer 122. The semiconductor layer 122 may have the same conductivity type as the substrate 100. Figure 5 In this embodiment, it is p-type. For example, semiconductor layer 122 may include p-type polysilicon, and metal layer 124 may include a conductive metal material or alloy. When metal layer 124 contacts semiconductor layer 122, a contact potential difference is formed between them. For semiconductor layer 122 formed of p-type polysilicon semiconductor material, when a low potential V is applied to metal layer 124... L When (for example, ground potential GND), due to the presence of a contact potential difference of approximately -0.4V, the potential on semiconductor layer 122 is approximately (V). L -0.4)V. And low potential V L In comparison, approximately (V) on semiconductor layer 122 LA potential of -0.4V can better keep the second region of the substrate 100 under the transfer gate TX in a cutoff state, thereby preventing photogenerated charge or noise charge in the photosensitive area from being transferred to the floating diffusion region 130 during exposure, reset and reset level sampling of the pixel unit, thereby further reducing noise.

[0038] Figure 5 An embodiment of the structure of the grating PG is also shown. For example... Figure 5 As shown, the grating PG may include a semiconductor layer 126 formed on a gate insulating layer and a metal layer 128 formed on the semiconductor layer 126. The semiconductor layer 126 may have a different conductivity type than the substrate 100. Figure 5 In this embodiment, the substrate 100 is p-type and the semiconductor layer 126 is n-type. For example, the semiconductor layer 126 may include n-type polysilicon, and the metal layer 128 may include a conductive metal material or alloy. In some embodiments, the material or thickness of the metal layer 128 may be selected such that the probe light can at least partially penetrate the grating PG, or the metal layer 128 may only cover a portion of the semiconductor layer 126. For example, the metal layer 128 may be a contact point or have one or more openings formed thereon to expose at least a portion of the semiconductor layer 126, so that the probe light can penetrate the grating PG and illuminate the photosensitive area of ​​the semiconductor substrate 100. Similarly, when the metal layer 128 contacts the semiconductor layer 126, a contact potential difference is formed between them. For the semiconductor layer 126 formed of n-type polysilicon semiconductor material, when a high potential V is applied to the metal layer 128... H At that time, due to the presence of a contact potential difference of approximately +0.4V, the potential on semiconductor layer 126 is approximately (V H +0.4)V. And high potential V H In comparison, approximately (V) on semiconductor layer 122 H A potential of +0.4V can better form a depletion layer in the first region (i.e., the photosensitive area) of the substrate 100 under the grating PG, thereby facilitating the formation of photogenerated charges during exposure, which also improves the signal-to-noise ratio of the sensing signal.

[0039] Refer to the above Figures 1 to 5 In the described embodiments, a specific conductivity type is used as an example, such as a p-type substrate, an n+ region for the charge storage region and the floating diffusion region, and the detected photogenerated charge being electrons. However, it should be understood that the opposite conductivity type can also be used, for example, an n-type substrate, a p+ region for the charge storage region and the floating diffusion region, and the detected photogenerated charge being holes. When the opposite conductivity type is used, other related aspects of the invention can be modified accordingly, and such modifications will be apparent to those skilled in the art under the teachings of this invention.

[0040] Figure 6is a schematic diagram of a non-invasive blood oxygen detector 200 according to an embodiment of the present application. As shown in Figure 6 The non-invasive blood oxygen detector 200 can include a controller 210, a light source 220, and a photoelectric sensor 230. The light source 220 can include, for example, a light emitting diode (LED) light source, and it can also include a driving circuit for driving the LED. The controller 210 can control the light source 220 to emit probe light. After the probe light is irradiated to a detection object, for example, a human body, reflected light or transmitted light can be generated, which can be detected by the photoelectric sensor 230. The photoelectric sensor 230 can include an array of pixel units as described above with respect to Figures 3 to 5 The non-invasive blood oxygen detector 200 can include a controller 210, a light source 220, and a photoelectric sensor 230. The light source 220 can include, for example, a light emitting diode (LED) light source, and it can also include a driving circuit for driving the LED. The controller 210 can control the light source 220 to emit probe light. After the probe light is irradiated to a detection object, for example, a human body, reflected light or transmitted light can be generated, which can be detected by the photoelectric sensor 230. The photoelectric sensor 230 can include an array of pixel units as described above with respect to

[0041] Figure 6 The non-invasive blood oxygen detector 200 shown can be integrated into a portable electronic device, Figure 7 is a schematic diagram of a portable electronic device 300 including a non-invasive blood oxygen detector 200 according to an embodiment of the present application. Such a portable electronic device 300 can be, for example, a mobile phone, a smart watch, a tablet computer, or a smart wearable device, etc. In this way, a user can conveniently detect blood oxygen concentration and heart rate at any time and any place, and the history record of the blood oxygen concentration and the heart rate can be stored in a local memory, or can also be stored in a cloud server, or can be sent to a hospital information system (HIS) of a hospital where the user is treated, so that when the blood concentration or the heart rate of the user is abnormal, the relevant medical structure or the medical staff responsible for the user can be timely notified.

[0042] The basic principles of the present application are described above in conjunction with specific embodiments, but it should be noted that the advantages, advantages, effects, etc. mentioned in the present application are only examples and are not limiting, and these advantages, advantages, effects, etc. cannot be considered as the must-have of each embodiment of the present application. In addition, the above specific details disclosed are only for the purpose of example and for the purpose of understanding, and are not limiting, and the above details do not limit the present application to the must-use of the above specific details.

[0043] The block diagrams of the devices, apparatuses, devices, systems involved in the present application are only illustrative examples and are not intended to require or imply that the connection, arrangement, configuration must be as shown in the block diagrams. As those skilled in the art will recognize, the devices, apparatuses, devices, systems can be connected, arranged, configured in any manner. Each block shown in the diagram can be subdivided into multiple sub-blocks, each of which can implement the relevant functions or steps, so that multiple sub-blocks can implement the functions of a large block before subdivision. Alternatively, multiple blocks shown in the diagram can be combined into one block, which can implement the functions of the multiple blocks before combination. In the present application, words such as "include", "contain", "have" and the like are open-ended words, which mean "include but not limited to", and can be used interchangeably. The words "or" and "and" used herein mean the word "and / or", and can be used interchangeably unless the context clearly indicates otherwise. The word "such as" used herein means the phrase "such as but not limited to", and can be used interchangeably.

[0044] It should also be noted that in the devices, apparatuses and methods of the present application, each component or each step can be decomposed and / or recombined. These decompositions and / or recombinations should be considered as equivalent solutions of the present application.

[0045] The above description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present application. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other aspects without departing from the scope of the present application. Thus, the present application is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0046] The above description has been given for the purpose of illustration and description. Furthermore, this description does not intend to limit the embodiments of the present application to the forms disclosed herein. Although a number of example aspects and embodiments have been discussed above, those skilled in the art will recognize certain variations, modifications, changes, additions and sub-combinations thereof.

Claims

1. A pixel cell for blood oxygen detection, comprising: a semiconductor substrate; a photo gate covering a first region of the semiconductor substrate, the first region serving as a light sensing area of the pixel cell; a transfer gate spaced apart from the photo gate by a distance and covering a second region of the semiconductor substrate, wherein the photo gate and the transfer gate and the semiconductor substrate are formed with a gate insulating layer, a semiconductor layer and a metal layer respectively, and a contact potential difference between the semiconductor layer and the metal layer corresponding to the photo gate has an opposite sign to a contact potential difference between the semiconductor layer and the metal layer corresponding to the transfer gate; a floating diffusion region formed in a third region of the semiconductor substrate adjacent to the second region covered by the transfer gate. 2.The pixel cell of claim 1, further comprising: a charge storage region formed in a fourth region of the semiconductor substrate between the photo gate and the transfer gate.

3. The pixel cell of claim 1, wherein, No doped implantation region is formed in the fourth region of the semiconductor substrate between the photo gate and the transfer gate.

4. The pixel cell of claim 2, wherein, The semiconductor substrate has a first conductivity type, and the floating diffusion region and the charge storage region have a second conductivity type.

5. The pixel cell of claim 4, wherein, The transfer gate comprises: a first semiconductor layer formed on the gate insulating layer and formed of a semiconductor material having the first conductivity type; and a second metal layer formed on the first semiconductor layer and formed of a conductive metal material, thereby forming a first contact potential difference between the second metal layer and the first semiconductor layer.

6. The pixel cell of claim 4, wherein, The photo gate comprises: a third semiconductor layer formed on the gate insulating layer and formed of a semiconductor material having the second conductivity type; and a fourth metal layer formed on the third semiconductor layer and formed of a conductive metal material, thereby forming a second contact potential difference between the fourth metal layer and the third semiconductor layer.

7. The pixel cell of claim 1, wherein, During a light sensing operation, the photo gate receives a first potential to form a depletion region in the first region of the semiconductor substrate, and the transfer gate receives a second potential to turn off the second region of the semiconductor substrate; During a read operation, the transfer gate receives the first potential to turn on the second region of the semiconductor substrate, and the photo gate receives the second potential, and an electric field formed by the first potential and the second potential causes photo-generated charges formed in the first region of the semiconductor substrate to move into the floating diffusion region. 8.A non-invasive blood oxygen detector, comprising: a light source for emitting probe light; a photoelectric sensor comprising an array of pixel cells according to any one of claims 1 to 7 for detecting transmitted light or reflected light formed after the probe light irradiates a detection object; and a control unit for determining blood oxygen content of the detection object based on the detected transmitted light or reflected light. 9.A portable electronic device comprising the non-invasive blood oxygen detector of claim 8. The electronic device is a mobile phone, a tablet computer, or a smart wearable device.

10. The electronic device of claim 9, wherein, ​

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