High-gain NPN type heterojunction photoelectric detector and preparation method thereof

Abstract

The invention discloses a high-gain NPN-type heterojunction photoelectric detector and a preparation method thereof, and relates to the field of photoelectric detectors, and the NPN-type heterojunction photoelectric detector comprises a first N-type semiconductor layer, a P-type semiconductor layer and a second N-type semiconductor layer which are sequentially arranged between a positive electrode and a negative electrode. The first N-type semiconductor layer and the second N-type semiconductor layer are made of the same material, and the doping concentration of the P-type semiconductor layer is larger than that of the first N-type semiconductor layer. According to the photoelectric detector, the photo-generated current of the photoelectric detector is remarkably improved by applying the bias voltage to the photoelectric detector, high responsivity is achieved, and decoupling of the restriction relation between the responsivity and the response time is achieved. By designing the NPN-type heterostructure detector, the potential barrier at the bottom and the depletion region of the middle layer can effectively reduce dark current, and high specific detection rate is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of photodetectors, and particularly to an NPN-type heterojunction photodetector with high gain and a preparation method thereof. Background Art

[0002] Infrared detection technology is a key means of obtaining target information by capturing infrared electromagnetic waves radiated or reflected by an object. The existing mainstream photovoltaic detectors are limited by high manufacturing costs and the need for low-temperature operation. Photoconductive quantum well infrared photodetectors face inherent bottlenecks such as low specific detectivity and slow response speed, and although thermophotovoltaic infrared detectors have the advantage of low cost, their performance indicators are weak, which limits the design of future high-performance detection devices. On the other hand, with the breakthrough progress of micro-nano processing technology and new semiconductor materials, infrared detector materials are evolving from traditional bulk materials to new two-dimensional materials.

[0003] Among them, two-dimensional transition metal sulfides have an adjustable bandgap, a bandgap that varies with the number of layers, high electron mobility, easy manufacturability, and good chemical stability, providing multiple possibilities for the material combination of detection devices. Coupled with the current mature heterojunction growth process, two-dimensional materials can complement each other's defects and play their respective advantages, making them one of the materials for preparing high-performance detection devices.

[0004] Responsivity, response time, and specific detectivity are the main indicators for measuring the performance of a detector. Among them, responsivity measures the sensitivity of the detector to the input signal, response time measures the dynamic response speed of the detector to signal changes, and specific detectivity measures the minimum detectable signal ability of the detector in a noise background. However, for traditional device structure designs, there is generally a constraint that responsivity, response time, and specific detectivity cannot all reach excellent levels simultaneously. How to break through the constraints of the three indicators of photodetectors to achieve high-performance detection has always been the mainstream research direction.

[0005] Therefore, a photodetector design with ultra-high responsivity, ultra-high specific detectivity, and relatively fast response speed is proposed. Summary of the Invention

[0006] In view of the problems existing in the above-mentioned prior art, the present invention is proposed.

[0007] Therefore, the problem to be solved by the present invention is how to perform detection in the near-infrared band and at the same time achieve excellent responsivity, specific detectivity, and response time.

[0008] To solve the above technical problems, in a first aspect, the present invention provides the following technical solution: An NPN-type heterojunction photodetector with high gain, which includes a first N-type semiconductor layer, a P-type semiconductor layer, and a second N-type semiconductor layer sequentially arranged between positive and negative electrodes; a first heterojunction barrier is formed between the first N-type semiconductor layer and the P-type semiconductor layer to block the injection of electrons into the first N-type semiconductor layer; a second heterojunction barrier is formed between the second N-type semiconductor layer and the P-type semiconductor layer to block the injection of holes into the second N-type semiconductor layer; a bias voltage is applied to the positive and negative electrodes.

[0009] As a preferred solution of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the doping concentrations of the first N-type semiconductor layer and the second N-type semiconductor layer are greater than the doping concentration of the P-type semiconductor layer, so that part of the P-type semiconductor layer is in the depletion region.

[0010] As a preferred solution of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the electron affinities of the first N-type semiconductor layer and the second N-type semiconductor layer are less than the electron affinity of the P-type semiconductor layer, and the valence band energy of the first N-type semiconductor layer is lower than the valence band energy of the P-type semiconductor layer.

[0011] As a preferred solution of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the materials of the first N-type semiconductor layer and the second N-type semiconductor layer are both HfS 2 , the material of the P-type semiconductor layer is NiS 2 ; the doping concentration of the first N-type semiconductor layer is greater than or equal to 10 15 cm 3 and less than or equal to 10 16 cm 3 , the doping concentration of the second N-type semiconductor layer is greater than or equal to 10 16 cm 3 , the doping concentration of the P-type semiconductor layer is less than or equal to 10 14 cm 3 .

[0012] As a preferred solution of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: it includes a first electrode layer, a first transparent glass layer, and a second transparent glass layer sequentially arranged on the surface of the above-mentioned first N-type semiconductor layer, wherein the refractive index of the first transparent glass layer is greater than that of the second transparent glass layer; a second electrode layer and a metal reflection layer are sequentially arranged on the surface of the above-mentioned second N-type semiconductor layer.

[0013] As a preferred embodiment of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the materials of the first electrode layer and the second electrode layer are graphene, the material of the first transparent glass layer is VITRON_IG2, and the material of the second transparent glass layer is MgF 2 , and the material of the metal reflection layer is silver.

[0014] As a preferred embodiment of the NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the thickness ranges of the first N-type semiconductor layer and the second N-type semiconductor layer are 20 nm to 100 nm, and the thickness range of the P-type semiconductor layer is 80 nm to 200 nm.

[0015] On the other hand, the present invention also provides a method for manufacturing an NPN-type heterojunction photodetector with high gain. This method is applicable to manufacturing the above-mentioned NPN-type heterojunction photodetector with high gain. The manufacturing method includes sequentially forming a first N-type semiconductor layer, a P-type semiconductor layer, and a second N-type semiconductor layer between the positive and negative electrodes of the photodetector; wherein, the materials of the first N-type semiconductor layer and the second N-type semiconductor layer are the same, and the doping concentration of the P-type semiconductor layer is greater than that of the first N-type semiconductor layer.

[0016] As a preferred embodiment of the method for manufacturing an NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the material of the first N-type semiconductor layer is HfS 2 , using HfCl 4 and H 2 S as precursors, growing on the surface of one of the electrodes by atomic deposition method, and introducing Cl 2 gas for N-type doping.

[0017] As a preferred embodiment of the method for manufacturing an NPN-type heterojunction photodetector with high gain according to the present invention, wherein: the material of the P-type semiconductor layer is NiS 2 , using Ni(CO) 3 and sulfur powder as raw materials, growing on the surface of the first N-type semiconductor layer by chemical vapor deposition, and introducing FeCl 3 vapor for P-type doping.

[0018] The beneficial effects of the present invention are as follows: By applying a bias voltage to the photodetector, the photocurrent of the photodetector is significantly increased, achieving high responsivity. According to the principle of the NPN heterojunction photodetector, the response time of the device is determined by the speed of the hole concentration accumulated at the junction, that is, by the carrier lifetime of the middle light-absorbing layer. The responsivity can increase with the increase of the bias voltage, decoupling the constraint relationship between the responsivity and the response time. By designing the NPN heterostructure detector, the bottom barrier and the depletion region of the middle layer can effectively reduce the dark current, achieving a high specific detectivity. Brief Description of the Drawings

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0020] Figure 1 It is a schematic structural diagram of the photodetector of the present invention.

[0021] Figure 2 It is a schematic diagram of the heterojunction energy band of the photodetector of the present invention.

[0022] Figure 3 It is a schematic diagram of the movement of electrons and holes in the heterojunction energy band of the photodetector of the present invention.

[0023] Figure 4 It is a schematic diagram of the change of the absorption curve of the photodetector of the present invention with the thickness of the second transparent glass layer (MgF 2 )

[0024] Figure 5 It is a contour map of the potential change in the semiconductor region of the photodetector of the present invention in the absence of light.

[0025] Figure 6 It is a contour map of the potential change in the semiconductor region of the photodetector of the present invention under illumination.

[0026] Figure 7 It is a graph showing the influence of the doping concentration of the second N-type semiconductor layer on the responsivity of the photodetector of the present invention.

[0027] Figure 8 It is a schematic diagram showing the influence of the thickness of the second N-type semiconductor layer on the responsivity of the photodetector of the present invention.

[0028] Figure 9 It is a schematic diagram showing the influence of the thickness of the second N-type semiconductor layer on the specific detectivity of the photodetector of the present invention.

[0029] Figure 10 Schematic diagram showing the influence of the carrier lifetime in the P-type semiconductor region on the responsivity and specific detectivity of the photodetector of the present invention.

[0030] Figure 11 Schematic diagram showing the influence of the carrier lifetime in the P-type semiconductor region on the responsivity of the photodetector of the present invention.

[0031] Figure 12 Graph showing the relationship between the responsivity and external quantum efficiency of the photodetector of the present invention under a bias voltage.

[0032] Figure 13 Graph showing the relationship between the noise equivalent power and specific detectivity of the photodetector of the present invention under a bias voltage.

[0033] Figure 14 Response time curve of the photodetector of the present invention.

[0034] In the figure: 1, the first N-type semiconductor layer; 2, the P-type semiconductor layer; 3, the second N-type semiconductor layer; 4, the first electrode layer; 5, the first transparent glass layer; 6, the second transparent glass layer; 7, the second electrode layer; 8, the metal reflection layer. Detailed implementation manners

[0035] To make the above objects, features, and advantages of the present invention more apparent and understandable, the following describes the detailed implementation manners of the present invention with reference to the accompanying drawings of the specification.

[0036] In the following description, many specific details are set forth to facilitate a thorough understanding of the present invention. However, the present invention may be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0037] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation manner of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or alternative embodiments that exclude each other with other embodiments.

[0038] Referring to Figures 1 to 14 , which is an embodiment of the present invention. This embodiment provides an NPN-type heterojunction photodetector with high gain. The NPN-type heterojunction photodetector with high gain includes a first N-type semiconductor layer 1, a P-type semiconductor layer 2, and a second N-type semiconductor layer 3 sequentially disposed between the positive and negative electrodes.

[0039] Specifically, referring to Figure 1, The structure of the NPN heterojunction photodetector with high gain from bottom to top is successively a metal reflection layer 8, a second electrode layer 7, a second N-type semiconductor layer 3, a P-type semiconductor layer 2, a first N-type semiconductor layer 1, a first electrode layer 4, a second transparent glass layer 6, and a first transparent glass layer 5.

[0040] The material of the metal reflection layer 8 is silver, and the thickness is 400 nm, which is used to reflect incident light and increase the optical path of the device.

[0041] Among them, the materials of the first electrode layer 4 and the second electrode layer 7 are graphene, and the thicknesses are both 0.35 nm, serving as the two electrodes of the photodetector.

[0042] The materials of the first N-type semiconductor layer 1 and the second N-type semiconductor layer 3 are both HfS 2 , the thicknesses are both 20 nm, and the band gaps are both 2.45 eV. Among them, the doping concentration of the first N-type semiconductor layer 1 is 10 15 cm -3 , and the doping concentration of the second N-type semiconductor layer 3 is 10 17 cm -3 .

[0043] The material of the P-type semiconductor layer 2 is NiS 2 , the thickness is 80 nm, the band gap is 0.37 eV, and the doping concentration is 10 14 cm -3 .

[0044] The material of the first transparent glass layer 5 is MgF 2 , and the thickness is 350 nm.

[0045] The material of the second transparent glass layer 6 is VITRON_IG2, and the thickness is 775 nm.

[0046] The working principle of the present invention is as follows: The constructed NPN vertical structure is as Figure 1 , in the dark state, due to HfS 2 (P-type semiconductor layer 2) has a larger band gap compared to the NiS 2 layer (the first N-type semiconductor layer 1 and the second N-type semiconductor layer 3), a high valence band barrier is formed between the junctions as Figure 2 and Figure 3 , which makes it impossible for holes to be transmitted in any direction, so the device can only form a current through the drift motion of electrons.

[0047] However, electrons need to continuously climb the barrier during the transmission process in the upper-layer HfS 2 (the first N-type semiconductor layer 1), plus NiS 2The layer (P-type semiconductor layer 2) is a P-doped layer. Since the electron concentration is low, the migration of electrons is always hindered under dark conditions. Additionally, due to the barrier of the lower heterojunction (P-type semiconductor layer 2 - second N-type semiconductor layer 3), some electrons with lower energy cannot cross over. This indicates that the dark current level of the device is not high, approximately 10 -6 A, and by changing the thickness of the lower HfS 2 (second N-type semiconductor layer 3), the dark current can be reduced to 10 -8 A.

[0048] Under illumination, only the NiS in the middle layer 2 (P-type semiconductor layer 2) absorbs photons to generate electron-hole pairs, such as Figure 3 . Under the influence of the applied bias voltage, these carriers drift towards both ends respectively. Since the potential barriers at both ends of the valence band are high, holes cannot be transported and accumulate at the junction position, and they attract electrons in the emitter HfS 2 layer (second N-type semiconductor layer 3), forming a photovoltaic voltage similar to the optical gating effect, which is manifested as raising the original conduction band and changing the depletion width, thereby enabling electrons to be transported more smoothly. The photo-generated electrons also improve the electron transport ability of the NiS 2 layer (P-type semiconductor layer 2). The accelerated electron flow reaches the upper HfS 2 -NiS 2 junction (first N-type semiconductor layer 1 - P-type semiconductor layer 2). The momentum and energy they possess enable them to easily cross the inter-junction barrier. It can be seen that the influencing factor of the responsivity is the bias voltage intensity under a certain photovoltaic voltage effect, while the influencing factor of the response time is the generation and disappearance of the photovoltaic voltage effect. Coupled with the device design that can greatly reduce the dark current, the decoupling of the responsivity, response speed, and specific detectivity constraint relationship is finally achieved within a certain applied voltage range.

[0049] On the other hand, in order to improve the device performance of the NPN-type heterojunction photodetector of the present invention, optimization is mainly carried out from two aspects: optical performance and electrical performance.

[0050] Specifically, as Figure 4 shown, the second transparent glass layer 6 (MgF 2 ) and the first transparent glass layer 5 (VITRON_IG2) form a full anti-reflection effect, and then form a simple resonant cavity with the metal reflection layer 8. Through a large number of experiments and theoretical calculations, it is obtained that changing the thickness of the second transparent glass layer 6 (MgF 2 ) can achieve the absorption enhancement effect of a specific wavelength.

[0051] For the above-mentioned second transparent glass layer 6 (MgF 2)The selection of the thickness can be made according to the actually required detection wavelength. The prerequisite is that the thickness of the first transparent glass layer 5 (VITRON_IG2) is maintained at 775 nm. By changing the thickness of the second transparent glass layer 6 (MgF 2 ), in the range from 200 nm to 500 nm, high-efficiency specific absorption of the overall device for wavelengths of 1000 nm, 1200 nm, 1400 nm, 1600 nm, and 1800 nm can be achieved, and the absorption rate can be increased from less than 40% to more than 80%.

[0052] Therefore, the thickness of the second transparent glass layer 6 (MgF 2 ) is locked at 350 nm, the wavelength is 1600 nm, and the light intensity is 30 W / m 2 . Under this condition, to further optimize the device performance and explore the working mechanism, from Figure 5 and Figure 6 , it can be seen that the potential drop is mainly concentrated in the lower layer HfS 2 (the second N-type semiconductor layer 3). Therefore, the accelerated movement of electrons mainly occurs in this region. So, explore the influence of the doping concentration of the lower layer HfS 2 (the second N-type semiconductor layer 3) on the responsivity, and the influence of the thickness of the lower layer HfS 2 (the second N-type semiconductor layer 3) on the responsivity and specific detectivity, as well as the extremely crucial influence of the carrier lifetime of the light-absorbing layer NiS 2 (the P-type semiconductor layer 2) on the response time of the responsivity.

[0053] The results show that, as Figure 7 when the bias voltage is -3V, for HfS 2 (the first N-type semiconductor layer 1 and the second N-type semiconductor layer 3), with higher doping, the responsivity is significantly improved from 12000 A / W to 24000 A / W. The increase in the emitter electron concentration causes more electrons in the device to flow through the device under the acceleration of the applied bias voltage under illumination.

[0054] It should also be noted that in order to keep the dark current low, the doping of the emitter needs to be regulated. So, the upper layer HfS 2 (the first N-type semiconductor layer 1) is relatively lightly doped to 10 15 cm -3 , but still higher than the doping concentration of 10 2 cm 14 of the middle layer NiS -3 (the P-type semiconductor layer 2). The purpose is to ensure that the depletion layer is mainly distributed in the NiS 2 layer (the P-type semiconductor layer 2) and to ensure that electrons in the upper N region do not easily pass through the built-in electric field under the action of the applied voltage, thereby suppressing the dark current.

[0055] Such asFigure 8 and Figure 9 When the underlying HfS 2 (the second N-type semiconductor layer 3) is gradually reduced from the original thickness of 80 nm to 20 nm, when the bias voltage is -3 V, the responsivity has a significant increase, from 2.4×10 4 A·W -1 increased to 9.93×10 5 A·W -1 , which is due to the increase in the electric field strength in this region. However, this approach will also amplify the dark current from the order of 10 -7 to the order of 10 -6 . Therefore, it is not advisable to overly reduce the thickness of the underlying HfS 2 (the second N-type semiconductor layer 3).

[0056] Due to the large increase in responsivity, the specific detectivity also has a certain degree of improvement, and can reach 2×10 18 Jones at a thickness of 20 nm. So far, the specific detectivity and responsivity exceeding those of the photoconductive type have been obtained, but the response time under the current conditions is relatively long, about 400 μs. According to the operating mechanism of the device, it is considered that the carrier lifetime in the intermediate layer is the main influencing factor and is discussed as follows Figure 10 and Figure 11 . A long carrier lifetime can increase the cumulative hole concentration in the equilibrium state under low light intensity. Therefore, increasing the carrier lifetime can effectively further improve the responsivity and specific detectivity. However, increasing the carrier lifetime will also cause an increase in the response time of the device.

[0057] To avoid this phenomenon, the light intensity can be increased while shortening the carrier lifetime to maintain the photovoltaic effect, and the voltage intensity can be enhanced to further improve the responsivity. Finally, the designed structure of the device is a heavily doped N-region semiconductor layer (the second N-type semiconductor layer 3) with a thickness of 20 nm and a doping concentration of 10 17 cm -3 . The carrier lifetime of NiS 2 is 10 μs. Under irradiation with an intensity of 1600 nm and 30 W / m 2 , as shown in Figure 12 and Figure 13 , the responsivity and specific detectivity of the device increase with the increase of voltage. As shown in Figure 14 , the final response time of the device is about 40 μs, where the rise response time of the device is 40 μs and the fall time is 45 μs. So far, the present invention has successfully prepared a high-performance photodetector device with a different design principle and mechanism from the traditional ones.

[0058] As an alternative embodiment, the present invention also provides a method for fabricating an NPN-type heterojunction photodetector with high gain, which is applicable to fabricating the above-mentioned NPN-type heterojunction photodetector with high gain. The fabrication method includes: S1: Fabricate the metal reflective layer 8.

[0059] Specifically, deposit silver with a thickness of 400 nm on a glass substrate, and treat the surface of the silver layer by argon plasma (power 50 W, time 5 min) to remove the oxide layer and enhance the surface activity.

[0060] S2: Fabricate the second electrode layer 7.

[0061] Specifically, place the silver substrate (metal reflective layer 8) in a PECVD reaction chamber, evacuate to 3 Pa, heat to 600 °C, introduce a CH 4 / H 2 mixed gas (ratio 1:4), apply microwave plasma (power 300 W), react for 10 min, and grow a 0.35-nm-thick vertically oriented graphene layer to obtain the second electrode layer 7.

[0062] S3: Fabricate the second N-type semiconductor layer 3.

[0063] Specifically, use atomic layer deposition (ALD) method, with HfCl 4 and H 2 S as precursors, grow a 20-nm-thick HfS 2 on the surface of graphene (second electrode layer 7). Introduce Cl 2 gas to achieve N-type doping, and regulate the carrier concentration to 10 17 cm -3 .

[0064] S4: Fabricate the P-type semiconductor layer 2.

[0065] Specifically, through chemical vapor deposition (CVD), use Ni(CO) 2 and sulfur powder as raw materials (temperature 450 °C) to grow an 80-nm-thick NiS 3 on the surface of HfS 2 , introduce FeCl 3 vapor (mass ratio 1%) to achieve P-type doping, and the hole concentration reaches 10 14 cm -3 .

[0066] S5: Fabricate the first N-type semiconductor layer 1.

[0067] Specifically, use the atomic layer deposition (ALD) method again to ensure the heterojunction symmetry to reduce the interface stress, with HfCl 4 and H 2S is the precursor, and 20 nm thick HfS is grown on the surface of NiS 2 (P-type semiconductor layer 2). 2 Then, Cl 2 gas is introduced to achieve N-type doping, and the carrier concentration is adjusted to 10 15 cm -3 .

[0068] S6: Prepare the first electrode layer 4.

[0069] Specifically, the device is then placed in a PECVD reaction chamber, evacuated to 3 Pa, heated to 600 °C, and CH 4 / H 2 mixed gas (ratio 1:4) is introduced, a microwave plasma (power 300 W) is applied, and the reaction is carried out for 10 min to grow a 0.35 nm thick vertically oriented graphene layer as the top electrode, obtaining the first electrode layer 4.

[0070] S7: Prepare the first transparent glass layer 5 and the second transparent glass layer 6.

[0071] Specifically, the pretreated MgF2 (350 nm) and VITRON_IG2 (775 nm) are bonded in a vacuum chamber.

[0072] For the upper-layer graphene, plasma activation is carried out (Ar / Q 2 mixed gas, power 20 W, time 30 s), and hydroxyl (-OH) groups are generated on the surface. The VITRON_IG2 (775 nm)-MgF2 (350 nm) and graphene are bonded in a vacuum chamber (10-3 Pa), and a pressure of 5 Mpa is applied at room temperature for 1 h, and they are combined by van der Waals hydrogen bonds.

[0073] In summary, the beneficial effects of the present invention are as follows: 1. By designing a new optoelectronic detection device with a new response mechanism, the present invention decouples the restrictive relationship between the responsivity and the response speed with a new mechanism, and at the same time maintains a small dark current, achieving a high specific detectivity.

[0074] 2. The present invention uses a double-layer glass to form a full antireflection absorption in a specific wavelength band, and can ensure the absorption enhancement method for the specified wavelength by changing the thickness of the MgF 2 layer.

[0075] 3. The overall device has no complex Wiener structure or expensive materials, and since the MgF 2 layer is located at the top, it can conveniently meet the customer's need to prepare different thicknesses to achieve the selection of the working band.

[0076] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A high-gain NPN heterojunction photodetector, characterized in that: include, A first N-type semiconductor layer (1), a P-type semiconductor layer (2) and a second N-type semiconductor layer (3) are sequentially arranged between the positive and negative electrodes; A first heterojunction barrier is formed between the first N-type semiconductor layer (1) and the P-type semiconductor layer (2), for preventing electrons from being injected into the first N-type semiconductor layer (1); A second heterojunction barrier is formed between the second N-type semiconductor layer (3) and the P-type semiconductor layer (2), for blocking holes from being injected into the second N-type semiconductor layer (3); The positive and negative electrodes have a bias voltage.

2. The NPN heterojunction photodetector with high gain as claimed in claim 1, characterized in that: The doping concentration of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3) is greater than the doping concentration of the P-type semiconductor layer (2), so that a portion of the P-type semiconductor layer (2) is in a depletion region.

3. The NPN heterojunction photodetector with high gain as claimed in claim 2, characterized in that: The electron affinity of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3) is smaller than the electron affinity of the P-type semiconductor layer (2), and the valence band energy of the first N-type semiconductor layer (1) is lower than the valence band energy of the P-type semiconductor layer (2).

4. The NPN heterojunction photodetector with high gain as claimed in claim 3, characterized in that: The materials of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3) are both HfS2, and the material of the P-type semiconductor layer (2) is NiS2; the doping concentration of the first N-type semiconductor layer (1) is greater than or equal to 10 15 cm 3 and less than or equal to 10 16 cm 3 The doping concentration of the second N-type semiconductor layer (3) is greater than or equal to 10 16 cm 3 The doping concentration of the P-type semiconductor layer (2) is less than or equal to 10 14 cm 3 .

5. The NPN heterojunction photodetector with high gain as claimed in claim 4, characterized in that: A first electrode layer (4), a first transparent glass layer (5) and a second transparent glass layer (6) are sequentially arranged on the surface of the first N-type semiconductor layer (1), wherein the refractive index of the first transparent glass layer (5) is greater than that of the second transparent glass layer (6); and a second electrode layer (7) and a metal reflective layer (8) are sequentially arranged on the surface of the second N-type semiconductor layer (3).

6. The NPN heterojunction photodetector with high gain as claimed in claim 5, characterized in that: The material of the first electrode layer (4) and the second electrode layer (7) is graphene, the material of the first transparent glass layer (5) is VITRON_IG2, the material of the second transparent glass layer (6) is MgF2, and the material of the metal reflective layer (8) is silver.

7. The NPN heterojunction photodetector with high gain as claimed in claim 5, characterized in that: The thickness of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3) ranges from 20 nm to 100 nm, and the thickness of the P-type semiconductor layer (2) ranges from 80 nm to 200 nm.

8. A method for preparing a high-gain NPN heterojunction photodetector, characterized in that: Suitable for preparing an NPN heterojunction photodetector with high gain as claimed in any one of claims 1 to 7, the preparation method comprising: A first N-type semiconductor layer (1), a P-type semiconductor layer (2) and a second N-type semiconductor layer (3) are sequentially formed between the positive and negative electrodes of the photodetector; The doping concentration of the P-type semiconductor layer (2) is lower than the doping concentration of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3).

9. The method for preparing a high-gain NPN heterojunction photodetector according to claim 8, characterized in that: The material of the first N-type semiconductor layer (1) is HfS2, which is grown on the surface of one of the electrodes by an atomic deposition method using HfCl4 and H2S as precursors, and Cl2 gas is introduced for N-type doping.

10. The method for preparing a high-gain NPN heterojunction photodetector according to claim 9, characterized in that: The material of the P-type semiconductor layer (2) is NiS2, using Ni(CO)3 and sulfur powder as raw materials, and is grown on the surface of the first N-type semiconductor layer (1) by chemical vapor deposition, and FeCl3 vapor is introduced for P-type doping.

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