An NPN-type heterojunction photodetector with high gain and a preparation method thereof

Abstract

The present invention discloses a high-gain NPN heterojunction photodetector and a method for preparing the same, relating to the field of photodetectors. The NPN heterojunction photodetector comprises a first N-type semiconductor layer, a P-type semiconductor layer, and a second N-type semiconductor layer, sequentially disposed between positive and negative electrodes; 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 greater than the doping concentration of the first N-type semiconductor layer. The present invention significantly increases the photocurrent of the photodetector by applying a bias voltage to the photodetector, achieving high responsiveness and decoupling the constraint between responsiveness and response time. By designing an NPN heterostructure detector, the potential barrier at the bottom and the depletion region in the middle layer can effectively reduce dark current, achieving a high specific detection rate.

Description

Technical Field

[0001] The present invention relates to the technical field of photoelectric detectors, in particular to an NPN heterojunction photoelectric detector with high gain and a preparation method thereof. Background Art

[0002] Infrared detection technology is a key means of acquiring target information by capturing infrared electromagnetic waves radiated or reflected by an object. The current mainstream photovoltaic detector design is limited by high manufacturing costs and low-temperature operation requirements. Photoconductive quantum well infrared photodetectors face inherent bottlenecks such as low specific detection rate and slow response speed. Photothermoelectric infrared detectors, while offering the advantage of low cost, suffer from weak performance indicators. This poses a limitation for the future design of high-performance detection devices. Furthermore, with breakthroughs in micro-nanofabrication technology and new semiconductor materials, infrared detector materials are evolving from traditional bulk materials to novel two-dimensional materials.

[0003] Two-dimensional transition metal sulfides, with their tunable band gap, band gap variation with layer number, high electron mobility, ease of fabrication, and excellent chemical stability, offer a wide range of potential combinations for detection device materials. Furthermore, the mature heterojunction growth process allows two-dimensional materials to complement each other's shortcomings and leverage their strengths, making them a promising candidate for the development of high-performance detection devices.

[0004] Responsivity, response time, and specific detectivity are the primary performance metrics for detectors. Responsivity measures the detector's sensitivity to the input signal, response time measures its dynamic response to signal changes, and specific detectivity measures the detector's minimum detectable signal in a noisy environment. However, traditional device designs often struggle to achieve optimal responsivity, response time, and specific detectivity simultaneously. Breaking through these three constraints to achieve high-performance detection has been a major research focus.

[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 above problems in the prior art, the present invention is proposed.

[0007] Therefore, the problem to be solved by the present invention is how to detect in the near-infrared band while achieving excellent responsivity, specific detection rate and response time.

[0008] To solve the above technical problems, in the first aspect, the present invention provides the following technical solutions: an NPN heterojunction photodetector with high gain, comprising 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, for blocking electrons from being injected 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, for blocking holes from being injected into the second N-type semiconductor layer; and a bias voltage is applied to the positive and negative electrodes.

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

[0010] As a preferred solution of the NPN heterojunction photodetector with high gain described in the present invention, the electron affinity of the first N-type semiconductor layer and the second N-type semiconductor layer is smaller 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 high-gain NPN heterojunction photodetector described in the present invention, the first N-type semiconductor layer and the second N-type semiconductor layer are both made of HfS2, the P-type semiconductor layer is made of NiS2; 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 heterojunction photodetector with high gain described in the present invention, 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 reflective layer sequentially arranged on the surface of the above-mentioned second N-type semiconductor layer.

[0013] As a preferred solution of the NPN heterojunction photodetector with high gain described in the present invention, the material of the first electrode layer and the second electrode layer is graphene, the material of the first transparent glass layer is VITRON_IG2, the material of the second transparent glass layer is MgF2, and the material of the metal reflective layer is silver.

[0014] As a preferred solution of the high-gain NPN heterojunction photodetector described in the present invention, the thickness of the first N-type semiconductor layer and the second N-type semiconductor layer ranges from 20nm to 100nm, and the thickness of the P-type semiconductor layer ranges from 80nm to 200nm.

[0015] On the other hand, the present invention also provides a method for preparing an NPN heterojunction photodetector with high gain, which is suitable for preparing the above-mentioned NPN heterojunction photodetector with high gain. The preparation method includes forming a first N-type semiconductor layer, a P-type semiconductor layer and a second N-type semiconductor layer in sequence between the positive and negative poles of the photodetector; wherein 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 greater than the doping concentration of the first N-type semiconductor layer.

[0016] As a preferred solution of the preparation method of a high-gain NPN heterojunction photodetector described in the present invention, the material of the first N-type semiconductor layer is HfS2, and HfCl4 and H2S are used as precursors. It is grown on the surface of one of the electrodes by atomic deposition, and Cl2 gas is introduced for N-type doping.

[0017] As a preferred solution of the preparation method of a high-gain NPN heterojunction photodetector described in the present invention, the P-type semiconductor layer is made of NiS2, and is grown on the surface of the first N-type semiconductor layer by chemical vapor deposition using Ni(CO)3 and sulfur powder as raw materials, and FeCl3 vapor is introduced for P-type doping.

[0018] The present invention has the following beneficial effects: by applying a bias voltage to the photodetector, the present invention significantly increases the photocurrent of the photodetector, achieving high responsivity. Based on the principle of NPN heterojunction photodetectors, the device's response time is determined by the speed of hole concentration accumulation at the junction, that is, by the carrier lifetime of the intermediate light-absorbing layer. The responsivity increases with increasing bias voltage, achieving a decoupling of the restrictive relationship between responsivity and response time. By designing an NPN heterostructure detector, the bottom potential barrier and the depletion region of the intermediate layer can effectively reduce dark current, achieving a high specific detectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 Schematic diagram of the structure of the photoelectric detector of the present invention.

[0021] Figure 2 Schematic diagram of the heterojunction energy band of the photodetector of the present invention.

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

[0023] Figure 4 Schematic diagram of the absorption curve of the photodetector of the present invention changing with the thickness of the second transparent glass layer (MgF2).

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

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

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

[0027] Figure 8 Schematic diagram showing the effect of the thickness of the second N-type semiconductor layer on the device responsivity in the photodetector of the present invention.

[0028] Figure 9 Schematic diagram showing the effect of the thickness of the second N-type semiconductor layer on the device specific detectivity in the photodetector of the present invention.

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

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

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

[0032] Figure 13The figure is a relationship curve between the noise equivalent power and the specific detection rate of the photodetector under bias voltage of the present invention.

[0033] Figure 14 This is a response time curve diagram of the photodetector of the present invention.

[0034] In the figure: 1. first N-type semiconductor layer; 2. P-type semiconductor layer; 3. second N-type semiconductor layer; 4. first electrode layer; 5. first transparent glass layer; 6. second transparent glass layer; 7. second electrode layer; 8. metal reflective layer. DETAILED DESCRIPTION

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

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

[0037] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it individually or selectively refer to an embodiment that is mutually exclusive of other embodiments.

[0038] Reference Figures 1 to 14 , is an embodiment of the present invention, which provides an NPN heterojunction photodetector with high gain. The NPN 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, which are sequentially arranged between positive and negative electrodes.

[0039] Specifically, refer to Figure 1 The structure of the high-gain NPN heterojunction photodetector is, from bottom to top, a metal reflective 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 metal reflective layer 8 is made of silver and has a thickness of 400 nm. It is used to reflect incident light and increase the optical path of the device.

[0041] The first electrode layer 4 and the second electrode layer 7 are made of graphene, both with a thickness of 0.35 nm, and serve as two electrodes of the photodetector.

[0042] The first N-type semiconductor layer 1 and the second N-type semiconductor layer 3 are both made of HfS2, with a thickness of 20 nm and a band gap of 2.45 eV. 15 cm -3 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 NiS2, with a thickness of 80nm, a band gap of 0.37eV, and a doping concentration of 10 14 cm -3 .

[0044] The first transparent glass layer 5 is made of MgF2 and has a thickness of 350 nm.

[0045] The second transparent glass layer 6 is made of VITRON_IG2 and has a thickness of 775 nm.

[0046] The working principle of the present invention is:

[0047] The constructed NPN vertical structure is as follows Figure 1 In the dark state, since NiS2 (P-type semiconductor layer 2) has a smaller band gap than the HfS2 layer (first N-type semiconductor layer 1 and second N-type semiconductor layer 3), a high valence band barrier is formed between the junctions. Figure 2 and Figure 3 , which makes it impossible for holes to be transmitted in any direction, so the device can only form current through the drift motion of electrons.

[0048] However, electrons need to continuously climb the potential barrier during transmission in the upper HfS2 layer (first N-type semiconductor layer 1). In addition, the NiS2 layer (P-type semiconductor layer 2) is a P-doped layer with a low electron concentration, so the migration of electrons is always hindered under dark conditions. In addition, some lower-energy electrons cannot cross the potential barrier of the lower heterojunction (P-type semiconductor layer 2-second N-type semiconductor layer 3). This means that the dark current level of the device is not high, about 10 -6 A and by changing the thickness of the lower HfS2 (the second N-type semiconductor layer 3), the dark current can be reduced to 10 -8 A.

[0049] Under light, only the middle layer of NiS2 (P-type semiconductor layer 2) absorbs photons to generate electron-hole pairs. Figure 3Under the influence of an applied bias, these carriers drift toward opposite ends, causing electrons and holes to drift toward the opposite ends. Due to the high potential barriers at both ends of the valence band, holes cannot transport themselves and instead accumulate at the junction. This attracts electrons in the emitter HfS2 layer (second N-type semiconductor layer 3), generating a photovoltage similar to a photogating effect, manifesting as a shift in the inherent conduction band and depletion width, thereby facilitating smoother electron transport. The photogenerated electrons further enhance the electron transport capacity of the NiS2 layer (P-type semiconductor layer 2). The accelerated electrons flow to the upper HfS2-NiS2 junction (first N-type semiconductor layer 1-P-type semiconductor layer 2), where their momentum and energy enable them to easily cross the interjunction barrier. Therefore, the responsivity is influenced by the bias voltage strength at which a certain photovoltage effect is achieved, while the response time is influenced by the generation and disappearance of the photovoltage effect. Combined with the device design's ability to significantly reduce dark current, the device achieves a decoupling of the constraints governing responsivity, response speed, and specific detectivity within a specific applied voltage range.

[0050] On the other hand, in order to improve the device performance of the NPN heterojunction photodetector of the present invention, optimization is mainly carried out from two aspects: optical performance and electrical performance.

[0051] Specifically, such as Figure 4 As shown, the second transparent glass layer 6 (MgF2) forms a full antireflection effect with the first transparent glass layer 5 (VITRON_IG2), and then forms a simple resonant cavity with the metal reflective layer 8. Extensive experiments and theoretical calculations have shown that varying the thickness of the second transparent glass layer 6 (MgF2) can enhance absorption at specific wavelengths.

[0052] The thickness of the second transparent glass layer 6 (MgF2) can be selected based on the desired detection wavelength. This presupposes maintaining the thickness of the first transparent glass layer 5 (VITRON_IG2) at 775nm. By varying the thickness of the second transparent glass layer 6 (MgF2) from 200nm to 500nm, the device achieves highly efficient specific absorption at wavelengths of 1000nm, 1200nm, 1400nm, 1600nm, and 1800nm, increasing the absorption rate from less than 40% to over 80%.

[0053] Therefore, the thickness of the second transparent glass layer 6 (MgF2) is locked at 350nm, the wavelength is 1600nm, and the light intensity is 30W / m 2 In order to further optimize the device performance and explore the working mechanism, Figure 5 and Figure 6It can be seen that the potential drop is primarily concentrated in the lower HfS2 layer (the second N-type semiconductor layer 3). Therefore, electron acceleration primarily occurs in this region. Therefore, we investigated the effects of the doping concentration of the lower HfS2 layer (the second N-type semiconductor layer 3) on responsivity, the thickness of the lower HfS2 layer (the second N-type semiconductor layer 3) on responsivity and specific detectivity, and the impact of the carrier lifetime of the critical light-absorbing layer NiS2 (the P-type semiconductor layer 2) on responsivity and response time.

[0054] The results show that if Figure 7 When the bias voltage is -3V, the response of HfS2 (the first N-type semiconductor layer 1 and the second N-type semiconductor layer 3) is significantly improved from 12000A / W to 24000A / W under higher doping. The increase in the emitter electron concentration allows more electrons to flow through the device under the acceleration of the external bias under light.

[0055] It should also be noted that in order to keep the dark current low, the emitter doping needs to be regulated, so the upper HfS2 (first N-type semiconductor layer 1) is relatively lightly doped to 10 15 cm -3 , but still higher than 10% of the middle layer NiS2 (P-type semiconductor layer 2) 14 cm -3 The doping concentration is designed to ensure that the depletion layer is mainly distributed in the NiS2 layer (P-type semiconductor layer 2) and that electrons in the upper N region do not easily pass through the built-in electric field under the action of an external voltage, thereby suppressing dark current.

[0056] like Figure 8 and Figure 9 When the thickness of the lower HfS2 (second N-type semiconductor layer 3) is gradually reduced from the original 80nm to 20nm, the response is significantly improved when the bias voltage is -3V, 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 area, but this approach will also amplify the dark current from 10 -7 The magnitude increased to 10 -6 Therefore, it is not advisable to reduce the thickness of the lower HfS2 (the second N-type semiconductor layer 3) excessively.

[0057] Due to the large increase in responsiveness, the specific detectivity is also improved to a certain extent, reaching 2×10 at a thickness of 20nm. 18 Jones. So far, the specific detectivity and responsivity have exceeded those of the photoconductive type, but the response time under the current conditions is relatively long, about 400μs. Based on the mechanism of device operation, it is believed that the carrier lifetime of 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 accumulated hole concentration in the equilibrium state under low light intensity conditions. Therefore, increasing the carrier lifetime can effectively further improve the responsivity and specific detectivity. However, increasing the carrier lifetime also increases the device response time.

[0058] To avoid this phenomenon, the light intensity can be increased while shortening the carrier lifetime to maintain the photovoltage effect, while increasing the voltage intensity to further improve the responsiveness. The final device design structure is a heavily doped N-region semiconductor layer (second N-type semiconductor layer 3) with a thickness of 20nm and a doping concentration of 10 17 cm -3 The carrier lifetime of NiS2 is 10μs. At 1600nm, 30W / m 2 Under the intensity of irradiation, Figure 12 and Figure 13 The responsivity and specific detectivity of the device shown increase with increasing voltage. Figure 14 The final response time of the device shown is about 40μs, with a rise time of 40μs and a fall time of 45μs. Thus, the present invention successfully fabricates a high-performance photoelectric detection device using a different design principle and mechanism than the traditional one.

[0059] As an optional embodiment, the present invention further provides a method for preparing an NPN heterojunction photodetector with high gain, which is suitable for preparing the above-mentioned NPN heterojunction photodetector with high gain, and the preparation method comprises:

[0060] S1: preparing a metal reflective layer 8.

[0061] Specifically, silver with a thickness of 400 nm was deposited on a glass substrate, and the surface of the silver layer was treated with argon plasma (power 50 W, time 5 min) to remove the oxide layer and enhance the surface activity.

[0062] S2: preparing the second electrode layer 7 .

[0063] Specifically, a silver substrate (metal reflective layer 8) was placed in a PECVD reaction chamber, evacuated to 3 Pa, heated to 600°C, introduced with a CH4 / H2 mixed gas (ratio 1:4), applied with microwave plasma (power 300 W), and reacted for 10 minutes to grow a 0.35 nm thick vertically oriented graphene layer to obtain the second electrode layer 7.

[0064] S3: preparing a second N-type semiconductor layer 3.

[0065] Specifically, the atomic layer deposition (ALD) method was used to grow 20nm thick HfS2 on the surface of graphene (second electrode layer 7) using HfCl4 and H2S as precursors. Cl2 gas was introduced to achieve N-type doping, and the carrier concentration was controlled to 10 17 cm -3 .

[0066] S4: preparing a P-type semiconductor layer 2.

[0067] Specifically, 80nm thick NiS2 was grown on the surface of HfS2 using Ni(CO)3 and sulfur powder as raw materials (temperature 450℃) by chemical vapor deposition (CVD). FeCl3 vapor (mass ratio 1%) was introduced to achieve P-type doping, and the hole concentration reached 10 14 cm -3 .

[0068] S5: preparing a first N-type semiconductor layer 1.

[0069] Specifically, atomic layer deposition (ALD) was used again to ensure heterojunction symmetry to reduce interface stress. HfCl4 and H2S were used as precursors to grow 20nm thick HfS2 on the surface of NiS2 (P-type semiconductor layer 2). Cl2 gas was introduced to achieve N-type doping, and the carrier concentration was controlled to 10 15 cm -3 .

[0070] S6: preparing the first electrode layer 4.

[0071] Specifically, the device was placed in a PECVD reaction chamber, evacuated to 3 Pa, heated to 600°C, introduced with a CH4 / H2 mixed gas (ratio 1:4), applied with microwave plasma (power 300 W), and reacted for 10 minutes to grow a 0.35 nm thick vertically oriented graphene layer as the top electrode to obtain the first electrode layer 4.

[0072] S7: preparing a first transparent glass layer 5 and a second transparent glass layer 6.

[0073] Specifically, pre-treated MgF2 (350nm) and VITRON_IG2 (775nm) were bonded in a vacuum chamber.

[0074] The upper graphene layer was plasma activated (Ar / Q2 mixed gas, power 20W, time 30s) to generate hydroxyl (-OH) groups on the surface. VITRON_IG2 (775nm)-MgF2 (350nm) was laminated to the graphene in a vacuum chamber (10-3Pa) and a pressure of 5MPa was applied at room temperature for 1 hour, allowing van der Waals hydrogen bonding.

[0075] In summary, the beneficial effects of the present invention are:

[0076] 1. The present invention designs a photoelectric detection device with a new response mechanism, decouples the restrictive relationship between responsivity and response speed with a new mechanism, while maintaining a small dark current and achieving a higher specific detection rate.

[0077] 2. The present invention utilizes double-layer glass to form full anti-reflection absorption in a specific wavelength band, and can ensure enhanced absorption of the specified wavelength by changing the thickness of the MgF2 layer.

[0078] 3. The device as a whole does not have a complex Wiener structure or expensive materials. Moreover, since the MgF2 layer is located at the top, it can easily meet the customer's needs for preparing different thicknesses to achieve the selection of the working band.

[0079] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in 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), so that photogenerated holes generated under light are accumulated in the P-type semiconductor layer (2), and the conduction band barriers of the first heterojunction barrier and the second heterojunction barrier are reduced through a light gating effect; The first N-type semiconductor layer (1) and the second N-type semiconductor layer (3) are both made of HfS2, have a thickness of 20 nm, and a band gap of 2.45 eV; wherein the doping concentration of the first N-type semiconductor layer (1) is 10 15 cm -3 , the doping concentration of the second N-type semiconductor layer (3) is 10 17 cm -3 The material of the P-type semiconductor layer (2) is NiS2, with a thickness of 80 nm, a band gap of 0.37 eV, and a doping concentration of 10 14 cm -3 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 the P-type semiconductor layer (2) is partially in a depletion region for absorbing photons to generate electron-hole pairs; The positive and negative electrodes have a bias voltage.

2. The high-gain NPN heterojunction photodetector according to claim 1, wherein: 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).

3. The high-gain NPN heterojunction photodetector according to claim 2, wherein: 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).

4. The high-gain NPN heterojunction photodetector according to claim 3, wherein: 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.

5. 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 4, 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 concentrations of the first N-type semiconductor layer (1) and the second N-type semiconductor layer (3).

6. The method for preparing a high-gain NPN heterojunction photodetector according to claim 5, wherein: 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 is then introduced with Cl2 gas for N-type doping.

7. The method for preparing a high-gain NPN heterojunction photodetector according to claim 6, wherein: The material of the P-type semiconductor layer (2) is NiS2, and Ni(CO)3 and sulfur powder are used as raw materials. It 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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