Schottky barrier diode

Abstract

To provide a Schottky barrier diode that has a high breakdown voltage and can obtain sufficient reverse surge current capability by using Ga2O3-based materials.SOLUTION: A Schottky barrier diode 1 comprises an n type semiconductor substrate 10 which is made of n type Ga2O3-based materials, and has an n type semiconductor layer 12; a plurality of p type semiconductor parts 20 which are arranged apart on a surface of the n type semiconductor substrate 10 in cross-sectional view; and an anode electrode 30 which is arranged on a surface of the n type semiconductor substrate 10 and the p type semiconductor parts 20, wherein the n type semiconductor substrate 10 is formed at least in a region contacting the anode electrode 30 on the surface of the n type semiconductor substrate 10, and further has an n type high-resistance region 13 having lower impurity density and higher resistance than the n type semiconductor layer 12.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a Schottky barrier diode. [Background technology]

[0002] Conventionally, a Schottky barrier diode that reduces the surface electric field by using an MPS (JBS) structure is known (see, for example, Patent Document 1).

[0003] As shown in FIG. 5( a), a conventional Schottky barrier diode with an MPS (JBS) structure (conventional Schottky barrier diode 800) is a Schottky barrier diode with a so-called MPS (JBS) structure, which is made of a silicon-based material and includes a semiconductor substrate 810 having an n-type substrate layer 811, an n-type semiconductor layer 812, and a plurality of p-type semiconductor regions 820 formed on the surface of the n-type semiconductor layer 812, an anode electrode 830 in contact with the p-type semiconductor regions 820 and the n-type semiconductor layer 812, and a cathode electrode 840.

[0004] In the conventional Schottky barrier diode 800, by introducing the MPS (JBS) structure, the depletion layers generated from the adjacent p-type semiconductor region 820 and n-type semiconductor layer 812 punch through each other, and the region with the maximum electric field can be pushed downward (directly below the p-type semiconductor region 820) from the surface of the semiconductor substrate 810. Therefore, the surface electric field of the semiconductor substrate 810 can be reduced, and the leakage current can be reduced. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2-151067 Summary of the Invention [Problem to be solved by the invention]

[0006] Recently, Ga2O3-based materials have been attracting attention as semiconductor materials. Because Ga2O3-based materials have a very high critical electric field value Ec, semiconductor devices using Ga2O3-based materials have the potential to be semiconductor devices with high breakdown voltage.

[0007] However, when attempting to manufacture Schottky barrier diodes using Ga2O3-based materials, there are problems such as the extremely high critical electric field value Ec, which lowers the Schottky barrier due to image forces and increases leakage current, and the high critical electric field value Ec makes the Schottky barrier thinner, which increases leakage current due to thermonic emission, thermonic field emission, and field emission due to the tunneling effect, making it impossible to obtain sufficient reverse surge resistance.

[0008] It is possible to consider introducing an MPS (JBS) structure, like a conventional Schottky barrier diode, into a semiconductor substrate using a Ga2O3-based material. However, in a semiconductor substrate using a Ga2O3-based material, the effective mass of holes is extremely heavy and the mobility is almost zero, making it difficult to form a p-type semiconductor region in an n-type semiconductor substrate by diffusion. This makes it difficult to introduce an MPS (JBS) structure and makes it difficult to appropriately reduce leakage current.

[0009] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a Schottky barrier diode that uses a Ga2O3-based material to have a high breakdown voltage and to be able to obtain sufficient reverse surge resistance. [Means for solving the problem]

[0010] The Schottky barrier diode of the present invention comprises an n-type semiconductor substrate made of an n-type Ga2O3-based material and having an n-type semiconductor layer; a plurality of p-type semiconductor portions arranged at a distance on the surface of the n-type semiconductor substrate in cross section; and an anode electrode arranged on the surfaces of the n-type semiconductor substrate and the p-type semiconductor portions, wherein the n-type semiconductor substrate further has an n-type high-resistance region formed at least in a region on the surface of the n-type semiconductor substrate that contacts the anode electrode and has a lower impurity concentration and higher resistance than the n-type semiconductor layer. [Effects of the Invention]

[0011] The Schottky barrier diode of the present invention includes an n-type semiconductor substrate made of a Ga2O3-based material, and therefore can utilize Ga2O3-based materials with high critical electric field values, making it possible to realize a semiconductor device with high breakdown voltage.

[0012] Furthermore, the Schottky barrier diode of the present invention includes a plurality of p-type semiconductor portions spaced apart on the surface of an n-type semiconductor substrate when viewed in cross section, eliminating the need to form a p-type region (p-type semiconductor region) within the n-type semiconductor substrate. Furthermore, a depletion layer is easily generated from the pn junction between the plurality of spaced apart p-type semiconductor portions and the n-type semiconductor substrate, and is easily extended across the surface of the n-type semiconductor substrate. This allows the surface electric field of the n-type semiconductor substrate in the region where the n-type semiconductor substrate contacts the anode electrode to be reduced, thereby reducing leakage current, without forming a p-type region (p-type semiconductor region) within the n-type semiconductor substrate. Therefore, sufficient reverse surge resistance can be achieved.

[0013] Furthermore, according to the Schottky barrier diode of the present invention, the n-type semiconductor substrate has an n-type high-resistance region formed at least in a region of the surface of the n-type semiconductor substrate that contacts the anode electrode and has a lower impurity concentration and higher resistance than the n-type semiconductor layer. Therefore, even when a high reverse bias voltage is applied, the Schottky barrier can be made thick enough to prevent thermoelectrons from crossing the Schottky barrier and to prevent the tunneling effect from occurring. Therefore, thermoelectron field emission and field emission are less likely to occur, and leakage current can be reduced. As a result, sufficient reverse surge resistance can be obtained from this perspective as well. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view showing a Schottky barrier diode 1 according to an embodiment. [Figure 2] 2 is a schematic graph illustrating the potential in the depth direction along the dashed-dotted line A-A' of the Schottky barrier diode 1 according to the embodiment. In Fig. 2, the dashed line indicates the potential in the depth direction at zero bias, and the solid line indicates the potential in the depth direction at reverse bias. [Figure 3] 2 is a schematic graph shown to explain the reverse characteristics of the Schottky barrier diode 1 according to the embodiment. [Figure 4] 2 is a schematic graph shown to explain the forward characteristics of the Schottky barrier diode 1 according to the embodiment. [Figure 5] 5A is a cross-sectional view showing a conventional Schottky barrier diode 800, and FIG. 5B is a cross-sectional view showing a Schottky barrier diode 801, which is the conventional Schottky barrier diode 800 made of a Ga2O3-based material. [Figure 6] FIG. 1 is a cross-sectional view showing a Schottky barrier diode 900 having a conventional MOS-SBD structure. DETAILED DESCRIPTION OF THE INVENTION

[0015] The Schottky barrier diode of the present invention will be described below based on the embodiments shown in the drawings. Note that the embodiments described below do not limit the invention according to the claims. Furthermore, not all of the elements and combinations thereof described in the embodiments are necessarily essential to the solution of the present invention.

[0016] [Embodiment] 1. Configuration of Schottky barrier diode 1 according to the embodiment As shown in FIG. 1, the Schottky barrier diode 1 according to the embodiment includes an n-type semiconductor substrate 10, a p-type semiconductor section 20, an anode electrode 30, and a cathode electrode 40.

[0017] The n-type semiconductor substrate 10 is made of an n-type Ga2O3-based material, and has an n-type substrate layer 11, an n-type semiconductor layer 12, and a high-resistance region 13. The thickness of the n-type semiconductor substrate 10 is, for example, 100 μm.

[0018] In this specification, when a component is described as "made of... material," it does not mean that the component is made of only the pure material, but that it may contain trace amounts of impurities (donors, etc.).

[0019] The n-type substrate layer 11 is ++ The cathode electrode 40 is in ohmic contact with the Ga2O3-based material.

[0020] The n-type semiconductor layer 12 is formed on the upper surface of the n-type substrate layer 11. + The n-type semiconductor layer 12 is an epitaxial layer made of a Ga2O3-based material. The n-type semiconductor layer 12 contains donor impurities such as Si and Sn. The impurity concentration of the n-type semiconductor layer 12 is lower than that of the n-type substrate layer 11.

[0021] The high resistance region 13 is formed in a region on the surface of the n-type semiconductor substrate 10 that is in contact with the anode electrode 30 and the end of the p-type semiconductor portion 20. -The high resistance region 13 has a lower impurity concentration and a higher resistance than the n-type semiconductor layer 12. Furthermore, the impurity concentration of the high resistance region 13 is two orders of magnitude lower than the impurity concentration of the n-type semiconductor layer 12. For example, when the n-type semiconductor layer 12 is 10 22 cm ―3 When the impurity concentration is on the order of 10 20 cm ―3 It has an impurity concentration of the order of 10 ...

[0022] The thickness of the high resistance region 13 is preferably a thickness that does not cause a tunnel effect even when a reverse bias is applied, and is as thin as possible. 22 cm ―3 When the high-resistance region 13 has an impurity concentration of the order of 1000 .mu.m, the thickness of the high-resistance region 13 is preferably in the range of 5 nm to 50 nm, for example. This is because if the thickness of the high-resistance region 13 is less than 5 nm, the Schottky barrier becomes too thin when a relatively high reverse bias is applied, making it difficult to prevent thermionic field emission and field emission, and therefore difficult to reduce leakage current, and if the thickness of the high-resistance region 13 exceeds 50 nm, the high-resistance region 13 becomes too thick, resulting in increased forward loss.

[0023] The impurity concentration of the high-resistance region 13 is lower than the impurity concentration of the n-type semiconductor layer 12. Therefore, the high-resistance region 13 has a high internal resistance. The high-resistance region 13 can also be formed, for example, by implanting impurities that cause high resistance (for example, N ions or Mg ions) into a specific region of the n-type semiconductor layer 12.

[0024] The impurity concentration of the n-type semiconductor layer 12 is sufficiently higher (resistance value is lower) than the impurity concentration of the high-resistance region 13. Therefore, the resistance value of the current path that flows from the anode electrode 30 through the high-resistance region 13, the n-type semiconductor layer 12, and the n-type substrate layer 11 to the cathode electrode 40 in the Schottky barrier diode 1 according to the embodiment is not significantly different from the resistance value of the current path that flows from the anode electrode through the semiconductor base to the cathode electrode in a general Schottky barrier diode made of a silicon material.

[0025] In other words, the sum of the total amount of impurities per unit area in the depth direction of the n-type semiconductor layer of a general Schottky barrier diode made of a silicon material is approximately equal to the sum of the total amount of impurities per unit area in the depth direction of the high-resistance region 13 in the Schottky barrier diode 1 according to the embodiment, and the sum of the total amount of impurities per unit area in the depth direction of the n-type semiconductor layer 12 below the high-resistance region 13.

[0026] As long as the condition for the sum of the above total amounts of impurities is satisfied, the thickness of the n-type semiconductor substrate, the thickness of the n-type semiconductor layer, the thickness of the high-resistance region, as well as the impurity concentrations of the n-type semiconductor substrate, the impurity concentration of the n-type semiconductor layer, and the impurity concentration of the high-resistance region can be adjusted to set appropriate thicknesses and impurity concentrations.

[0027] The p-type semiconductor portion 20 is made of a NiO-based material, a CuO-based material, a CuO-based material, or a SnO-based material (p-type oxide semiconductor material). The p-type semiconductor portion 20 can be formed by depositing a p-type oxide semiconductor material on the surface of the n-type semiconductor substrate 10 at a distance from the surface when viewed from the cross section. A hetero pn junction is formed between the p-type semiconductor portion 20 and the n-type semiconductor layer 12.

[0028] The anode electrode 30 is disposed on the surfaces of the n-type semiconductor substrate 10 and the p-type semiconductor portion 20. The anode electrode 30 is composed of a metal layer 31 for connection to other electronic components, and a Schottky barrier metal layer 32 formed below the metal layer 31 in a portion that contacts the n-type semiconductor substrate 10 (between adjacent p-type semiconductor portions 20 in cross section). The metal layer 31 is composed of, for example, a stacked film of Ti, Ni, and Au. The Schottky barrier metal layer 32 forms a Schottky junction with the n-type semiconductor substrate 10. The anode electrode 30 may be a single layer of a Schottky barrier metal layer.

[0029] The cathode electrode 40 is a metal film formed on the surface of the n-type substrate layer 11.

[0030] 2. Potential in the depth direction of Schottky barrier diode 1 Next, we will explain the potential in the depth direction of the Schottky barrier diode 1. Generally, when a voltage is applied in the reverse direction to a Schottky barrier diode, the potential of the anode electrode 30 and the n-type semiconductor layer 12 decreases, while the potential of the region near the boundary between the anode electrode 30 and the n-type semiconductor layer 12 increases, forming a Schottky barrier (in the case of the embodiment, see the portion in FIG. 2 where the potential is high between the anode electrode 30 and the n-type semiconductor layer 12).

[0031] In a Schottky barrier diode without a high-resistance region, a Schottky barrier of a certain thickness is formed at zero bias (the same Schottky barrier as the dashed line in Figure 2 is formed). Furthermore, when a reverse bias is applied, the Schottky barrier becomes thinner than at zero bias due to a high critical electric field value Ec (not shown). This results in an increase in the number of electrons that overcome the Schottky barrier or pass through it due to the tunneling effect, compared to at zero bias, resulting in a large leakage current due to thermionic field emission and field emission.

[0032] In contrast, in the Schottky barrier diode 1 according to the embodiment, an n-type high-resistance region 13 having a lower impurity concentration than the n-type semiconductor layer 12 is formed in a region of the surface of the n-type semiconductor substrate 10 that is in contact with the anode electrode 30, and therefore, as shown in Fig. 2, even when a relatively high reverse bias is applied, the thickness of the Schottky barrier remains almost unchanged from that at zero bias (see the solid line in Fig. 2). Therefore, the number of electrons that overcome the Schottky barrier and the number of electrons that pass through due to the tunneling effect do not change significantly from that at zero bias, and leakage current due to thermionic field emission and field emission can be reduced even when a reverse bias is applied, compared to a typical Schottky barrier diode.

[0033] 3. Reverse characteristics of Schottky barrier diode 1 Next, the reverse characteristics of the Schottky barrier diode 1 will be described using the graph in Fig. 3. Fig. 3 is a graph schematically showing the reverse characteristics of Example A1 (Schottky barrier diode 1 according to the embodiment), Comparative Example A2 (MPS structure), and Comparative Example A3 (MOS-SBD structure). In Fig. 3, the reverse characteristics of Example A1 are shown by a solid line, the reverse characteristics of Comparative Example A2 are shown by a dashed line, and the reverse characteristics of Comparative Example A3 are shown by a two-dot chain line.

[0034] Example A1 is a Schottky barrier diode having a configuration similar to that of the Schottky barrier diode 1 according to the embodiment. Comparative Example A2 is a Schottky barrier diode in which the conventional Schottky barrier diode 800 is made of a Ga2O3-based material (see FIG. 5(b)). Comparative Example A3 is a Schottky barrier diode having a MOS-SBD structure made of a Ga2O3-based material (described later).

[0035] In order to take advantage of the wide band gap and high breakdown voltage advantages of Ga2O3-based materials, the impurity concentrations of n-type substrate layer 11 and n-type semiconductor layer 12 are made higher in Example A1 compared to when they are made of Si-based materials. Also, in Comparative Examples A2 and A3, the impurity concentrations of n-type substrate layers 811, 911 and n-type semiconductor layers 812, 912 are made higher compared to when they are made of Si-based materials (see FIG. 5(a)) as in Example A1.

[0036] 6, a Schottky barrier diode 900 having a conventional MOS-SBD structure has a structure in which a trench 914 is formed in the surface of an n-type semiconductor substrate 910 made of a Ga2O3-based material, the metal constituting the anode electrode 930 is also disposed inside the trench 914, and a high-k oxide film 950 (hafnium oxide film) is formed along the edge of the trench 914. In FIG. 6, reference numeral 911 denotes an n-type substrate layer, reference numeral 912 denotes an n-type semiconductor layer, and reference numeral 940 denotes a cathode electrode.

[0037] 3, even when a relatively large voltage is applied in the reverse direction, the reverse current Ir in Example A1 remains low up to a certain voltage. This is thought to be because the depletion layers generated at the pn junctions between the adjacent p-type semiconductor portion 20 and n-type semiconductor substrate 10 extend and pinch off the relatively thinly doped n-type semiconductor layer 12, thereby reducing the surface electric field and, as a result, reducing the leakage current.

[0038] On the other hand, in Comparative Example A2, the reverse current Ir increases as a relatively large voltage is applied in the reverse direction. This is thought to be because, by using a Ga2O3-based material, the impurity concentrations of n-type substrate layer 811 and n-type semiconductor layer 812 are increased compared to when using a Si-based material (see FIG. 5(a)), and therefore the depletion layers generated from the pn junctions between adjacent p-type semiconductor region 820 and n-type semiconductor base 810 (n-type semiconductor layer 812) are less likely to extend and are less likely to be pinched off.

[0039] In Comparative Example A3, the reverse current Ir remains low up to a certain voltage, as in Example A1. This is thought to be because the trench shortens the surface spacing, and the high-k oxide film 950 weakens the electric field, reducing the surface electric field and, as a result, reducing the leakage current.

[0040] Next, the forward characteristics of the Schottky barrier diode 1 will be described using the graph in Fig. 4. Fig. 4 is a graph schematically showing the forward characteristics of Example A1 (Schottky barrier diode 1 according to the embodiment), Comparative Example A2 (MPS structure), and Comparative Example A3 (MOS-SBD structure). Note that in Fig. 4 as well, the forward characteristics of Example A1 are shown by a solid line, the forward characteristics of Comparative Example A2 are shown by a dashed line, and the forward characteristics of Comparative Example A3 are shown by a two-dot chain line.

[0041] 4, when a voltage is applied in the forward direction, the slope (the reciprocal of the resistance value) of the forward current If with respect to the forward voltage Vf in Example A1 is large. Therefore, it can be seen that Example A1 has a small resistance value and a small forward loss. This is because, although Example A1 has a high-resistance region 13 with a lower impurity concentration than the n-type semiconductor layer 12, the forward loss when current flows via the high-resistance region 13 is compensated for by increasing the impurity concentration of the n-type semiconductor layer 12 to reduce the forward loss when current flows via the n-type semiconductor layer 12, and therefore, it is possible to prevent a reduction in the forward current of the Schottky barrier diode as a whole.

[0042] On the other hand, the slope of the forward current If with respect to the forward voltage Vf (the reciprocal of the resistance value) in Comparative Example A2 is smaller than that in Example A1, and the resistance value is larger than that in Example A1, resulting in larger forward loss. This is thought to be because the p-type semiconductor region 820 in Comparative Example 1 hardly contributes to conduction, reducing the effective area of ​​the Schottky barrier diode and increasing forward loss.

[0043] Furthermore, the slope of the forward current If with respect to the forward voltage Vf (the reciprocal of the resistance value) in Comparative Example A3 is smaller than those in Example A1 and Comparative Example A2, and the resistance value is larger than those in Example A1 and Comparative Example A2, resulting in a larger forward loss. This is thought to be because in Comparative Example A3, the high-k oxide film 950 is formed along the edge of the trench 914, preventing holes from being supplied from the metal in the trench 914 to the n-type semiconductor layer, resulting in a smaller forward current.

[0044] 4. Effects of Schottky Barrier Diode 1 According to the Embodiment The Schottky barrier diode 1 according to the embodiment includes an n-type semiconductor substrate 10 made of a Ga2O3-based material, and therefore can utilize a Ga2O3-based material with a high critical electric field value, making it possible to realize a semiconductor device with high breakdown voltage.

[0045] Furthermore, the Schottky barrier diode 1 according to the embodiment includes a plurality of p-type semiconductor sections 20 spaced apart from one another on the surface of the n-type semiconductor substrate 10 in a cross-sectional view, eliminating the need to form a p-type region (p-type semiconductor region) within the n-type semiconductor substrate. Furthermore, a depletion layer is easily generated from the pn junction between the n-type semiconductor substrate 10 and the plurality of spaced apart p-type semiconductor sections 20. This facilitates the depletion layer to extend over the surface of the n-type semiconductor substrate. Therefore, without forming a p-type region (p-type semiconductor region) within the n-type semiconductor substrate 10, the surface electric field of the n-type semiconductor substrate 10 in the region where the n-type semiconductor substrate 10 contacts the anode electrode 30 can be reduced, thereby reducing leakage current. As a result, a sufficient reverse surge resistance can be obtained.

[0046] Furthermore, in the Schottky barrier diode 1 according to the embodiment, the n-type semiconductor substrate 10 has an n-type high-resistance region 13 formed at least in a region of the surface of the n-type semiconductor substrate 10 that contacts the anode electrode 30. The n-type high-resistance region 13 has a lower impurity concentration and higher resistance than the n-type semiconductor layer 12. Therefore, even when a high reverse bias voltage is applied, the Schottky barrier can be made thick enough to prevent thermoelectrons from crossing the Schottky barrier and to prevent the tunneling effect from occurring. This makes it difficult for thermoelectron field emission and field emission to occur, thereby reducing leakage current. Therefore, from this perspective as well, sufficient reverse surge resistance can be achieved.

[0047] Furthermore, in the Schottky barrier diode 1 according to the embodiment, the thickness of the high-resistance region 13 is such that the tunneling effect does not occur even when a reverse bias is applied, making it difficult for field emission due to the tunneling effect to occur, thereby reducing leakage current.

[0048] Furthermore, in the Schottky barrier diode 1 according to the embodiment, the high-resistance region 13 is in contact with the anode electrode 30 (Schottky barrier metal layer 32) and the p-type semiconductor portion 20, and therefore, even when a reverse bias is applied, it is possible to more effectively prevent thermionic emission, thermionic field emission, and field emission due to the tunneling effect. Furthermore, because the p-type semiconductor portion 20 is also in contact with the n-type semiconductor layer 12, the region where avalanche breakdown occurs can be the region where the p-type semiconductor portion 20 and the n-type semiconductor layer 12 contact each other (see region R in FIG. 1 ). This prevents avalanche breakdown from occurring in the region where the anode electrode 30 (Schottky barrier metal layer 32) and the high-resistance region 13 contact each other. This makes it possible to more reliably reduce leakage current.

[0049] Furthermore, in the Schottky barrier diode 1 according to the embodiment, the impurity concentration of the high-resistance region 13 is two orders of magnitude lower than the impurity concentration of the n-type semiconductor layer 12. In other words, since the impurity concentration of the n-type semiconductor layer 12 is relatively high, the resistance of the n-type semiconductor layer 12 is low, which can compensate for the decrease in the amount of current due to the high resistance of the high-resistance region 13. As a result, the resistance of the current flowing from the anode electrode 30 through the high-resistance region 13 and the n-type semiconductor layer 12 to the cathode electrode 40 can be reduced, and an increase in forward loss in the entire Schottky barrier diode can be prevented.

[0050] Although the present invention has been described based on the above embodiment, the present invention is not limited to the above embodiment and can be embodied in various forms without departing from the spirit of the present invention, and for example, the following modifications are also possible.

[0051] The positions, connections, numbers, etc. described in the above embodiments are merely examples and can be changed within the scope that does not impair the effects of the present invention. [Explanation of symbols]

[0052] 1... Schottky barrier diode, 10... n-type semiconductor substrate, 12... n-type semiconductor layer, 13... high resistance region, 20... p-type semiconductor portion, 30... anode electrode

Claims

1. n-type Ga ２ O ３ an n-type semiconductor substrate made of a silicon-based material and having an n-type semiconductor layer; a plurality of p-type semiconductor portions disposed apart from each other on a surface of the n-type semiconductor substrate when viewed in cross section; an anode electrode disposed on the surfaces of the n-type semiconductor substrate and the p-type semiconductor portion, the n-type semiconductor substrate further includes an n-type high-resistance region formed at least in a region of the surface of the n-type semiconductor substrate that is in contact with the anode electrode, the n-type high-resistance region having a lower impurity concentration and higher resistance than the n-type semiconductor layer.

2. 2. The Schottky barrier diode according to claim 1, wherein the high resistance region has a thickness such that no tunneling effect occurs even when a reverse bias is applied.

3. the high-resistance region is in contact with the anode electrode and the p-type semiconductor portion, 3. The Schottky barrier diode according to claim 1, wherein the p-type semiconductor portion is also in contact with the n-type semiconductor layer.

4. 3. The Schottky barrier diode according to claim 1, wherein the impurity concentration of the high-resistance region is two orders of magnitude smaller than the impurity concentration of the n-type semiconductor layer.

Images