JPS61288474A - Semiconductor device - Google Patents

Abstract

PURPOSE:To prevent the characteristics from deteriorating due to the running of carriers in the impurity-doped semiconductor, by providing with non-doped semiconductor having a small energy gap between the P-N junction. CONSTITUTION:In a case where non-doped GaAs 30 is put between P-doped Al0.5Ga0.5As 29 and N-doped Al0.5Ga0.5As 31, two-dimensional hole gas 40 is produced on the hetero interface between the GaAs 30 and the P<-> Al0.5Ga0.5As 29, and two-dimensional electron gas 39 on the hetero interface between the GaAs 30 and the N<-> Al0.5Ga0.5As. In the non-doped GaAs 42, the potential gradient is created, and thus extremely large internal electrical field may be generated. Accordingly, when electrons 37 and holes 38 are created by light irradiation 44 in the non-doped GaAs layer 42, each carrier flows into the hetero interfaces at an almost saturated speed by the large internal electrical field.

Description

[Detailed description of the invention] (Industrial application field) The present invention provides a photovoltaic cell that is easy to manufacture and has high performance.

The present invention relates to semiconductor devices such as photodetectors, N-type and P-type field effect transistors, phototransistors, non-bias photodetectors using combinations thereof, non-bias phototransistors, complementary circuits, and optoelectronic circuits.

(Summary of the Invention) The present invention provides a photovoltaic semiconductor device using a PN junction formed on a substrate, in which a band energy gap between the PN junctions is smaller than that of both P-type and N-type semiconductors. and a non-doped or lightly doped third semiconductor layer is provided, and a P-type ohmic contact and an N-type ohmic contact are respectively formed with respect to the third semiconductor layer to configure a photovoltaic device. By doing so, the internal electric field acting on the photoexcited carrier is large, and by using a two-dimensional electron gas or two-dimensional hole gas, there is less characteristic deterioration due to scattering and recombination of fine objects, and the planar shape is easy to manufacture and has a large light-receiving area. The purpose of the present invention is to provide a high-performance photovoltaic device that can be expanded.

The present invention provides a photovoltaic semiconductor device formed on a substrate, in which a non-doped or lightly doped semiconductor having a smaller band energy gap than both P-type and N-type semiconductors is provided between the PN junctions. 3 semiconductor layers are arranged, two N-type ohmic electrodes are formed on the third semiconductor layer, and a bias voltage is applied between the electrodes to configure a photocurrent detection device. It has a large internal electric field acting on the carrier, uses two-dimensional electron gas and two-dimensional hole gas, has little characteristic deterioration due to fine object scattering and recombination, and is easy to manufacture in a planar shape that allows for a large light-receiving area. The present invention provides a photocurrent detection device.

The present invention provides a photovoltaic semiconductor device formed on a substrate, in which a non-doped or lightly doped semiconductor having a smaller band energy gap than both P-type and N-type semiconductors is provided between the PN junctions. 3 semiconductor layers are arranged, two N-type ohmic electrodes are formed on the third semiconductor layer, a bias voltage is applied between the electrodes to constitute a photocurrent detection device, and the same crystal P-type and N-type ohmic electrodes are formed on the third semiconductor layer in other regions to configure a photovoltaic device, and the photovoltaic device is connected to the bias of the photocurrent detection device. By using it as a power source and fully configuring the non-bias photocurrent detection device, the internal electric field acting on the photoexcited carrier is large, and by using two-dimensional electron gas and two-dimensional hole gas, there is little characteristic deterioration due to impurity scattering φ recombination. It is an object of the present invention to provide a photovoltaic device which has a planar shape that is easy to manufacture, has a high performance with a large light-receiving area, and can be configured as an integrated LY in a planar shape that is easy to manufacture on the same crystal.

(Prior art and problems to be solved by the invention) (a)
The first photovoltaic semiconductor device using a PN junction
As shown in Figure 1, conventional P-type semiconductor processing.

A device was provided in which each of the N-type semiconductors 2 was provided with a direct contact electrode 4.5. (Reference: J, R, Da
vis et al: Conf, Rec, 7
th PhotovoltaSpec, Conf,
(9618) p, 85) However, in this element, the electromotive force is generated between the P-type semiconductor 1 and the N-type semiconductor 2, so it is 4゛ in the figure.

5, the contact electrodes essentially had to be placed on opposite sides of the PN junction surface 3, which was disadvantageous in increasing the light receiving surface for higher performance.

In addition, carriers such as electrons and holes generated by light irradiation must be conducted by gold diffusion in the impurity crystal in the direction perpendicular to the PN junction surface, as shown in the principle diagram shown in Figure 12, and the series resistance is large and There is also a large loss of carriers due to recombination, etc., making it difficult to increase the photo-electromotive force conversion efficiency. In the figure, 6 indicates excitation light, 7 indicates photoexcited electrons, 8 indicates photoexcited holes, 9 indicates an N-type semiconductor, and 10 indicates a P-type semiconductor.

(b) Also, a photocurrent detection device using the photoconductive effect
Conventionally, as shown in FIG. 13, two N-type ohmic electrodes 13 and 14 are simply formed on a semiconductor n to detect the excitation current generated by the excitation light 11.

(References: R, H, Bube et al.:
Ph)rs, Rev, 128 (1962) 53
2) In this case, there was a drawback that the photocurrent flowing through the semiconductor was smaller than the scattering of electrons due to impurities in the semiconductor.

(c) On the other hand, as a non-bias photodetector that does not require external application of a bias voltage for photodetection,
As the principle is shown in the figure, there is a system in which electrons 16 generated by excitation light 15 are collected by the internal electric field of the crystal, and the electromotive force between the channel and the substrate at this Peter interface is used as a bias voltage. In the figure, 17 is a photoexcitation hole, 18 is N
-A9aAa side, 19 is non-doped (P-9N-) G
On the aAa side, KA indicates a hetero interface, and 21 indicates a two-dimensional electron gas. (References: C, Y, Chen et al:
Appl, Phys, Lett, 41(3)
p282 This is because, as shown in Fig. 15, the process is complicated due to the drawback of having to provide electrodes on the substrate side, and the planar configuration is difficult because the electrodes must be placed on the substrate side as shown in Figure 15. It was difficult to do so.

In addition, conventional general semiconductor photocells, photodetectors, phototransistors, electronic circuits, etc. required different crystal layer structures and electrode installation methods, so they were combined into one brainer type on the same crystal substrate. The drawback was that it was difficult to do.

(d) N type and P type generated at the semiconductor hetero interface
Both types of transistors using carriers of the form (References:
T, Mimura et al: Jpn, J
, Appl, Phya.

20 (1981) p 598, H, L, St.
omer et al: Appl.

Ph1s, Latt, 44(lυ1. (1984
) Complementary circuit construction method according to p. 1062) A method that can be easily inferred from conventional individual devices is to stack individual devices in multiple layers as shown in Figure 16, but this It has many disadvantages in terms of crystal growth, such as deterioration of the crystal quality of the upper layer crystal, complexity in the process due to its non-planar shape, and breakage of wiring due to steps. In the figure, 85 is a substrate, 86 is P-AtGaAa, and 87 is a non-doped (
p-, N-) GaAa, 88 is N-AtGaAa.

89 is a two-dimensional hole gas, 90 is a two-dimensional electron gas, 91
, 93 is a source and drain electrode, 92 is a gate electrode, 9
4.96 is the source and drain electrodes, 95 is the gate electrode,
97 indicates a P-channel FET, and 98 indicates an N-channel PET.

(Means for Solving the Problems) An object of the present invention is to prevent characteristic deterioration caused by carriers traveling in an impurity-doped semiconductor in a PN junction photovoltaic device, and to provide a planar type electrode. To realize a simple and high-performance photovoltaic device by removing restrictions on the process and element structure due to installation location, and
The aim is to use this to realize high-performance non-biased photocurrent detectors and non-biased phototransistors. At the same time, it is also possible to realize a complementary circuit by forming simple planar type N-type and P-type field effect transistors with controllable threshold values using the same crystal, and to use the same crystal for the various elements mentioned above. Combined on a crystal,
The goal is to realize optoelectronic circuits.

Ninth, to achieve the above object, the present invention provides a non-doped third semiconductor device with a small energy gap between PN junctions, and utilizes the conduction of two-dimensional gas generated at the hetero interface, which is photoexcited by the internal electric field of the crystal. The first main feature is that the accelerated carriers are made to travel parallel to the crystal film plane and through the non-doped semiconductor. The present invention differs from the conventional technology in that the crystal layer structure is a third semiconductor device, and the ohmic electrodes can all be placed on the same surface of the crystal.

(Embodiments) Next, the present invention will be described in detail.The embodiments are merely illustrative, and it goes without saying that various changes and improvements can be made without departing from the spirit of the present invention.

(I) Figure 1 is a schematic diagram of the crystal structure of an example according to the present invention, and Figure 2 is its band diagram.0 As in the example shown in Figure 1, non-doped GaAs 3
0 is P-doped Ato, 5 Ga 6.5 AB 29
and N-doped At□, 5 caO, 5As 31, as shown in Fig.
A two-dimensional hole gas 40 is generated at the hetero interface on the Gao, 5 AS 29 side, and a two-dimensional electron gas 39 is generated at the hetero interface on the N - At6.5 Ga O, SA side. Aha board,
A indicates the light irradiation surface.

Here, a potential gradient as shown in FIG. 2 occurs in the non-doped G&As 42, and an extremely large internal electric field is therefore generated there. As an example, assuming that the non-doped QaAs layer thickness is 100 OA and the GaAs band energy gap is 1-4 eV, the calculated average internal electric field reaches 140 KVlon. 41
is N-AfflaAs, 43 is p-AtGaA
Show s'z.

Therefore, electrons 37. When a hole is generated, each carrier flows into the heterointerface at almost saturation velocity due to the large internal electric field. Therefore, P-type and N-type ohmic contacts are made to the non-doped GaAs channel layer by ion implantation using Be ions, St ions, etc., and AuZnNi/Ti/Au + AuGe.
When formed on the plane of a crystal film using 70mm /Ni etc. and electrically connected, a voltage of up to 1.4 V (band energy gap of GaAs) is generated between these two electrodes as shown in Figure 3. A photovoltaic force is generated. In the figure, 45 is a P-type ohmic electrode, 46 is an N-type ohmic electrode, 47 is a mesa separation, 48 is an excitation light, and 49 is a light irradiation semiconductor.

Unlike conventional PN junction photovoltaic cells, carriers that conduct through the heterointerface parallel to the plane of the crystal film have a channel in non-doped QaAs, so impurity scattering is small, the resistance is low, and recombination is also difficult. Therefore, the photo-electromotive force conversion efficiency is extremely high. Another difference from conventional devices is that the total number of carriers generated is proportional to the light-receiving area, and because of the planar configuration, the light-receiving surface can be easily made arbitrarily large.

(6) On the other hand, as shown in FIG. 4, two N-type ohmic electrodes 55 and 56i are formed on the crystal, and a bias voltage 52 is applied between these electrodes 55 and 56. When irradiation light 50 is applied to each sandwiched semiconductor region to excite carriers, a photocurrent 53 flows between the two electrodes. Regarding carrier generation, conduction to the heterointerface, and conduction on the Peter interface, as in (1), carriers that are photoexcited by the large internal electric field of the crystal are first accelerated, and then the carriers are transferred to the non-doped crystal with less impurity scattering. Since it flows at high speed through the hetero interface, the detection speed and detection efficiency are improved compared to general ones.

(III) In Figure 5, a non-biased photodetector is constructed by using the same crystal and the photocell shown in (I) as the bias voltage source of the photocurrent detector shown in (G). This is an example of a case. In the figure, 57 is an excitation light, 58 is a mesa separation, 59 is an N-type ohmic electrode, 60 is a P-type ohmic electrode, 61.62 is an N-type ohmic electrode, 63, 6
4 indicates a light-irradiated semiconductor. That is, the N-type ohmic electrode 59. The photovoltaic force generated between the P-type ohmic electrode ω is expressed as N
A bias is applied between the ohmic electrodes 61 and 62. Here, the photovoltaic voltage 'k between the electrodes 59 and 60 is 1.4
If the distance between the electrodes is 5 μm, an electric field of about 3 KV/an acts on the carriers at the hetero interface, and the carriers can be sufficiently accelerated.

In this way, a bias-free photodetector can be easily realized simply by combining the wires between the ohmic electrodes arranged on a plane.

(■ In Fig. 6, the same crystal is used to form the photodetector shown in ■, and a Schottky gate electrode 69 made of sTi+Au or the like is installed on the semiconductor plate between these two ohmic electrodes 66 and 67. This is an example of configuring an N-type field effect transistor or a phototransistor. 65 indicates a mesa isolation.

In this case, the seventh band diagram under the gate electrode is
As shown in the figure, the mechanism for controlling the two-dimensional gas at the hetero interface using the applied gate voltage is the same as that of a normal two-dimensional electron gas heterostructure FET.

Therefore, if the gate electrode is made highly transparent, a high-performance phototransistor can be formed.

If a phototransistor is not irradiated with light, it functions in the same way as a normal field effect transistor.As with 0, a non-biased phototransistor can be constructed by combining it with a photovoltaic cell formed in the same crystal. .

FIG. 7 shows a band diagram. In the figure, 70 is a gate electrode, 71 is N-AtGaAa, 72 is non-doped (p"', N-) GaAs, and 73 is P-A.
tGaAs, 143 is a two-dimensional electron gas, 144 is an excitation light, and 145 is a photoexcited electron.

(V) Based on the same principle as shown in Fig. 7, the concentration of the two-dimensional hole gas generated at the hetero interface with the P-type crystal is controlled by applying a negative gate voltage (see Fig. 8). P-type transistors can also be constructed from the same crystal.In Fig. 8, 99 is a gate electrode, and 100 is a P-type transistor.
-AtGaAs, 101 is non-doped (p-, N-)
GaAs, 102 aAa to N-A.

103 is a two-dimensional electron gas, and 104 is a two-dimensional hole gas. In this case, all P-type ohmic electrodes are used. Since N-type and P-type transistors can be formed in a planar shape on the same crystal, a complementary circuit can be constructed much more easily than the conventional one (see FIG. 9). In FIG. 9, 105 is a substrate, 106 is P-ALGaAa, and 107 is non-doped A/[aA
s, 108 is N-AtGaAs, 109
is non-doped As, 110 is non-doped G
aAs 1111 is non-doped QaAs% 112 is a two-dimensional hole gas, 113 is a two-dimensional electron gas, 114
, 116 is a P-type ohmic electrode, 115 is a gate electrode, 117 and 119 are N-type ohmic electrodes, 118 is a gate electrode, 120 and 121 are P ion implantation regions, 1
Reference numerals 22 and 123 indicate N ion implantation regions, and 124 indicates isolation between elements.

In the complementary circuit configuration using this method, the doping concentration for N-type and P-type is controlled, or the amount of etching under the gates of both N-type and P-type transistors is controlled (first
(See Figure 0) has the advantage of matching threshold values and achieving high performance. In FIG. 10, 125 is a substrate;
126 is N-A9As, 127 is non-doped AtGaAs, 128 is non-doped GaAs,
129 is non-doped A stain aAs.

130 is P-AtGaAas, 131 is non-doped GaAs, 132 and 134 are P-type ohmic electrodes.
% 133 is a gate electrode, 135 and 137 are N-type ohmic electrodes, 136 is a gate electrode, 138 is a two-dimensional hole gas, 139 is a two-dimensional electron gas, 140 and 14
1 indicates a process-etch, and 142 indicates isolation between elements. Furthermore, a method of controlling the threshold value by using the Nif gate metal and sinking the metal through heat treatment can also be applied. (References: IEEE, EDL, Vol, BD
L-5 No., 7 1984 p241) Furthermore, as described later, by placing a P-type semiconductor in the upper layer and an N-type semiconductor in the lower layer, the influence of leakage current due to parallel conduction due to P-type contamination can be removed. There is also an advantage in reducing the power consumption of the circuit.

(6) As described above, the photocell, photocurrent detector, N-type and P-type field effect transistors, and phototransistor can all be formed into a planar shape using the same crystal, so these can be combined on the same unit. can be formed to construct various optoelectronic circuits.

(To) The layer structure of the crystal used above is the 9th layer structure.
As shown in Fig. 1 and Fig. 10, At near the hetero interface
When the impurity concentration of O,5Ga 0.5As is reduced, 1
Suppresses the effects of impurity diffusion and scattering! Ij is effective for improving device performance. Further, the At composition ratio of the mixed crystal may be other than the sail 5. Furthermore, P.

For N semiconductors, if the doping concentration is distributed such that it is higher nearer to the hetero interface and lower closer to the hetero interface, and the PN semiconductor layer is depleted by supplying carriers to the hetero interface, ohmic contact can be made. In this case, there is no need to worry about parallel conduction except for high-mobility two-dimensional gases.

In addition, it is generally assumed that the MBE crystal has P-type contamination, and as a method to prevent parallel conduction of the P-type, there is a method of making the upper layer of the crystal tp type and the lower layer '(zN type). If the P-type ohmic is not made too deep in the crystal thickness direction, the P-type parallel conduction can be easily removed. ALo, sGa 6.5
Non-doped GaAa 7m'ft for protection on A8 surface
When provided as a cap, it can prevent surface oxidation and the like.

(Effects of the Invention) As explained above, according to the present invention, the internal electric field acting on the photoexcited carrier is large, and the two-dimensional electron gas
High-performance photovoltaic devices, photocurrent detection devices, and phototransistors that have little characteristic deterioration due to particle scattering and recombination by using dimensional hole gas, as well as non-bias photodetectors and non-bias photodetectors made by combining these nine devices. Phototransistors and the like can be formed into a planar shape that is easy to manufacture on the same crystal. This is a light sensor.

Can be applied to optical power sources, etc.

Furthermore, a high-performance N type using a high-mobility two-dimensional gas as a carrier on the same crystal.

By forming P-type field effect transistors with threshold matching, high-performance complementary circuits can also be constructed in a planar type. By combining these various optoelectronic elements on the same crystal, a high-performance optoelectronic circuit can be realized, which has the effect of being applicable to optical communication devices such as computers and repeaters.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the crystal structure of the photovoltaic device according to the present invention, FIG. 2 is a band diagram showing the principle of the photovoltaic device that best expresses the first feature of the present invention, and FIG. FIG. 4 is a block diagram of a photovoltaic device according to the present invention. FIG. 4 is a block diagram of a light and current detection device according to the present invention. FIG. 5 is a block diagram of a non-bias photocurrent detection device according to the present invention. FIG. 7 is a band diagram showing the principle of the (optical) field effect transistor according to the present invention, and FIG. 8 is a complementary circuit that best represents the second feature of the present invention. Figure 9 is an example of a complementary circuit configuration according to the present invention, Figure 10 is another example of a complementary circuit configuration according to the present invention, and Figure 11 is a conventional PN junction diagram. A schematic diagram of a photovoltaic device; Fig. 12 is a band diagram showing the principle of a conventional PN junction photovoltaic device; Fig. 13 is a schematic diagram of a conventional photocurrent detector;
Figure 4 is a band diagram showing the principle of a conventional non-bias photocurrent detector, Figure 15 is a schematic diagram of a conventional non-bias photocurrent detector, and Figure 16 is a conventional complementary photocurrent detector.
A schematic diagram of the circuit configuration is shown. 1.10・・・・・・・・・・・・P-type semiconductor 2
．． 9, 13.14...N-type semiconductor 3......PN junction surface 4, 32, 45, 60.91, 93.114. 1
16.132゜134・・・・・・・・・・・・・・・
...P-type ohmic electrode 5, 22.23.33.46
．． 55.56.59,61,62,66°67,94.
96, 117, 119, 135, 137...
・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・N-type ohmic electrode 6, 11.15
．． 34.44.48.50.57, 144...
・・・・・・・・・・・・・・・・・・・・・・・・
...Excitation light 7, 16, 37.145...Photoexcited electron 97...P
Channel FET98 ・・・・・・・・・・・・・・・
...N-channel FET8.17.38...
...Photoexcitation hall, 24.36.49, 54.
63, 64・・・・・・・・・・・・・・・・・・
−・・・・・・・・・・・・・・・・・・・・・・・・
...Light-irradiated semiconductor δ...
...... Conductive layers 47, 51, 58.65...
・Mesa separation 18.31, 41.71.88. 102.
108. 126・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・-・N-AtGaAs
107, 109, 127, 129...
・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
・Non-doped (p″+, , f) AtGaAs19.
30.42,72,82. 101, 110. 1
11. 128゜131・・・・・・・・・・・・−
...Non-doped (p-, f) GaAs20...
・・・・・・・・・・・・・・・Hetero interface 12
0, 121...r ion implantation region 12
2, 123...T ion implantation region 21
．． 39,90. 103. 113. 139. 14
3 ・・・・・・・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・Two-dimensional electronic gas chrysanthemum, 89
, 104, 112, 138...
・・・・・・・・・・・・・・・・・・Two-dimensional hole gas 124.142・・・・・・・・・Separation between elements 26.5
2・・・・・・・・・・・・Bias voltage n, 5
3・・・・・・・・・・・・Photocurrent 140.14
1・・・・・・Recess Etch 29.43,73
,86,100,106,130=---
・－・・−−・・・−・・・・・・・・・・・・・・・
・・・・・・・・・P-AtGaAs visit, 85.105
．． 125...Buffer single layer, board etc. 69.70,92
．． 95.99. 115. 118. 133. 13
6・・・・・・・・・・・・・・・・・・・・・・・・
Gate electrode patent applicant Nippon Telegraph and Telephone Corporation Figure 1 Figure 3 Figure 4 Figure 5 Figure 8

Claims (8)

[Claims] (1) In a photovoltaic semiconductor device using a PN junction formed on a substrate, a P-type and an N-type are connected between the PN junctions.
A non-doped or lightly doped third semiconductor layer having a band energy gap smaller than that of any of the type semiconductors is provided, and a P-type ohmic contact and an N-type ohmic contact are provided to the third semiconductor layer. 1. A semiconductor device characterized in that a photovoltaic device is constructed by forming each of these. (2) In a photovoltaic semiconductor device formed on a substrate, a non-doped or lightly doped third semiconductor having a smaller band energy gap than both P-type and N-type semiconductors is placed between the PN junctions. A photocurrent detection device is constructed by disposing a semiconductor layer, forming two N-type ohmic electrodes on the third semiconductor layer, and applying a bias voltage between the electrodes. Semiconductor equipment. (3) In the photovoltaic semiconductor device formed on the substrate, a non-doped or lightly doped third semiconductor having a smaller band energy gap than both the P-type and N-type semiconductors is placed between the PN junctions. A semiconductor layer is provided, two N-type ohmic electrodes are formed on the third semiconductor layer, a bias voltage is applied between the electrodes to constitute a photocurrent detection device, and a photocurrent detection device is constructed. P-type and N-type ohmic electrodes are formed on the third semiconductor layer in other regions to configure a photovoltaic device, and the photovoltaic device is connected to a bias power source of the photocurrent detection device. 1. A semiconductor device characterized in that it is used as a non-bias photocurrent detection device. (4) A non-doped or lightly doped third semiconductor layer with a smaller band energy gap than both P-type and N-type semiconductors is disposed between the PN junctions, and two semiconductor elements can be separated by device isolation. two P-type ohmic electrodes are formed on the third semiconductor layer of one of the semiconductor elements, and a layer is formed on the surface of the semiconductor crystal sandwiched between the P-type ohmic electrodes. Schottky electrodes are provided separately to form a P-type field effect transistor, and further, N-type regions are formed on both sides of the other semiconductor element, N-type ohmic electrodes are formed in these regions, and between the N-type ohmic electrodes 2. The semiconductor device according to claim 1, wherein a Schottky electrode is provided apart from this electrode to constitute an N-type field effect transistor to constitute a complementary circuit. (5) Forming a phototransistor using the Schottky electrode formed in the N-type field effect transistor as a transparent electrode,
In combination with a photovoltaic device formed on the same substrate,
The semiconductor device according to claim 1, which comprises a non-biased phototransistor. (6) Al_xGa_(_
1_-_x_)As, Al_yGa_(_1_-_y_
) As (0<x, y≦1), and non-doped GaAs is used as the third semiconductor. The semiconductor device described. (7) Near the hetero interface of P-type and N-type semiconductors 40-1
7. The semiconductor device according to claim 1, 2, 3, 4, 5, or 6, characterized in that a range of 50 Å is non-doped. (8) The doping of the P-type and N-type semiconductors is characterized by having a profile such that the doping becomes larger closer to the hetero interface and smaller as the distance from the interface increases. , 3rd term, 4th term, 5th term, 6th term
7. The semiconductor device according to item 7.