JPH0148663B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to the structure of a novel photodetecting conductor element having wavelength selectivity, that is, having sensitivity only in a specific wavelength region.

Conventional Structures and Their Problems In recent years, with the development of the fields of optical communication and optical information processing, there has been an increasing demand for small and highly sensitive photodetecting elements. In particular, in a wavelength division multiplexing optical communication system that simultaneously transmits light of different wavelengths, a photodetector element with steep wavelength selectivity is strongly desired. In other fields such as remote sensing, there is also a demand for elements that selectively detect light in a specific wavelength range.

Conventionally, various photodiodes and phototransistors have been used as small, highly sensitive photodetecting elements. However, these elements have a property of being sensitive over a fairly wide wavelength range determined by the light absorption characteristics of the semiconductor material used. Therefore, in order to obtain wavelength selectivity as mentioned above,
It is necessary to place a spectroscopic device such as a spectroscope using a prism or a diffraction grating or an interference filter in front of the photodetector, which makes the device complex and large, and the optical loss caused by the spectroscopic device causes a substantial loss of energy. There were drawbacks such as decreased sensitivity.

Here, the structure and operating principle of typical photodetecting elements that have been used conventionally will be explained.

FIG. 1 is a cross-sectional view schematically showing the structure of a heterojunction phototransistor, which is known as a particularly highly sensitive photodetecting element. Further, FIG. 2 is a top view corresponding to FIG. 1. In these figures, 1 is a semiconductor substrate, 2 is a collector layer, 3 is a base layer, and 4 is a semiconductor substrate.
is an emitter layer, and 11 and 12 are electrodes. As in a normal transistor, the base layer and other layers have different conductivity types. Note that the element is formed into a plateau shape by so-called mesa etching. Semiconductors have a forbidden band width inherent to their materials, but in this case, the emitter layer 4 and the base layer 3
The emitter layer 4 is made of different semiconductor materials.
The forbidden band width of the base layer 3 is configured to be sufficiently wider than that of the base layer 3. Here, the light to be detected is made to enter from the emitter layer 4 side. Of the incident light, it passes through the emitter layer 4 and passes through the base layer 3.
The absorbed light generates charges in the base layer 3 and lowers the base potential. As a result, the collector current increases due to normal transistor action,
It will operate as a highly sensitive photodetecting element.

It goes without saying that at this time, a DC voltage in the direction of reverse biasing the collector junction is applied between the collector layer 2 and the emitter layer 4 via the electrodes 11 and 12. By the way, emitter injection efficiency is a major factor that determines the amplification factor of a transistor. As described above, when the forbidden band width of the emitter layer 4 is sufficiently larger than the forbidden band width of the base layer 3, the emitter injection efficiency becomes large, and a high amplification factor, that is, high sensitivity can be obtained.

Now, the wavelength dependence of sensitivity in this conventional element is as follows. In general, semiconductors have the property of absorbing light with a wavelength shorter than the absorption edge wavelength λg (unit: μm), which is approximately given by λg = 1.24/Eg, where the forbidden band width is Eg (unit: electron volt). There is. Therefore, in the device of this example, light having a wavelength shorter than λg of the emitter layer 4 is absorbed by the emitter layer and does not reach the base layer 3. Also, light with a wavelength longer than λg of the base layer 3 is not absorbed by either the emitter layer or the base layer (Eg of the emitter layer is larger than Eg of the base layer, so
λg of the emitter layer is on the shorter wavelength side than λg of the base layer). However, light having a wavelength between λg of the emitter layer 4 and λg of the base layer 3 is absorbed by the base layer 3 after passing through the emitter layer 4. That is, this element exhibits sensitivity to light in the above wavelength range.

In such an element, in order to obtain steep wavelength selectivity, it is considered that λg of the emitter layer and the base layer should be made close to each other, that is, Eg should be made close to each other. However, in that case, the emitter injection efficiency described above decreases, the amplification factor of the transistor decreases, and the sensitivity decreases.

By the way, the inventors have discovered that a photodetecting element with high sensitivity and steep wavelength selectivity can be obtained by a novel structure including a semiconductor light absorption layer.

Purpose of the Invention In view of the above-mentioned conventional problems, the present invention provides a light emitting device that exhibits arbitrary wavelength selectivity, particularly steep wavelength selectivity, and has high sensitivity without using a spectroscopic device. The purpose is to provide semiconductor devices.

Structure of the Invention The present invention comprises a semiconductor having a forbidden band width narrower than the forbidden band width of the semiconductor constituting the emitter layer and wider than the forbidden band width of the semiconductor constituting the base layer in a conventional heterojunction phototransistor. By adding a light absorption layer, a highly sensitive wavelength selective photodetecting semiconductor element is realized.

DESCRIPTION OF EMBODIMENTS Specific embodiments of the present invention will be described with reference to the drawings. In the diagrams showing the element structure of each embodiment, the same reference numerals are given to the constituent elements common to the conventional example and each embodiment.

FIG. 3 is a cross-sectional view schematically showing the structure of a photodetecting semiconductor element in the first embodiment of the present invention.
The top view is omitted because it is the same as FIG. 2, which is a top view of the conventional example. In FIG. 3, 1 is a semiconductor substrate, 2 is a collector layer, 3 is a base layer, 4 is an emitter layer, 5 is a light absorption layer, and 11 and 12 are electrodes. This is the same as the conventional example in that the conductivity type of the base layer 3 and the conductivity types of the other layers and the substrate are different from each other. Also, as in the conventional example, the forbidden band width of the emitter layer is wider than the forbidden band width of the base layer.

In this embodiment, the light absorption layer 5 is made of a semiconductor having a forbidden band width narrower than that of the emitter layer 4 and wider than that of the base layer 3. As a result, the wavelength dependence of sensitivity in this example is as follows. In this embodiment as well, the light to be detected is made incident from the top surface of the element, that is, from the side of the light absorption layer 5. At this time, the absorption edge wavelength λg of the light absorption layer 5 (the definition of λg is given in the section on the structure of the conventional example and its problems). (as explained above) is absorbed by the light absorption layer 5 and does not reach the emitter layer 4 and the base layer 3. Further, light having a wavelength longer than λg of the base layer 3 is not absorbed by any of the light absorbance 5, the emitter layer 4, and the base layer 3. This is because the forbidden band width Eg of the light absorption layer and the emitter layer is wider than that of the base layer, and therefore the λg of the light absorption layer and the emitter layer is on the shorter wavelength side than the λg of the base layer.

On the other hand, when the wavelength of the incident light is between λg of the light absorption layer 5 and λg of the base layer 3, the incident light passes through the light absorption layer 5 and the emitter layer 4 and then reaches the base layer 3.
It is absorbed by. This is because the Eg of the light absorption layer is smaller than that of the emitter layer and larger than the Eg of the base layer, so λg of the emitter layer is on the shorter wavelength side than λg of the light absorption layer, and λg of the light absorption layer is This is because the wavelength is on the shorter wavelength side than λg of the base layer.

As can be seen from the above, the element of this example exhibits sensitivity because the base layer absorbs only light having a wavelength between λg of the light absorption layer and λg of the base layer. That is, the sensitivity wavelength range can be arbitrarily set by the forbidden band width Eg of the light absorption layer and the base layer. In particular, the Eg of the light absorption layer and base layer
If they are made close to each other, it is possible to realize a photodetecting element with a narrow sensitivity wavelength range, that is, steep wavelength selectivity. In the case of this example, since the Eg of the emitter layer is sufficiently larger than the Eg of the base layer,
High emitter injection efficiency can be obtained regardless of the degree of wavelength selectivity, and a large amplification factor, that is, high sensitivity can be obtained.

FIG. 4 shows an example of the wavelength dependence of sensitivity (solid line) in the photodetector semiconductor element of this example in comparison with that of the comparative example (dashed line). Sensitivity is indicated by the magnitude of collector current per unit amount of incident light. In this case, the Eg of the base layer, emitter layer, and light absorption layer are 0.99 electron volt, 1.35 electron volt, and 1.01 electron volt, respectively. Converting these to the absorption edge wavelength λg is approximately 1.25 μm, respectively.
They are 0.92μm and 1.23μm. The thickness of each layer is 3 μm for the collector layer, 1 μm for the base layer, and 2 μm for the emitter layer. As can be seen from Figure 4,
The half-width of the single-wave sensitivity curve (solid line) of this device is approximately
It is extremely small at 0.05 μm, providing steep wavelength selectivity. Also, the maximum sensitivity is approximately
It is extremely large at 400A/W. The structure of the device of the comparative example is the same as that of the above example except that it does not include a light absorption layer. In this case, the element has a thickness of 0.9 to 1.25 μm, based on the characteristics shown by the broken line in Figure 4.
It can be seen that the sensitivity is shown over a wide wavelength range.

By the way, in principle, in order to obtain steep wavelength selectivity, the light absorption layer and the base layer should be
It is considered that the closer Eg is, the better. However, the inventors have discovered that if this difference in Eg is too small, the wavelength selectivity and sensitivity are reduced. Figure 5 shows an element similar to the element whose characteristics are shown in Figure 4, with only the Eg of the light absorption layer set to 0.995.
Change to electron volt, light absorption layer and base layer Eg
This is the characteristic when the difference in is made smaller. In this case, it can be seen that the half-width is on the contrary increased to about 0.07 μm, and the wavelength selectivity is decreased, and the sensitivity is also considerably decreased. This is because the thickness of the light absorption layer and base layer is finite, so the light absorption spectrum (wavelength dependence) of these layers is not ideal, and the property that the vicinity of the absorption edge wavelength λg gradually changes. This is thought to be for the purpose of showing. In other words, for this reason, if the Eg (λg) of the light absorption layer and the base layer are very close,
Even at wavelengths between the two λg, there is some absorption by the light absorption layer, and a situation occurs in which the base layer's light absorption in this wavelength region becomes insufficient, resulting in a decrease in sensitivity in this wavelength region. It is thought that the half width becomes relatively large.
After considering this point in detail, we found that in order to obtain steep wavelength selectivity, the above-mentioned Eg difference should be reduced to the Eg of the base layer.
It has been found that it is appropriate to set the content to 1 to 3%.

Now, in the above explanation, Eg of the collector layer 2 in FIG. 3 was not mentioned. In this example, Eg of the collector layer is considered to have no essential effect on the operation of the device. However, if we match the Eg of the collector layer with the Eg of the base layer,
It has been experimentally found that the sensitivity is particularly high. This is because the light that cannot be absorbed by the base layer is absorbed in the collector layer and generates a charge, and some of this charge is injected into the base layer due to the potential gradient existing at the base-collector junction. This is thought to be because it contributes to an increase in collector current in the same way as light absorption in the layer. Therefore, it is desirable that the Eg of the collector layer match the Eg of the base layer.

In this embodiment, the semiconductor substrate 1 and the collector layer 2 are separated, but a structure in which the semiconductor substrate also serves as the collector layer may be used without affecting the operation of the device.

In addition, in this embodiment, the semiconductor substrate 1 and the collector layer 2
However, a buffer layer made of a semiconductor of the same conductivity type as both may be provided between them, and has no relation to the operation of the element.

Although examples of the thickness of each layer are shown in the description of this embodiment, these are not necessarily limited to these values. The collector layer, emitter layer, and light absorption layer are about 1 to 10 μm, and the base layer is about 0.1 to 2 μm.
A value of about m is appropriate.

FIG. 6 is a sectional view schematically showing the structure of a photodetecting semiconductor element in a second embodiment of the present invention.

In this embodiment, a light absorption layer 5 is provided on the semiconductor substrate 1,
The emitter layer 4, the base layer 3, and the collector layer 2 are sequentially laminated, and the order is reversed from that of the first embodiment. In this case, the light to be detected is from the semiconductor substrate 1
Inject it from the side. Therefore, the electrode 1 on the substrate side
1 is provided at the periphery so as not to block the incident light.
Further, the forbidden band width of the semiconductor substrate 1 is equal to or wider than the forbidden band width of the light absorption layer 5. The other components are completely the same as in the first embodiment. The operation of this element is exactly the same as that of the first embodiment, and there is no difference in its characteristics.

FIG. 7 is a sectional view schematically showing the structure of a photodetecting semiconductor element in a third embodiment of the present invention.

In this embodiment, a light absorption layer 5 is provided on the semiconductor substrate 1,
A collector layer 2, a base layer 3, and an emitter layer 4 are laminated in this order. That is, this embodiment differs from the first and second embodiments in that the light absorption layer 5 is provided on the collector layer 2 side. In the case of this embodiment, since the light to be detected is incident from the semiconductor substrate 1 side,
The electrode 11 on the substrate side is provided at the periphery as in the second embodiment. Furthermore, in this embodiment, since the incident light reaches the base layer 3 through the semiconductor substrate 1 and the collector layer 2 in addition to the light absorption layer 5, the forbidden band width of this substrate and the collector layer is equal to the forbidden band width of the light absorption layer. , or larger so as not to interfere with the original operation of the element. The other components are exactly the same as those in the first embodiment, and their operations and characteristics are also the same.

FIG. 8 is a sectional view schematically showing the structure of a photodetecting semiconductor element in a fourth embodiment of the present invention.

In this embodiment, an emitter layer 4, a base layer 3, a collector layer 2, and a light absorption layer 5 are sequentially laminated on a semiconductor substrate 1, and the structure is such that the order of each layer is reversed in the third embodiment. There is. In the case of this embodiment, the light to be detected is incident from the top surface of the element, that is, from the absorption layer 5 side. The forbidden band width of collector layer 2 is
As in the case of the third embodiment, the forbidden band width is equal to or wider than the forbidden band width of the light absorption layer 5. The other components are exactly the same as those in the first embodiment, and their operation and characteristics are also the same.

In the third and fourth embodiments described above, it is also possible for one layer to serve as both the collector layer 2 and the light absorption layer 5, and in that case, there is no effect on the operation and characteristics of the device.

Next, the semiconductor materials used in the above embodiments will be described. When manufacturing a photodetecting semiconductor device according to the present invention, it is necessary to sequentially stack semiconductor layers having different forbidden band widths. In such a case, in order to improve the crystallinity of each layer, it is essential to match the lattice constants of the substrate and the crystals of each layer with high precision. The inventors have discovered that gallium-aluminum-arsenic semiconductors and indium-gallium-arsenic-phosphorus semiconductors are semiconductor materials that satisfy these conditions and have properties suitable for the photodetector semiconductor element of the present invention. I found it.

The former is a mixture of gallium arsenide (GaAs) and aluminum arsenide (AlAs), and where the content ratio of aluminum arsenide is x, Ga _1-x Al _x A _s (O≦x≦
1). Crystals of this system have a lattice constant that is almost constant regardless of the value of x, while the forbidden band width varies significantly from 1.43 eV when x = O to 2.16 eV when x = 1. So,
It is very suitable for the device of the present invention.

The latter is a mixture of indium phosphide (InP), indium arsenide (InAs), gallium phosphide (GaP), and gallium arsenide (GaAs), where x is the content ratio of gallium to indium, and y is the content ratio of arsenic to phosphorus. In _1-x Ga _x As _y P _1-y (O≦x, y≦
1). In this type of crystal, by appropriately selecting the values of x and y, the forbidden band width can be changed significantly while keeping the lattice constant constant. For example, if the values of x and y are approximately y=
If it is selected to satisfy the relationship 2.2x, its lattice constant will always match the lattice constant of indium phosphorus when x=0 and y=0. Furthermore, the forbidden band width at this time changes significantly from 1.35 electron volts when x=0 and y=0 to 0.75 electron volts when x=0.47 and y=1, which is extremely suitable for the device of the present invention. .

Note that these semiconductor crystals can be easily formed by various epitaxial growth methods.

Effects of the Invention As described above, the present invention provides a conventional heterojunction phototransistor with a forbidden band width narrower than the forbidden band width of the semiconductor constituting the emitter layer and wider than the forbidden band width of the semiconductor constituting the base layer. By adding a light-absorbing layer made of a semiconductor that has the same properties, we have realized a highly sensitive photodetecting semiconductor element that exhibits wavelength selectivity in any wavelength range, especially steep wavelength selectivity, and can be used in conjunction with a spectrometer. This has the unprecedented effect of providing a simple means for detecting light in a specific wavelength range without any interference.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of a conventional photodetecting semiconductor element (heterogeneous junction phototransistor), FIG. 2 is a top view corresponding to FIG. 1, and FIG.
FIG. 4 is a cross-sectional view showing the structure of the photodetection semiconductor device of the example of the present invention; FIG. FIG. 5 is a characteristic curve diagram showing another example of the wavelength dependence of sensitivity in the photodetecting semiconductor device according to the embodiment of the present invention, and FIGS. 3. It is a sectional view showing the structure of the photodetecting semiconductor element of the fourth example. 1... Semiconductor substrate, 2... Collector layer, 3...
base layer, 4... emitter layer, 5... light absorption layer,
11, 12...electrodes.

Claims (1)

[Claims] 1. A heterojunction transistor having a structure in which a collector layer, a base layer, and an emitter layer made of a semiconductor having a forbidden band width wider than the forbidden band width of the semiconductor constituting the base layer are successively connected. , a light absorption layer made of a semiconductor having a forbidden band width narrower than a forbidden band width of a semiconductor constituting the emitter layer and wider than a forbidden band width of a semiconductor constituting the base layer, as an emitter layer of the heterojunction transistor; A photodetecting semiconductor element characterized in that it is provided on the side. 2. A patent claim characterized in that the difference in forbidden band width between the semiconductor constituting the base layer and the semiconductor constituting the light absorption layer is 1% to 3% of the forbidden band width of the semiconductor constituting the base layer. The photodetecting semiconductor device according to scope 1. 3. The photodetecting semiconductor device according to claim 1 or 2, wherein the base layer and the collector layer are made of semiconductors having the same forbidden band width. 4. Claim 1, wherein each of the collector layer, base layer, emitter layer, and light absorption layer is made of a semiconductor whose main component is gallium arsenide, aluminum arsenide, or gallium/aluminum arsenide. 3. The photodetecting semiconductor device according to any one of items 1 to 3. 5. Each of the collector layer, base layer, emitter layer, and light absorption layer is made of a semiconductor whose main component is indium phosphide, indium arsenide, gallium phosphide, gallium arsenide, or a mixture of at least two of these. A photodetecting semiconductor device according to any one of claims 1 to 3. 6 A heterojunction transistor having a structure in which a collector layer, a base layer, and an emitter layer made of a semiconductor having a forbidden band width wider than a forbidden band width of a semiconductor constituting the base layer are successively connected, and the collector layer is The forbidden band width of the semiconductor constituting the base layer is made wider than the forbidden band width of the semiconductor constituting the base layer,
A light absorption layer made of a semiconductor having a forbidden band width that is equal to or narrower than the forbidden band width of the semiconductor constituting the collector layer and wider than the forbidden band width of the semiconductor constituting the base layer is used as the collector layer of the heterojunction transistor. A photodetecting semiconductor element characterized in that it is provided on a layer side. 7. A patent claim characterized in that the difference in forbidden band width between the semiconductor constituting the base layer and the semiconductor constituting the light absorption layer is 1% to 3% of the forbidden band width of the semiconductor constituting the base layer. The photodetecting semiconductor device according to scope 6. 8. Claim 6, wherein each of the collector layer, base layer, emitter layer, and light absorption layer is made of a semiconductor whose main component is gallium arsenide, aluminum arsenide, or gallium/aluminum arsenide. Or the photodetecting semiconductor device according to item 7. 9. Each of the collector layer, base layer, emitter layer, and light absorption layer is made of a semiconductor whose main composition is indium phosphide, indium arsenide, gallium phosphide, gallium arsenide, or a mixture of at least two of these. A photodetecting semiconductor device according to claim 6 or 7.