JPH077372A - Surface acoustic wave convolver - Google Patents

Abstract

PURPOSE:To improve a temperature characteristic of the surface acoustic wave convolver having a laminated structure of a piezoelectric body layer/an insulator layer/a semiconductor layer. CONSTITUTION:The surface acoustic wave convolver of a laminated structure of a piezoelectric body layer 1/an insulator layer 2/a semiconductor layer 3 is obtained. Impurity density of the semiconductor layer 3 is formed so that the impurity density becomes small in the part on the surface of the semiconductor layer for coming into contact with the insulator layer 2, and it becomes such an impurity density distribution as the impurity density increases gradually, as for the direction extending from the surface of the semiconductor layer to the inside of the semiconductor layer (the direction extending from the part for coming into contact with the insulator layer 2 to the inside of the semiconductor layer 3, that is, the depth direction of the semiconductor layer).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to surface acoustic waves (hereinafter referred to as S
The present invention relates to a convolver (abbreviated as AW), and more particularly, to an improvement of a SAW convolver for suppressing temperature characteristics and suppressing self-convolution.

[0002]

2. Description of the Related Art FIGS. 4A and 5A show a typical structure of a conventional monolithic SAW convolver having a laminated structure of a piezoelectric layer / insulator layer / semiconductor layer.

In the figure, 1 is a piezoelectric layer, 2 is an insulator layer, 3 is a semiconductor layer, 4 is a first conductivity type semiconductor layer, 5 is a second conductivity type semiconductor layer, 6 is a semiconductor epitaxial layer, and 7 is a semiconductor epitaxial layer. Gate electrode, 8 is a back surface electrode, 9 is a comb-shaped electrode (input electrode), 10 is a high-concentration semiconductor substrate, 11 is an input terminal, 12
Is an output terminal.

The SAW convolver having the structure shown in FIG.
The semiconductor layer 3 is characterized by using a laminated structure of semiconductor epitaxial layer / high-concentration semiconductor substrate. In this case, the impurity density distribution of the semiconductor layer 3 is as shown in FIG.
It becomes like (b). The impurity density is the same except for the vicinity of the interface between the semiconductor epitaxial layer 6 and the high-concentration semiconductor substrate 10.
The distribution is uniform in the semiconductor epitaxial layer 6 and the high-concentration semiconductor substrate 10, respectively. In the vicinity of the interface between the semiconductor epitaxial layer 6 and the high-concentration semiconductor substrate 10, the impurity density gradually increases from the semiconductor epitaxial layer 6 toward the high-concentration semiconductor substrate 10. The impurity density gradually changes in the vicinity of the interface between the semiconductor epitaxial layer 6 and the high-concentration semiconductor substrate 10 because the high-concentration semiconductor is deposited in the manufacturing process of depositing the semiconductor epitaxial layer 6 on the high-concentration semiconductor substrate 10. This is because impurities are thermally diffused from the substrate 10 to the semiconductor epitaxial layer 6 side. Further, when the insulator layer 2 is formed by thermal oxidation of the semiconductor layer 3 or when there is a step of heat-treating the semiconductor layer 3 at a high temperature, the impurities from the high-concentration semiconductor substrate 10 are also heated to the semiconductor epitaxial layer 6 side. The cause is the diffusion. However, in the conventional structure shown in FIG. 4A, as described above, the impurity density distribution in the semiconductor epitaxial layer 6 is uniform except in the vicinity of the interface between the semiconductor epitaxial layer 6 and the high-concentration semiconductor substrate 10. It is a big feature that it is like this. The SAW element having the conventional structure shown in FIG. 4A is characterized by high convolution efficiency as a convolver. For a more detailed description of the SAW element having the conventional structure shown in FIG. 1, see, for example, JP-A-63-62281.

In the SAW element having the conventional structure shown in FIG. 5A, the semiconductor layer 3 has a laminated structure of a first conductivity type semiconductor layer / a second conductivity type semiconductor layer / a high-concentration semiconductor substrate. Is a feature. Usually, the second-conductivity-type semiconductor layer 5 is a semiconductor epitaxial layer, and the first-conductivity-type semiconductor layer 4 is subjected to ion implantation or thermal diffusion of an impurity of a type different from that of the second-conductivity-type semiconductor layer 5 to a second type. It is often formed by implanting or diffusing on the surface of the conductive type semiconductor layer 5. The impurity density distribution of the SAW element having such a conventional structure is as shown in FIG. The impurity density is measured in the first conductivity type semiconductor layer 4, and in the vicinity of the interface between the first conductivity type semiconductor layer 4 / the second conductivity type semiconductor layer 5 and the interface between the second conductivity type semiconductor layer 5 / high-concentration semiconductor substrate 10. Except for this, the distribution is uniform in the second conductivity type semiconductor layer 5 and the high-concentration semiconductor substrate 10, respectively.

The impurity density distribution is not uniform in the first conductivity type semiconductor layer 4 and in the vicinity of the interface between the first conductivity type semiconductor layer 4 and the second conductivity type semiconductor layer 5, as described above. This is because the semiconductor layer 4 is formed by ion implantation of impurities or thermal diffusion. Further, the impurity density distribution in the vicinity of the interface of the second conductivity type semiconductor layer 5 / high-concentration semiconductor substrate 10 is not uniform and gradually changes, as in the case of the conventional structure of FIG. In the process of forming the epitaxial layer 6 and the process of thermal oxidation and high temperature heat treatment of the semiconductor layer 3,
This is because the impurities are thermally diffused from the high-concentration semiconductor substrate 10 to the second conductivity type semiconductor layer 5 side. However, in the SAW element having the conventional structure shown in FIG. 5A, the impurity density distribution in the second conductivity type semiconductor layer 5 is high in the vicinity of the interface with the first conductivity type semiconductor layer 4 and in the high concentration as described above. The major feature is that it is uniform except near the interface with the semiconductor substrate. In the SAW element having such a conventional structure, not only the convolution efficiency as a convolver is large, but also the large convolution efficiency can be obtained even when the DC bias voltage applied to the gate electrode 7 is 0V. The feature is that the convolver can operate without the need for an external DC power supply. A more detailed description of the conventional structure of the SAW device of FIG. 5 (a), for example, JP 62-64113 Patent or "zero-bias operation type prototype monolithic ZnO / SiO ₂ / Si convolver" 1986 Autumn See Proceedings of the Japan Society of Applied Physics, page 905.

[0007]

The SAW device having the conventional structure as described above has an advantage of high convolution efficiency, which is extremely advantageous in practical use.
There is a problem that the convolution efficiency changes with temperature. FIG. 6 shows an example of the temperature characteristic of the convolution efficiency (hereinafter, represented by the symbol F _T ) in the SAW convolver having the conventional structure described above.

FIG. 6 shows ZnO / SiO ₂ / n-Si / n ^+.
This is an example of the case where the SAW mode that propagates is a Sezawa wave and the gate length is 40 mm in a -Si laminated structure element. For both the conventional SAW convolver of FIG. 4A and the conventional SAW convolver of FIG. 5A, the temperature characteristic of the convolution efficiency F _T is qualitatively as shown in FIG. As can be seen in FIG. 6, the convolution efficiency F _T usually decreases with increasing temperature. Therefore, when the SAW convolver having the conventional structure is applied to a predetermined application, an AGC circuit (automatic gain adjustment circuit) is required as a peripheral circuit of the SAW convolver, or the operating temperature range is limited to a narrow range. There is. That is, the SAW convolver having the conventional structure has a problem that the operating temperature range is narrow, or an AGC circuit is required as a peripheral circuit, which increases cost and makes it difficult to downsize the peripheral circuit. It was

An object of the present invention is to provide a SAW convolver having a laminated structure of piezoelectric layer / insulator layer / semiconductor layer,
It is to propose a SAW convolver having a new structure that improves the temperature characteristics of convolution efficiency. Another object of the present invention is to enable suppression of a self-convolution signal only by adding a simple means in the SAW convolver.

[0010]

In order to achieve the above-mentioned object, a first aspect of the present invention provides a surface acoustic wave convolver, comprising:
The impurity density of the semiconductor layer is formed so that the impurity density is small on the surface of the semiconductor layer in contact with the insulator layer and gradually increases from the surface of the semiconductor layer toward the inside of the semiconductor layer. What is done is summarized.

A second invention is a surface acoustic wave convolver having a laminated structure of a piezoelectric layer / insulator layer / first conductivity type semiconductor layer / second conductivity type semiconductor layer, wherein the impurity density of the second conductivity type semiconductor layer is The impurity density distribution is such that the impurity density is small in the portion in contact with the first conductivity type semiconductor layer and gradually increases from the portion toward the inside of the second conductivity type semiconductor layer. The gist is to form it. Furthermore, the third invention is characterized in that a sound absorbing material for attenuating the strength of surface acoustic waves is provided on the piezoelectric layer.

[0012]

By forming the impurity density distribution of the semiconductor layer as described above, the nonlinear constant on the surface of the semiconductor layer can be increased with increasing temperature, and as a result, the convolution efficiency of the convolver can be improved. The temperature characteristic of the convolver can be improved by preventing the temperature characteristic from decreasing as the temperature rises. Further, the sound absorbing material absorbs the reflection of the surface acoustic wave and suppresses the self-convolution signal.

[0013]

Embodiments of the present invention shown in the drawings will be described below.
1 and 2 show an embodiment of the present invention. FIG. 1 (a) shows a SAW convolver having a structure improved from the conventional structure of FIG. 4 (a), and FIG. 2 (a) shows a SAW convolver having a structure improved from the conventional structure of FIG. 5 (a). It is a thing.

In the figure, the same reference numerals as those in FIGS. 4A and 5A represent the same or similar members, and the difference from the conventional structure is the impurity density distribution in the semiconductor layer 3. Figure 1
In the SAW convolver having the laminated structure of the piezoelectric layer / insulator layer / semiconductor layer shown in (a), the impurity density of the semiconductor layer 3 is changed to the semiconductor layer surface in contact with the insulator layer 2 as shown in FIG. It is characterized in that the impurity density is small in the portion, and the impurity density distribution is such that the impurity density gradually increases from the surface of the semiconductor layer toward the inside of the semiconductor layer. On the other hand, in the conventional structure shown in FIG. 4A, the impurity density of the semiconductor layer 3 varies from the semiconductor layer surface portion in contact with the insulator layer 2 to the semiconductor epitaxial layer 6 / high-concentration semiconductor substrate 10 and near the interface. Up to the above, the impurity density is uniform, which is different from the impurity density distribution of the semiconductor of the embodiment of FIG.

On the other hand, in the embodiment shown in FIG. 2A, the piezoelectric layer /
FIG. 2B shows a SAW convolver having a laminated structure of an insulator layer / a first conductivity type semiconductor layer / a second conductivity type semiconductor.
As shown in, the impurity density of the second conductivity type semiconductor layer 5 is gradually reduced in the direction in which the impurity density is small in the portion in contact with the first conductivity type semiconductor layer 4 and goes from the portion to the inside of the second conductivity type semiconductor layer. In addition, the second conductivity type semiconductor layer is formed so as to have an impurity density distribution that increases the impurity density. On the other hand, in the conventional structure shown in FIG. 5A, the impurity density of the second conductivity type semiconductor layer 5 is
The impurity density is uniform from the vicinity of the portion in contact with the first conductivity type semiconductor layer 4 to the vicinity of the interface of the second conductivity type semiconductor layer 4 / high-concentration semiconductor layer substrate 10. In that respect, FIG.
This is different from the impurity density of the semiconductor of the example.

In the embodiment of FIGS. 1 and 3, the impurity density distribution of the semiconductor layer 3 is formed to have a distribution different from that of the conventional structure of FIGS. 4 and 5 as described above. The constant is made to increase with increasing temperature, resulting in the convolution efficiency F _T of the convolver.
There prevents decreases with increasing temperature, to reduce the temperature variation of the F _T, in order to improve the temperature characteristics of the F _T. Hereinafter, that point will be described in order.

Convolution efficiency F _{T of the} convolver
Is generally expressed by the following equation.

F _T = 20logγ ₂ −αLg−30 + Le (1)

Here, γ ₂ is a nonlinear constant on the surface of the semiconductor layer, α is a propagation loss of surface wave, and Lg is a gate length. L
Since e is a term that is determined irrespective of temperature, and since the emphasis here is on explaining the temperature change of F _T , the expression of Le is omitted.

From the equation (1), the convolution efficiency F
_It can be seen that the temperature change of T is affected by the temperature change of the nonlinear constant γ _{2 and} the temperature change of the propagation loss α of the surface wave (the gate length Lg is constant). Of these, SAW propagation loss α
Is generally known to increase with increasing temperature in the case of the structure of piezoelectric layer / insulator layer / semiconductor layer.
This is because the loss of the piezoelectric body itself increases as the temperature rises, and the loss in the semiconductor layer due to the electroacoustic effect also increases as the temperature rises. A qualitative temperature change of the propagation loss α is shown in FIG. The temperature change of the propagation loss α is qualitatively the same in both the conventional structure and the present invention. For details on the temperature change of the propagation loss α,
For example, see the following references:

S. Mitsutsuka, et. al “Propagation lo
ss of surface acoustic waves on a monolithic meta
l-insulator semiconductor structure "Journal of A
pplied Physics, vol.65, No.2, January 1989, pp651-66
1

The fact that the propagation loss α increases as the temperature rises as shown in FIG. 3 (a) means that the convolution efficiency F _T tends to decrease with the temperature rise from the equation (1). Indicates. The actual temperature change of F _T depends on what the other parameter, the non-linear constant γ ₂ , changes with temperature.

Next, regarding the temperature change of the nonlinear constant γ ₂ ,
The case of the conventional structure and the case of the present invention will be compared. The nonlinear constant γ ₂ generally has the following relationship with the impurity density N of the semiconductor at the position of the depletion edge when the surface of the semiconductor layer of the convolver is in the depleted state or the inverted state.

[0023]

[Equation 2] γ ₂ ∝ 1 / N (2)

Since F _T has a large value when the semiconductor layer surface is in a depleted or weakly inverted state, it may be considered that the equation (2) holds in the operating state of the convolver. That is, in the operating state of the convolver, it can be considered that the nonlinear constant γ ₂ is inversely proportional to the impurity density N at the depletion edge. Therefore,
The temperature change of γ ₂ depends on the temperature change of the impurity density N at the depletion edge. By the way, the depletion width of the semiconductor layer changes when the temperature changes. The depletion width generally decreases with increasing temperature. Therefore, the position of the depletion edge in the semiconductor layer approaches the surface of the semiconductor layer (the interface between the insulator layer 2 and the semiconductor layer 3) from the inside of the semiconductor as the temperature rises. On the other hand, the position of the depletion edge is in the region where the impurity density N is uniform in the semiconductor epitaxial layer 6 in the conventional structure of FIG. 4A, and in the conventional structure of FIG. Within the two-conductivity-type semiconductor layer 5, the impurity density N is also in a uniform region. Therefore, in the conventional structure, even if the temperature rises and the position of the depletion end approaches the semiconductor layer surface side, the impurity density N at the position of the depletion end is constant. Therefore, from the relationship of the equation (2), the nonlinear constant γ ₂ of the conventional structure is almost constant even if the temperature changes. The qualitative temperature characteristic of γ ₂ of the conventional structure is shown by the dotted line in FIG. 3 (b). On the other hand, in the case of FIG. 1 of the present invention, the position of the depletion edge is in the region in the semiconductor layer 3 where the impurity density N gradually increases in the depth direction, and the position of the depletion end of FIG. Then, again, the position of the depletion edge is in the region in the second conductivity type semiconductor layer 5 where the impurity density N gradually increases in the depth direction. Therefore, in the present invention, when the temperature rises and the position of the depletion end approaches the semiconductor surface side, the impurity density N at the position of the depletion end gradually decreases. Therefore, from the relation of the equation (2), the nonlinear constant γ ₂ of the present invention increases as the temperature rises. The qualitative temperature characteristic of γ ₂ in the structure of the present invention is shown by the solid line in FIG.

The above temperature change of the propagation loss α (FIG. 3)
When (a)) and the temperature change of the non-linear constant γ ₂ (FIG. 3B) are combined, the temperature change of the convolution efficiency F _T is qualitatively shown in FIG. become that way. That is, in the present invention, the temperature change of the convolution efficiency F _T can be improved (the decrease due to the temperature rise of F _T can be prevented) as compared with the case of the conventional structure. This is because, in the present invention, the nonlinear constant γ ₂ can be increased as the temperature rises, so that the effect due to the temperature change of the propagation loss α can be canceled. The above is the reason why the temperature characteristic of the convolution efficiency can be improved in the present invention as compared with the conventional structure.

In forming the SAW convolver having the structure of the present invention, as a method of gradually changing the impurity density in the semiconductor layer 3 of FIG. 1 and the second conductivity type semiconductor layer 5 of FIG. 2 in the depth direction. Can be formed by gradually changing the amount of impurities in the process of epitaxially growing the semiconductor layer on the high-concentration semiconductor substrate, or by epitaxially growing the low-concentration semiconductor layer on the high-concentration semiconductor substrate,
It can also be formed by a method of thermally diffusing impurities by performing high temperature heat treatment. However, in the present invention,
It is important that the impurity density distribution is a desired distribution, and the method of forming such a distribution is not particularly limited.

By the way, the material of each component of the present invention is
The same structure as the conventional structure can be used. That is, as the piezoelectric layer layer 1, ZnO or AlN, the insulating layer 2
As the material, SiO ₂ or SiNx can be used, and as the semiconductor layer 3, Si, GaAs, or the like can be used. Also, each electrode has A
1 or Au can be used.

In addition, in the SAW convolver having the structure as shown in FIGS. 1 and 2, in addition to the convolution signal between the signals input to the two comb electrodes 9 (hereinafter referred to as IDT), the self There is an unwanted signal called the convolution signal. What is a self-convolution signal?
One I.D. D. The surface wave generated by T. D. This signal is generated by being reflected by T, and corresponds to the convolution signal of its own. This is shown in FIG. In FIG. 7, the input signal P
_The desired convolution signal P _out is generated from the surface acoustic waves S ₁ and S ₂ generated by ₁ and P ₂ , but in addition, between S ₁ and its reflected wave S _1r , and between S ₂ and its reflected wave S _2r. It will be clear that the convolution signal also occurs during the period. The latter two signals are self-convolution signals.

By the way, such a self-convolution signal appears as spurious noise on the output of the convolver and has an unfavorable effect of reducing the dynamic range of the convolver. In particular,
In the SAW convolver having the structure as shown in FIGS. 1 and 2, when the length of the gate electrode 7 becomes shorter, the opposing I.D. D. The degree of attenuation of the surface wave transmitted between the T and T becomes small, and the opposing I.D. D. Since the intensity of the surface wave reflected from T becomes large, the magnitude of the self-convolution signal becomes large, and the spurious noise due to the self-convolution signal becomes remarkable.

FIG. 8 shows an embodiment of the present invention for suppressing the above-mentioned self-convolution signal. In addition to the sound absorbing material 13 installed at the end of the element, a piezoelectric film 1 / insulator 2 / semiconductor 3 laminated substrate is provided. The sound absorbing material 14 is characterized in that it is provided on the piezoelectric film 1 in a region between each of the comb-shaped electrodes (input electrodes) 9 and the gate electrode 7 arranged on the piezoelectric film 1.

As described above, the feature of the embodiment shown in FIG. 8 is that, in addition to the sound absorbing material 13 installed at the end of the device, the region between the comb-shaped electrode (input electrode) 9 and the gate electrode 7 on the substrate is provided. This is to newly install the sound absorbing material 14. The role of the sound absorbing material 13 installed at the end of the SAW element is to prevent the surface wave from being reflected from the end surface of the element. Therefore, at the part of the sound absorbing material 13 installed at the end of the element, the surface wave is almost completely removed. Attenuated. However, in the portion of the sound absorbing material 14 newly installed in the present invention, the surface wave is not completely attenuated but is propagated after being attenuated to some extent. It is possible by adjusting the coating width and the coating amount of the sound absorbing material 14. For example, if the coating width of the sound absorbing material 14 is narrowed or the coating amount is reduced, the degree of attenuation of the surface wave in the sound absorbing material 14 can be reduced.

Next, in FIG. 8, the reason why the self-convolution signal can be suppressed by newly installing the sound absorbing material 14 will be described. The reason is that by installing the sound absorbing material 14 between the input electrode 9 and the gate electrode 7, it is possible to increase the propagation loss of the surface wave that propagates between the input electrode 9 and the gate electrode 7. Reflected wave S
_{This is because 1r} and S _2r can be reduced. In FIG. 7, the propagation loss of the surface wave propagating between the input electrode (IDT) 9 and the gate electrode 7 is L (dB). Figure 8
In the case of, the presence of the sound absorbing material 14 causes attenuation of L (dB) (propagation loss). For simplicity, it is assumed that the input 1 side and the input 2 side both receive the same propagation loss.

If there is no sound absorbing material 14 as shown in FIGS. 1 and 2, the convolution output P _out (dBm) is obtained.
In FIG.

## _EQU00003 ## P _out = F _To + P ₁ + P ₂ (3) Here, F _To is the convolution efficiency (dBm), and P ₁ and P ₂ are I.V. D. It is an input electrode (dBm) to T. At this time, the self-convolution output P _s1 (dBm) due to the influence of input 1 is

## _EQU00004 ## P _s1 = F _To + 2P ₁ + R−L ₀ (4) Here, R is the I.V. D. Surface wave reflectance at T (dB)
Is. Further, L ₀ is a propagation loss (dB) of the surface wave while being transmitted through the gate. The self-convolution with input 2 is (4), and the subscript may be changed from 1 to 2. For the sake of simplicity, consider only the effect of input 1 on self-convolution.

From (3) and (4), the suppression ratio C _o ≡log (P _s1 / P _out ) (dB) of the self-convolution signal.
Is

[Number 5] _{C o = log (P s1 /} P out) = P 1 -P 2 + R-L 0 (5)

Next, when the sound absorbing material 14 is present as shown in FIG. D. Since the attenuation of L (dB) occurs between T and the gate electrode, the convolution output P _out (dBm)
Is

[Equation 6] P _out = F _To + P ₁ + P ₂ -2L (6) Further, the self-convolution output P _s1 by the input 1
(DBm) is

P _s1 = F _To + 2P ₁ + R−L ₀ −3L (7) Therefore, the suppression ratio C≡log (P _s1 / P _out ) (dB) of the self-convolution signal in the case of the structure of FIG. ,
From (6) and (7),

Equation 8] _{C = log (P s1 / P} out) = P 1 -P 2 + R-L 0 -L (8)

Comparing (5) and (8), in the case of FIG. 8, the self-convolution suppression ratio is C- _Co times higher than that of the structures of FIGS. 1 and 2, that is, (5), (8) )Than,

[Equation 9] C−C _o = −L (dB) (9) The size can be increased. From the equation (7), it can be seen that the larger L is, the smaller the self-convolution signal in FIG. 8 can be made than the structures in FIGS. That is, in FIG.
It can be understood that the self-convolution signal can be made smaller and suppressed by providing a propagation loss to the surface wave thereat, by making it smaller than that of the structures shown in FIGS.

The sound absorbing material 14 may be made of the same material as the sound absorbing material 13 installed at the end of the element, or may be made of a different material. As the material of the sound absorbing material 14 and the sound absorbing material 13, the material generally used in the surface acoustic wave element can be used, and for example, silicone rubber can be used. However, the material of the sound absorbing material 14 and the sound absorbing material 13 is not particularly limited. The SAW convolver to which the sound absorbing material 14 can be applied may be a general piezoelectric / insulator / semiconductor laminated structure convolver, or an elastic convolver as shown in FIG. 9, in which case the piezoelectric substrate 15 is The same material as that of the conventional structure can be used, and for example, LiNbO ₃ can be used. 9, the same symbols as those in FIG. 8 represent the same or similar members.

[0038]

As described above, according to the present invention,
SA having a laminated structure of piezoelectric layer / insulator layer / semiconductor layer
In the W convolver, the temperature characteristic of convolution efficiency can be improved. Therefore, the operating temperature range of the SAW convolver can be expanded more than ever before. Moreover, as in the case of application of the conventional structure, the AG
Since it is not necessary to provide the C circuit, it is possible to reduce the size and cost of the peripheral circuit more than ever before. Furthermore, the self-convolution signal can be suppressed simply by adding a simple means such as a sound absorbing material. Note that as a specific application example of the SAW convolver according to the present invention, a spread spectrum communication device, a correlator, a radar, an image processing device, a Fourier transformer, etc. can be used.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention and a diagram showing an impurity density distribution of a semiconductor.

FIG. 2 is a sectional view showing another embodiment of the present invention and a diagram showing an impurity density distribution of a semiconductor.

FIG. 3 is a characteristic diagram showing a comparison of temperature characteristics between an element having a conventional structure and an element of the present invention.

FIG. 4 is a cross-sectional view showing a structure of a conventional SAW convolver and a diagram showing an impurity density distribution.

FIG. 5 is a diagram showing a structure and an impurity density distribution of another conventional SAW convolver.

FIG. 6 is a diagram showing temperature characteristics of convolution efficiency of a conventional SAW convolver.

FIG. 7 is an explanatory diagram of self-convolution signal generation.

FIG. 8 is a top view and a cross-sectional view of still another embodiment of the present invention.

9 (a) and 9 (b) are a top view and a cross-sectional view showing a modified example of the embodiment of FIG.

[Explanation of symbols]

 1 Piezoelectric layer 2 Insulator layer 3 Semiconductor layer 4 First conductivity type semiconductor layer 5 Second conductivity type semiconductor layer 13 Sound absorbing material 14 Sound absorbing material

Claims (10)

[Claims] 1. A surface acoustic wave convolver having a laminated structure of a piezoelectric layer / insulator layer / semiconductor layer, wherein the impurity density of the semiconductor layer is small at a portion of the semiconductor layer surface in contact with the insulator layer, A surface acoustic wave convolver having an impurity density distribution in which the impurity density gradually increases from the surface of the semiconductor layer toward the inside of the semiconductor layer. 2. A surface acoustic wave convolver having a laminated structure of a piezoelectric layer / insulator layer / first conductivity type semiconductor layer / second conductivity type semiconductor layer, wherein the impurity density of the second conductivity type semiconductor layer is The impurity density is small in the portion in contact with the conductivity type semiconductor layer, and the impurity density distribution is such that the impurity density in the second conductivity type semiconductor layer gradually increases from that portion toward the inside of the second conductivity type semiconductor layer. A surface acoustic wave convolver, characterized in that 3. The surface acoustic wave convolver according to claim 1, wherein a sound absorbing material for reducing the intensity of the surface acoustic wave is provided on the piezoelectric layer of the surface acoustic wave convolver. 4. The surface acoustic wave convolver according to claim 2, wherein a sound absorbing material for reducing the intensity of the surface acoustic wave is provided on the piezoelectric layer of the surface acoustic wave convolver. 5. The surface acoustic wave convolver according to claim 3, wherein the sound absorbing material is arranged in a region on the piezoelectric layer between the input electrode and the output electrode. 6. The surface acoustic wave convolver according to claim 4, wherein the sound absorbing material is arranged in a region on the piezoelectric layer between the input electrode and the output electrode. 7. The surface acoustic wave convolver according to claim 5, wherein a sound absorbing material is arranged on an edge of the piezoelectric layer. 8. The surface acoustic wave convolver according to claim 6, wherein the sound absorbing material is arranged on an edge of the piezoelectric layer. 9. A surface acoustic wave convolver having at least a piezoelectric substrate, wherein a sound absorbing material for reducing the intensity of surface acoustic waves is provided in a region on the piezoelectric substrate between an input electrode and an output electrode. Surface acoustic wave convolver. 10. The surface acoustic wave convolver according to claim 9, wherein a sound absorbing material is arranged on an edge of the piezoelectric substrate.