JPS62195910A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To improve the reliability and efficiency and to keep the advantage of the monolithic structure as it is by forming an impurity concentration distribution increased toward an insulation layer near the insulation layer of the semiconductor layer in the monolithic structure including the lamination layer structure comprising an n(p) channel semiconductor layer, the insulation layer and the piezoelectric layer. CONSTITUTION:The monolithic structures is provided to an element 10, where the n(p) channel semiconductor layer 12, the insulation layer 14 and the piezoelec tric layer 16 and laminated in the order. The gradient impurity concentration distribution region 18 having an impurity concentration distribution increased toward the insulation layer 14, as shown in figure is formed between the semicon ductor layer 12 and the insulator layer 14. Thus, a depletion layer capacitance (C)-applied voltage (V) characteristic curve in response to the distribution curve is obtained from the impurity concentration distribution region 18. Thus, even when the applied electric field is changed slightly, since the depletion layer capacitance is changed remarkably, the efficiency and sensitive are improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a surface acoustic wave device, and more particularly to a surface acoustic wave device that can be used, for example, in an acoustoelectric convolver, a parametric oscillator, and the like.

(Prior Art) Surface acoustic wave elements are used in a variety of ways, such as filters and oscillators, and one of their fields of use is, for example, as a spread spectrum demodulator in a reception system for spread spectrum (SS) communications.
There is an f1 elastic wave (SAW) convolver. In a SAW convolver, surface acoustic waves generated by interdigital transducers (IDTs) at both ends of the substrate are propagated toward the center of the substrate and interact between the two transducers. It produces a weak electric field due to nonlinear action.

FIG. 7 is an illustrative view showing an example of a conventional elastic convolver which is the background of the present invention. The elastane ink convolver includes a signal input electrode 2 and a reference signal input electrode 3 arranged on a piezoelectric substrate 1, and the surface acoustic waves generated by these two input electrodes 2 and 3 are compressed by a beam width compressor 4 and output. By utilizing the nonlinear strain of the piezoelectric substrate 1 directly under the waveguide serving as the electrode 5, a convolution signal is extracted from the output electrode 5.

FIG. 8 is an illustrative view showing an example of a conventional separation medium type convolver, which is the background of the present invention. In the separation medium type convolver, a semiconductor 6 is placed above a piezoelectric substrate 1, which is a propagation medium for surface waves, with a slight gap. An alternating current electric field leaks from the surface of the piezoelectric substrate 1 as the surface waves propagate from the two human-powered electrodes 2 and 3, and the leaked alternating current electric field and the semiconductor 6
A signal is extracted from the output electrode 5 by interaction with carriers therein.

FIG. 9 is an illustrative diagram showing an example of a monolithic convolver which is the background of the present invention. In this monolithic convolver, a semiconductor layer 6', an insulator layer 7, and a piezoelectric material N1' are laminated in this order. A signal input electrode 2 and a reference signal input electrode 3 are formed on the piezoelectric layer 1', and a gate electrode 8 is formed between them. That is, the monolithic convolver has electrode 8/piezoelectric layer 1'/
MIS (Metal) of insulator layer 7/semiconductor layer 6'
In5ulator Sem1 conductor)
Has a structure. Then, by utilizing the nonlinear interaction between the piezoelectric layer 1' and the semiconductor layer 6', the depletion layer capacitance between the insulator layer 7 and the semiconductor layer 6' is generated by the surface acoustic waves transmitted to the piezoelectric layer 1'. An output signal is extracted from the gate electrode 8 by changing the .

(Problems to be Solved by the Invention) In the elastic convolver shown in FIG. 7, the electric field generated by the weak nonlinearity of the piezoelectric substrate 1 is directly extracted by the metal electrodes 2 and 3 on the surface of the substrate, so the efficiency is very high. bad.

The separation medium type convolver shown in FIG. 8 utilizes nonlinear interaction between the piezoelectric body and the semiconductor to take out the output electrode from the electrode on the semiconductor, so it is efficient. However, a minute air gear (for example, 1
000 people), it is not only difficult and expensive to manufacture, but also susceptible to vibration and lacking in reliability.

The monolithic convolver shown in FIG. 9 has a monolithic structure, so it has high reliability and is easy to manufacture. Furthermore, since the depletion layer capacitance of the semiconductor is used to obtain output through its nonlinear distortion, it is more efficient than the elastane convolver.

However, since the impurity concentration of the semiconductor layer 6' is constant or uniform as shown in FIG. 10, a large change in depletion layer capacitance cannot be expected, so the efficiency is not very good.

Therefore, the main object of the present invention is to provide a surface acoustic wave element having a monolithic structure and high efficiency.

(Means for Solving the Problems) Simply put, the present invention provides a monolithic structure including a laminated structure of an n (p) type semiconductor layer, an insulator layer, and a piezoelectric layer. This is a surface acoustic wave device that has an impurity concentration distribution that increases as it gets closer to the insulator layer.

(Function) Due to the impurity concentration distribution that increases as it approaches the insulator layer in the semiconductor layer, the slope of the depletion layer capacitance-voltage characteristic curve becomes very steep, and even when a weak alternating current electric field is applied, the depletion layer capacitance changes. is big.

(Effects of the Invention) According to the present invention, the advantages of a monolithic structure such as high reliability and easy manufacturing are maintained, and the efficiency is significantly improved compared to the conventional seven-nolithic convolver. A surface elastic device can be obtained.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

(Example) FIG. 1 is an illustrative cross-sectional view showing one embodiment of the present invention.

The surface acoustic wave device 10 has n (p) type semiconductor layers 12 . It has a monolithic structure in which the insulator layer 14 and the piezoelectric layer 16 are stacked in this order. The n (p) type semiconductor layer 12 is made of a semiconductor material such as Si, and the insulator layer 14 is made of an oxide of the semiconductor material, such as Sin.
The piezoelectric layer 16 is made of Zn◯, for example.

When an oxide of the material of the semiconductor layer 12 is used as the insulator layer 14, the insulator layer can be obtained by simply oxidizing the semiconductor layer, which not only simplifies the process, but also reduces the gap between the two layers 12 and 14. A good quality interface, that is, a good depletion layer can be obtained. However, the insulator layer 14 may be formed of other materials, or may be formed of the same material as the piezoelectric layer 16. In this case, there is no particular need to form the insulator layer 14, and the piezoelectric layer 16 can be used instead or in combination. Therefore, the effort of forming a separate insulator layer 14 can be saved, leading to further cost reduction.

Characteristically, the insulator layer 1 of the n (p) type semiconductor layer 12
A gradient impurity concentration distribution region 18 having an impurity concentration distribution such as, for example, a super-step junction, which increases as it approaches the insulator layer 14, is formed near the insulating layer 14. In this impurity concentration distribution region 18, for example, as shown in FIG. 2, the impurity concentration increases exponentially as it approaches the insulator layer 14 from the n (p) type semiconductor layer 12. Therefore, in this impurity concentration distribution region 18, a depletion layer capacitance (C)-applied voltage (■) characteristic curve corresponding to this distribution curve is obtained, and therefore, when the applied electric field changes slightly, the depletion layer capacitance changes suddenly or significantly. Therefore, a surface acoustic wave element with higher efficiency or sensitivity than that of a conventional monolithic structure can be obtained.

FIG. 3 is an illustrative cross-sectional view showing another embodiment of the present invention. In this embodiment, the n (p) type semiconductor layer 12 is n”
, (p'') type semiconductor [12a and n layered thereon
(p) type semiconductor layer 12b.

Then, as shown in FIG. 4, a gradient impurity concentration distribution is formed in the n (p) type semiconductor layer L2b, which becomes thicker as it approaches the semiconductor layer 14.

Regarding the semiconductor region other than the range that contributes to the formation of a depletion layer, if the resistivity is large, the loss will be large, so in this example, in order to increase the conductivity, n (
p) type semiconductor layer 12, n゛(p゛) type semiconductor Jti1
2a is provided. Further, if it is necessary to attach a ground electrode, an n''Cpl type semi-shaped conductor layer 12a is effective in order to make an ohmic contact, since an n (p) type semiconductor causes a Schottky contact, which is undesirable.

Furthermore, in this embodiment, the semiconductor substrate is n'' (p
If a ) type semiconductor is used, the impurity concentration near the insulator layer 14 cannot be controlled, so an epitaxial layer is grown as an n (p) type semiconductor layer 12b on the n''(p'') type semiconductor layer 12a. Then, impurities are implanted or diffused there.

In the embodiment shown in FIG. 3 as well, due to the steep impurity concentration gradient, a steep C-V
characteristics, and therefore the degree of variation of the depletion layer capacitance with respect to the applied voltage is very large.

FIG. 5 is an illustrative view showing an example of an acousto-electric convolver as a preferred embodiment of the present invention. This convolver 1
00 has the same monolithic structure as the surface acoustic wave device 10 shown in FIG.
, n (1)) type semiconductor layer 112b, insulator layer 114, and piezoelectric layer 116 are stacked in this order. Piezoelectric layer 1
On top of 16 is an ink disk electrode 120 for signal human power.
and reference signal manual ink digital electrode 122,
They are arranged at both ends of the piezoelectric layer 116 in the length direction (SAW propagation direction) at intervals. A gate electrode 124 is formed between these two ink digital electrodes 120 and 122, and an n''(p'')
A ground electrode 126 is formed on the lower surface of the shaped semiconductor layer 112a.

In the n (p) type semiconductor layer 112b, a gradient impurity concentration distribution as shown in FIG. 4 is formed as in the previous embodiment of FIG.

A bias voltage is applied to the gate electrode 124, and in that state, the two input ink digital electrodes 120 and 1
When a signal is manually inputted from each of the piezoelectric layers 116 and 22, the piezoelectric layer 116
Due to the surface acoustic waves transmitted to ZnO as a
The depletion layer capacitance of the (p) type semiconductor layer 112b changes.

A voltage corresponding to this change in depletion layer capacitance is applied to the gate electrode 12.
2 through a coupling capacitor.

Note that, as in this embodiment, Zn is used as the piezoelectric layer 116.
When O is used, it is possible to obtain a large electromechanical coupling coefficient of 2 in Sezawa waves or Rayleigh waves, and also to realize a piezoelectric layer with a large nonlinear coupling coefficient, and therefore a wide bandwidth and a large T product can be obtained.

Note that the ink digital electrodes 20 and 22 are connected to the sixth A
It may be modified as shown in FIGS. 6D to 6D.

In the example of FIG. 6A, an ink disk electrode 20 (22) is formed on the top surface of the piezoelectric layer 16, and a counter electrode 28 is formed on the top surface of the insulator layer 14, facing this interdigital electrode.
is formed.

In the example shown in FIG. 6B, an ink digital electrode 20 (22) is formed on the upper surface of the insulating layer 14, and a counter electrode 28 is formed on the upper surface of the piezoelectric layer 16.

In the example shown in FIG. 6C, the ink digital electrode 20 (2
2) is formed on the upper surface of the insulator layer 14, and the counter electrode 28 is formed on the upper surface of the semiconductor layer 12 (12b).

In the case shown in FIG. 6B or 6C, the 6D
As shown in the figure, the counter electrode 28 may be omitted.

The arrangement of the interdigital electrodes and the counter electrodes may be appropriately selected so that the electromechanical coupling coefficient is larger than 2 depending on the cut angle of the semiconductor substrate and the propagation direction of the surface acoustic wave. In either case, when the counter electrode 28 is provided, the counter electrode 28 must be present at least directly above or below the ink digital electrode 20 (22).

Furthermore, the piezoelectric layer 16 needs to be located directly below or directly above at least the crossing width portion of the ink digital electrode 20 or 22.

In each of the above-described embodiments, the case where the impurity concentration gradually increases continuously in an exponential manner, such as in a super-step junction, has been described as an example of a gradient impurity concentration distribution. However, it goes without saying that this may be a concentration distribution that increases linearly, a concentration distribution that increases discontinuously, or a concentration distribution that increases gradually.

[Brief explanation of drawings]

FIG. 1 is an illustrative cross-sectional view showing one embodiment of the present invention. FIG. 2 is a diagram showing the impurity concentration distribution of the embodiment shown in FIG. FIG. 3 is an illustrative cross-sectional view showing another embodiment of the present invention. FIG. 4 is a diagram showing the impurity concentration distribution of the embodiment shown in FIG. FIG. 5 is a perspective view showing an example of a sound 9N and air convolver as a preferred embodiment of the present invention. 6A to 6D are principal part illustrative views showing other examples of the arrangement structure of ink digital electrodes and their related counter electrodes. FIG. 7 is an illustrative view showing an example of an elastane ink convolver, which is the background of the present invention. FIG. 8 is an illustrative view showing an example of a separation medium type convolver which is the background of the present invention. FIG. 9 is an illustrative diagram showing an example of a monolithic convolver which is the background of the present invention. The xo-th direction is a diagram showing the impurity concentration distribution of the semiconductor layer of the monolithic convolver of FIG. 9. In the figure, 10 is a surface acoustic wave element, 12 and 112 are n
The (p) type semiconductor layers 12a and 112a are n"(p"
) type semiconductor layer, 12b, 112b are n(p) type semiconductor layers, 14, 114 are insulator layers, 16, 116 are piezoelectric layers, 18 are gradient impurity concentration distribution regions, 20, 22, 120
, 122 is an interdigital electrode, 28 is a counter electrode, 1
00 is an acoustoelectric convolver, 124 is a gate electrode, 12
6 indicates a ground electrode. Patent applicant Murata Manufacturing Co., Ltd. Agent Patent attorney Yama 1) Yoshito (and 1 other person) Figure 1 Figure 2 1 DQn Takumi 1816

Claims (1)

[Scope of Claims] 1. A surface elastic material in which an n(p) type semiconductor layer, an insulator layer, and a piezoelectric layer are formed in a laminated manner in this order, and at least two or more electrodes are arranged on the piezoelectric layer. A surface acoustic wave device, characterized in that the n(p) type semiconductor layer near the insulator layer has an impurity concentration distribution that increases as it approaches the insulator layer. 2 The n(p) type semiconductor layer includes an n^+(p^+) type semiconductor region and an n(p) type semiconductor region formed thereon, and the impurity is added to the n(p) type semiconductor region. The surface acoustic wave device according to claim 1, which has a concentration distribution. 3. The surface acoustic wave device according to claim 2, wherein the n(p) type semiconductor region includes an n(p) type epitaxial layer. 4. Claim 1, wherein the piezoelectric layer contains ZnO
The surface acoustic wave device according to any one of Items 1 to 3. 5. The surface acoustic wave device according to any one of claims 1 to 4, wherein the insulator layer is formed of the same material as the piezoelectric layer. 6. The surface acoustic wave device according to any one of claims 1 to 4, wherein the insulator layer contains an oxide of a material for the n(p) type semiconductor layer. 7. The electrode has a comb-shaped structure,
A surface acoustic wave device according to any one of claims 1 to 4. 8. According to any one of claims 1 to 5, wherein a gate electrode is formed on the piezoelectric layer between at least two of the electrodes, thereby being configured as an acoustoelectric convolver. The surface acoustic wave device described above.