JPH03145319A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To reduce the power consumption and to improve the temperature characteristic by applying a reverse bias voltage to a high-density layer of second conduction type formed to a strip on the surface part of a silicon substrate. CONSTITUTION:When a reverse bias voltage +VB is deeply applied to an n<+> (P<+>) diffused area 31 as the high-density layer of second conduction type formed to a strip on the surface part of a silicon substrate 30 of first conduction type, a depletion layer 38 is spread in a p-type silicon substrate 30 as a high-resistance area. Since carrier scarcely exists in this depletion area 38, the carrier density is very reduced, and therefore, the propagation loss of SAW is very reduced without charging of carrier, namely, flow of a current. Thus, the power consumption is reduced to such degree that the current consumption can almost be ignored, and the temperature characteristic is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in a surface acoustic wave device suitable as a narrowband interference wave suppression filter used in a spread spectrum communication system.

[Summary of the Invention] In a surface acoustic wave device having a high concentration conductivity type layer formed in a strip shape on the surface of a high resistance first conductivity type silicon substrate, a reverse bias voltage is applied to the high concentration conductivity type layer. This makes it possible to vary the propagation loss of surface acoustic waves while consuming almost no current.

[Prior Art] In a surface acoustic wave device having a metal/ZnO/oxide/Si structure, a surface acoustic wave (SAW) is generated by forming a no or P0 region in Si and flowing a forward current, that is, injecting carriers. ) has been proposed to change the propagation loss of the SAW, making it possible to operate as a type of SAW variable attenuator.
This device is useful as a component of various functional devices.

FIG. 5 shows an example of such a SAW device, which is used, for example, as a SAW element for one channel having one center frequency with a narrowband interference signal removal device.

In the figure, 1 is a P"(n") Si single crystal substrate, 2 is a p(n) type Si epitaxial film layer formed on the substrate, and 3 is a thermal oxide film layer further formed on it. , 4
is a ZnO piezoelectric thin film formed on the thermal oxide film layer, 5
, 6.7 are metal electrodes formed thereon, which are respectively an input surface wave comb transducer, an output surface wave comb transducer, and a gate electrode. Reference numeral 8 denotes a high concentration impurity diffusion region formed in the p(n) type Si epitaxial film layer 2 under the fully bent electrode of the transducer, and serves to improve the excitation efficiency of the transducer. 9 is an n''(P'') impurity diffusion region formed in the p(n) type Si epitaxial film layer 2 under the gate electrode 7, and a first pn diode array is formed along the SAW propagation path. ing. The operation of this pn diode array is such that the carrier density in the epitaxial film layer is controlled by controlling the diode bias, and the interaction between the SAW and the carriers increases the attenuation constant of the SAW to 100 dB/
It also plays a role in significantly changing the value of am. In other words, it has the ability to turn channels on and off quickly.

10 is the p (n) type Si outside the input quadrature transducer.
This is an n(p) impurity diffusion region formed in the epitaxial film layer 2, and a second pn diode array is formed. 11 is a resistor connected to the pn diode array, 1
2 is a DC power supply. 13 is a voltage signal monitor terminal where the input signal is converted to SAW and detected by the second pn diode array, and the intensity (power) of the input signal within that channel (frequency range) is expressed as a voltage change l[Il '
This is the terminal that is used. 14 is a bias control terminal of the first pn diode array.

FIG. 6 shows the relationship between the power of the input signal and the bias shift amount of one monitor terminal. The bias shift amount is the SAW power (
It is proportional to the square of the input signal power).

In this way, the SAW potential is square-law detected by the pn diode array provided outside the input transducer, and the input signal strength is obtained as a baseband signal.

Therefore, information on the frequency spectrum intensity distribution of the input signal can be easily obtained by configuring a plurality of channels of the above-mentioned SAW elements each having a different center frequency connected in parallel.

FIG. 7 shows an adaptive narrowband interference suppression filter system (AIS F system: AdaptiveInt) consisting of n channels of SAW elements having the above-described structure connected in parallel.
erference 5uppression File
ter 5yste+*) is shown below.

The part in FIG. 7 consisting of the input quadrature transducer group 17 and the second pn diode array group 20 is a part that monitors the intensity distribution of the input spectrum. The input and output quadrature transducer groups 17 and 18 function as a sorting filter that classifies input signals according to their frequencies, propagates them, and synthesizes them again.

The first p on the SAW propagation path provided for each channel
An n-diode array group 19 controls the attenuation constant of the SAW of each channel.

The operation of the AI SF system shown in FIG. 7 is as follows.

The input signal is converted into a SAW by the input quadrature transducer group 17, and the bias shift amount of the second pn diode array group 20 is monitored and the first signal for propagation control is converted into a SAW signal.
Bias control circuit 2 controls the bias voltage of the pn diode.
The function of the bias control circuit 21 is to control the second pn diode 20 according to the signal strength of each channel.
The purpose is to amplify the amount of bias shift of , and bias the pn diode 19 of the channel having a large shift amount in the opposite direction. Or, rather than just amplifying
By setting a certain threshold value, the comparator turns the bias of the first pn diode 19 ON (forward bias) and OF
F (reverse direction bias) function.

[Problems to be Solved by the Invention] According to the operating principle of the conventional SAW device described above, carriers are injected, so a current flows and power consumption cannot be ignored. Furthermore, since the interaction between the carriers and the potential of the SAW is utilized, the mobility of the carriers at high temperatures deteriorates, so there is a drawback that the loss of the SAW increases at high temperatures.

[Objective of the Invention] Therefore, the object of the present invention is to provide a SAW that has low power consumption to the extent that the current consumption is almost negligible, has good temperature characteristics, and has an operating principle in which the loss of the SAW is variable without flowing current.
We are in the process of providing equipment.

[Means for Solving the Problems] In order to achieve the above object, the SAW device of the present invention includes a high-resistance first conductivity type silicon substrate and a high concentration second conductivity type layer formed in a strip shape on the surface of the silicon substrate. and a piezoelectric film formed on the silicon substrate, and means for applying a reverse bias voltage to the high concentration second conductivity type layer.

[Function] When a reverse bias voltage is applied to the high concentration second conductivity type layer,
A depletion layer spreads in the high-resistance first conductivity type silicon, and the carrier density becomes extremely small. Therefore, the propagation loss of the SAW can be extremely reduced without injecting carriers, that is, without flowing current.

[Examples] The present invention will be described below with reference to examples shown in the drawings. FIG. 1 shows an embodiment of a SAW device according to the present invention. In the figure, 30 is a first conductivity type silicon substrate, for example 50
A p (n) type Si substrate of ~5000Ω■, 31 is a highly concentrated second conductivity type layer formed in a strip shape in order to configure a diode array in the SAW propagation path on the surface of the substrate. n"(p") diffusion region, 32 is an ohmic electrode, 33 is Sio, a piezoelectric film formed on the Si substrate 30 with or without an insulating film 34, 35
is a metal gate electrode provided almost directly above the region 31 on the piezoelectric film 33, 36 and 37 are input and output transducers, respectively, and 39 and 40 are high concentration electrodes for improving the excitation efficiency of the transducer. This is an impurity diffusion region.

A voltage VG is applied to the gate electrode 35 so that the silicon surface forms a flat band.

Figure 2 shows surface waves (Acousto) - electricity (elect).
ric) effect (hereinafter referred to as AE) and the relationship between the resistivity of the silicon substrate. In this example, the silicon substrate is of p-type, but the same characteristics can be obtained even if the silicon substrate is of n-type. As is clear from Figure 2, in order to increase the SAW loss, the specific resistance of the silicon substrate must be 50 to 5000 Ω.
Preferably.

Now, in the embodiment shown in FIG. 1, when zero bias voltage or a slight forward bias voltage -VB is applied to the n''(p'') diffusion region 31, the propagation loss of the SAW increases as is clear from FIG. Extremely large.

Next, when a reverse bias voltage +vB is deeply applied to the diffusion layer 31, a depletion layer 38 spreads into the p-type silicon substrate 3o, which is a high resistance region. Since almost no carriers exist in this depletion layer 38, the loss due to the AE interaction becomes negligibly small. As described above, since the diode bias that controls the propagation loss of the SAW is a reverse bias, the AE loss can be made extremely small even though no current flows in the silicon substrate 30.

Figure 3 shows the n diffusion region 31 constituting the diode array.
The relationship between the depletion layer width and the AE loss when the depletion layer 38 is expanded by applying a reverse bias voltage of 1,000 B is shown.

It is clear from the figure that when the depletion layer expands by about 10 μm, the AE loss can be almost ignored.

FIG. 4 shows the carrier distribution when +Va=+10V is applied to the n+ diffusion region 31 to create a reverse bias state, where the vertical axis represents the depth and the horizontal axis represents the distance.

From the same figure, it can be seen that the depletion layer spreads by 10.mu.m or more, and the propagation loss of the SAW can be extremely reduced with a reverse bias of approximately +IOV.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a SAW device in which the SAW loss is variable and the current consumption is very small, so that the SAW device has low power consumption and good temperature characteristics.

Note that the SAW device of the present invention can be used as a narrowband interference suppression filter by having the multi-channel configuration shown in FIG. 7, for example.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, and FIG. 2 is a schematic diagram showing an embodiment of the present invention.
A characteristic diagram showing the relationship between the SAW loss due to the effect and the resistivity of the silicon substrate, Figure 3 is a characteristic diagram showing the relationship between the depletion layer width and AE loss when a reverse bias voltage is applied, and Figure 4 is a characteristic diagram showing the relationship between the depletion layer width and the AE loss when a reverse bias voltage is applied. Fig. 5 is a schematic diagram showing an example of a conventional SAW device, and Fig. 6 is a characteristic diagram showing the relationship between input signal power and bias shift amount of one monitor terminal in the device shown in Fig. 5. , FIG. 7 is a diagram showing a narrowband interference suppression filter system consisting of n-channel SAW elements. 30...P(n) type silicon substrate, 31
......n"(p+) diffusion region, 33...
...Piezoelectric film, 35...Gate electrode, 38...Depletion layer.

Claims (1)

[Scope of Claims] A high-resistance first conductivity type silicon substrate; a high concentration second conductivity type layer formed in a strip shape on the surface of the silicon substrate; a piezoelectric film formed on the silicon substrate; A surface acoustic wave device comprising: means for applying a reverse bias voltage to the highly concentrated second conductivity type layer.