JPH06204782A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To obtain a surface acoustic wave device in which the sensitivity of a detection section is enhanced by using a thin epitaxial layer whose specific resistivity is sufficiently low. CONSTITUTION:The thickness of an epitaxial layer 21 formed on a silicon single crystal substrate 20 is thin as 3-5mum while its specific resistivity is 10OMEGA.cm or below and p<+>, n<-> impurity diffusion regions 22 as a diode are formed in the epitaxial layer 21 at an interval of 4mum. The device is in ON state when no bias is applied to the diode or a reverse bias is applied to the diode and the device is in OFF state when a reverse deeper bias is applied to the diode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device suitable for a device for removing a narrow band interference signal used in a spread spectrum communication system.

[0002]

2. Description of the Related Art FIG. 3 shows a structure of a conventional surface acoustic wave (SAW) element for one channel having one center frequency with a device for removing a narrow band interference signal. 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, 3 is a thermal oxide film layer further formed thereon, Reference numeral 4 is a ZnO piezoelectric thin film formed on the thermal oxide film layer, and reference numerals 5, 6 and 7 are metal electrodes formed thereon, which are a surface acoustic wave comb-shaped transducer for input, a surface acoustic wave comb-shaped transducer and a gate electrode, respectively. . 8 is p under the metal electrode of the transducer
It is a high-concentration impurity diffusion region formed in the (n) type Si epitaxial film layer 2 and serves to improve the excitation efficiency of the transducer. Reference numeral 9 is an n ⁺ (p ⁺ ) impurity diffusion region formed in the p (n) type Si epitaxial film layer 2 below 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 SAW is caused by the interaction between the SAW and the carriers.
Plays the role of greatly changing the damping constant of 100 dB / cm or more. In other words, there is a function to turn the channel on and off at high speed. Reference numeral 10 is an n (p) impurity diffusion region formed in the p (n) type Si epitaxial film layer 2 outside the input transducer, and forms a second pn diode array. Reference numeral 11 is a resistor connected to the pn diode array, and 12 is a DC power supply. Reference numeral 13 is a voltage signal monitor terminal in which the input signal is converted to SAW and detected by the second pn diode array, and the strength (power) of the input signal in the channel (frequency range) is observed as a voltage change. Is. Reference numeral 14 is a bias control terminal of the first pn diode array.

[0003]

In the conventional SAW device as described above, in order to reduce the AE loss and realize the ON state in which the signal passes, the carrier is injected from the diode to reduce the specific resistance, or the diode is injected into the diode. It is necessary to apply a reverse bias deeply to widen the depletion layer to increase the specific resistance. As a substrate, an AE of 50 to 5000 [Ω-cm) is required.
It was necessary to use a high-loss specific resistance. AE loss is also related to the thickness of the epitaxial layer,
At least 5 μm to obtain a sufficient amount of AE loss
The epitaxial layer having the above thickness was required. vice versa,
Since the interference wave detecting section for adaptive signal processing is formed on the same substrate, there is a drawback that the input signal is largely attenuated and sufficient detection sensitivity cannot be obtained. Therefore, it is necessary to provide an additional process such as forming an epitaxial layer in two steps or forming a high-concentration buried layer. (For example, JP-A-3-2971
(See No. 20)

An object of the present invention is to provide a SAW device having a structure and an operation mode in which the specific resistance is sufficiently low and the sensitivity of the detecting portion can be increased by using a thin epitaxial layer.

[0005]

In order to achieve the above object, a SAW device of the present invention comprises a semiconductor substrate and an epitaxial layer formed on the semiconductor substrate, and a plurality of P and N are formed on the surface thereof. An epitaxial layer having a diode composed of regions, an oxide film layer formed on the epitaxial layer, a piezoelectric film layer formed on the oxide film layer, and an electrode formed on the piezoelectric film layer When,
A surface acoustic wave device consisting of, wherein the relationship between the thickness of the epitaxial layer and the spacing of each region of the diode,
The gist is that the diode is turned on when a zero bias or a slight reverse bias is applied to the diode, and turned off when a larger reverse bias is applied. In this case, the thickness of the epitaxial layer and the interval between the regions of the diode are,
It is preferably about 3 μm and 4 μm.

[0006]

In the SAW device of the present invention, a substrate having a lower specific resistance and a thinner epitaxial layer is used,
Since the weak inversion state is used, the state in which no bias is applied to the diode or a slight reverse bias is applied is the on state, and the state in which a larger reverse bias is applied is the off state.

[0007]

Embodiments of the present invention shown in the drawings will be described below.
FIG. 1 shows an embodiment of a SAW device according to the present invention, where 20 is p
⁺ (N ⁺ ) Si single crystal substrate, 21 is a p (n) type Si epitaxial film layer, and the epitaxial film layer 21 having a concentration of about 1 × 10 ¹⁵ / cm ^{3 is formed} on the p ⁺ (n ⁺ ) Si single crystal substrate 20. Is 3
It is formed in the order of μm. On the surface of the substrate 20, n ⁺ (p ⁺ ) impurity diffusions 22 serving as diodes are formed at intervals of about 4 μm. Further, an oxide film 23 and a ZnO piezoelectric film 24 are formed, and input / output transducers 25, 26 and a gate electrode 27 are formed on the surface thereof.

In the above embodiment, the ON state for passing a signal between the input and output transducers is realized when the diode 22 is set to 0 V or the reverse bias state of 1 to 2 V in which the depletion layer spreads a little. That is, since the specific resistance of the epitaxial layer 21 is 10 Ω · cm or less, the magnitude of the AE loss is sufficiently smaller than that of FIG.

Next, the off state is realized by further applying a deep reverse bias to the diode 22. When the reverse bias is set to 5 V, the surface of the semiconductor 21 is in a weak inversion state, and the n ⁺ region 22 has a spacing of 4 μm, so the depletion layers are connected to each other and the minority carriers generated on the surface are -10 ¹⁴ / cm ^3.
It becomes a state where it is accumulated at a certain concentration. At this time, the magnitude of the AE loss is as large as -60 dB / cm or more.

Since the on / off operation is performed as described above, the thickness of the epitaxial layer of the filter portion does not affect the characteristics. Therefore, it becomes possible to match the epitaxial layer to the conditions required in the detection section, and the magnitude of the AE loss in the detection section can be made sufficiently small. As a result, the SAW power in the detection unit can be increased, and as a result, the detection signal strength can be increased.

Next, referring to FIG. 2, turning on the conventional device,
The difference in the off state will be described. Previous AIS (Adap
tiv Interference Suppeerssion) and variable depletion layer type AIS filters use the original AE loss of a thick epitaxial layer, which has a high specific resistance, in the off state where the signal is cut off (state of FIG. 2 (B)). By injecting carriers from this state (B), the state (A) is produced, and by expanding the depletion layer, the state (C) is produced to realize the on state. On the other hand, in the present invention, the state (A) is the on state, and the reverse bias is applied to the diode from this state to realize the state (B) by making the weak inversion state and the off state is created. Become.

[0012]

As described above, according to the present invention, since the weak inversion state of the semiconductor surface is used to realize the off state, the thick epitaxial layer as in the past is not necessary and the bias is applied. Since it is in the ON state in the initial state, the attenuation amount of the SAW of the detection unit can be reduced, and as a result, the detection sensitivity of the interference wave can be increased.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing the resistivity dependence of AE loss.

FIG. 3 is a schematic diagram showing an example of a conventional SAW device.

[Explanation of symbols]

20 p ⁺ (n ⁺ ) Si single crystal substrate 21 p (n) Si epitaxial film layer 22 n ⁺ (p ⁺ ) impurity diffusion region constituting a diode 23 oxide film 24 ZnO piezoelectric film 25, 26 input / output transducer 27 gate electrode

Claims (2)

[Claims] 1. A semiconductor substrate, an epitaxial layer formed on the semiconductor substrate, the epitaxial layer having a diode formed of a plurality of P and N regions on the surface thereof, and formed on the epitaxial layer. An oxide film layer, a piezoelectric film layer formed on the oxide film layer, and an electrode formed on the piezoelectric film layer, and a thickness of the epitaxial layer. The relationship with the interval of each region of the diode,
A surface acoustic wave device characterized in that it is turned on when a zero bias or a slight reverse bias is applied to the diode and turned off when a larger reverse bias is applied. 2. The surface acoustic wave device according to claim 1, wherein the thickness of the epitaxial layer and the distance between the regions of the diode are approximately 3 μm and 4 μm, respectively.