Surface acoustic wave functional device

Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices

Reexamination Certificate

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C310S31300R

Reexamination Certificate

active

06194808

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a surface acoustic wave functional element, such as surface acoustic wave amplifier or surface acoustic wave convolver, by use of interaction between a surface acoustic wave propagating through the piezoelectric substrate and electrons in the semiconductor.
BACKGROUND ART
Known as one conventional functional element by use of interaction between a surface acoustic wave and electrons in the semiconductor is a surface acoustic wave functional element of such a structure as to perform an interaction over the propagation path all in width of a surface acoustic wave. With respect to a surface acoustic wave amplifier as one example of surface acoustic wave functional element, for example, three structures have been proposed: direct type amplifier (FIG.
2
), separate type amplifier (
FIG. 3
) and monolithic type amplifier (FIG.
4
). The first direct type amplifier is of a structure that a piezoelectric semiconductor substrate
11
such as CdS or GaAs in possession of both piezoelectricity and semiconductivity is used to install thereon an input electrode
4
, an output electrode
5
and an electrode
8
for applying a DC electric field to the piezoelectric semiconductor substrate
11
, thereby amplifying a surface acoustic wave. However, no piezoelectric semiconductor in possession of both large piezoelectricity and large electron mobility has been found thus far. The second separate type amplifier is an amplifier of a structure with an input electrode
4
and an output electrode
5
provided on a piezoelectric substrate
1
of a large piezoelectricity and a semiconductor
12
of a large electron mobility disposed via a gap
13
as well. In this type of amplifiers, the amplification gain is largely affected by the flatness of the surface of the semiconductor and the piezoelectric substrate and by the magnitude of the gap. To obtain the amplification gain equal to practical use, it is required to make the gap as small as possible and keep it constant all over the operating region, so that there is an extreme difficulty in industrial production. On the other hand, the third monolithic type amplifier is an amplifier of a structure with an input electrode
4
and an output electrode
5
provided on a piezoelectric substrate
1
and a semiconductor
12
formed via a dielectric layer
14
rather than a gap
13
as well. According to 1970s studies by Yamanouchi and others (K. Yamanouchi et al., Proceedings of the IEEE, 75, p. 726 (1975)), an electron mobility for InSb of 1600 cm
2
/Vs has been obtained in a structure with SiO coated onto the LiNbO
3
substrate and a 50 nm thick InSb film deposited thereon and a gain of 40 dB has been obtained at a central frequency of 195 MHz under application of an extremely high DC voltage of 1100 V in a surface acoustic wave amplifier using this film. Since no good film quality of InSb was obtained, however, there has been a problem of too high driving voltage and too small amplification gain at a low voltage in consideration of applications to an actual portable apparatus.
Next, a surface acoustic wave convolver can be referred to as another application by use of interaction between a surface acoustic wave and electrons in a semiconductor. At present, surface acoustic wave convolvers arrest attention greatly as correlators for CDMA (Code Division Multiple Access) scheme of spread spectral communication. Since former times, digital LSI and analog LSI have been examined as CDMA correlators, but either of them was extremely large in power consumption, thus forming an extremely large barrier against applications to a handy device requiring a low power consumption. Thus, a surface acoustic wave convolver of zero consumed power in principle begins to be examined to practical use with advantages taken of low power consumption and no need for synchronism. In studies of a surface acoustic wave convolver, a convolution output of −59 dBm has been obtained at the system of InSb/LiNbO
3
, for example according to K. Yamanouchi, S. Mitsui and K. Shibayama, IEEE MTT-S Intern. Microwave Symp. Digest, p. 31 (1980).
To ensure applicabilities of a monolithic type amplifier to actual portable telephone or the like, however, it is required to obtain a better amplification gain at a practically low voltage of at least 9 V or lower and to implement it in a feasible process as well. In other words, a lower voltage than the former technique by two factors or more must be intended. Besides, as regards a surface acoustic wave convolver, a still greater efficiency must be attained.
In a former structure of surface acoustic wave functional elements, it has been required to lessen the thickness of a semiconductor film greatly in using such a semiconductor as InSb of a large electron mobility to match the electric impedance of a surface acoustic wave with that of the semiconductor. With a thin film thickness, however, the semiconductor film is poor in crystallinity and becomes smaller in electron mobility, so that no functional element better in characteristics has been obtained.
Besides, in convolvers, because of a small thickness of the semiconductor layer, there has been a problem for a method of taking out an output in a direction of thickness that no high efficiency is obtained and moreover a problem that the sheet resistance reduces, thus leading to a short circuit in the electric field of a surface acoustic wave has occurred for a greater thickness of the semiconductor layer. Furthermore, in a structure of forming a semiconductor layer above the propagation path, the loss of a surface acoustic wave has increased, thus causing a decrease in amplification gain and a lowering of efficiency.
Still further, no attention whatever has been paid to the presence of a buffer layer, the position of a grounded electrode and interaction in shape of a strip electrode.
DISCLOSURE OF THE INVENTION
It is one object of the present invention to provide a surface acoustic wave functional element easy of industrial production, having a semiconductor film good in film quality as the active layer with the semiconductor so arranged that an interaction between a surface acoustic wave and the semiconductor occurs sufficiently.
As a result of intensive examination for solving the above task, the inventors implemented a surface acoustic wave amplifier of great amplification characteristics and a surface acoustic wave convolver of an extremely high efficiency under a low voltage wherein the semiconductor layer was improved in crystallinity by inserting between the piezoelectric substrate and an active layer a buffer layer lattice-matched to the active layer and further an interaction is enabled to occur in the semiconductor layer by disposing a semiconductor layer beside the propagation path and using a grating electrode to convey the electric field of a surface acoustic wave to the semiconductor layer.
An extremely great amplification characteristic and an extremely high efficiency of convolution output under low voltage was implemented in the present invention 1) because an extremely good active layer could be formed on the piezoelectric substrate by inserting a buffer layer in the growth of a semiconductor layer, 2) because the absence of a semiconductor layer on the propagation path of the piezoelectric substrate could minimize the loss of a surface acoustic wave, 3) because the electrode width and the electrode interval in grating electrodes disposed on the propagation path were selected so as to suppress the reflection and 4) because the efficiency of interaction between a surface acoustic wave and electrons could be improved by the formation of an output electrode alternating the grating electrode for an electric surface wave convolver.
Here, the active layer means a layer in which electrons to interact with the surface acoustic wave propagated.
By properly selecting the relatively positional relation among the semiconductor layer (active layer) improved in film quality as a result of such insertion of a buffer layer, the grating electrode a

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