Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-02-07
2001-03-06
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06198197
ABSTRACT:
TECHNICAL FIELD
The present invention relates to surface acoustic wave functional elements, such as a surface acoustic wave amplifier and a surface acoustic wave convolver, in which surface acoustic waves propagating in a piezoelectric substrate interact with carriers in a semiconductor, and to an electronic circuit including such a surface acoustic wave functional element.
BACKGROUND ART
Recently, a mobile communication apparatus such as a portable telephone has been down-sized and devised to operate at lower voltages with reduced power consumption. With this progress, intensive investigation has been made to develop monolithic elements which can be mounted in a portable apparatus. However, since a bandpass filter and a duplexer are bigger in size than other high frequency components, there is little advantage to fabricate these elements monolithically together with other elements. Moreover, it is very difficult to fabricate a power amplifier as a monolithic element. For this reason, a duplexer, a power amplifier, a bandpass filter, a low noise amplifier arranged upstream of the bandpass filter, etc. have been developed as respective discrete elements and fabricated as respective modules. When these discrete elements are fabricated as modules, wiring for connecting a plurality of parts and circuitry for matching impedance are formed, and therefore the discrete elements as units are very large in size.
On the other hand, there have been made various studies on amplification of surface acoustic waves. In order to amplify the surface acoustic wave, it is known to propagate the surface acoustic wave in a surface of a piezoelectric substrate and couple the electric field generated by the wave with carriers in a semiconductor. Actual surface acoustic wave amplifiers are classified into three types according to the types of combination of the piezoelectric material for propagating the surface acoustic wave and the semiconductor: (1) a direct type amplifier (FIG.
3
); (2) a separation type amplifier (FIG.
4
); and (3) a monolithic type amplifier (FIG.
5
). As shown in
FIG. 3
, the direct type amplifier is an amplifier having the structure which has a substrate
7
composed of a material, such as CdS or GaAs, with both piezoelectric characteristics and semiconductor characteristics simultaneously, on which input and output electrodes
4
and
5
are provided, with the substrate
7
being sandwiched by electrodes
6
for applying a direct current electric field to the substrate
7
. However, a piezoelectric semiconductor with large piezoelectric properties and large mobility has not been found so far. As shown in
FIG. 4
, the separation type amplifier is an amplifier having the structure in which a semiconductor
3
′ of a large mobility is disposed on a piezoelectric substrate
1
of large piezoelectric property with a gap
8
. Input and output electrodes
4
and
5
are provided on the substrate
1
, and electrodes
6
for applying a direct current electric field to the semiconductor
3
′ are provided on both sides of the semiconductor
3
′. In the amplifier of this type, surface flatness of the semiconductors and the piezoelectric substrate and the size of the gap
8
have a great effect on the amplification gain. In order to obtain a practically acceptable amplification gain, the gap
8
must be made as small as possible and maintained constant over an operation range and so that industrial fabrication of the amplifiers with such a gap is very difficult. As shown in
FIG. 5
, the monolithic type amplifier is an amplifier having the structure in which a semiconductor
3
′ is formed on a piezoelectric substrate
1
via a dielectric film
9
without a gap. Input and output electrodes
4
and
5
are provided on the piezoelectric substrate
1
, and electrodes
6
for applying a direct current electric field to the semiconductor layer
3
′ are provided on both sides of the semiconductor layer
3
′. The monolithic type amplifier can achieve a high gain and be used in a high frequency region. Moreover, the monolithic type amplifier is said to be suitable for mass production. However, application of these surface acoustic wave amplifiers to a mobile communication apparatus such as a portable telephone has not been studied yet.
In order to realize a monolithic type amplifier, a semiconductor film of good electric characteristics must be formed on a piezoelectric substrate and the semiconductor film must be sufficiently thin so that there can occur efficient interaction between the surface acoustic wave and the carriers in the semiconductor. According to the study by Yamanouchi et al. of Tohoku University in 1970s (Yamanouchi K., et, al., Proceedings of the IEEE, 75, p726 (1975)), an electron mobility of InSb of 1,600 cm
2
/Vsec was achieved using the structure in which SiO is coated on a LiNbO
3
substrate to a thickness of 30 nm and then InSb thin film is evaporation-deposited on the substrate to a thickness of 50 nm. When a DC voltage of 1,100 V was applied to a surface acoustic wave amplifier having the semiconductor films, an amplification gain of net gain 40 dB was obtained at a center frequency of 195 MHz. Furthermore, based on their theoretical calculation, Yamanouchi et al. predicted that in an InSb thin film of 50 nm thick, the maximum electron mobility is 3,000 cm
2
/Vsec because of surface scattering of carriers (Yamanouchi et al., Shingaku Gihou, US78-17. CPM78-26, p19 (1978)). That is, the monolithic type amplifier faces the trouble that a thin film semiconductor layer having good electric characteristics is difficult to be formed on a piezoelectric substrate. Moreover, a conventional structure requires a dielectric film such as SiO in order to prevent deterioration of InSb and a LiNbO
3
substrate because of diffusion of oxygen from the LiNbO
3
substrate. Moreover, when a surface acoustic wave amplifier is used as an amplifier of a high frequency portion of a portable apparatus and a bandpass filter, the surface acoustic wave amplifier is useless if it gives no amplifying effect at a driving voltage of 3 to 6 V. A conventional monolithic amplifier needs a high voltage and there was no surface acoustic wave amplifier that could be driven at low voltages. Furthermore, there is the problem that a surface acoustic wave convolver, which makes use of interaction between a surface acoustic wave and electrons like the surface acoustic wave amplifier, gives an insufficient gain.
In general, an amplification gain, G, of a surface acoustic wave amplifier is given by the following equation:
G
=
A
k
2
ϵ
p
σ
h
(
μ
E
-
v
)
where A=constant, k
2
=an electromechanical coupling coefficient, &egr;p=an equivalent dielectric constant of a piezoelectric substrate, &sgr;=a conductivity, h=a film thickness of an active layer, &mgr;=an electron mobility, E=an applied electric field, and v=a velocity of a surface acoustic wave. In order to obtain a large amplification gain at a low voltage of a practical level, it is necessary that; (1) a semiconductor thin film is formed which has a high electron mobility and whose film thickness is as thin as possible; and that (2) a piezoelectric substrate is used whose k
2
is as large as possible.
The present inventors have made intensive investigation on the above problems and as a result found that an active layer which is a thin film and has good electric characteristics can be obtained by inserting a buffer layer between the piezoelectric substrate and the active layer. The present inventors also found that an electromechanical coupling coefficient k
2
of the piezoelectric substrate far larger than that of a bulk can be achieved by using a multilayer piezoelectric thin film substrate of at least three layers. Furthermore, the present inventors confirmed that a surface acoustic wave amplifier is fabricated using the semiconductor layer or the piezoelectric thin film subst
Kanno Yasuhito
Kuze Naohiro
Shibata Yoshihiko
Yamanouchi Kazuhiko
Asahi Kasei Kogyo Kabushiki Kaisha
Dougherty Thomas M.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
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