Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1998-07-22
2000-04-04
Dougherty, Thomas M.
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
H01L 4108
Patent
active
060465247
DESCRIPTION:
BRIEF SUMMARY
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 productio
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Kanno Yasuhito
Kuze Naohiro
Shibata Yoshihiko
Yamanouchi Kazuhiko
Asahi Kasei Kogyo Kabushiki Kaisha
Dougherty Thomas M.
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