Surface acoustic wave parametric device

An surface acoustic wave device, especially amplifier which includes input and output means for electrical signals provided so as to be spaced on a surface of a piezoelectric material which comprises a laminate in combination with a semiconductor, and an electrode provided in a surface signal wave propagation path between said input and output means for supplying pumping power.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an surface acoustic wave device, and more 
particularly to an surface acoustic wave device operative by a parametric 
amplifying effect. 
2. Description of the Prior Art 
An surface acoustic wave (elastic surface wave) amplifier is a solid-state 
amplifier which generally employs a piezoelectric semiconductor or a 
suitable combination of a semiconductor and a piezoelectric material, and 
attains a desired amplification effect by interaction between an surface 
acoustic wave (hereinafter referred to as "surface wave") and a 
semiconductor charge. 
In a conventional surface wave amplifier, a piezoelectric material such as 
lithium niobate and a semiconductor are disposed oppositely so as to be 
spaced a distance of about 1,000 .ANG., and a d.c. voltage is applied to 
the semiconductor in a direction along the surface of the semiconductor 
which defines the region to accomplish an amplifying operation by the 
interaction between a semiconductor charge and a surface wave caused by 
such application of the d.c. voltage. 
This conventional amplifier, however, has the defect that continuous wave 
operation cannot be obtained due to Joule heat generated by the d.c. 
current. It has another difficulty in integration because of its 
non-monolithic structure. 
There have been other conventional devices than that of d.c. 
voltage-application type, which utilize a parametric amplification effect. 
One of them is so constructed that a semiconductor material such as Si is 
provided in the propagation path of travelling waves on a piezoelectric 
material through a conductive liquid and so operated that a surface signal 
wave and pumping wave (propagated wave) are propagated in the same 
direction at a portion of the piezoelectric material and a space 
capacitance nonlinearity at a portion of the semiconductor is utilized to 
attain amplification of the surface signal wave. 
This device, however, has such disadvantages that since the frequency of 
the pumping wave is much higher than that of the surface signal wave, 
there is some difficulty in the preparation of transducers for generating 
the pumping waves, and that since necessary pumping power is increased due 
to generation of harmonic components of the pumping wave, there is caused 
some disadvantage in actual operation. It has further disadvantages in 
integration, too. 
Another type of conventional parametric amplifier is so operated that with 
respect to a surface wave signal propagating on a piezoelectric material, 
an exciting voltage is applied to a metal electrode provided in the 
propagation path for exciting the piezoelectric material at the relevant 
portion, thereof, thereby amplifying the surface signal wave propagating 
through the piezoelectric material due to a nonlinear effect of the 
piezoelectric material. 
OBJECT OF THE INVENTION 
It is an object of the present invention to provide an elastic surface wave 
device which is capable of producing continuous waves. 
It is another object of the present invention to provide an elastic surface 
wave device which is capable of attaining conversion in an ultra-high 
frequency band effectively. 
It is still another object of the present invention to provide an elastic 
surface wave device which is capable of eliminating the minute processing 
heretofore necessitated. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an elastic 
surface wave device comprising: a laminate composed of a semiconductor 
layer and a piezoelectric material layer; a first electrode provided on 
the propagating path of surface acoustic waves in said laminate; and means 
for supplying a high frequency pumping electric power to said electrode, 
thereby causing a surface charge layer capacitance formed at the surface 
portion of said semiconductor layer to vary.

PREFERRED EMBODIMENTS OF THE INVENTION 
Referring now to the drawings, there are illustrated preferred embodiments 
of the present invention. 
In FIG. 1, character S designates a semiconductor substrate made of, for 
example, silicon (Si) etc., I a piezoelectric film made of zinc oxide 
(ZnO) etc., and I' a protective film such as a silicon dioxide (SiO.sub.2) 
film. The protective film I' acts as a stabilizing film for the surface of 
the semiconductor substrate S. 
The semiconductor substrate S, the protective film I' and the piezoelectric 
film I comprise a laminate. 
The laminate may alternatively have such construction that a piezoelectric 
material is used as a substrate and a semiconductor film is formed in a 
suitable manner on the piezoelectric substrate. 
The semiconductor substrate S may be either p-type or n-type. In accordance 
with the type employed, a polarity of a d.c. base voltage as mentioned 
later may be determined so as to produce a space charge layer at the 
surface portion of the semiconductor substrate S. 
M.sub.1 on the piezoelectric film I is a metal electrode for supply of a 
d.c. bias voltage from a variable voltage source E and an a.c. electrical 
signal from a signal source F. The metal electrode M.sub.1 is formed in a 
thin film, for example by evaporation. 
M.sub.2 is an electrode for an ohmic contact with the semiconductor 
substrate S. 
The electrode M.sub.1 is connected to the variable voltage source E through 
a choke coil CH for choking off a.c. current. 
This electrode M.sub.1 is further connected to the signal source I through 
a capacitor C for preventing a d.c. current flow in this branch. 
Character A designates a sound wave (surface wave) absorber, for example 
made of silicone grease. This sound wave absorber A acts to absorb one of 
surface waves B and B' propagating in the opposite direction, i.e. surface 
wave B' in the embodiment of FIG. 1, so as to operate the device as a 
unidirectional surface elastic wave transducer. 
The resulting surface wave device in accordance with the present invention 
will operate as follows: 
Explanation will be given referring to the case where an a.c. electrical 
signal is converted to a surface signal wave. 
A suitable value of d.c. bias voltage is first applied from the variable 
voltage d.c. power source E to the electrode M.sub.1 to impart to the 
semiconductor substrate S at its interfacial surface portion a surface 
charge layer forming a nonlinear capacitance. 
When an a.c. electrical signal having a frequency f is then supplied from 
the a.c. electrical signal source F to the electrode M.sub.1, surface 
signal waves B and B' of a frequency f/2 are generated by the parametric 
interaction between the power of the a.c. electrical signal and the 
non-linear surface charge layer capacitance. These surface signal waves B 
and B' are propagated in directions right and left of the electrode 
M.sub.1 as viewed in FIG. 1. 
At this time, if the frequency of the a.c. electrical signal to be supplied 
to the electrode M.sub.1 is selected so as to be 2f, the frequency of the 
surface signal waves B and B' converted is f. Thus, when the frequency of 
the a.c. electrical signal is varied over a wide range, the surface signal 
waves B and B' having the corresponding frequency are generated. 
The amplitude of the converted surface signal waves B and B' varies 
depending upon the length of the electrode M.sub.1 in the propagation 
direction of the surface signal waves and the gain of the nonlinearity at 
the surface portion of the substrate. 
The gain of the nonlinearity is determined by the surface charge layer 
capacitance, which is in turn determined by a d.c. bias voltage value and 
the amount of the a.c. electrical signal power to be supplied. 
Accordingly, in practical implementation, the frequency and power of the 
a.c. electrical signal and the gain of the nonlinearity may be suitably 
selected to produce a surface signal wave of the corresponding frequency 
and amplitude. 
Furthermore, when a sound wave absorber A is mounted on one end of the 
piezoelectric material as shown in FIG. 1, the surface signal wave B' 
propagating in the direction towards the absorber is absorbed and not 
reflected, so that there is provided an unidirectional elastic surface 
wave transducer. 
The foregoing description is given referring only to the conversion from an 
a.c. electrical signal to a surface signal wave, the conversion in the 
opposite direction, i.e., from a surface signal wave to an a.c. electrical 
signal is accomplished in substantially the same manner. 
Though the piezoelectric material I is made of zinc oxide in the 
illustrated embodiment, it may alternatively be made of other 
piezoelectric materials such as lithium niobate (LiNbO.sub.3), aluminum 
nitride (AlN), cadmium sulfide (CdS), zinc sulfide (ZnS), etc. 
In the light of the foregoing description, it is easily understood that in 
accordance with the present invention, a semiconductor and a piezoelectric 
material are formed into a laminate using them as a substrate and a film 
provided thereon, respectively, for providing a monolithic formation so 
that it can improve the reproducibility and attain the advantages of 
integration. 
The present invention has a further advantage that since the configuration 
of the electrode mounted on the surface of the piezoelectric material may 
be freely designed and it may be of a planar sheet, there is no need to 
subject the material to elaborate processing so long as it can produce a 
necessary space charge layer. 
As the present invention accomplishes the conversion between an a.c. 
electrical signal and a surface signal wave utilizing a parametric effect, 
the frequency range can be widened so that the conversion can be effected 
even in a ultra high frequency band, with excellent efficiency, where 
conventional techniques cannot achieve the conversion. 
FIG. 2 shows another embodiment of the present invention, wherein the 
elastic surface wave device is applied to an amplifier. In the figure, 
like members are designated by like numerals or characters. 
Numerals 1 and 2 designate input and output means, respectively, for 
electrical signals, and they include comb electrodes as shown in FIG. 3. 
At the input means 1, an electrical signal is converted to an elastic 
surface wave, and at the output means 2 the surface signal wave is 
converted to an electrical signal. 
An electrode M.sub.1 is connected to high frequency power source 3 through 
a capacitor C for blocking d.c. current and through an inductor CH to a 
variable d.c. voltage source E, the high-frequency power source serving as 
a supply of pumping power. 
The so constructed elastic surface wave amplifier in accordance with the 
present invention will operate as follows: 
Upon supply of an electrical signal to the input means 1, the electrical 
signal is converted to a surface signal wave, which is then propagated at 
the surface portion of a piezoelectric material I towards the output means 
2. The frequency of the surface signal wave being propagated is now 
assumed to be f. Then, when a pumping power having a frequency 2f is 
supplied to the electrode M.sub.1 on the piezoelectric material I from a 
high-frequency power source 3 as well as a d.c. bias voltage from a d.c. 
power source E, the surface signal wave is amplified by a parametric 
interaction due to the surface charge layer capacitance nonlinearity at 
the surface portion of a semiconductor S under the electrode M.sub.1. The 
so amplified surface signal wave is converted to an electrical signal by 
the output means 2 and drawn out. 
The gain in the above-mentioned amplifying operation is expressed as a 
function of the length of the electrode M.sub.1 in the propagation 
direction of the surface wave, the gain .epsilon. of the nonlinearity at 
the surface portion of the semiconductor substrate, and the frequency of 
the pumping power, etc. Accordingly, the gain can be varied by varying the 
values of the respective parameters. 
The gain .epsilon. of the nonlinearity is determined by a surface charge 
layer capacitance nonlinearity of the semiconductor, which is in turn 
determined by a value of a d.c. bias voltage applied and the amount of the 
pumping power. Accordingly, in a practical application, these two 
parameters may be advantageously varied to regulate the gain. 
Thus, in the elastic surface wave amplifier in accordance with the present 
invention, the amplification depends highly on the surface charge layer 
capacitance nonlinearity at the semiconductor substrate S of silicon etc. 
The effect due to this nonlinearity is by substantially superior as 
compared with that of the conventional technique which utilizes 
nonlinearity of the piezoelectric material itself, and results in various 
advantages sole that the required pumping power can be much reduced to 
obtain a given gain. 
As the parameter amplifying effect of the elastic surface wave amplifier in 
accordance with the present invention is a kind of a positive feedback 
amplifying effect, an electrical quality factor Q is required to be 
improved to enhance the gain. One example of the frequency response of the 
gain with varying Q values is shown in FIG. 4. As apparent from the 
figure, a frequency bandwidth can be varied as well as the gain. Thus, the 
amplifier of the present invention further has a variable 
bandwidth-amplifying effect. 
For supply of the pumping power having a frequency of 2f, one 
high-frequency power source 3 is employed in the embodiment shown in FIG. 
2, but two or more high-frequency power sources may be employed so long as 
the beat frequency thereof is equal to 2f. For example, when 
high-frequency power sources having frequencies of f.sub.1 and f.sub.2, 
respectively, are employed, a power having frequency components of f.sub.1 
.+-.f.sub.2 and nf.sub.1 .+-.mf.sub.2 is produced due to the nonlinearity 
at the surface portion of the semiconductor substrate. 
If the frequency of the resulting power is equal to 2f, it can accomplish 
the amplification of the surface signal wave as when one high-frequency 
power source is employed. 
Though the foregoing description is given referring only to the pumping 
power having a frequency 2f, the frequency of the pumping power is not 
limited to 2f and it may be 4f, 6f, 8f . . . . 
In effect, however, the frequency of the pumping power is preferred to be 
2f because the pumping power having a frequency 2f provides the maximum 
gain E of the nonlinearity. In this connection, it is to be noted that 
when the frequency of the pumping power is changed, the center frequency 
of the frequency band capable of being amplified is variable. In brief, 
the elastic surface wave amplifier in accordance with the present 
invention has a variable bandwidth-amplifying function. Accordingly, it 
can work also as a band pass filter variable in frequency bandwidth and 
center frequency. 
Though, in the embodiment as shown in FIG. 2, the electrode M.sub.1 is 
provided intermediate the input means 1 and the output means 2 and only 
the surface signal wave which is transmitted through the electrode M.sub.1 
and amplified is drawn out of the output means 2, the elastic surface wave 
amplifier in accordance with the present invention can essentially produce 
a reflected signal wave at a portion of the electrode M.sub.1 as well as 
the transmitted signal wave and utilize the reflected wave signal for 
amplification effect as the transmitted signal wave. Accordingly, if the 
output is disposed intermediate the input means 1 and the electrode 
M.sub.1, an electrical output can be drawn out by the reflected signal 
wave. In this case, however, it is required to employ a unidirectional 
elastic surface wave transducer as the output means. 
The semiconductor to be employed may be either p-type or n-type. The 
polarity of the d.c. power source for the bias voltage is chosen depending 
on the type selected, namely, p-type or n-type so as to produce a space 
charge layer at a surface portion of the semiconductor. 
The protective film I' on the surface of the semiconductor substrate S as 
illustrated in FIGS. 1 and 2 may be omitted if the piezoelectric film I is 
an insulator and there are not too many surface states between the 
protective film I' and the semiconductor substrate S. 
Further it is to be noted that, instead of applying a d.c. bias voltage to 
the electrode M.sub.1, a suitable impurity ion may be implanted into a 
superficial portion of the semiconductor substrate S in the vicinity of 
the protective film I' or the piezoelectric film I to produce a suitable 
space charge layer capacitance at the surface of the semiconductor 
substrate S. In this case, the capacity of the space charge layer 
capacitance may be controlled by selecting a kind or amount of ion to be 
implanted without externally applying a d.c. bias voltage. A power source 
for applying a d.c. bias voltage may also be omitted in the case where the 
space charge layer capacitance has a suitable capacity without external 
application of the bias voltage, according to the kind of an ion which 
happens to be mingled with the materials in the manufacturing process of 
the device, or an intrinsic property of the piezoelectric film employed. 
Although the electrode M.sub.2 is used as an ohmic electrode with respect 
to the semiconductor substrate S in the foregoing embodiments, if the 
specific resistance at a portion of the semiconductor substrate near the 
surface thereof on the side of the electrode M.sub.2 is low, a contact 
impedance at a high frequency between the semiconductor substrate and the 
electrode M.sub.2 is low and therefore the electrode for the ohmic contact 
is not required. In the present invention, the electrode M.sub.2 may 
generally be made of a low resistance material such as electrified paste, 
electrified rubber, etc. as well as a metal. 
In general, to vary the space charge layer capacitance at the surface of 
the semiconductor effectively by pumping, the specific resistance 
(resistivity) at the interfacial portion of the semiconductor is 
preferably large and therefore, a preferable range of the specific 
resistance is considered to be from several ohm centimeters to several 
hundred ohm centimeters. However, if bulk crystal having a large specific 
resistance is used as a semiconductor substrate, a large bulk resistance 
is put in the path of the pumping current and the pumping efficiency is 
undesirably lowered. To solve this problem, it is desirable to employ an 
epitaxial substrate having an epitaxial growth of a semiconductor layer 
having a large specific resistance on the bulk crystal having a small 
specific resistance. 
Further it is to be noted that parametric acoustic wave generation is 
effected only where the amplification gain of the parametric amplification 
operation is large enough and the pumping power is higher than a certain 
threshold value. In this case, a component having a frequency f/2, half of 
the pumping frequency f, is markedly amplified and constantly outputted 
among waves occurring at a portion corresponding to the pumping electrode 
M.sub.1 due to naturally existing thermal noise or other causes without 
application of an input of elastic surface wave signal. The threshold 
value of the pumping power is lowered as the nonlinear characteristic of 
the space charge layer capacitance at the semiconductor surface becomes 
stronger, as the length of the pumping electrode M.sub.1 in the direction 
of travelling of the elastic surface wave becomes longer or as the pumping 
frequency f (or the frequency f/2) becomes higher. In case there is a 
reflection of the elastic surface wave in the travelling direction of the 
elastic surface wave, the larger the reflection coefficient is, the lower 
the threshold value of the pumping power is. 
As the elastic surface wave amplifier in accordance with the present 
invention is so adapted that the surface wave signal may be amplified 
through the parametric amplification effect caused by supplying an a.c. 
pumping power from a high-frequency power source without causing a d.c. 
current to flow through the semiconductor, it has such an excellent effect 
that it can attain a continuous wave without Joule heat generation. 
Further, as the present invention utilizes the surface charge layer 
nonlinearity of the conductor obtained by causing a surface charge layer 
capacitance formed at the surface portion of the semiconductor substrate 
to vary, layer nonlinearity effect can be expected and accordingly the 
pumping power necessary to obtain a given gain can be reduced. 
Still further, as the present invention utilizes the parametric amplifying 
effect by the surface charge layer nonlinearity, undesirable noise 
generation due to a thermal noise derived from resistors etc. can well be 
eliminated and amplification can be accomplished with an excellent S/N 
ratio.