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DESCRIPTION
1. Technical Field
This invention relates to electrically tunable surface acoustic wave
reflector resonators, and more particularly to control of surface acoustic
wave resonator frequency by variation of the carrier concentration in
piezoelectric and semiconductive substrates of resonator reflector element
arrays.
2. Background Art
Surface acoustic wave (SAW) devices are used as principal frequency control
elements in oscillator circuits for a variety of purposes. In some cases,
the purpose is simply to provide a small, competent device capable of
being controlled so that its parameters remain essentially constant within
a desired tolerance, to provide oscillator frequency control to a desired
degree of accuracy. In other cases, parameters of the SAW device may be
altered by a strain or other phenomenon, such as in response to force,
pressure, temperature and the like, to thereby provide a phenomenon
transfer which is compatible with related frequency-responsive or digital
circuitry. In some cases, it is desirable to provide voltage tuning of the
SAW resonator in order to change a mode, to provide temperature stability,
or for trimming purposes. A voltage tunable surface acoustic wave
reflective resonator is described in Cross, P. S. et al, Electronically
Variable Surface-Acoustic-Wave Velocity and Tunable SAW Resonators,
Applied Physics Letters, Vol. 28, No. 1, Jan. 1976, pp 1-3. The device
described therein has a plurality of reflector elements disposed in arrays
on either side of a tunable SAW delay line which has input and output
acoustoelectric transducers spanning an interaction region that includes a
tuning transducer. The substrate of that device is lithium niobate, which
has a relatively high electromechanical coupling constant, yielding a
theoretical maximum tuning range of 4.5%. However, the achievable tuning
range is limited by the ratio of the length of the tuning transducer along
the direction of wave propagation to the effective propagation length
within elements of the reflector arrays. Thus, achievable tuning ranges
are on the order of 1.4%.
The problem with the voltage tunable, lithium niobate SAW resonator is that
it is impossible to provide such a resonator in an oscillator
configuration integrally formed therewith in a monolithic fashion because
the lithium niobate does not have semiconductive properties. Provision of
voltage tunable devices capable of implementation on semiconductive
substrates is disclosed in my commonly owned, copending U.S. patent
application Ser. No. 11,612, filed on Feb. 12, 1979 and entitled CARRIER
CONCENTRATION CONTROLLED SURFACE ACOUSTIC WAVE VARIABLE DELAY DEVICES, now
U.S. Pat. No. 4,233,573. In said patent, the entirety of which is
incorporated herein by reference, a voltage tunable delay line includes a
segmented rectifying contact on a SAW delay line which may either be
disposed on a substrate of semiconductive and piezoelectric material, or
on a nonpiezoelectric semiconductive material having a piezoelectric
surface layer thereon. Application of voltage to the segmented electrode
to enhance carrier concentration in a semiconducting semiconductive
material or to deplete carrier concentration in a semi-insulating
semiconductive material alters the acoustic velocity and thus the
frequency of the device. As described in said patent, such device may be
used as the tuning element disposed between the reflectors of a resonator
of the type disclosed in Cross et al, supra. However, because of the much
lower eletromechanical coupling constant of a semiconductive piezoelectric
material, or of a thin piezoelectric layer, the tuning range of such
devices may be limited to a fraction of a percent. And, with the further
limitation pointed out by Cross et al, supra, that the overall tuning
range is limited by the fraction of cavity length that the tuning element
occupies, a SAW resonator employing voltage tuning of said patent may be
further limited by that fractional cavity length, which may not be
adequate in many applications. Furthermore, the larger the spacing between
the input and output transducers, the more modes which may be supported
within the tunable resonator. Therefore, to reduce the number of modes
which are supportable, the physical extent of the tuning element between
the input and output transducers should be as small as possible.
DISCLOSURE OF INVENTION
Objects of the invention include provision of a carrier concentration
controlled SAW resonator with improved tuning range, capable of
integration with semiconductor circuits.
According to the present invention, an electrically controlled surface
acoustic wave reflector resonator includes a piezoelectric and
semiconductive substrate having two sets of segmented rectifying contacts
disposed on opposite sides of a transducer region, each segmented contact
serving as a reflector array, and a variable voltage applied to each
segmented contact to control carrier concentration within the substrate,
thereby to control the velocity of the SAW wave beneath the reflector
elements, and therefore the resonant frequency of the resonator. According
further to the present invention, the transducer region may comprise a
pair of transducers which are acoustically coupled to each other for use
as a series tuning element at which the frequency of lowest insertion loss
is the tuning phenomenon, or having a single transducer coupled directly
to the elements of an oscillator (such as in the traditional Pierce,
Colpitts or Clapp configurations), such that the oscillation is at a
frequency of correct phase of feedback provided through the single
transducer disposed between the reflector elements of the resonator.
The invention may take a variety of forms as described in the
aforementioned patent, and is readily implemented utilizing known
microcircuit processing techniques, in the light of the teachings which
follow herein. Other objects, features and advantages of the present
invention will become more apparent in the light of the following detailed
description of exemplary embodiments thereof, as illustrated in the
accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a simplified schematic diagram of a tunable resonator known to
the prior art;
FIG. 2 is a simplified diagram of a tunable resonator in accordance with
the present invention having acoustically coupled input and output ports;
FIG. 3 is a simplified schematic diagram of a tunable resonator in
accordance with the invention having but a single transducer;
FIG. 4 is a partial plan view of tunable reflector elements in accordance
with the present invention; and
FIG. 5 is a partial, sectioned side elevation view of the portion disclosed
in FIG. 4.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to FIG. 1, a tunable resonator of the general type described
by Cross et al, as modified to use carrier concentration control of the
type described in my aforementioned patent, includes a substrate 40 having
two arrays of reflector elements 6, 7 disposed thereon on opposite sides
of an interaction region which includes input and output interdigital
transducers 44, 42 and a segmented rectifying contact 35, the fingers of
the contact being tilted slightly, if desired, to reduce reflections. A
tuning voltage is applied by a variable tuning voltage source 37 and a
nearby, ohmic ground contact 55. The input transducer 44 is connected to
the output of an amplifier 62 and the output transducer 42 is connected to
the input of the amplifier 62. Therefore, the amplifier 62 is provided
with a resonant, acoustic feedback path between the transducers 44, 42
which includes the tuning region between these transducers, in that
portion of the substrate subsisting the segmented contact 35. As described
in my aforementioned patent, variation of the voltage beneath the contact
35 will vary the carrier concentration in the region between the
transducers 44, 42, thereby adjusting the acoustic velocity in a different
manner, but for the same purpose as described in Cross et al. The
semiconductive piezoelectric substrate 40 may either comprise a material
which is both semiconductive and piezoelectric such as gallium arsenide,
or it may comprise a non-semiconductive piezoelectric film (such as zinc
oxide) on a semiconductive substrate (such as silicon). Depending upon the
substrate, the variable tuning voltage of the source 37 may be selected so
as to control depletion of carriers or enhancement of carrier
concentration in the semiconductive material of the substrate 40, so as to
selectively control the semiconducting/semi-insulating properites thereof,
thereby controlling the velocity of the acoustic wave and thus the
frequency of the oscillator formed by the amplifier 62 with its
acoustoelectric feedback. As is known, the output may be taken at output
terminals 76.
The reflector elements 6, 7 may comprise deposited aluminum, each element
having a length (in the propagation direction) of a quarter wavelength,
each element being spaced one-quarter wavelength from the next. Because
the depletion depth of interest is on the order of a wavelength, the
fingers of the segmented contact 35 may be separated by on the order of a
wavelength or so, to ensure continuity of enhancement or depletion in the
successive regions associated with each segment, but may be other than an
odd number of wavelengths apart so as to avoid presenting apparent short
circuits which could tend to reduce the difference between the potential
in a substrate with the substrate nonconducting and the potential in a
substrate with the substrate conducting. Although shown dirctly on the
substrate 40, the tuning voltage source 37 may more commonly be formed
separately; on the other hand, the invention, employing a semiconductive
substrate, is readily adapted for monolithic integration with an amplifier
62, directly on the substrate 40, as shown in FIG. 1 and as further
described in my aforementioned patent.
As described briefly hereinbefore, the problem with the configuration of
FIG. 1 is that there must be a significant length of the tuning region
beneath the contact 35 along the direction of acoustic wave propagation in
order to have a significant tuning capability. And, this is particularly
true where the particular piezoelectric semiconducting substrate 40 has a
relatively low electromechanical coupling constant, such as gallium
arsenide. This causes the transducers 42, 44 to be separated to such an
extent that a large number of modes may be sustainable. These modes may
include, typically, spurious side lobes, or other closely spaced cavity
modes.
Referring now to FIG. 2, a piezoelectric semiconductive substrate, which
may take any of the forms described in my aforementioned patent, has a
plurality of reflector elements 10, 11, formed thereon, which elements
comprise rectifying junctions, such as Schottky barriers formed by
depositing aluminum on the substrate 9. Associated with each of the
reflector arrays 10, 11 is an ohmic ground contact 12, 13 to permit
impressing a voltage, from a variable tuning voltage source 15, between
the reflector elements 10, 11 and the ground contacts 12, 13, thereby to
control the carrier concentration in a semiconducting or semi-insulating
region of the substrate 9. An input interdigital transducer 16 and an
output interdigital transducer 17 provide acoustically coupled feedback to
an amplifier 18, so as to form an oscillator, the output of which may be
taken at output terminals 19. The transducers may be formed in various
ways as set forth in my aforementioned patent. According to the invention,
the segments 10, 11 are not tilted, but rather are perfectly normal to the
direction of wave propagation, thereby to provide reflective arrays on
either side of the transducer region which includes the input and output
transducers 16, 17. The design of the reflector elements 10, 11 and of the
transducers 16, 17 may be in accordance with teachings well known in the
art for surface acoustic wave reflector resonators. The difference in
accordance with the invention is that tuning is achieved by adjusting the
velocity throughout the acoustic region of the reflective arrays 10, 11 by
carrier control concentration of the type described in my aforementioned
patent. This permits having the transducers 16, 17 located immediately
adjacent one another, as close as a quarter of a wavelength from each
other, if desired. In the invention, the dimensions and spacing of the
reflector elements in each of the arrays 10, 11 are chosen for suitable
matching with the substrate for the desired design frequency, in contrast
with the segmented electrode 35 of my copending patent, in which the
segmented electrode is designed to provide minimal surface shorting while
at the same time providing sufficient carrier concentration control. And,
the arrays 10, 11 may be disposed immediately adjacent the transducers 16,
17 in accordance with well known design criteria, so that a substantial
portion of the length of the cavity formed by the resonators is tunable by
carrier concentration control. This provides maximum tuning, even though
the preferred semiconductive piezoelectric substrate which permits
integrated circuit fabrication is utilized, which has a lower inherent
tuning range capability than other materials such as lithium niobate.
In FIG. 2, the oscillator formed by the amplifier 18 and the transducer 16,
17 has its frequency controlled by an acoustic wave having a corresponding
velocity at which there is minimum insertion loss in the acoustic coupling
between the transducers 16, 17 in the feedback path of the oscillator. The
embodiment of FIG. 3 is identical to that of FIG. 2 except that a single
acoustoelectric interdigital transducer 20 is utilized to couple the
resonator to an oscillator circuit 21, the resonator 20 being connected
between any of the elements of the active device of the oscillator 21
(such as base to collector, collector to emitter, emitter to base, of a
bipolar transistor) so as to form a Colpitts, Pierce or Clapp type of
oscillator in a fashion which is well known in the art. In this case, the
resonator has an acoustic wave launched therein at the frequency of
oscillation of the oscillator 21, which quickly stabilizes at a frequency
which has a correct phase in dependence upon the construction and tuning
of the resonator.
In the embodiments of FIGS. 2 and 3, it is to be noted that the reflector
elements are connected together so as to permit application of the tuning
voltage thereto. It has been found that this does not materially affect
the operating characteristics of a properly designed and built resonator,
only a minor reduction in the resonant quality factor (Q) resulting
therefrom. Also, it is immaterial whether the ohmic ground contacts 12, 13
are provided on the same side of the arrays 10, 11 as the connection of
the reflector elements in each array (as in FIG. 2) or if the ohmic
contacts 12, 13 are provided on the unconnected sides of the reflector
elements (as in FIG. 3). On the other hand, the ohmic contacts 12, 13 may
be made to the underside of the substrate if the substrate is
semiconducting throughout. For instance, if a conducting n-plus-type GaAs
substrate with an n-type epitaxial layer is used, the ohmic contact may
preferably be made to the underside surface of the n-plus-type material.
Other contact locations and methods may be used in various embodiments.
Referring briefly to FIG. 4, the details of a portion of the array 11 and
ground contact 13 are shown. And in FIG. 5 a typical illustration of
operation, similar to that disclosed in FIG. 6 of the aforementioned
patent, is given for the case of an n-type gallium arsenide epitaxial
substrate in which the tuning voltage applied to the elements of the
reflector array 11 causes depletion of carriers, shown by the dotted line,
in dependence on the magnitude of the voltage.
The present invention permits voltage tuning of a SAW reflective resonator
formed on a semiconductive substrate, and thus compatible with the
formation of integrated circuits directly on the same substrate therewith.
The invention may be practiced in all of the variety of forms described in
my aforementioned patent, insofar as formation of the reflector elements
and the types of substrates are involved, as well as the depletion and
enhancement modes described therein. However, the rectifying contacts in
all cases in the present invention are preferably formed by deposition of
aluminum, or other suitable metal, in reflector arrays 10, 11 as described
hereinbefore.
Although the invention has been shown and described with respect to
exemplary embodiments thereof, it should be understood by those skilled in
the art that the foregoing and various other changes, omissions and
additions in the form and detail thereof may be made therein and thereto,
without departing from the spirit and the scope of the invention.
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