US3462617A

US3462617A - Current function generator

Info

Publication number: US3462617A
Application number: US610638A
Authority: US
Inventors: Masakazu Shoji
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1967-01-20
Filing date: 1967-01-20
Publication date: 1969-08-19
Anticipated expiration: 1986-08-19

Description

Aug. 19, 1969 MASAKAZU SHOJI 7 3,462,617

CURRENT FUNCTION GENERATOR Filed Jan. 20, 1967 2 Sheets-Sheet 1 uvvewron M. SHOJ/ ATTO NEV Aug,- 1969- MASAKAZU SHOJI I 3,462,617

:CURRENT FUNCTION GENERATOR 2 Sheets-Sheet 2 Filed Jan. 20, 1967 FIG. 7

FIG. 6

United States Patent Office 3,462,617 Patented Aug. 19, 1969 U.S. Cl. 307-460 7 Claims ABSTRACT OF THE DISCLOSURE Two-valley semiconductor devices exhibiting a traveling high field domain upon application of a bias voltage have a current characteristic that varies with doping level and/or cross-sectional area. Specific current waveforms are produced by variations of these parameters along the length of the device.

BACKGROUND OF THE INVENTION This invention relates to current function generators and, more particularly, to such devices utilizing the Gunn effect.

Current functions, that is, currents of specialized or particular waveforms, are of utility in a number of applications such as, for example, analog computers, logic circuits, test equipment, and as current sources for display devices. In many applications, high frequency or high speed is required of the function generator. Desired waveforms and speed may be obtained by relatively com-.

plex circuitry involving the use of numerous passive and active devices such as transistors and tunnel diodes. Obviously, a reduction in circuit complexity and the number of elements used without sacrificing either speed or fidelity of waveform are highly desirable.

SUMMARY OF THE INVENTION The present invention is a high speed current function generator that utilizes the Gunn effect to achieve high speed of operation and extreme simplicity. It is based upon the discovery that a two-valley semiconductor, i.e., Gunn effect, device, such as a properly doped gallium arsenide crystal, produces an output current that is substantially an exact replica of the geometric shape of the crystal.

In a Gunn effect device, upon application of suflicient voltage, a narrow high-field domain is nucleated at the cathode and travels toward the anode with a substantially constant velocity and current density. During the transit of this domain from cathode to anode, the instantaneous current flowing through the device is given by the product of the current density and the cross-sectional area of the device at the location of the domains. As a consequence, the output current waveform depend upon the crosssectional area of the device and is substantially a replica thereof. As will be discussed more fully hereinafter, the current Waveform can also be made to follow variations in doping concentration, which variations produce changes in the current density during passage of the domain through the sample. Because of the dependence of current waveform upon the geometry or doping concentration, it is possible to produce a variety of Waveforms that cannot be produced by the switching actions which typify many of the prior art devices.

The present invention will be more readily understood from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of one illustrative embodiment of the invention;

FIG. 2 depicts the current waveform produced in the arrangement of FIG. 1;

FIG. 3 is a diagram of another illustrative embodiment of the invention;

FIG. 4 depicts the current waveform of the arrangement of FIGURE 3;

FIGS. 5 and 6, respectively, depict a particular crystal shape and the resulting waveform; and

FIGS. 7 and 8, respectively, depict still another crystal shape and the resulting waveform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is shown an embodiment of the invention for generating a triangular current waveform. The arrangement of FIG. 1 comprises a crystal 11 of gallium arsenide or other suitable Gunn effect type material. In the case of gallium arsenide a suitable dopant such as oxygen may be used to provide sufficient carriers for achieving Gunn oscillations. Crystal 11 has a thickness s, a length L, and a height b which varies between b and b as shown. At one end (x=o) of crystal 11 is a first contact 12 of suitable material such as, for example, indium,.and another such contact 13 is at the other end (x=L) of crystal 11. Connected in series between

contacts

12 and 13 is a suitable voltage source 14 which may take any of a number of suitable forms and an output or load resistor 16 at the ends of which are

output contacts

17 and 18. Load resistor 16 is intended to represent any of a number of possible broadband loads or outputs which have sufficient bandwidth capacity to pass the current waveform produced without degradation.

In operation, upon application of a voltage from source 14 in excess of the Gunn effect threshold, a high field domain is nucleated adjacent contact 12, i.e., the cathode. As is known, where crystal 11 is uniformly doped, this domain travels toward contact 13, i.e., the anode, with a substantially constant velocity v and substantially constant current density I as long as the applied voltage is sufficient to produce a domain sustaining field E The current density I is given by J 87love With the thickness s held constant, the instantaneous current varies directly with the height b for a uniformly doped crystal. Thus the current waveform is 'a replica of the shape of crystal 11. FIG. 2 depicts the current waveford produced by the arrangement of FIG. 1. For the waveshape of FIG. 2, it is necessary that the bias voltage from source 14 be sufliciently large to sustain the traveling domain throughout the length of the sample. If the voltage is insufficient to sustain the domain at b the domain collapses and a sawtooth waveform results instead of that shown in FIG. 2. Where the voltage is sufficient to sustain the domain throughout the length L, a current spike occurs when the domain arrives at the anode, i.e., contact 13. This spike is shown in dashed lines in FIG. 2. This current spike may be eliminated when necessary.

For a waveform as shown in FIG. 2, for a fundamental frequency of, for example, megacycles, the circuitry associated with the device of FIG. 1 should be sufficiently broadband to support the tenth harmonic, i.e., 1,000 megacycles. This is in contrast to other Gunn effect devices where the associated circuitry is narrowband, passing primarily the single frequency of interest. In general, the load circuitry to which the output of a function generator is applied is sufficiently broadband to sustain the current waveform without degradation.

In FIG. 3 there is shown a variation of the arrangement of FIG. 1 which produces the waveform shown in FIG. 4. Crystal 21, as can be seen, differs from crystal 11 of FIG. 1 only in the variations in height. In all other respects the arrangements are the same, hence the same reference numerals have been used. The bandwidth requirements for the function generator of FIG. 3 and waveform of FIG. 4 differ somewhat from those of FIGS. 1 and 2. For each different waveform the bandwidth requirements must be determined on the basis of passing the highest harmonic necessary to maintain the particular waveform. This determination is, in every case, within the capabilities of those skilled in the art.

FIGS. 5 through 8 show other examples of crystal shapes 31 and 41 and their associated waveforms. It can be seen that many possible waveforms are possible through application of the principles of the present invention.

As was pointed out before, and as can be seen from Equations 1 and 2, changes in doping which produce changes in J can be used to accomplish the same ends. The particular crystal shapes shown in FIGS. 1, 3, 5, and 7 would then represent the doping profile rather than changes in the b dimension. Techniques for producing variations in doping levels are known to workers in the art.

In one embodiment of the invention, crystal samples were made from n-type oxygen doped GaAs having an electron concentration of 4 to 7 10 /cm. The crystals were cut into 40 x 40 mil squares from to mils thick and ground to the desired shape. The shaped samples were cleaned, etched, and alloyed with pure indium contacts. In operation the crystals were biased by a 50 nanosecond pulse train of a mercury relay pulse having a repetition frequency of 120 Hz. The current waveforms through a 4-ohm monitoring resistor were observed on a sampling oscilloscope.

From the foregoing, it is readily apparent that a wide variety of waveforms may be generated utilizing the principles of the present invention. All that is necessary is a single active element, i.e., a Gunn efiect crystal, cut to the desired shape, and associated circuitry having sufficient bandwidth to pass the desired waveform. This is in contradistinction to high speed function generators of the prior art, which usually consists of complex combinations of both active and passive elements.

Numerous arrangement utilizing the principles of the present invention may occur to workers in the art without departure from the spirit and scope of the present invention.

What is claimed is:

1. Apparatus comprising:

a semiconductor wafer of the type that is capable of forming and propagating traveling electric field domains in response to the application of a sufficient bias voltage;

cathode and anode contacts connected to the wafer;

means for applying sufiicient voltage to the contacts to cause traveling domains to propagate through the wafer, the velocity of propagation being a substantially constant velocity v said apparatus being characterized by means for generating current waveforms each of which waveforms has an amplitude I that varies significantly throughout a substantial portion of its period as a predetermined function of time t;

said waveform generating means comprising substantial variations in the product n A with distance throughout a substantial portion of the region through which the domain propagates, wherein n is the doping concentration, A is the cross-sectional area of the device and v t is approximately the distance in the region of domain propagation from the point where the domain is formed;

I(t) being a linear function of the product n A;

said apparatus being connected to a circuit of suflicient bandwidth to maintain the waveform of the current through said device.

2. A two-valley semiconductor device as claimed in claim 1 wherein the doping level is substantially uniform along the length of the device and the cross-sectional area of said device varies along the length thereof.

3. A two-valley semiconductor device as claimed in claim 2 wherein one dimension only of said device varies along the length thereof.

4. A two-valley semiconductor device as claimed in claim 1 wherein the cross-sectional area of said device is substantially constant along the length thereof and the doping level varies.

5. The apparatus of claim 1 wherein the current waveforms that are generated are sinusoidal and the variation in the product n A is sinusoidal.

6. The apparatus of claim 1 wherein the current waveforms are triangular and the product n A increases linearly to an intermediate point in the region through which the domain propagates and decreases linearly from thereon.

7. The apparatus of claim 1 wherein the current waveforms are sawtoothed in shape and the product n A increases linearly throughout the -region through which the domain propagates.

References Cited UNITED STATES PATENTS 8/1956 Shockley. 1/1968 Gunn.

OTHER REFERENCES JOHN W. HUCKERT, Primary Examiner JERRY D. CRAIG, Assistant Examiner U.S. Cl. X.R.