US3469209A

US3469209A - Microwave bulk-effect negative-resistance device having a high perimeter to area ratio

Info

Publication number: US3469209A
Application number: US650540A
Authority: US
Inventors: Charles H Mosher
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-06-30
Filing date: 1967-06-30
Publication date: 1969-09-23
Anticipated expiration: 1986-09-23
Also published as: DE1766643A1; FR1571163A; GB1226387A

Description

34 v v 4 33 INVENTOR. X I CHARLESHMQSHE-R Sept. 23, 1969 c. H. MOSHER 3,469,209

MICROWAVE BULK-EFFECT NEGATIVE RESISTANCE DEVICE HAVING A HIGH PERIMETER TO AREA RATTO Filed June 50, 1967 FIG. I

PRIOR ART ACTIVE REGION F ILL FIG.II

United States Patent US. Cl. 331-107 Claims ABSTRACT OF THE DISCLOSURE A microwave bulk-effect negative-resistance semicond uctive device is disclosed. The bulk-effect semiconductive device is formed and arranged as a ring of semiconductive material or as a ring-shaped array of semiconductwo segments. The ring-shape serves to remove semiconductive material from the central region of the device where the microwave current density is low due to skin effect. In this manner, the current density is nearly constant across the device to limit transverse domain propagation which can cause burnout. The radial thickness of the ring or segments of the ring-shaped device is preferably less than a skin depth at the operating frequency of the device and the circumference is preferably less than a half wavelength to prevent unwanted modes of oscillation. The ring-shape increases the power handling capa'bilities of transit-time-mode Gunn-effect devices and improves the efficiency of Gunn devices operated in the limited space charge accumulation mode.

DESCRIPTION OF THE PRIOR ART Heretofore, two bulk-effect negative-resistance semiconductive devices have been operated in parallel to obtain higher power output at L-band. Such devices are described in an article titled, High-Peak-Power Gallium Arsenide Oscillators, appearing in the IEEE Transactions on Electron Devices, vol. ED-l3, No. 1, pages 105-110, January 1966. These devices provided 205 watts peak power at 1540 mHz. The devices were each about 1 square millimeter in cross-sectional area, about 100 microns thick, and had a resistance of 0.89. It turns out that the devices cannot be operated without failure above a certain voltage which is about three times the threshold bias voltage, -i.e., the bias voltage at which microwave oscillations will start. Also, the DC current for a given device remains approximately constant with increased bias voltage above threshold voltage. Thus, the obvious way to increase the output power is to decrease the resistance of the devices such that, at their operating voltage, they conduct more DC current. Given a certain resistivity and thickness of the semiconductive material, the current is increased by increasing the cross-sectional area of the semiconductive device. When the cross-sectional area of the semiconductive device is increased above a certain value, as of 1-2 square millimeters at L-band to provide a low field DC resistance of less than 0.59, it was found that the devices would burn out.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved bulk-effect negative-resistance microwave semiconductive device.

One feature of the present invention is the provision of a bulk-effect negative-resistance semiconductive device wherein the active semiconductive material is hollow in the central region and confined to a region near the perimeter of the device as defined by the highest density microwave magnetic field lines circumscribing the device in operation whereby unwanted burn-outs of the semi- ICC conductive device are minimized while providing low field device resistances less than 0.59 for high power operation.

Another feature of the present invention is the same as the preceding feature wherein the radial width of the semiconductive material is less than 1.5 skin depths.

Another feature of the present invention is the same as any one or more of the preceding features wherein the semiconductive device is formed by an array of smaller semiconductive elements disposed in the region near the perimeter of the composite device, whereby each element of the array can be individually selected and tested for improving the probability of having a composite device which is free of semiconductive defects.

Another feature of the present invention is the same as the preceding feature wherein the individual semiconductive elements are shaped in the form of a segment of a ring with rounded corners, whereby a composite ringshaped array of elements is obtained without sharp corners which would otherwise tend ot promote burnout of the device at high power levels.

Another feature of the present invention is the same as any one or more of the preceding features wherein the semiconductive material has a cross sectional area less than one half of the area bounded by the perimeter defined by the highest density of enclosing microwave magnetic field lines.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of DC current I versus DC bias voltage V for a bulk-effect negative-resistance semiconductive device incorporating features of the present invention,

FIG. 2 is a perspective view of a semiconductive wafer of the prior art and depicting the burnout region,

FIG. 3 is a plot of instantaneous current density J, as a high electric field domain is forming versus radius for the structure of FIG. 2,

FIG. 4 is a perspective view of a semiconductive device incorporating features of the present invention,

FIG. 5 is a plan view of an alternative device of the present invention,

FIGS. 6 and 7 are plan views of alternative devices of the present invention,

FIG. 8 is a longitudinal sectional view of a composite semiconductive device of the present invention,

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line 9-9 in the direction of the arrows,

FIG. 10 is a schematic circuit diagram of a high power microwave oscillator employing a device of the present invention, and

FIG. 11 is a sectional view of a device similar to that of FIG. 4 depicting an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a typical plot of DC current I versus DC bias voltage V as applied across terminals of a bulk-effect negative-resistance semiconductive device. It is found that with increased bias voltage the DC current increases until a certain threshold value of voltage V is reached. At the threshold value of voltage, the device will interact with microwave fields of a circuit to produce microwave oscillations and to convert DC power into microwave power. Such bulk-effect devices include Gunn devices operable in any one of a number of modes. The bulk-effect conversion process is due predominately to the properties of the bulk semiconductive material as contrasted with other types of negative-resistance devices which rely primarily upon the properties of a p-n junction for the power conversion process. Typical of such junction devices is the tunnel diode. The advantage of bulk-effect negative-resistance devices is that they are capable of operating to much higher power levels because the power is dissipated in the three dimensional bulk of the semiconductive material as compared with power dissipation in a thin p-n junction region.

Above the threshold voltage V the DC current remains nearly constant with increasing voltage V. Thus, the power output may be increased by increasing the bias voltage. However, the bias voltage should not be increased above about 3V as this will shorten operating life. This, then, sets the upper limit on power output for a given device and it is seen that the power output can then only be increased by increasing the DC current which can be done by lowering the DC low field resistance, i.e., resistance below threshold V For a semiconductive material having a given resistivity and thickness, the resistance can only be decreased by increasing the cross-sectional area of the device. However, it was discovered that when the resistance was lowered below 0.582 that the devices failed by burnouts which occurred most frequently in a ring-like region 1 of the device 2, as shown in FIG. 2.

It is believed that the bumouts occur in this region because the instantaneous current density J is not uniform across the bulk material. More particularly, due to skin effect, when the device 2 has a diameter of about 2 skin depths or more, as shown in FIG. 3, the current density I will be less near the central region of the device 2 as compared with the current density J at the perimeter of the device. The disturbance which will become the high field domain will tend to propagate transversely toward the central region of the device as indicated by the peaked inwardly directed wavefronts A sketched on the diagram of FIG. 3. The result is that excessive electric fields and current densities are produced resulting in a breakdown of the material with a resultant burnout. The adverse effects of transverse propagation of disturbances have been observed for the operation in the various domain modes. It has not been observed, to date, in the (LSA) mode of operation.

Referring now to FIG. 4, there is shown a bulk-effect negative-resistance semiconductive device of the present invention. The device 3 is essentially the same as the prior art large area Gunn effect devices except the central region of bulk semiconductive member has been removed to place the active area of the device near its perimeter as contrasted with a solid cross-sectional area device 2, as shown in FIG. 2. The area near the perimeter can be made sufiiciently large to produce a low resistance device 3, i.e., resistance less than 0.59. The device will have an effectively uniform current density J across the ring of semiconductive material 4 since its radial thickness is selected to be less than 1.5 skin depths.

The parameter of interest is that the semiconductive material should be confined to an area of the device near its perimeter as defined by the loop of most intense microwave magnetic field circumscribing the device. For a circular disk of semiconductive material, the loop of most intense microwave magnetic field which circumscribes the semiconductive structure coincides with the perimeter of the disk. In other geometries, the loop defined perimeter is more difficult to determine. To obtain a substantial improvement over a solid disk of semiconductive material, at least half of the cross-sectional area bounded by the loop defined perimeter of the semiconductive structure should be removed. Thus, for a circular cross-section semiconductive structure a ring, as shown in FIG. 4, offers the optimum geometry. The ratio of areas bounded to that remaining at the perimeter can be reduced to the expression:

where P is the perimeter of the disk, and A is the area occupied by the ring of semiconductive material 4. If the disk were solid, as in FIG. 2, the ratio of Eq. 1 would be unity. Thus, for a circular ring-shaped semiconductive structure 4 of the present invention the ratio of Eq. 1 should be at least 2 and preferably as high as possible consistent with a preferred operating condition that the perimeter of the structure be less than one half an electrical wavelength long at the operating frequency in the semiconductive material in order to avoid possible circumferential modes of oscillation in the device 3.

A suitable semiconductive material for the ring 4 includes n-type GaAs having no dislocations or deep donors in the crystal lattice and having a positive temperature coelficient of resistivity.

A pair of

annular electrodes

5 and 6, as of nickel-tin, are alloyed to the ends of the semiconductive material 4 in the manner as described in an article titled, Microwave Phenomena in Bulk GaAs, IEEE Transactions on Electron Devices, vol. ED-13, No. 1, at pages 94-105, January 1966.

The axial thickness of the semiconductive body 4 is determined by its intended mode of operation. In a transit time mode, this thickness is about 3-4 mils at L-band and about 0.5 mil at X-band.

Referring now to FIG. 5, there is shown, in plan view, an alternative rectangular ring-shaped semiconductive device 8 wherein the area of the semiconductive structure is less than one half the area bounded by the perimeter defined by the loop of highest intensity microwave magnetic field H. Roughly, the highest intensity magnetic field is at the perimeter of the device 8. The central area, which is removed, is at least equal to the remaining area of the ring and preferably much greater than the area of the ring.

Referring now to FIGS. 6 and 7, there are shown alternative embodiments of the present invention. In these embodiments, the semicondutive devices 13 and 14, respectively, are made up of a number of segments 15 of bulk-effect negative-resistance semiconductive material arranged in and confined to a region near the perimeter defined by the most intense closed loop of microwave magnetic field H.

The segments 15 are preferably rounded on the corners to prevent concentration of circumferential magnetic fields and, thus, axial microwave currents. The individual segments 15 each include their own electrodes for applying DC and microwave potentials. The individual segments 15 are connected in parallel to form the composite devices 13 and 14, respectively.

One advantage of the segmented geometries of FIGS. 6 and 7 is that the individual semiconductive segments 15 can be separately tested and selected for proper operating characteristics. In the single ring-shaped semiconductive elements of FIGS. 4 and 5, it is often difficult to grow a single perfect crystal of sufficient size to permit cutting out the relatively large rings of FIGS. 4 and 5. On the other hand, relatively small segments 15 of perfect material are more easily produced. Moreover, with the segmented ring-shapes of FIGS. 6 and 7, transverse current domain propagation, as indicated in FIG. 3, is impeded by the breaks in the semiconductive material.

Also, as indicated in the embodiment of FIG. 7, the segments 15 need not form a complete circle but need only form a generally C-shaped array or structure. Such a C-shaped semiconductive structure will define a sufficiently large hollow interior region. The intense loop of microwave magnetic field as indicated by the dotted line H of FIG. 7, should not dip too far into the central region of the structure as this will substantially decrease the area enclosed by the intense magnetic field and reduce the ratio of bounded area to actual area of the device.

Referring now to FIGS. 8 and 9, there is shown a packaged bulk-effect negative-resistance device made up of a plurality of individual segments 15 arranged in a circular array near the perimeter defined by the closed loop of most intense microwave magnetic field. The segments 15 are each about 3 to 4 mils thick and 40 mils on a side (40 mils square) and there are about 12 segments 15 in the array to form an L-band device 17. Each segment 15 has a pair of electrodes on opposite sides. The electrodes on one side are soldered to the fiat end of a stud 18 which is made of a good electrical and thermally conductive material such as Te-Cu to serve as one microwave terminal and as a heat sink.

An array of conductive tabs 19, as of gold, 40 mils in width and 1 mil thick and 80 mils in length interconnect the upper electrodes of the segments 15 with the metallized end of a ceramic cylinder 21. The cylinder 21 is brazed to a shoulder of the stud 18 and surrounds the array of segments 15. A kovar cap 22 is soldered at its lip 23 to the outer ends of the tabs 19 to form the other terminal of the device 17. The stud 18 is provided with external threads to permit it to be screwed into a microwave circuit.

Briefly, the circuit of FIG. comprises a pair of half wave open circuited resonant sections. of

transmission lines

24 and 25 contained in a conductive housing 26. Tuning capacitors 27 are provided at the open circuited ends of the

transmission lines

24 and 25 for tuning their resonant frequencies and for shifting the position of their microwave voltage nulls which typically occur centrally of their lengths.

The bulk-effect negative-resistance device 17 of FIGS. 8 and 9 is screwed through the housing 26 for connection across the resonant line 24 at a point near the voltage null point to provide a low impedance match to the device 17. An output coaxial line 31 has its center conductor 32 connected to the output resonant line 25 near its midpoint for impedance matching. A low impedance source of bias voltage 33, which may be pulsed is connected to the microwave voltage null point of the input resonant line 24 for reducing microwave coupling to the source 33 and for applying operating bias voltage across the bulk-effect device 17 A conductive septum 34 extends partially across the housing 26 to define an inductive coupling iris 35 between the two

resonant lines

24 and 25 for controlling the degree of coupling therebetween.

In operation, the microwave fields of the input resonator 24 interact with the charge carriers in the bulk-effect device 17 to produce a microwave output signal that is coupled to a load via output coaxial line 31. A peak power output of 500 watts at 1 gHz. has been produced by a device 17 having about a dozen segments arranged as shown in FIGS. 8 and 9 and operated in a Gunn-effect domain mode. A similar device 17 having the segments 15 dimensioned for operation in the limited space charge accumulation (LSA) mode should provide higher output power with increased efficiency as compared with operation in the LSA mode with a solid non-ring shaped device. Operation on the (LSA) mode is described in an article titled, A New Mode of Operation for Bulk Negative Resistance Oscillators, John A. Copeland, Proc. IEEE, vol. 54, #10, pp. 14791480, October 1966. The ring-shaped LSA mode device may not have advantages with respect to device failure but should provide increased efficiency due to skin depth consideration.

The gist of the present invention is to confine the active region of the bulk-effect negative-resistance device to a region near its perimeter. As thus far described, this has been accomplished, in practice, by removing the semiconductive material from all regions but the perimeter. As an alternative, where the dimensions of the device do not permit removal of the central region, one or both of the

electrodes

5 and 6 for applying the bias and microwave potentials may be removed from the central region, as shown in FIG. 11. Since the active region of the semiconductive device 4 is confined to regions which experience a bias voltage in excess of V the central region of the semiconductive material 4 will not be active.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A bulk-effect negative-resistance semiconductive microwave device including, means forming semiconductive structure having an active region exhibiting bulk-effect negative-resistance with the application of a certain bias voltage across said semiconductive structure, means forming a pair of electrode structures affixed to opposite sides of said semiconductive structure for applying the certain bias voltage and for providing a pair of microwave equipotential surfaces on opposite sides of said semiconductive structure to support a microwave potential and a bias potential thereacross for electromagnetic interaction with charge carriers in said bulk-eflFect structure, the improvement wherein, said semiconductive structure is formed and arranged such that the active semiconductive material is confined to a region of the structure which is near the perimeter thereof, and wherein the central region of said structure is free of said active semiconductive material.

2. The apparatus of claim 1 wherein said region located near the perimeter of said semiconductive structure and within which said active semiconductive material is confined has a radial thickness less than 1.5 skin depths for the frequency of the microwave potential to be interacted with the device.

3. The apparatus of claim 1 wherein said semiconductive structure includes a plurality of segments of active semiconductive material disposed in said region near the perimeter of said structure.

4. The apparatus of claim 1 wherein said semiconductive device has a low field DC resistance of less than 0.59.

5. The apparatus of claim 1 wherein said active semiconductive material is confined to a ring-shaped region of space.

6. The apparatus of claim 1 wherein said semiconductive structure is a segmented ring-shaped structure of semiconductive material.

7. The apparatus of claim 1 wherein the microwave current flowing through said semiconductive structure in operation produces a microwave magnetic field to fix a perimeter defined by the most intense closed loop of microwave magnetic field which encircles the device and wherein the area bounded by the perimeter defined by the microwave magnetic field is more than twice the area occupied by said active semiconductive structure.

8. The apparatus of claim 1 including an electrically conductive stud, and wherein one of said electrode structures which is affixed to said active semiconductive structure is bonded to the end of said stud for making an electrical connection thereto and for heat sinking the semiconductive device.

9. The apparatus of claim 8 wherein said semiconductive structure includes a circular array of semiconductive segments.

10. The apparatus of claim 8 wherein the perimeter of said semiconductive structure is less than one-half an electrical wavelength long at the operating microwave frequency of the bulk-effect device.

No references cited.

JOHN KOMINSKI, Primary Examiner U.S. Cl. X.R.