The present invention relates to Gunn diodes used for oscillation of microwaves and millimeter waves, and is especially related to Gunn diodes which realize improvements in thermal characteristics, yield factor of good products and easy assembly to planar circuits, fabricating methods thereof and structures for assembly of the same.
The present invention also relates to NRD guide Gunn oscillators that are comprised by combining a NRD guide (Non Radiative Dielectric Waveguide) circuit and Gunn diodes.
Gunn diodes for oscillation of microwaves or millimeter waves are usually comprised of compound semiconductors such as gallium arsenide (GaAs) or indium phosphide. It is the case with such compound semiconductors that the electron mobility is several thousands of cm2/Vxc2x7sec and thus large in a low electric field while the mobility is decreased in case a large electric field is applied since accelerated electrodes transit to a band of large effective mass and thus causes generation of negative differential mobility within the bulk. Consequently, a negative differential conductance is caused in the current-voltage characteristics and leads to thermodynamic instability. Therefore, a domain is generated which transits from the cathode side to the anode side. Repetition of this phenomenon results in vibrating current (oscillation).
The oscillating frequency of a Gunn diode is determined by the distance of transit of the domain. In case of Gunn diodes for millimeter waves, this distance of transit needs to be extremely short (1 to 2 xcexcm). In addition, the product of an impurity concentration and a distance of transit for the domain (active layer) needs to be set to be a specified value (e.g. 1xc3x971012/cm2) to obtain sufficient oscillating efficiency, while the impurity concentration of the active layer becomes rather high in high frequency zones like those of millimeter waves since the oscillating frequency is non-ambiguously determined by the thickness of the active layer. The current concentration during operation is determined by the product of the impurity concentration of the active layer and a saturation electron speed, and in zones of the millimeter waves, the temperature of the active layer is increased owing to the increase in current concentration, whereby the oscillating efficiency is decreased.
In order to solve such problems, measures had been taken with conventional Gunn diodes for millimeter waves such as employing a mesa-type arrangement to use elements including the active layer of extremely small sizes, having diameters of approximately several tens of xcexcm, and assembling the diodes within pill-type packages comprised with a heat portion made of diamond or similar material of favorable thermal conductivity which greatly affects oscillating efficiency on which the most important performance indices are dependent.
A sectional view of gallium arsenide Gunn diode element 100 of conventional mesa-type arrangement is shown in FIG. 29.
On to a semiconductor substrate 101 of high concentration n-type gallium arsenide, there are sequentially laminated, through MBE method, a first contact layer 102 of high concentration n-type gallium arsenide, an active layer 103 of low concentration n-type gallium arsenide, and a second contact layer 104 of high concentration n-type gallium arsenide, and it is employed a mesa-type arrangement in order to reduce the transit space for the electrons.
Thereafter, a rear surface of the semiconductor substrate 101 is laminated, a cathode electrode 105 is formed onto this rear surface of the semiconductor substrate 101 while an anode electrode 106 is formed on the surface of the second contact layer 104, and by performing element separation, the Gunn diode element is completed.
The Gunn diode element 100 thus obtained is built-in in a pill-type package 110 as shown in FIG. 30. This pill-type package 110 comprises a heat sink electrode 111 and a cylinder 112 of glass or ceramics that serves as an enclosure for enclosing the Gunn diode element 100, wherein the cylinder 112 is brazed by hard-soldering to the heat sink electrode 111. The Gunn diode element 100 is electro-statically attracted by a bonding tool of sapphire material or the like (not shown) and is adhered to the heat sink electrode 111.
Further, the Gunn diode element 100 and a metal layer provided at a tip of the cylinder 112 are connected by a gold ribbon 113 through thermo-compression bonding or the like. After connecting the gold ribbon 113, a lid-like metallic disk 114 is brazed onto the cylinder 112 to complete the building-in to the pill-type package 110.
An example of a structure for assembling the Gunn diode that has been built-in in the pill-type package 110 to a microstrip line 120 is shown in FIG. 31. One of the two electrodes 111, 114 of the pill-type package 110 is pierced through a hole formed in a flat insulating substrate 121 of e.g. alumina and is electrically connected to a ground electrode 122 formed on a rear surface of the flat insulating substrate 121, while the other one is connected by a gold ribbon 123 to a signal line 124 formed on the plate substrate 121 as a microstrip line.
NRD guide circuits are paid attention to as transmission lines for microwaves, especially of millimeter wave zones of not less than 30 GHz, since they present lower insertion losses than compared to microwave strip lines, and since manufacturing of transmission line is easier than compared to waveguides.
This NRD guide circuit is arranged in that a dielectric strip line, in which propagation of electromagnetic waves is performed, is pinched between two parallel plates of conductive metal. Since the opposing distance between the parallel plates is set to be not more than half of the free space wavelength of the used frequency, electromagnetic waves are intercepted and its radiation is restricted at portions other than the dielectric strip line, electromagnetic waves can be propagated with low losses along the dielectric strip line.
Oscillators arranged of such a NRD guide circuit and Gunn diodes of 35 GHz and 60 GHz zone have been developed which are capable of producing output power which are equivalent to those of waveguides.
FIG. 32(a) is a view showing an arrangement of a conventional NRD guide Gunn oscillator. This is arranged in which a mount 320 is provided in a space between parallel plates 201, 202, being mounted with a dielectric strip line 203 as well as Gunn diode 310. High frequency output oscillated by the Gunn diode 310 is derived to the dielectric strip line 203 via a resonator 330. FIG. 32(b) is a view showing a representative example of such resonator 330 comprised with a copper layer portion 331 patterned through etching a copper layer of a Teflon copper-clad laminate. By adjusting the width or length of the copper layer portion 331, the output frequency can be adjusted.
FIG. 33 is a view showing the arrangement of the mount 320. The Gunn diode 310 is set in a cylindrical portion 321, and bias voltage is applied thereto via a bias choke 340 connected to aside the cylindrical portion 321. The bias choke 340 is obtained by patterning through etching a Teflon copper-clad laminate and by hacking a portion of the laminated plate of the cylindrical portion 321 such that a copper layer portion remains to be connected to a lid for connecting portion 341. A cathode electrode of the Gunn diode 310 is connected onto a heat sink 322 of the mount 320. The heat releasing base 322 is insulated and separated from the lid 341 by a cylindrical ceramic 342, and the lid 341 is connected to an anode electrode of the Gunn diode 310 via a ribbon 343.
Conventional Gunn diode elements 100 (FIG. 30) are formed through chemical wet etching by employing a photoresist as an etching mask to obtain the above described mesa-type arrangement. However, since etching is progressed not only in the depth direction but also simultaneously in lateral directions in this etching method, it is presented a drawback during manufacture that control of the transit space of the electrons (active layer) is made very difficult, whereby ununiformity in electrical characteristics of Gunn diode element is caused.
It was also presented a drawback at the time of building-in the Gunn diode element in a pill-type package 110 that the bonding tool intercepted one""s field of view during adhesion of the Gunn diode element 100 to the heat sink electrode 111 so that the heat releasing sink 111 could not be directly viewed at. Consequently, the efficiency of building-in operation was quite poor.
Further, utilization of a gold ribbon 123 (FIG. 31) for assembling the pill-type package 110 incorporated with the Gunn diode element 100 to the microstrip line 120 arranged on the plate substrate 121 resulted in generation of parasitic inductance, whereby ununiformity in electrical characteristics was caused during the assembly.
Manufacture of the above described NRD guide Gunn oscillator is difficult since it employs a special mount 320, and the operating efficiency was very poor since the substrate needed to be hacked to expose the lid 341 of the bias choke 340.
Utilization of the ribbon 343 for connecting the anode electrode of the Gunn diode 310 to the lid 341 resulted in generation of parasitic inductance, whereby variations in characteristics were caused.
It is an object of the present invention to provide Gunn diodes, fabricating methods thereof and structures for assembling the same which solve the above described problems which are caused during, fabricating building-in and assembly.
It is another object of the present invention to provide a NRD guide Gunn oscillator free of the above described problems.
For this purpose, the Gunn diode according to the first invention is a Gunn diode which is formed by sequentially laminating a first semiconductor layer, an active layer and a second semiconductor layer onto a semiconductor substrate, comprising first and second electrodes arranged on the second semiconductor layer for impressing voltage on the active layer, and a concave portion which is cut from around the first electrode in a direction of the second semiconductor layer and the active layer and which subdivides the second semiconductor layer and the active layer to which the first electrode is connected as a region which functions as a Gunn diode.
The Gunn diode according to a second invention is so arranged that a conductive film is provided within the concave portion for shorting between the second electrode and the first semiconductor layer of the first invention.
The Gunn diode according to the third invention is so arranged that the first and second electrodes are formed of an underlying electrode layer and conductive protrusions successive to the underlying electrode layer such that their upper surfaces assume a substantially identical level height.
The Gunn diode according to the fourth invention is so arranged that the conductive protrusion of the first electrode is formed substantially in a central portion and in that the conductive protrusions of the second electrode are formed at both sides thereof in the first to third inventions.
The Gunn diode according to the fifth invention is so arranged that an area for the first electrode is set to be not more than {fraction (1/10)} of an area for the second electrode in the first to fourth inventions.
The Gunn diode according to the sixth invention is so arranged that there are provided at least two first electrodes and concave portions which have been cut from around the first electrodes in the first to fifth inventions.
The Gunn diode according to the seventh invention is so arranged that the semiconductor substrate, the first semiconductor layer, the active layer and the second semiconductor layer are formed of compound semiconductors such as gallium arsenide or indium phosphide in the first to sixth inventions.
The Gunn diode according to the eighth invention is so arranged that the second semiconductor layer and the active layer being successive to the second electrode are substituted as a single semiconductor layer or a conductive layer in the first to seventh inventions.
The Gunn diode according to the ninth invention is so arranged that a third electrode is formed on a rear surface of the semiconductor substrate, in that the third electrode and first electrode are used for impressing voltage on the active layer, and in that the second electrode is made to be for the spacers in the first to eighth inventions.
The fabricating method for a Gunn diode according to the tenth invention is so arranged that it comprises a first step of sequentially laminating and forming a first semiconductor layer which serves as a first contact layer, an active layer, and a second semiconductor layer which serves as a second contact layer onto a semiconductor substrate, a second step of forming first and second electrodes of specified shapes onto the second contact layer, and a third step of removing the second semiconductor layer and the active layer through dry etching wherein the first and second electrodes are used as masks.
The fabricating method for a Gunn diode according to the eleventh invention is so arranged that the second step includes a step of forming, after forming an underlying electrode layer for the first and second electrodes of specified shapes, conductive protrusions on the underlying electrode layer such that their heights are substantially identical with each other in the tenth invention.
The fabricating method for a Gunn diode according to the twelfth invention is so arranged that the semiconductor substrate, the first semiconductor layer, the active layer and the second semiconductor layer are formed of compound semiconductors such as gallium arsenide or indium phosphide in the tenth or eleventh inventions.
The structure for assembly of the Gunn diode of the thirteenth invention is so arranged that a surface ground electrode is formed on a surface of a microstrip substrate obtained by forming a signal electrode on a surface of a semi-insulating plate substrate and a ground electrode on a rear surface thereof, wherein the surface ground electrode is connected to the ground electrode on the rear surface through a via hole, and that the first and second electrodes of the Gunn diode of the first to eighth inventions are respectively connected and mounted to the signal electrode and the surface ground electrode.
The structure for assembly of the Gunn diode of the fourteenth invention is so arranged that the first and second electrodes of the Gunn diode of the first to eighth inventions are respectively connected and mounted to a signal electrode and a pair of ground electrodes of a coplanar waveguide obtained by forming the signal electrode and the pair of ground electrodes on a surface of a semi-insulating plate substrate.
The structure for assembly of the Gunn diode of the fifteenth invention is so arranged that one end of the signal electrode is open at length L from a portion to which the first electrode of the Gunn diode is connected, wherein a first electrode portion of the length L acts as a resonator and wherein an oscillating frequency is determined by the length L.
The structure for assembly of the Gunn diode of the sixteenth invention is so arranged that fourth and fifth electrodes are formed at a heat sink made of an insulating substrate, wherein the first electrode of the Gunn diode of the ninth invention is directly connected and mounted to the fourth electrode of the heat sink and the second electrode of the Gunn diode to the fifth electrode of the heat sink.
The structure for assembly of the Gunn diode of the seventeenth invention is so arranged that a hole is formed on a microstrip substrate obtained by forming a signal electrode on a surface of a semi-insulating plate substrate and a ground electrode which concurrently acts as a heat sink on a rear surface thereof, the hole extending from the surface to the ground electrode on the rear surface, wherein the fifth electrode of the heat sink of the sixteenth invention is connected to the ground electrode and wherein the third electrode of the Gunn diode of the sixteenth invention is connected to the signal electrode of the microstrip line through a conductive line within the hole.
The structure for assembly of the Gunn diode of the eighteenth invention is so arranged that an oscillating circuit, which oscillates at a specified frequency, is arranged of the signal electrode, the ground electrode and the Gunn diode, or by further adding a dielectric resonator thereto, in the thirteenth to seventeenth inventions.
The structure for assembly of the Gunn diode of the nineteenth invention is so arranged that a portion of the signal electrode that functions as an electrode of the oscillating circuit is at least partially covered by a plate substrate of conductive material, and in that the conductive portion of the plate substrate is connected to the ground electrode in the eighteenth invention.
The structure for assembly of the Gunn diode of the twentieth invention is so arranged that a resistivity of the plate substrate of the microstrip line or coplanar waveguide is not less than 106 xcexa9cm, and a thermal conductivity is not less than 140 W/mK in the thirteenth to nineteenth inventions.
The structure for assembly of the Gunn diode of the twenty-first invention is so arranged that the plate substrate of the microstrip line or the coplanar waveguide is made of at least one of AlN, Si, SiC or diamond in the thirteenth to twentieth inventions.
The NRD guide Gunn oscillator of the twenty-second invention is obtained by disposing two parallel plates of metal at a distance that is not more than half a free space wavelength of an used frequency and combining a NRD guide circuit pinching and holding a dielectric strip line between the parallel plates and a Gunn diode, wherein the NRD guide Gunn oscillator comprises a plate substrate of insulating or semi-insulating material on which surface there are formed a signal electrode connected to a signal line and a ground electrode insulated with respect to the signal electrode, a Gunn diode being formed with an anode electrode and a cathode electrode on a same plane wherein one of the electrodes is connected to the signal electrode of the plate substrate and the other one is connected to the ground electrode, and a heat sink for supporting a rear surface of the plate substrate with respect to the other parallel plate, wherein a tip of the signal line of the plate substrate is electromagnetically combined to the dielectric strip line.
In the twenty-third invention, the plate substrate to which the Gunn diode is connected and mounted is parallel with respect to the parallel plate, and the signal line is electromagnetically combined thereto in a vertical direction with respect to the dielectric strip line in the twenty-second invention.
In the twenty-fourth invention, the plate substrate to which the Gunn diode is connected and mounted is parallel with respect to the parallel plate, a progressing direction of electromagnetic waves of the signal line is identical with a progressing direction of electromagnetic waves of the dielectric strip line, and the signal line is electromagnetically combined to a base end portion of the dielectric strip line in the twenty-second invention.
In the twenty-fifth invention, a posture of the parallel substrate to which the Gunn diode is connected and mounted is changed from a parallel one to a vertical one with respect to the parallel plate in the twenty-third or twenty-fourth invention.
In the twenty-sixth invention, the signal line is a suspended microstrip line, microstrip waveguide or coplanar line in the twenty-second to twenty-fifth inventions.
In the twenty-seventh invention, the parallel substrate comprises an electrode for grounding on a rear surface thereof, and the electrode for grounding is connected to the ground electrode through a via hole in the twenty-second to twenty-sixth invention.
The NRD guide Gunn oscillator of the twenty-eighth invention is obtained by disposing two parallel plates of metal at a distance that is not more than half a free space wavelength of an used frequency and combining a NRD guide circuit pinching and holding a dielectric strip line between the parallel plates and a Gunn diode, wherein the NRD guide Gunn oscillator comprises a plate substrate of insulating or semi-insulating material on which surface there are formed two signal electrodes connected to both ends of a signal line and a ground electrode insulated with respect to the respective signal electrodes, two Gunn diodes being respectively formed with an anode electrode and a cathode electrode on a same plane wherein one of the electrodes is connected to the signal electrodes of the plate substrate and the other one is connected to the ground electrode, and a heat sink for supplying a rear surface of the plate substrate with respect to the other parallel plate, wherein a substantially central portion of the signal line of the plate substrate is electromagnetically combined to the dielectric strip line.
In the twenty-ninth invention, a length of the signal line is set to be substantially half of a guide wave length of the signal line or an integer multiple thereof in the twenty-eighth invention.
In the thirtieth invention, the plate substrate to which the Gunn diodes are connected and mounted is vertical with respect to the parallel plate, and the substantially central portion of the signal line is electromagnetically combined with an end portion of the dielectric strip (line) in the twenty-eighth or twenty-ninth inventions.
In the thirty-first invention, a posture of the plate substrate to which the Gunn diodes are connected and mounted is changed from a vertical one to a parallel one with respect to the parallel plate in the thirtieth invention.
In the thirty-second invention, the signal line is a suspended microstrip line, microstrip line or coplanar line in the twenty-eighth to thirty-first inventions.
In the thirty-third invention, the plate substrate comprises an electrode for grounding on a rear surface thereof, and the electrode for grounding is connected to the ground electrode through a via hole in the twenty-eighth to thirty-second inventions.
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