Abstract:
Indium phosphor (InP) Gunn diode that realizes improvements in thermal characteristics, yield factor of good products and easy assembly to planar circuits is provided. 
     In a Gunn diode of the present invention, contact layers are interposing an active layer. An anode electrode and a cathode electrode are formed on the uppermost contact layer. A high resistance region around the cathode electrode is formed at least in an uppermost contact layer by ion implantation using the cathode and anode electrode as a mask. 
     A region under the cathode electrode functions as a Gunn diode and a region under the anode electrode function as a conductive path from the anode electrode to the active layer. These two regions are defined by the high resistance region.

Description:
BACKGROUND OF THE INVENTION 
     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, and manufacturing methods thereof. 
     Gunn diodes for oscillation of microwaves or millimeter waves are usually comprised of compound semiconductors such as gallium arsenide (GaAs) or indium phosphide (InP). It is the case with such compound semiconductors that the electron mobility is several thousands of cm 2 /V·sec 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). 
     An oscillating frequency ft can be determined from transit distance L of the domain and average drift velocity Vd of the electrons with an equation ft=Vd/L in a microwave range. Energy relaxation time consists of time needed for the electron to increase and decrease energy at Γ valley is a main cause for fixing the upper limit of oscillating frequency in a millimeter wave range. It is reported that the relaxation time constant of GaAs is twice as a mode of fundamental frequency of InP and a cut-off frequency of GaAs and InP is 100 GHz and 200 GHz respectively (M. A. di Forte-poisson et al.: Proc. IPRM&#39;89, p.551 (1989)). Since the upper limit of the oscillating frequency of GaAs Gunn diode ranges 60 GHz through 70 GHz in the practical application, higher frequency bands such as 77 GHz band that is used for car mount radar is appropriate to InP Gunn diode. 
     In case of Gunn diodes for millimeter waves, this distance of transit needs to be extremely short (1 to 2 μm). 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. 1×10 12 /cm 2 ) 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 unambiguously 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 structure to use elements including the active layer of extremely small sizes, having diameters of approximately several tens of μm, and assembling the diodes within pill-type packages comprised with a heat portion made of diamond or similar material of favorable thermal conductivity. 
     A sectional view of InP Gunn diode element  100  of conventional mesa-type structure is shown in FIG.  8 . On to a semiconductor substrate  101  of heavily doped n-type InP, there are sequentially laminated, through MOCVD method, a first contact layer  102  of heavily doped n-type InP, an active layer  103  of lightly doped n-type InP, and a second contact layer  104  of heavily doped n-type InP, and it is employed a mesa-type structure in order to reduce the transit space for the electrons. 
     Thereafter, a lower surface of the semiconductor substrate  101  is laminated, a cathode electrode  105  is formed onto the 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.  9 . 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 TiC 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 . 
     Conventional InP Gunn diode elements  100  are formed through chemical wet etching by employing a photoresist as an etching mask to obtain the above described mesa-type structure. 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. 
     Also there is a disadvantage that an alloy electrode such as AuGe, which is used for the anode electrode  106 , reacts with In at relatively low temperature, thereby causing deterioration of anode electrode obtaining ohmic contact. 
     There is another disadvantage that Gunn diode may be burned out since current is concentrated to surface of mesa structure due to the instability of the surface of mesa structure in the active layer  103  of InP. 
     There is still another disadvantage that the bonding tool intercepted one&#39;s field of view, at the time of building-in the Gunn diode element in a pill-type package  110 , during adhesion of the Gunn diode element  100  to the heat sink electrode  111  so that the heat sink electrode  111  could not be directly viewed at. Consequently, the efficiency of building-in operation was quite poor. 
     Further, utilization of a gold ribbon for assembling the pill-type package  110  incorporated with the Gunn diode element  100  to the microstrip line arranged on the plate substrate resulted in generation of parasitic inductance, whereby ununiformity in electrical characteristics was caused during the assembly. 
     It is an object of the present invention to provide Gunn diodes and manufacturing methods thereof which solve the above described problems. 
     SUMMARY OF THE INVENTION 
     For this purpose, according to the first aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InGaAs are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the second semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating a region covered with the first electrode from the other in the second semiconductor layer, so as to let regions under the first electrode in the second semiconductor layer and the active layer work as a Gunn diode is provided. 
     According to the second aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP, a second semiconductor layer of heavily doped n-type InP and a third semiconductor layer of heavily doped InGaAs are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the third semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating regions covered with the first electrode from the others in the third and second semiconductor layer, so as to let regions under the first electrode in the second semiconductor layer and the active layer work as a Gunn diode is provided. 
     According to the third aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InGaAs, an active layer of lightly doped n-type InP, a second semiconductor layer of heavily doped n-type InP and a third semiconductor layer of heavily doped InGaAs are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the third semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating regions covered with the first electrode from the others in the third and second semiconductor layer, so as to let regions under the first electrode in the first semiconductor layer and the active layer work as a Gunn diode is provided. 
     According to the fourth aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InGaAs, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InP are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the second semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating a region covered with the first electrode from the other in the second semiconductor layer, so as to let regions under the first electrode in the first semiconductor layer and the active layer work as a Gunn diode is provided. 
     The Gunn diode according to the fifth aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type GaAs, an active layer of lightly doped n-type GaAs and a second semiconductor layer of heavily doped n-type GaAs are formed on a semiconductor substrate of heavily doped n-type GaAs in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the second semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating a region covered with the first electrode from the other in the second semiconductor layer, so as to let regions under the first electrode in the second semiconductor layer and the active layer work as a Gunn diode is provided. 
     According to the sixth aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InP are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the second semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating a region covered with the first electrode from the other in the second semiconductor layer, so as to let regions under the first electrode in the second semiconductor layer and the active layer work as a Gunn diode is provided. 
     According to the seventh aspect of the present invention is so arranged that the first electrode and the second electrode are formed with refractory metal such as WSi, Mo or the like in said first to third aspects. 
     The Gunn diode according to the eighth aspect of the present invention is so arranged that the first semiconductor layer is composed of In 0.53 Ga 0.47 As for lattice matching with the semiconductor substrate of InP in said third and fourth aspects. 
     The Gunn diode according to the ninth aspect of the present invention is so arranged that the first electrode and the second electrode are composed of a base electrode and one or more conductive bumps on the base electrode respectively, and the conductive bumps have their tops at a substantially same level in said first to sixth aspects. 
     The Gunn diode according to the tenth aspect of the present invention is so arranged that the conductive bump of the first electrode is formed in the central part of the Gunn diode and the conductive bumps of the second electrode are formed therebetween in said ninth aspect. 
     The Gunn diode according to the eleventh aspect of the present invention is so arranged that the area of the second electrode is larger than that of the first electrode by 10 to 1000 times in said first to tenth aspects. 
     The Gunn diode according to the twelfth aspect of the present invention is so arranged that two or more electrode are formed as the first electrode along with respective high resistance regions in said first to eleventh aspects. 
     The Gunn diode of the thirteenth aspect of the present invention further comprises a third electrode formed on the lower surface of the semiconductor substrate, wherein the first electrode and the third electrode are used for applying voltages to the active layer and the second electrode is used as a mounting spacer to fix the Gunn diode in said first to twelfth aspects. 
     A method for manufacturing a Gunn diode of the fourteenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type InP having a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InGaAs formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the second semiconductor layer, and step of forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     A method for manufacturing a Gunn diode of the fifteenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type InP having a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP, a second semiconductor layer of heavily doped n-type InP and a third semiconductor layer of heavily doped n-type InGaAs formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the third semiconductor layer, and step of forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     A method for manufacturing a Gunn diode of the sixteenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type InP having a first semiconductor layer of heavily doped n-type InGaAs, an active layer of lightly doped n-type InP, a second semiconductor layer of heavily doped n-type InP and a third semiconductor layer of heavily doped n-type InGaAs formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the third semiconductor layer, and step of forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     A method for manufacturing a Gunn diode of the seventeenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type InP having a first semiconductor layer of heavily doped n-type InGaAs, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InP formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the second semiconductor layer, and step of forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     A method for manufacturing a Gunn diode of the eighteenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type GaAs having a first semiconductor layer of heavily doped n-type GaAs, an active layer of lightly doped n-type GaAs and a second semiconductor layer of heavily doped n-type GaAs formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the second semiconductor layer, and step of forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     A method for manufacturing a Gunn diode of the nineteenth aspect of the present invention comprises step of preparing a semiconductor substrate of heavily doped n-type InP having a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InP formed thereon in said order, step of forming a first electrode of smaller area and a second electrode of larger area on the second semiconductor layer, and step forming a high resistance region, by ion implantation using the first electrode and the second electrode as a mask, at least deeply to reach the lower surface of the second semiconductor layer. 
     In a method for manufacturing a Gunn diode of the twentieth aspect of the present invention, the step of forming the first and the second electrode comprises step of forming base electrodes of the first electrode and the second electrode respectively and step of forming conductive bumps, having their tops at a substantially same level, on the base electrodes in said fourteenth to nineteenth aspects. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 illustrates a Gunn diode of the first embodiment of the present invention, FIG.  1 ( a ) is a top view and FIG.  1 ( b ) is a sectional view; 
     FIG. 2 is a sectional view showing manufacturing method of the Gunn diode in FIG. 1; 
     FIG. 3 is a sectional view of the Gunn diode of second embodiment; 
     FIG. 4 is a sectional view of the Gunn diode of third embodiment; 
     FIG. 5 is a sectional view of the Gunn diode of fourth embodiment; 
     FIG. 6 is a sectional view of the Gunn diode of fifth embodiment; 
     FIG. 7 is a perspective view showing a mounting structure of the Gunn diode according to sixth embodiment; 
     FIG. 8 is a sectional view of the conventional mesa structural Indium Phosphor (InP) Gunn diode; 
     FIG. 9 is a sectional view of the Gunn diode mounted on pill type package. 
    
    
     DETAILED DESCRIPTION 
     Embodiment 1 
     FIGS.  1 ( a ) and  1 ( b ) are diagrams showing a structure of a InP Gunn diode  10 A according to a first embodiment of the present invention, wherein FIG.  1 ( a ) is a top view and FIG.  1 ( b ) is a sectional view. FIG. 2 is a view showing manufacturing steps. 
     Onto a semiconductor substrate  11  of heavily doped n-type InP having an impurity concentration of 1×10 18  to 2×10 18  atom/cm 3 , there are sequentially laminated through MOCVD method a first contact layer  12  of heavily doped n-type InP having an impurity concentration of 2×10 18  atom/cm 3  and a thickness of 0.5 μm, an active layer  13  of lightly doped n-type InP having an impurity concentration of 1.2×10 16  atom/cm 3  and a thickness of 1.6 μm, and a second contact layer  14  of heavily doped n-type In 0.53 Ga 0.47 As having an impurity concentration of 1×10 19  atom/cm 3  and a thickness of 0.2 μm. 
     Secondly, refractory metal such as WSi is deposited on the overall surface of the second contact layer  14 . Then, on to the WSi layer, a mask (not shown) is formed with a metallic film of Ti/Pt/Au. With using the mask, anode electrode  15  and cathode electrode  16  are formed with etching the WSi layer through gas such as SF 6  (refer to FIG.  2 ( a )). As shown in FIG. 1, the cathode electrode  16  is surrounded with the anode electrode  15 . The planar shape of the anode electrode  15  is oblong and the planar shape of the cathode electrode  16  is round, while these might alternatively be oval or substantially square. 
     The reason for employing WSi for the electrodes  15  and  16  is that WSi is a refractory metal with thermal stability, and also can obtain ohmic contact with the second contact layer  14  consists of InGaAs without alloying. Other than the WSi, Mo or the like may be used as a refractory metal. 
     In a case where no refractory metal such as WSi, Mo or the like is used, onto the second contact layer  14 , there is a patterned a photoresist that is opened at regions on which a cathode electrode and an anode electrode are to be formed, and a metal film (underlying electrode layer) of AuGe, Ni, Au or the like that is in ohmic contact with the second layer  14  is vapor-deposited thereon. After removing the photoresist, a heat treatment (sintering) is performed, and anode electrode  15  and cathode electrode  16  are formed on the second contact layer  14  in a separate manner. 
     Photoresist  17  is patterned as to leave a part of the surfaces of the anode electrode  15  and cathode electrode  16  open, and bumps (electrodes)  18 ,  19  which are conductive protrusions of Au or the like are formed in the open portions through precipitation by electrolytic plating or non-electrolytic plating (Refer to FIG.  2 ( b )). 
     Next, after exposing the second contact layer through removing the photoresist  17 , the anode electrode  15  and cathode electrode  16  are used as a mask and oxygen (O) ion is injected. The ion implantation is carried out three times in total providing acceleration energy is 40 KeV, 200 KeV and 300 KeV and each amount of dose is 1×10 14 /cm 2 , 2×10 14 /cm 2  and 3×10 14 /cm 2  to form almost uniform high resistance region  20  through heat treatment at 300 degrees. In this manner, high resistance region  20  can be accurately formed through ion implantation due to self-alignment using the anode electrode  15  and the cathode electrode  16  as a mask. 
     Further, while the resistance of the high resistance region  20  can be equal or higher than the active layer  13 , 0.1 Ω·cm or higher is preferable. In addition, the high resistance region  20  can be formed deeper to reach into the active layer  13 , or further into the first contact layer  12 . 
     In a Gunn diode  10 A according to the present embodiment, it is possible to avoid the occurrence of burnout due to the current concentration result from instability of the exposed surface, because the surface of the active layer  13  is covered with the second contact layer  14  and the high resistance region  20 , and not exposed. 
     Further, boron (B), iron (Fe), hydrogen (H) or the like can be used for ion implantation other than oxygen (O). 
     The area of the active layer  13  to which the cathode electrode  16  which has been sub-divided by the high resistance region  20  is connected is set to be an area (transverse cross section) with which a specified operating current of the Gunn diode can be obtained. That is, this area is set to be an area that can function as a Gunn diode. Further, an area of the active layer  13  to which the anode electrode  15  is connected is set to be larger enough than an area of the active layer  13  to which the cathode electrode  16  is connected, and the electric resistance of a semiconductor laminated portion below the anode electrode  15  is made to be smaller than the electric resistance of a semiconductor laminated portion below the cathode electrode  16 . With these arrangements, the portion below the anode electrode  15  is not made to function as a Gunn diode but to function as a resistance of substantially low value, and the anode electrode  15  is substantially connected to the first contact layer  12 . 
     The ratio of area to which the anode electrode  15  is connected to the area to which the cathode electrode  16  is connected in the active layer  13  needs to be not less than 10, and preferably 10 to 1000, since a ratio of below 10 would not be effective but only results in a decreased conversion efficiency. 
     Next, the lower surface of the semiconductor substrate  11  is polished to thin the semiconductor substrate  11  in accordance with the ordinary manufacturing processes for Gunn diodes such that the thickness of the whole Gunn diode becomes approximately 100 μm. Thereafter, if required, a metal film  21  of AuGe, Ni, Au, Ti, Pt or Au that is in ohmic contact with the semiconductor substrate  11  is vapor-deposited onto the lower surface of the semiconductor substrate  11 , and a heat treatment is performed (FIG.  2 ( d )), the metal film  21  that is formed on the lower surface of the semiconductor substrate  11  is not necessarily required. However, if the metal film  21  is formed, it might function as an anode electrode substituting the anode electrode  15  or along with the anode electrode  15  so that electrical resistance of the laminated semiconductor layers for the anode electrode can be decreased. Moreover, when the metal film  21  substitutes the anode electrode  15 , the anode electrode  15  can be functioned as a spacer at the mounting since the anode electrode  15  and the cathode electrode  16  are mounted on the same surface. 
     As explained so far, the InP Gunn diode  10 A according to the present embodiment is so arranged that the laminated semiconductor layers is divided, by the high resistance region  20  surrounding the cathode electrode  16 , into a portion which functions as a Gunn diode (i.e. a portion under the cathode electrode  16  in the second contact layer  14  and the active layer  13 ) and the other portion which functions as a low resistance region for applying voltage to the first contact layer  12  under said portion of the Gunn diode function. With this arrangement, both the anode electrode  15  and cathode electrode  16  can be formed on the upper surface of the second contact layer  14 . In other words, the anode electrode  15  and the cathode electrode  16  can be arranged on a same surface, whereby great advantages can be obtained in terms of easiness in implementation onto planer surfaces and superior heat dissipation. 
     Since ion implantation to define a region for determining an operational current (i.e. said portion of Gunn diode function) is performed through a self-alignment method by utilizing electrodes  15  and  16  formed above the region as a mask, thereby improving yield factor of good products. 
     Moreover, the electron is to be doped as hot electron from the cathode electrode  16  to the active layer  13  via the junction of InGaAs and InP (i.e. the junction of the second contact layer  14  and the active layer  13 ) having conduction band offset of 0.2 eV, thereby reducing required time for forming domain and increasing the oscillation efficiency. 
     Since the second contact layer  14  is composed by InGaAs, ohmic contact can be obtained with the electrodes (the anode electrode  15  and the cathode electrode  16 ) formed thereon without composing alloys therebetween. Accordingly, stable refractory metal such as WSi, Mo or the like can be employed for the electrodes. If the electrode is formed on a layer of InP, it is necessary to form the alloy of them to obtain ohmic contact so that AuGe, Au or the like is typically used for the electrode. In this case, however, Au in the electrode easily reacts with in the underneath layer so as to degrade the ohmic contact even at a relatively low temperature. In the embodiment in which the second contact layer of InGaAs and the electrodes of refractory metal are employed, such trouble does not arise. 
     Though the embodiment in which only one combination of the cathode electrode  16 , the bump  19  and the high resistance region  20  is formed in the central region of the diode is described above, two or more combinations may be formed between the bumps  18 . The combinations of the cathode electrode  16 , the bump  19  and the high resistance region  20  may be arranged in a matrix-like manner, staggered manner, radiated manner, concentric manner or the like. To form two or more combinations in one Gunn diode, the function of the mesa structure to accomplish the operation of the Gunn diode is distributed to the two or more regions. Therefore, the efficiency of heat radiation is greatly improved so that the oscillating efficiency and output power are also greatly increased. 
     Embodiment 2 
     FIG. 3 is a cross sectional view of Gunn diode  10 B according to the second embodiment of the present invention. Onto a semiconductor substrate  11  heavily doped n-type Inp having an impurities concentration of 1×10 18  to 2×10 18  atom/cm 3 , there are sequentially laminated through MOCVD method the first contact layer  12  of heavily doped n-type InP having an impurity concentration of 2×10 18  atom/cm 3  and a thickness of 0.5 μm, and the active layer  13  of lightly doped n-type InP having an impurity concentration of 1.2×10 16  atom/cm 3  and a thickness of 1.6 μm, and the second contact layer  14 ′ of heavily doped n-type Inp having an impurity concentration of 2×10 18  atom/cm 3  and a thickness of 0.2 μm, and the third contact layer  14 ″ of heavily doped n-type In 0.53 Ga 0.47  As having an impurity concentration of 1×10 19  atom/cm 3  and a thickness of 0.2 μm. 
     Onto the third contact layer  14 ″, the anode electrode  15  and the cathode electrode  16  of metallic films of WSi or the like that is in ohmic contact with the third contact layer  14 ″ are formed separately. Further, similar to said first embodiment, the high resistance region  20  which surrounds the cathode electrode  16  and reaches to the lower surface of the second contact layer  14 ′ is formed by performing ion implantation from the exposed surface of the third contact layer  14 ″. 
     In a Gunn diode  10 B according to the present embodiment, since the second contact layer  14 ′ and the third contact layer  14 ″ are laminated on the upper surface of the active layer  13  sequentially, it has the same arrangement as the first embodiment (described in relation to FIG.  1  and FIG. 2) except that the second contact layer  14 ′ and the active layer  13  under the cathode electrode  16  function as Gunn diode and has the same advantages as the first embodiment. 
     The high resistance region  20  is needed for the second contact layer  14 ′ and third contact layer  14 ″ without fail, and it may be formed deeper to reach into the active layer  13 , or further into the first contact layer  12 . 
     Moreover, the functions of the anode electrode  15  and the cathode electrode  16  are interchangeable (i.e.the electrode  15  as cathode and the electrode  16  as anode). 
     Embodiment 3 
     FIG. 4 is a cross sectional view of the Gunn diode  10 C according to the third embodiment of the present invention. In the embodiment, the first contact layer  12  of the second embodiment (shown in FIG. 3) is replaced by the first contact layer  12 ′ of heavily doped n-type In 0.53 Ga 0.47 As with the thickness of 0.2 μm. 
     Unlike to the first and second embodiment (shown in FIGS. 1 to  3 ), since the junction of the first contact layer  12 ′ of heavily doped n-type InGaAs and the active layer  13  of lightly doped n-type InP which functions as Gunn diode is positioned at the lower surface of the active layer  13  (i.e. at the side of the first contact layer  12 ′), smaller electrode positioned central is to be the anode electrode  15  while larger electrode positioned outside is to be the cathode electrode  16 . 
     The formation of the anode electrode  15 , the cathode electrode  16  and bump  18 ,  19  are carried out by the same manner as described in relation to FIG.  2 . And, the high resistance region  20  is formed by ion implantation from exposed surface of the third contact layer  14 ″ to the lower surface of the second contact layer  14 ′ in the same manner as described in FIGS. 2 and 3. 
     Further in the present embodiment, the first contact layer  12 ′ is composed of In 0.53 Ga 0.47 As for lattice matching with the semiconductor substrate  11  of InP, thereby not causing lattice defect in this epitaxial grown layer. 
     The Gunn diode  10 C according to the present embodiment has similar arrangement as the first or second embodiment (shown in FIG. 1 to FIG. 3) except that the anode electrode  15  and the cathode electrode  16  are interchanged and has the same function and advantages as those of these embodiments. 
     Embodiment 4 
     FIG. 5 is a cross sectional view of a Gunn diode  10 D according to the fourth embodiment of the present invention. In this embodiment, the third contact layer  14 ″ in the third embodiment (refer to FIG. 4) is not formed and the second contact layer  14 ′ of heavily doped n-type InP with an impurity concentration 2×10 18  atom/cm 3  is formed in a thickness of 0.3 μm on the active layer  13  of lightly doped n-type InP. Then onto there, the anode electrode  15  and the cathode electrode  16  are formed from a metal film consists of AuGe, Ni, Au or the like to obtain ohmic contact. 
     The high resistance region  20  is formed deeper to reach into the lower surface of the second contact layer  14 ′ as same as the first embodiment (shown in FIGS.  1  and  2 ). However it also possible to form it deeper to reach into the active layer  13 , or further into the first contact layer  12 ′. 
     In the embodiment, since the second contact layer  14 ′ of heavily doped n-type InP is not covered with a semiconductor layer of heavily doped n-type InGaAs, the anode electrode  15  and the cathode electrode  16  to be formed on the second contact layer  14 ′ required to be alloyed with the second contact layer  14 ′ so as to obtain ohmic contact. Thus, there is a disadvantage that material such as Au, which is used for the anode electrode  15  and the cathode electrode  16 , causes chemical reaction with In at relatively low temperature. However, in other aspects, it presents similar advantages as the first to third embodiments. 
     Embodiment 5 
     FIG. 6 is a cross sectional view of the Gunn diode according to the fifth embodiment of the present invention. Onto a semiconductor substrate  41  of n-type GaAs having an impurity concentration of 1×10 18  to 2×10 18  atom/cm 3 , there are sequentially laminated through MBE method a first contact layer  42  of n-type GaAs having an impurity concentration of 2×10 18  atom/cm 3  and a thickness of 1.5 μm, an active layer  43  of n-type GaAs having an impurity concentration of 1.2×10 16  atom/cm 3  and a thickness of 1.6 μm and a second contact layer  44  of n-type GaAs having an impurity concentration of 1×10 18  atom/cm 3  and the thickness of 0.3 μm. 
     The anode electrode  15  and the cathode electrode  16  of metallic film comprising AuGe, Ni, Au or the like that can obtain ohmic contact are formed on the upper surface of second contact layer  44  separately. 
     Hereinafter, the high resistance region  20 , which surrounds the cathode electrode  16  and reaches to the lower surface of the second contact layer  44 , is formed by ion implantation from exposed surface of the second contact layer  44  in the same manner as described in the first embodiment. Though gallium arsenide (GaAs) is used in this embodiment, it is possible to have same arrangements as those of the first to fourth embodiments and same advantages as of these embodiment could be obtained. It may possible to change said semiconductor substrate  41  to a heavily doped n-type InP substrate, the first contact layer  42  to a heavily doped n-type InP, the active layer  43  to lightly doped n-type InP and the second contact layer  44  to a heavily doped n-type InP respectively. 
     Embodiment 6 
     FIG. 7 is a view showing one example of an arrangement in which a Gunn diode  10  ( 10 A,  10 B) is assembled on a planar circuit substrate forming a microstrip line  30  to form an oscillator. A signal electrode  32  is formed on a plate substrate  31  of semi-insulating material such as aluminum nitride (AlN), silicone (Si), silicone carbide (SiC) or diamond having a favorable resistivity of not less than 106 Ω·cm and a thermal conductivity of not less than 140 W/mK, and the lower surface thereof is formed a ground electrode  33 .  34  denotes via holes filled with tungsten, that connect the ground electrode  33  on the lower surface and surface ground electrode  35  formed on the upper surface. 
     A bump  19  of an anode electrode of the Gunn diode  10  is connected to the signal electrode  32 , and bumps  18  of cathode electrodes are connected to the surface ground electrode  35 .  32 A denotes an electrode of a bias portion for supplying power source voltage to the Gunn diode  10 ,  32 B an electrode for making up an resonator formed by the microstrip line including the Gunn diode element  10  and  32 C an electrode of a signal output portion formed by the microstrip line. 
     The oscillator constructed by mounting Gunn diode  10  in such manner can obtain 60 mW oscillating power 77 GHz frequency with the length of the electrode  32 B of 400 μm. 
     In this structure for assembly, the Gunn diode  10  is set in a facing-down posture and the bumps  18 ,  19  are directly connected to the electrodes  35 ,  32  without employing a gold ribbon. With this arrangement, generation of parasitic inductance owing to connection through the gold ribbon can be eliminated, and an oscillator with no variations in characteristics can be realized. 
     Since heat generated in the Gunn diode  10  is dissipated, via the bumps  18 ,  19 , to the substrate  31  which also functions as a heat sink, thereby improving heat dissipating effects. Further, since the bumps  18  of the cathode electrode are positioned on both sides of the bump  19  in such an assembly of the Gunn diode  10 , it can be prevented that excessive mechanical load is applied to the anode electrode. 
     As explained so far, since defining a region that is to function as a Gunn diode is performed by self-aligning ion implantation utilizing electrodes formed above this region as a mask, variations in characteristics are restricted in the Gunn diode of the present invention. 
     Further, since the bumps of the cathode electrode and anode electrode can be formed on a same plane to assume identical height levels in the Gunn diode of the present invention, the Gunn diode can be assembled in a (facing-down) down posture. With this arrangement, the Gunn diodes do not need to be incorporated into conventional pill-type packages, whereby advantages in terms of manufacturing are presented to enable easy assembly to plate substrates. 
     Also, since it is not required to connect the Gunn diode to minute electrodes through means such as gold ribbons at the time of assembly, parasitic inductance do not occur and variations in circuit characteristics owing to variations in lengths of the gold ribbon or the like can be eliminated. 
     In a case where semiconductor substrate to form the cathode and the anode electrodes thereon is composed by InGaAs, refractory metal can be used as the electrodes. Therefore it is possible to avoid the disadvantage that In reacts with Au, thereby causing deterioration of anode electrode obtaining ohmic contact. 
     Further, the high resistance region is formed for defining a region of anode electrode and cathode electrode, thereby notches, cutouts, or the like for defining the regions is not necessary. Therefore, burned out of the diode by electric current concentration due to the instability of the exposed surface of the active layer is prevented. 
     Further, in a case where InGaAs layer is formed on the cathode side of the active layer, electrons are injected as hot electrons into the active layer of InP via a junction of InGaAs and Inp, thereby shortening required time for forming domain and improving oscillating efficiency. 
     Further, in the arrangement in which the function of mesa-type structure is divided into a plurality of regions, the heat dissipating efficiency is remarkably improved so that the conversion efficiency and the output power can be largely improved.