Patent Publication Number: US-2002011604-A1

Title: Semiconductor device for milliwave band oscillation, fabricating method therefor and oscillator therewith

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a semiconductor device in which different compound semiconductor elements are integratedly formed and to a milliwave/microwave band oscillator employing the semiconductor device.  
       [0002] In recent years, there have been developed systems using the milliwave band and the microwave band.  
       [0003] Particularly in systems using the milliwave band (30 to 90 GHz), transistors such as a HBT (hetero-junction bipolar transistor) and a HEMT (high-electron-mobility transistor) are used for an oscillation element and an amplifier, and a Schottky diode is used for a varactor and a mixer. Furthermore, those elements are fabricated on a same substrate for integration of the elements, which is disclosed in Japanese Patent Laid-Open Publication Nos. HEI 3-64929 and SHO 63-129656.  
       [0004] However, when a HBT or a HEMT is employed as an oscillation element in the milliwave band, it is required to reduce emitter width to 1 μm or less and gate width to 0.2 μm or less for a fine structure in order to cope with the increase in frequency of the transistor. In compliance with the dimensional reduction, it is also required to reduce the parasitic capacitance and the parasitic resistance.  
       [0005] For achievement of such a fine structure, complicated processes are required and a reduction in yield is caused. Furthermore, the amount of current is reduced when the emitter width and the gate width are reduced for the fine structure, and this has led to difficulty in obtaining a necessary output power from the oscillation element.  
       [0006] In order to solve these disadvantages, it can be considered to employ a diode having a negative resistance as an oscillation element instead of employing a HBT or a HEMT. One example is disclosed in Japanese Patent Laid-Open Publication No. HEI 1-112827. Its structure and fabrication method will be described with reference to FIGS. 5A through 5C.  
       [0007] With regard to the structure, as shown in FIG. 5C, an electrode constructed of a TiW film  806 /Au film  807  is provided on a p + -GaAs layer  805 , and an electrode constructed of Ti  808 /Au  809  is provided on an n + -GaAs layer  802 , so as to constitute an IMPATT (impact ionization avalanche transit time) diode having a negative resistance. Moreover, a microstrip patch constructed of a Ti film  810 /Au film  811  is formed on a semi-insulating GaAs substrate  801 . Furthermore, this example discloses capability of integrating another device on the substrate  801  and capability of integrating another device by utilizing the n + -GaAs layer  802 .  
       [0008] In the fabricating method of this example, first of all, as shown in FIG. 5A, an n + -GaAs layer  802  (having a density of 1×10 19  cm −3  and a thickness of 1.5 μm), an n-GaAs layer  803  (having a density of 2×10 17  cm −3  and a thickness of 0.25 μm), a p-GaAs layer  804  (having a density of 2×10 17  cm −3  and a thickness of 0.25 μm) and a p + -GaAs layer  805  (having a density of 1×10 19  cm −3  and a thickness of 0.2 μm) are successively epitaxially grown on the semi-insulating GaAs substrate  801 . Next, a photoresist is applied, and a circle of 5 μm is patterned to form an electrode constructed of a TiW film  806  (having a thickness of 100 nm)/Au film  807  (400 nm).  
       [0009] Next, as shown in FIG. 5A, the p + -GaAs layer  805 , the p-GaAs layer  804 , the n-GaAs layer  803  and the n + -GaAs layer  802  are etched by wet etching using the aforementioned electrode as an etching mask, and the etching is stopped within the n + -GaAs layer  802 . Next, a photoresist is applied and patterned into a square of a side of 75 μm, and an electrode constructed of a Ti film  808  (100 nm)/Au film  809  (400 nm) is formed as shown in FIG. 5B by the lift-off technology. At this time, the electrodes  808  and  809  are self-aligned with the electrodes  806  and  807 .  
       [0010] Next, as shown in FIG. 5C, the n + -GaAs layer  802  and part (about 100 nm in depth) of the substrate  801  is subjected to anisotropic plasma etching. Through this process, an IMPATT diode is isolated as a mesa on the semi-insulating substrate  801 . Subsequently, a microstrip patch, which is constructed of a Ti film  810  (100 nm)/Au film  811  (400 nm), is formed on the substrate  801  by the lift-off technology.  
       [0011] This document also discloses the following matters. Another device can be integratedly formed on the substrate  801  immediately before the formation of the microstrip patches  810  and  811 . In particular, an active device region can be formed by implanting ions into the semi-insulating substrate  801  separately from the regions corresponding to the IMPATT diode and the microstrip patch. Instead of this, an n + -type doped GaAs region  802  can be preserved to fabricate a device separately from the regions corresponding to the IMPATT diode and the microstrip patch by means of another photolithographic mask in the process of etching the n + -GaAs layer  802 .  
       [0012] The thus-fabricated IMPATT diode can cope with the milliwave band even when not made fine. Therefore, the fabricating process becomes easy in comparison with the HBT and HEMT. There is a further merit that a large output power is produced by the oscillation element.  
       [0013] However, the aforementioned prior art examples have the following disadvantages.  
       [0014] (1) In the conventional examples, ion implantation is used as a method for fabricating active devices and so on other than the IMPATT diode on the semi-insulating substrate. However, it is required to perform heat treatment i.e. annealing at a high temperature of about 600° C. after the ion implantation in order to activate the region in which ions have been implanted. This heat treatment disadvantageously causes deterioration in contact resistance and deterioration in the epitaxial structure (deterioration in the heterojunction and deterioration in density profile) of the IMPATT diode previously produced.  
       [0015] (2) In order to reduce the contact resistances of the electrodes  808  and  809 , the n + -GaAs layer  802 , which is the contact layer of the IMPATT diode, is highly doped into the n + -type. This high-density n + -GaAs layer  802  has a disadvantage that the Schottky characteristics, which are necessary for, for example, the gate electrode of a MESFET and the Schottky electrode of a Schottky diode, cannot be obtained when the n + -GaAs layer  802  is utilized for fabrication of the active devices other than the IMPATT diode.  
       [0016] As described above, in the aforementioned prior art examples, even if the diode that has a negative resistance and the Schottky diode are formed on a same substrate, the characteristic variation cannot be reduced, and moreover, no sufficient reproducibility can be obtained.  
       SUMMARY OF THE INVENTION  
       [0017] Accordingly, an object of the present invention is to provide a semiconductor device having a negative resistance diode and a Schottky diode integratedly formed on a same diode through easy fabricating processes without contact resistance deterioration and epitaxial structure deterioration.  
       [0018] In order to achieve the aforementioned object, the present invention provides a semiconductor device comprising: a negative resistance diode provided with an anode-ohmic electrode and a cathode-ohmic electrode having a negative resistance diode characteristic and an epitaxial structure; and a Schottky diode provided with an active layer having a negative resistance diode characteristic and an epitaxial structure, a Schottky electrode formed on the active layer, and an ohmic electrode formed on an ohmic electrode forming high-density layer, wherein the negative resistance diode and the Schottky diode are integratedly formed on a same substrate.  
       [0019] According to the present invention, the Schottky electrode is provided on the epitaxial active layer which has the negative resistance diode characteristic. The active layer can be constituted of a low density layer such as n-GaAs. The active layer not only allows the Schottky electrode to have the sufficient Schottky characteristic, but also needs no annealing at a high temperature (for example 600° C.) after ion implantation for forming other active devices. Therefore, the problems of the epitaxial structure deterioration, the contact resistance deterioration and the like can be resolved, allowing loss reduction and miniaturization to be achieved.  
       [0020] In the semiconductor device of one embodiment, the negative resistance diode and the Schottky diode are connected by a transmission line to be integratedly formed on the same substrate.  
       [0021] With this arrangement, the line length is reduced between the negative resistance diode and the Schottky diode since the negative resistance diode, the Schottky diode and the transmission line are integratedly formed on the same substrate. This arrangement is, therefore, very effective in loss reduction of line.  
       [0022] In the semiconductor device of another embodiment, the negative resistance diode is a Gunn diode.  
       [0023] The Gunn diode is an oscillation element having a reduced phase noise.  
       [0024] The present invention also provides a method for fabricating the semiconductor device, comprising the steps of: exposing the active layer by removing by etching an anode-ohmic electrode forming high-density layer or a cathode-ohmic electrode forming high-density layer having the negative resistance diode characteristic and the epitaxial structure; and forming the Schottky electrode on the active layer.  
       [0025] In this embodiment, the Schottky electrode can be formed on a low-density active layer such as n-GaAs layer, and therefore, the satisfactory Schottky characteristic of the Schottky electrode can be obtained and no high temperature annealing is required.  
       [0026] The present invention further provides a method for fabricating a semiconductor device, the semiconductor device having: a negative resistance diode having an epitaxial structure with a negative resistance diode characteristic, an anode-ohmic electrode and a cathode-ohmic electrode, the epitaxial structure having a heterostructure; and a Schottky diode provided with an active layer having with a negative resistance diode characteristic and an epitaxial structure, a Schottky electrode formed on the active layer, and an ohmic electrode formed on an ohmic electrode forming high-density layer, wherein the negative resistance diode and the Schottky diode are integratedly formed on a same substrate, the method comprising the steps of: exposing a wide-band gap layer by removing by etching a cathode-ohmic electrode forming high-density layer; exposing the active layer by removing by etching the wide-band gap layer; and forming the Schottky electrode on the active layer.  
       [0027] The wide bandgap layer is exposed by removing the cathode-ohmic electrode forming high-density layer by etching, and then is removed by selective etching to expose the active layer. Therefore, thickness of this active layer can be controlled by the thickness in epitaxial growth (within the wafer surface). Moreover, variation in the thickness of the active layer among wafers is also be reduced. Therefore, satisfactory reproducibility of the Schottky diode characteristic can be obtained.  
       [0028] In one embodiment of the method for fabricating the semiconductor device, the negative resistance diode is a Gunn diode.  
       [0029] The cathode structure of the Gunn diode is heterostructurally formed in general. Therefore, thickness of the active layer can be controlled within a wafer surface, and variation in thickness among wafers is also reduced. With this arrangement, the satisfactory reproducibility of the Schottky diode characteristic can easily be obtained.  
       [0030] The present invention provides an oscillator having a semiconductor device, comprising: a negative resistance diode provided with an anode-ohmic electrode and a cathode-ohmic electrode having a negative resistance diode characteristic and an epitaxial structure; a Schottky diode provided with an active layer having a negative resistance diode characteristic and an epitaxial structure, a Schottky electrode formed on the active layer, and an ohmic electrode formed on an ohmic electrode forming high-density layer; and a transmission line connecting the negative resistance diode and the Schottky diode, wherein the negative resistance diode, the Schottky diode and the transmission line are integratedly formed on a same substrate; and wherein the negative resistance diode is served as an oscillation element, the Schottky diode is provided for a varactor diode, and the transmission line is provided for an output line and a stub.  
       [0031] According to the oscillator of this embodiment, the transmission line connects the negative resistance diode and the Schottky diode on the same substrate, and is also provided for the output line and the stub. Therefore, line loss and the mounting loss such as wire-bonding loss can be reduced, so that the degradation in performance such as phase noise degradation can be prevented.  
       [0032] Moreover, in one embodiment of the present invention, the negative resistance diode is a Gunn diode.  
       [0033] In this embodiment, the Gunn diode is used as an oscillation element in the oscilltor, and therefore a reduced phase noise can be obtained so that the performance of the oscillator is improved. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0034] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
     [0035]FIG. 1 is a sectional view showing a structure of a semiconductor device according to a first embodiment of the present invention;  
     [0036]FIGS. 2A through 2D are sectional views for explaining a method for fabricating the semiconductor device of the first embodiment of the present invention in order of process;  
     [0037]FIG. 3 is a block diagram of a voltage-controlled oscillator that serves as a milliwave band oscillator according to a second embodiment of the present invention;  
     [0038]FIG. 4 is a sectional view for explaining one process in fabrication according to a modification example of the first embodiment;  
     [0039]FIGS. 5A through 5C are sectional views for explaining a structure of a conventional semiconductor device and a fabricating method therefor;  
     [0040]FIG. 6A is a block diagram of a transmitter provided with the semiconductor device of the first embodiment; and  
     [0041]FIG. 6B is a block diagram of a receiver provided with the semiconductor device of the first embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0042] The present invention will be described below with reference to the drawings.  
     [0043] A structure of Gunn diode and Schottky diode integrated circuit according to a first embodiment of this invention will be described with reference to FIG. 1. Thereafter, a fabricating method of the integrated circuit will be described with reference to FIG. 2.  
     [0044] As shown in FIG. 1, in the integrated circuit of this first embodiment, a Gunn diode GD is formed in a region A, a Schottky diode SD is formed in a region B, and a transmission line CP is formed in a region C.  
     [0045] That is, the Gunn diode GD formed in the region A has a cathode-ohmic electrode  108  and an anode-ohmic electrode  107 , which are made of AuGe/Ni/Au, and has an active layer  103  made of n-GaAs. On the other hand, the Schottky diode SD formed in the region B has an ohmic electrode  109  made of AuGe/Ni/Au, the active layer  103  and a conductive film  112  made of Ti/Au. The above-mentioned AuGe/Ni/Au is a laminate film in which an Ni film and an Au film are successively laminated on an AuGe film, and the above-mentioned Ti/Au is a laminate film in which an Au film is laminated on a Ti film.  
     [0046] As shown in FIG. 1, peripheries of the Gunn diode GD and the Schottky diode SD are etched, forming an isolated region SA between the Gunn diode GD and the Schottky diode SD.  
     [0047] The transmission line CP formed in the region C is constructed of a conductive film  112  and an Au film  113 . A silicon nitride film (not shown) is formed on this transmission line CP.  
     [0048] In the Gunn diode and Schottky diode integrated circuit of this embodiment, a Schottky electrode is formed on the active layer  103  of the Gunn diode GD. The Gunn diode GD is constructed in an epitaxial structure and possesses a negative resistance diode characteristic having. Therefore, the Gunn diode GD and the Schottky diode SD can be integratedly formed on a same substrate through a process that needs no heat treatment (annealing) at a high temperature (600° C.) without utilizing the ion implantation technology nor the contact layer of the IMPATT diode, dissimilarly to the prior art. Therefore, such problems as epitaxial structure deterioration and contact resistance deterioration can be resolved, and loss reduction and miniaturization can be achieved.  
     [0049] Moreover, in this embodiment, the Gunn diode GD having the diode characteristic of negative resistance and the Schottky diode SD are formed on a same GaAs substrate  101  and connected by the transmission line CP. That is, the line size can be reduced by producing the negative resistance diode (Gunn diode GD) that becomes an oscillation element and the Schottky diode SD on the same substrate. Therefore, the above arrangement is very effective in loss reduction. Moreover, by virtue of the negative resistance diode provided by the Gunn diode, the oscillation element of a reduced phase noise and the Schottky diode can be integratedly formed on the same substrate.  
     [0050] The fabricating process of the Gunn diode and Schottky diode integrated circuit of this first embodiment will be described next by sequentially referring to FIGS. 2A through 2D concurrently with a description provided for a detailed structure.  
     [0051] First of all, as shown in FIG. 2A, an n + -GaAs layer  102 , which becomes an anode-ohmic electrode forming high-density layer, is epitaxially grown to a thickness of 800 nm at an Si doping density of 5×10 18  cm −3  on a semi-insulating GaAs substrate  101  by the MBE (molecular beam epitaxial growth), MOCVD (metalorganic chemical vapor deposition) method or the like. Next, an n-GaAs layer  103 , which becomes an active layer, is epitaxially grown to a thickness of 2000 nm at an Si doping density of 2×10 16  cm −3 . Next, a cathode layer n-Al x Ga 1−x As (X=0.35) layer  104 , which is constructed of a wide bandgap layer, is epitaxially grown to a thickness of 50 nm at an Si doping density of 5×10 17  cm −3 . Further, an n-Al x Ga 1−x As layer (X=0.35→0)  105  is epitaxially grown to a thickness of 20 nm at an Si doping density of 5×10 17  cm −3 . Next, an n + -GaAs layer  106 , which becomes a cathode-ohmic electrode forming high-density layer  106 , is epitaxially grown to a thickness of 500 nm at an Si doping density of 5×10 18  cm −3 .  
     [0052] Next, as shown in FIG. 2B, the n + -GaAs layer  106 , the n-Al x Ga 1−x As layer  105  and the n-Al x Ga 1−x As layer  104  are removed by etching with a region that becomes the cathode of the Gunn diode GD masked with an SiN film, an SiO film or the like, exposing the active layer  103 .  
     [0053] With regard to this etching, time etching may be performed by using an etching method that has no selectivity of GaAs and AlGaAs. In the above case, the thickness of the active layer  103  cannot be controlled, and, in addition, the thickness varies also within the wafer surface. The variation in the thickness of this active layer  103  becomes a characteristic variation of the Schottky diode formed subsequently.  
     [0054] Accordingly, by using the etching method with selectivity, this thickness variation can be prevented. In concrete, the n + -GaAs layer  106  is removed by etching, and thereafter, the n-Al x Ga 1−x As layer  105  and the n-Al x Ga 1−x As layer  104  are removed by hydrofluoric acid. The AlGaAs etching selection ratio of hydrofluoric acid with respect to GaAs is not smaller than one hundred. Therefore, the thickness of the active layer  103  can be controlled to the thickness at the epitaxial growth within the wafer surface. Furthermore, the etching can be stopped by the n-Al x Ga 1−x As layer  105  or the n-Al x Ga 1−x As layer  104 , whose electron affinity is smaller than that of GaAs. In this case, a Schottky diode with a large Schottky barrier can be formed.  
     [0055] It is also possible to perform etching to the n-Al x Ga 1−x As layer  105  or the n-Al x Ga 1−x As layer  104  by the time etching. However, by using an etchant that contains an acid such as citric acid or sulfuric acid and hydrogen peroxide aqueous solution, the GaAs  106  can be etched at a high selection ratio with respect to AlGaAs.  
     [0056] Next, the Schottky region of the Schottky diode SD is masked by a photoresist pattern or the like without removing the SiN film or SiO film, the SiN film or the SiO film remaining in the region that becomes the cathode of the Gunn diode GD. The active layer  103  is then removed by etching so as to expose the anode-ohmic electrode forming high-density layer  102 , as shown in FIG. 2C.  
     [0057] When etching the Schottky region, as described above, the mask of SiN film or the SiO film is reused instead of newly forming a cathode region mask of the Gunn diode. Thereby, a smooth etching configuration can be obtained in such a manner that no irregular difference in level exists on the sidewall of the active layer  103  of the Gunn diode. If an irregular difference in level exists on the sidewall of the active layer  103 , then a frequency component unnecessary for the oscillation of the Gunn diode is generated. This consequently leads to an oscillation power reduction and an oscillation efficiency reduction.  
     [0058] At this time, if an InGaP layer having a thickness of, for example, 20 nm is interposed as an etching stopper layer between the active layer  103  and the anode-ohmic electrode forming high-density layer  102 , then the active layer  103  can be selectively removed by etching, although this arrangement is not adopted by this embodiment.  
     [0059] Next, as shown in FIG. 2C, AuGe (100 nm)/Ni (15 nm)/Au (100 nm) is formed by the vapor deposition method in the region where the anode-ohmic electrode  107  of the Gunn diode GD is formed, the region where the cathode-ohmic electrode  108  is formed and the region where the ohmic electrode  109  of the Schottky diode SD is formed. Then, an ohmic electrode alloying process is performed by heat treatment at a temperature of 390° C. Through this process, the anode-ohmic electrode  107 , the cathode-ohmic electrode  108  and the ohmic electrode  109  are formed. The above-mentioned AuGe (100 nm)/Ni (15 nm)/Au (100 nm) is a laminate film obtained by successively laminating an Ni layer of 15 nm and an Au layer of 100 nm on an AuGe layer of a layer thickness of 100 nm.  
     [0060] Next, resist patterning is performed to form a resist mask so that the Gunn diode GD in the region A and the Schottky diode SD in the region B are separated from each other. Then, the n + -GaAs layer  102  is etched for mesa isolation as shown in FIG. 2D. At this time, if isolation is performed by ion implantation instead of mesa isolation, then a difference in level becomes smaller than in the case of mesa isolation, and the subsequent resist coating patterning becomes easy.  
     [0061] Subsequently, a silicon oxide film or a silicon nitride film, which becomes a protective coat (not shown), is deposited to a thickness of 200 nm. Next, a resist  110  is left by resist patterning in a stepped portion ST of each device where the transmission line CP extends. Subsequently, heat treatment for reflow is performed at a temperature at which the resist  110  is softened to form the resist  110  shown in FIG. 2D. This resist  110  is to prevent disconnection of a transmission line  113  to be produced next in the stepped portion ST.  
     [0062] Next, a protecting film (not shown) is removed by etching from the anode-ohmic electrode  107  and the cathode-ohmic electrode  108  of the Gunn diode GD, from on the ohmic electrode  109  of the Schottky diode SD and from a Schottky electrode forming region  111  on the active layer  103 .  
     [0063] Next, a contact hole is formed, and a conductive film  112  constructed of Ti (100 nm) and Au (100 nm) is deposited on the entire surface by the vapor deposition method or the like.  
     [0064] This conductive film  112  plays not only a role of a power feed for plate-forming the Au film  113  that constitutes the transmission line CP but also a role of the Schottky electrode of the Schottky diode SD.  
     [0065] Although both the Schottky electrode and the power feed metal are concurrently formed with the conductive film  112  in this embodiment, it is also acceptable to form the Schottky electrode after forming the ohmic electrode  109  of the Schottky diode SD. Also, a high melting point metal of W (tungsten) or Mo (molybdenum), a high melting point nitride, a high melting point silicide, Al (aluminum) or the like can be used as a Schottky electrode material. It is preferable to select a material that can form a stable Schottky barrier.  
     [0066] Next, a resist of 15 μm in thickness is applied to perform patterning of the region where the transmission line  113  is formed, and thereafter, Au is plated to a thickness of 9 μm.  
     [0067] Subsequently, the above resist is removed, and the unnecessary conductive film  112  is removed by etching. The resist  110  that has been subjected to reflow is also removed, and thereby the transmission line CP is completed as shown in FIG. 1.  
     [0068] The thus-fabricated Schottky diode SD has a small capacitance per unit area and is able to be easily produced since the active layer of the Gunn diode is thick and has a low density, and this allows a varactor of a large capacitance variance to be easily obtained.  
     [0069] With this regard, both of the IMPATT diode of FIG. 5 and the Gunn diode of the present embodiment has such a structure as to oscillate in the milliwave band. However, the active layer of the IMPATT diode has a smaller thickness than that of the Gunn diode and requires a p-type n-type layer. This fact means that it is difficult to obtain a varactor diode of a small capacitance per unit area when the diode is fabricated by forming a Schottky electrode on the n-GaAs layer  803  of the IMPATT diode shown in FIG. 5. Specifically, the capacitance per unit area is large as exemplified by 1.5×10 −7  (F/cm 2 ) in FIG. 5 and 5×10 −8  (F/cm 2 ) in the present embodiment. This fact means that reduction of the device area is required in order to obtain a varactor of a small capacitance. However, fabrication of the varactor becomes difficult. Moreover, since the n-GaAs layer  803  has a small film thickness of 0.25 μm, the withstand voltage is low and the depletion layer less expands. Consequently, the capacitance variance is also reduced.  
     [0070] Although the transmission line CP is formed in a coplanar line in this embodiment, it may be formed in a microstrip line. Moreover, although the transmission line CP is formed with Au plating, Cu plating may be used so as to reduce fabrication cost.  
     [0071] The coplanar line is adopted for the transmission line in this embodiment. Particularly, when an NRD (non-radiative dielectric) guide is adopted in the milliwave band, a low-loss transmission line can be provided in comparison with the coplanar line and the microstrip line, and therefore this allows the prevention of performance degradation.  
     [0072] Moreover, in the fabricating process of this first embodiment, the resist  110  is formed at the stepped portion ST of each device and subjected to reflow so that the transmission line CP is not disconnected. However, instead of the reflow of the resist  110 , it is acceptable to apply and form a flattening film  114  of polyimide, benzocyclobutene, spin-on-glass or the like as shown in FIG. 4.  
     [0073] In the above case, after formation of a contact hole mask, the flattening film  114  is processed by dry etching so as to form a contact hole on the electrodes  107 ,  108 ,  111  and  109 . Subsequently, a transmission line CP constructed of the conductive film  112  and the Au film  113  is formed from the electrodes  107 ,  108 ,  111  and  109  onto the flattening film  114 . In this case, since the contact hole mask is formed on the flattening film  114 , photolithography of 1 μm or less can be easily achieved, which allows a minute contact hole to be formed. Therefore, the devices can be reduced in size for a fine structure.  
     [0074] Although a heterostructure of AlGaAs is employed as a cathode structure in the first embodiment, InGaP may be also used. This InGaP can be more easily subjected to selective etching in comparison with AlGaAs. Moreover, etching can be performed at a high selection ratio when a hydrochloric acid based etchant is used for this selective etching.  
     [0075] Although the GaAs/AlGaAs based semiconductor is employed in the aforementioned embodiment, other semiconductors that generate a negative resistance may be employed. For example, when an InP/InGaAs based semiconductor is employed instead of the GaAs/AlGaAs based semiconductor, characteristics such as efficiency at high frequencies of the Gunn diode are improved in comparison with the GaAs/AlGaAs system.  
     [0076] Next, a milliwave band oscillator that serves as a voltage-controlled oscillator according to a second embodiment of this invention will be described with reference to FIG. 3. According to the second embodiment, such a Gunn diode GD and a Schottky diode SD as described in the first embodiment are provided on a same substrate.  
     [0077] In this second embodiment, the Gunn diode GD formed in the region A of FIG. 1 is employed as an oscillation element  601 , and the Schottky diode SD formed in the region B of FIG. 1 is employed as a varactor diode to provide a variable capacitor  602 . The transmission line CP formed in the region C of FIG. 1 is employed as an output line  603  that has an impedance Z o  of 50 Ω. Then, the variable capacitor  602  and a λ/4 wavelength open stub  604  are connected together to constitute a resonator.  
     [0078] In the milliwave band (30 to 90 GHz), if the Gunn diode GD and the Schottky diode SD are formed of different wafers and mounted to constitute a VCO (voltage-controlled oscillator), the line loss and the mounting loss such as wirebond loss are increased, as a consequence of which the Q-value is reduced to lead to performance degradation such as phase noise degradation.  
     [0079] According to the oscillator of this second embodiment, therefore, the Gunn diode GD that serves as the oscillation element and the Schottky diode SD are formed on the same substrate so as to reduce distance between transmission lines, and thereby the line loss reduction and the miniaturization are achieved.  
     [0080] In this case, capacitance of the varactor diode constructed of the Schottky diode SD is also changed by a device area of the Schottky diode SD. For example, the capacitance is about 50 (fF) under no bias application since the device area is 100 μm 2  when the active layer  103  of the first embodiment is employed in this embodiment. The capacitance is reduced when the depletion layer is expanded by biasing the Schottky diode SD. On the other hand, the capacitance is increased in proportion to the device area. Therefore, the variable capacitance range of the variable capacitor  602  constructed of the Schottky diode SD can flexibly be changed.  
     [0081] Furthermore, the capacitance of the variable capacitor  602  can also be adjusted by changing a density of the active layer  103 . For example, when the density of the active layer  103  is increasingly inclined from a cathode interface toward an anode interface, the Schottky diode of a large capacitance variance can be obtained without impairing the characteristics of the Gunn diode. In this case, adjustment should be achieved within a range where the characteristics of the Gunn diode GD are not degraded. In addition, the active layer of the diode having a negative resistance is about 2000 nm in thickness in the milliwave band. The film thickness of the active layer is larger than thickness e.g. 500 nm of the collector layer of a bipolar transistor adapted for high frequency. Therefore, the expansion of the depletion layer is large. With this arrangement, the variable capacitance can be enlarged when the Schottky diode is employed as a varactor diode.  
     [0082] Next, a third embodiment of this invention will be described with reference to FIG. 6. FIG. 6A shows the construction of a millimeter wave transmitter provided with an oscillation circuit  910 , while FIG. 6B shows the construction of a millimeter wave receiver provided with an oscillation circuit  920 .  
     [0083] In the millimeter wave transmitter (and the millimeter wave receiver), a mixer  902  is connected to an oscillator  901  constructed of the voltage-controlled oscillator of the second embodiment. A filter  903  is connected to this mixer  902 , and a power amplifier  904  (a low noise amplifier  906 ) is connected between an antenna  905  and a filter  903 .  
     [0084] This mixer  902  is constructed of the Schottky diode SD formed in the region B of FIG. 1 of the first embodiment. The filter  903  is constructed of the transmission line CP formed in the region C of FIG. 1 of the first embodiment.  
     [0085] In this case, the high frequency characteristic is improved when thickness of the active layer  103  is reduced by control of etching in forming the Schottky diode SD, and thus the performance of the mixer  902  is also improved.  
     [0086] It is required to separately mount a transistor as the power amplifier  904  of the transmitter shown in FIG. 6A and the low noise amplifier  906  of the receiver shown in FIG. 6B. However, if a local signal from the oscillator  901  is sufficiently large, the power amplifier  904  and the low noise amplifier  906  are not necessary. This means that the transmitter and the receiver of the milliwave band can be made monolithic.  
     [0087] In the milliwave band, there occurs a problem that the mixer  902  cannot be operated with a large signal when the line loss is large between the oscillator  901  and the mixer  902 . Therefore, it is very effective in loss reduction to reduce the line dimension by fabricating the Gunn diode GD that serves as the oscillation element and the Schottky diode SD that constitutes the mixer  902  on the same substrate.  
     [0088] The invention being thus described, it will be obvious that the invention may be varied in many ways. Such variations are not be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.