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 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).
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 xcex93 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""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 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 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 xcexcm, 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""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.
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 In0.53Ga0.47As 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.