Abstract:
A bipolar transistor is disclosed, that comprises a collector layer of first conduction type, a base layer of second conduction type, formed on the collector layer, a prevention layer, formed on the base layer, for preventing carriers from being recombined, an emitter layer of first conduction type, formed on a first part of the prevention layer, and a base electrode, formed on a second part separated from the first part of the prevention layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heterojunction bipolar transistor and a fabricating method thereof, and in particular, to those used for microwave devices (such as mobile telephone units) and optical communication signal processing circuits. 
     2. Prior Arts 
     FIG. 1 is a sectional view showing the structure of a heterojunction bipolar transistor (hereinafter referred. to as HBT) disclosed in “Extended Abstracts of the 1992 International Conference on Solid State Devices and Materials, pages 316-318 and “Electronic Letters”, Vol. 28, No. 25, pages 2308-2309. As shown in FIG. 1, in the HBT according to the first prior art reference, a sub-collector layer  52  is grown on a substrate  51 . A collector layer  53  is grown on the sub-collector layer  52 . A base layer  54  is grown on the collector layer  53 . A part of the base layer  54  is coated with a very thin emitter layer  55 . The thickness of the emitter layer  55  is around 30 nm. An emitter contact layer  56  is grown on the emitter layer  55 . 
     A base electrode  61  is formed on an alloyed region  63 . The alloyed region  63  is formed by heat treating the emitter layer  55 . The base electrode  61  and the base layer  54  are ohmic contacted through the alloyed region  63 . A part of the emitter layer  55  without being covered by the emitter contact layer is depleted and acts as guard-ring. Tile guard-ring  70  is depleted by pinning the Fermi level on front surface of a semiconductor so as prevent carriers from being recombined in the vicinity of the front surface of the base layer  54 . 
     An emitter electrode  62  is formed on the emitter contact layer  56 . An insulator film  80  is formed so that the insulator film  80  coats the guard-ring  70 , the base electrode  61 , the base layer  54 , and the sub-collector layer  52 . 
     However, in the HBT shown in FIG. 1, since the thickness of the emitter layer  55  is around 30 nm, even if the alloyed region  63  is formed, the contact resistance between the base layer  54  and the base electrode  61  becomes large. Thus, the high frequency characteristic of the HBT deteriorates. 
     As a second prior art reference that solves such a disadvantage, an HBT that does not have an alloyed region  63  is disclosed in JPA 10-303214. 
     FIG. 2 is a sectional view showing the structure of the HBT described in the publication as the second prior art reference. For simplicity, in FIG. 2, similar portions to those in FIG. 1 are denoted by similar reference numerals. In the HBT shown in FIG. 2, a base electrode  61  is formed so that the base electrode  61  contacts to a base layer  54  and a part of a guard-ring  70 . Thus, the base electrode  61  and the base layer  54  are contacted without need to use an alloyed region  63 . As a result, since the contact resistance between the base layer  54  and the base electrode  61  becomes small, the high frequency characteristic of the HBT can be suppressed from deteriorating. 
     However, in the HBT shown in FIG. 2, since the base electrode  61  is contacted to the guard-ring  70 , when the HBT is driven, the surface potential of the guard-ring  70  varies. Thus, a part of the guard-ring  70  is not depleted, but becomes a neutral region. Consequently, a PN junction between the guard-ring  70  and the base layer  54  causes carriers to be recombined. As a result, since a leak current flows between the base and the emitter of the HBT, the current gain decreases. 
     On the other hand, when the base electrode  61  is not contacted to the guard-ring  70 , carriers are recombined in the vicinity of the front surface of the base layer  54  and an edge portion  70   a  of the guard-ring  70 . As a result, a leak current flows. 
     SUMMARY OF THE INVENTION 
     In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide an HBT and its fabricating method in which a contact resistance between the base layer and the base electrode is small and a leak current between the base and the emitter is suppressed. 
     According to an aspect of the present invention, there is provided a bipolar transistor, comprising: a collector layer of first conduction type; a base layer of second conduction type, formed on the collector layer; a prevention layer, formed on the base layer, for preventing carrier recombination; an emitter layer of first conduction type, formed on a first part of the prevention layer; and a base electrode, formed on a second part separated from the first part of the prevention layer. 
     The bipolar transistor may further comprise: a guard ring, formed on an outer periphery of the emitter layer and on the prevention layer. 
     The bipolar transistor may further comprise: an emitter electrode, formed on the emitter layer. 
     The bipolar transistor may further comprise: an emitter contact layer, formed between the emitter layer and the emitter electrode. 
     The bipolar transistor may further comprise: a sub-collector layer of first conduction type, formed below the collector layer. 
     The bipolar transistor may further comprise: a collector electrode, connected to the sub-collector layer. 
     The bipolar transistor may further comprise: an insulation side wall, formed on the guard-ring, for defining the size of the guard-ring. 
     The bipolar transistor may further comprise: an alloyed region, piercing the prevention layer, formed between the base layer and the base electrode. 
     The bipolar transistor may further comprise: a diffused region, piercing the prevention layer, formed between the base layer and the base electrode. 
     In the bipolar transistor, the emitter layer may be a III-V group compound semiconductor. 
     In the bipolar transistor, a V group element of the III-V group compound semiconductor may be phosphorus. 
     In the bipolar transistor, the III-V group compound semiconductor may be selected from a group consisting of InGaP, InGaAsP, InGaAlP, InGaAlAsP, AlGaP, AlGaAsP, GaP, GasP, and InP. 
     In the bipolar transistor, the emitter layer may have a laminate structure. 
     In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers. 
     In the bipolar transistor, the base layer may be a III-V group compound semiconductor. 
     In the bipolar transistor, a V group element of the III-V group compound semiconductor may be arsenic. 
     In the bipolar transistor, the III-V group compound semiconductor may be selected from a group consisting of GaAs, AlGaAs, InGaAs, InGaAs, InAlGaAs, and InAlAs. 
     In the bipolar transistor, the base layer may have a laminate structure. 
     In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers. 
     In the bipolar transistor, the prevention layer may be of first conduction type. 
     In the bipolar transistor, a potential gap of the prevention layer may be larger than a potential gap of the base layer so that the potential gap of the prevention layer functions as a potential barrier for majority carriers of the base layer. 
     In the bipolar transistor, when the bipolar transistor is driven, a portion of the prevention layer which is not coated with the emitter layer may be fully depleted. 
     In the bipolar transistor, when the bipolar transistor is driven, a portion of the prevention layer which is not coated with the emitter layer or the guard-ring layer may be fully depleted. 
     In the bipolar transistor, a potential barrier of the prevention layer for majority carriers of the emitter layer may be 100 meV or less. 
     In the bipolar transistor, the prevention layer may have a laminate structure. 
     In the bipolar transistor, the laminate structure may have a sloped-composition layer between two adjacent layers. 
     In the bipolar transistor, the alloyed region may contain palladium or platinum. 
     In the bipolar transistor, the diffusion region may be doped with at least one of zinc, beryllium, carbon, magnesium, and manganese. 
     In the bipolar transistor, the highest portion of the prevention layer may have an etching resistance against an etchant of the emitter layer. 
     In the bipolar transistor, the highest portion of the base layer may have an etching resistance against an etchant of the prevention layer. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best modes of embodiment thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a sectional view showing the structure of an HBT according to a first prior art reference; 
     FIG. 2 is a sectional view showing the structure of a modification of the structure shown in FIG. 1 as a second prior art reference; 
     FIG. 3 is a sectional view showing the structure of an HBT according to a first embodiment of the present invention; 
     FIGS. 4A to  4 D are sectional views showing the structure of the HBT shown in FIG. 3 at steps of a fabricating method according to the present invention; 
     FIG. 5 is a sectional view showing the structure of an HBT according to a second embodiment of the present invention; 
     FIG. 6 is a sectional view showing the structure of an HBT according to a third embodiment of the present invention; and 
     FIG. 7 is a sectional view showing the structure of an HBT according to a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, with reference to the accompanying drawings, embodiments of the present invention will be described. 
     [First Embodiment] 
     FIG. 3 is a sectional view showing the structure of an HBT according to a first embodiment of the present invention. The HBT shown in FIG. 3 is an InGaP/GaAs NPN type of HBT. A sub-collector layer  2  is grown on a substrate  1 . The sub-collector layer  2  is composed of n +  type of GaAs. The substrate  1  is composed of semi insulating GaAs. A collector layer  3  is grown on the sub-collector layer  2 . The collector layer  3  is composed of n type of GaAs. 
     A base layer  4  is grown on the collector layer  3 . The base layer  4  is composed of p +  type of GaAs. A prevention layer  5  is grown as a thin layer having a thickness of, for example, around 5 nm on the base layer  4  so that carriers are not recombined in the vicinity of the front surface of the base layer  4 . The prevention layer  5  is composed of n type of InGaAsP. 
     The prevention layer  5  and the base layer  4  are grown so that the potential gap of the prevention layer  5  is larger than that of the base layer  4 . As a result, the potential gap of the prevention layer  5  becomes a potential barrier for majority carriers of the base layer  4 . The thickness of the prevention layer  5  is decreased or the doping concentration is decreased corresponding to the thickness of the prevention layer  5  so that at least a part of the prevention layer other than two parts, that is, a first part which is covered with a emitter mesa (composed of the emitter layer  6  and emitter contact layer  7 ) and a second part which is covered with the guard-ring  20 , will be completely depleted in operation. 
     It is preferred that the potential barrier of the prevention layer  5  for majority carriers of the emitter layer  6  is small. As a permissible value, the potential barrier is 100 meV or less. To do that, the material of the prevention layer is properly selected corresponding to the material of the emitter layer  6 . 
     The emitter layer  6  is grown as a thin layer having the thickness of, for example, around 30 nm on the prevention layer  5 . The emitter layer  6  is composed of n type of InGaP. The emitter contact layer  7  is grown on the emitter layer  6 . The emitter contact layer  7  is composed of n +  type of GaAs. A base electrode  11  is spaced apart from the emitter mesa composed of the emitter layer  6  and the emitter contact layer  7  by around 500 nm. In addition, the base electrode  11  is directly formed on the prevention layer  5 . 
     The guard-ring  20  is formed around the emitter layer  6  as a depleted portion of the emitter layer  6 . The guard-ring  20  is depleted by pinning the Fermi level in front surface of the semiconductor. The guard-ring  20  prevents carriers from being recombined in the vicinity of the front surface of the base layer  4 . The thickness of the guard-ring  20  is, for example, around 50 nm. 
     An emitter electrode  12  is formed on the emitter contact layer  7 . In addition, an insulator film  80  is formed so that the insulator film  80  coats the guard-ring  20 , the base electrode  11 , the base layer  4 , and the sub-collector layer  2 . 
     According to the first embodiment of the present invention, carrier recombination due to exposure of the base layer  4  to the insulator film  80  is prevented. In addition, carrier recombination at edge  20   a  of the guard-ring  20  due to contact of the base layer  4  with the guard-ring  20  is prevented. Thus, a leak current due to such carrier recombinations can be suppressed. Consequently, the current gain does not deteriorate. That is an advantage over the second prior art reference. 
     In addition, according to the first embodiment of the present invention, since only the prevention layer  5  is interposed between the base layer  4  and the base electrode  11 , the contact resistance between the base layer  4  and the base electrode  11  is the same as that in the case they are directly contacted. Thus, the contact resistance can be remarkably decreased in comparison with the first prior art reference. 
     Consequently, the first embodiment overcomes the disadvantages of the first prior art reference and the second prior art reference. 
     FIGS. 4A to  4 D are sectional views showing the structure of the HBT shown in FIG. 3 at steps of the fabricating method according to the present invention. Next, with reference to FIGS. 4A,  4 B,  4 C, and  4 D, the fabricating method of the HBT shown in FIG. 3 will be described. First, the subcollector layer  2 , the collector layer  3 , the base layer  4 , the prevention layer  5 , the emitter layer  6 , and the emitter contact layer  7  are successively deposited on the substrate  1 . 
     Those layers are deposited by an epitaxial growth method such as an MOCVD (Metal Organic Chemical Vapor Deposition) method and an MBE (Molecular Beam Epitaxy) method. The thickness of the prevention layer  5  is preferably several nm so as to prevent the contact resistance between the base layer  4  and the base electrode  11  from becoming large. 
     In order to form the prevention layer  5  as a thin layer, it may be prevented that the prevention layer  5  is directly formed on the base layer  4 . Instead, the emitter layer  6  is formed on the base layer  4 . They are heat-treated and reacted in the solid phase so as to form the prevention layer  5 . The optimum temperature and time necessary for the heat treatment depend on the materials of the base layer  4  and the emitter layer  6 . For example, when the emitter layer  6  is composed of n type of InGaP and the base layer  4  is composed of p type of GaAs, the heat treatment is performed in the atmosphere of arsenic at 680° C. for around 10 minutes. 
     Thereafter, in order to form the emitter electrode  12 , a Ti layer, a Pt layer, and an Au layer (not shown) are successively deposited on the emitter contact layer  7 . A predetermined pattern is formed with a photo resist  30  on the Au layer. With the pattern as a mask, unnecessary portions of the Ti layer, the Pt layer, and the Au layer are removed by an ion milling method or the like. As a result, the emitter electrode  12  is formed (as shown in FIG.  4 A). Alternatively, as a mask material, an insulation material such as SiN x  or SiO 2  may be used instead of the photo resist  30 . 
     After the photo resist  30  is removed, using the emitter electrode  12  as a mask, the emitter contact layer  7  is etched by a reactive ion beam etching (RIBE) method or the wet etching method. If a so-called selective etching method in which the emitter layer  6  has a sufficient etching resistance against an etchant of the emitter contact layer  7  is used when forming the emitter layer  7 , the emitter layer  6  can be prevented from being removed. 
     Next, as shown in FIG. 4B, with a mask composed of a photo resist  31 , a part of the emitter layer  6  is removed. As a result, the emitter layer  6  is left below and around the emitter electrode  12 . The periphery of the lower portion of the emitter electrode  12  functions as a guard-ring  20 . It is preferred that the prevention layer  5  has a sufficient etching resistance against an etchant of the emitter layer  6 . 
     As shown in FIG. 4C, with a photo resist  32 , a mask with a pattern having a predetermined open portion is formed. Thereafter, a Pt layer, a Ti layer, a Pt layer, an Au layer, and a Ti layer are successively evaporated on the entire surface of the wafer so as to form the base electrode  11 . By a lift off method or the like, the mask and metals evaporated thereon are removed. As a result, the base electrode  11  is formed. 
     As shown in FIG. 4D, with a photo resist, unnecessary portions of the prevention layer  5 , the base layer  4 , and the collector layer  3  are etched out. Thereafter, an insulator film  80  is formed so that the insulator film  80  coats the guard-ring  20 , the base electrode  11 , the base layer  4 , and the sub-collector layer  2 . The insulator film  80  is formed by depositing an SiO 2  film having a thickness of around 100 to 200 nm by, for example, a chemical vapor deposition (CVD) method. Thereafter, the collector electrode  10  (as shown in FIG. 3) is formed on the sub-collector layer  2 . As a result of the above-described fabricating method, the HBT shown in FIG. 3 is obtained. 
     According to the present invention, the fabricating method for the HBT shown in FIG. 3 is not limited the steps shown in FIGS. 4A to  4 D. For example, in the fabricating method, the step for removing the unnecessary portions of the base layer  4  and the collector layer  3  may be exchanged with the step for forming the base electrode  11 . 
     In addition, the HBT shown in FIG. 3 may be of PNP type of which the conduction type of each layer is inverted. Alternatively, the HBT shown in FIG. 3 may be of InP type. Moreover, the materials of the layers of the HBT are not limited to those described above. 
     The material of the base layer  4  may be AlGaAs, InGaAs, InAlGaAs, and InAlAs as well as GaAs. The base layer  4  may be a laminate composed of these materials. In the laminate structure, a sloped-composition layer may be interposed between two adjacent layers. 
     The material of the prevention layer  5  may be InGaAlAsP, AlGaAsP, GaAsP, AlAsP, and InAsP as well as InGaAsP. Alternatively, the prevention layer  5  may be a laminate composed of those materials. In the laminate structure, a sloped-composition layer may be interposed between two adjacent layers. 
     The material of the prevention layer  6  may be InGaAsP, InGaAlP, InGaAlAsP, AlGaP, AlGaP, AlGaAsP, GaP, GaAsP, and InP as well as InGaP. Alternatively, the emitter layer  6  may be a laminate composed of those materials. In the laminate structure, a sloped-composition layer may be interposed between two adjacent layers. 
     In other words, as long as the prevention layer  5  is formed along with the emitter layer  6  and the base layer  4  by an epitaxial growth method or the like, in any combination of those materials of the those layers, a desired effect can be obtained. 
     However, in case the base layer  4  and the emitter layer  6  are heat-treated to form the prevention layer  5  after the emitter layer  6  has been formed on the base layer  4 , the material of the prevention layer  5  contains the materials of the base layer  4  and the emitter layer  6 . 
     [Second Embodiment] 
     FIG. 5 is a sectional view showing the structure of an HBT according to a second embodiment of the present invention. The HBT shown in FIG. 5 is different from the HBT shown in FIG. 4 in that an insulation side wall  21  having a thickness of around 100 to 300 nm is formed on a guard-ring  20  using the emitter contact layer  7  as a reference. 
     The insulation side wall  21  is used as an etching mask when forming the guard-ring  20 . After an emitter mesa is formed, an insulation layer is grown on the entire surface of the wafer. Thereafter, the insulation layer is anisotropically etched by a reactive ion etching (RIE) method or the like to form the insulation side wall. The other layer structure and the fabricating steps of the HBT according to the second embodiment are the same as those according to the first embodiment. For simplicity, in FIG. 5, similar portion to those in FIG. 3 are denoted by similar reference numerals. 
     According to the first embodiment, when the mask is misaligned, the length of a part of the guard-ring  20  may become short. In the short portion, carriers may be recombined. In contrast, according to the second embodiment, since the length of the guard-ring  20  does not vary, such a problem does not take place. 
     [Third Embodiment] 
     FIG. 6 is a sectional view showing the structure of an HBT according to a third embodiment of the present invention. The structure of the HBT shown in FIG. 6 is the same as the structure of the HBT shown in FIG. 3 except that the former further contains an alloyed region  22  that electrically connects a base electrode  11  and a base layer  4 . The alloyed region  22  may contain palladium or platinum. 
     For example, the alloyed region  22  is formed by heat-treating a base electrode  11  so as to alloy the lowest layer (Pt layer) of the base electrode  11  and a prevention layer  5 . Thus, the contact resistance between the base electrode  11  and the base layer  4  becomes small. 
     [Fourth Embodiment] 
     FIG. 7 is a sectional view showing the structure of an HBT according to a fourth embodiment of the present invention. The structure of the HBT shown in FIG. 7 is the same as the structure of the HBT shown in FIG. 3 except that the former further contains a diffusion region  23  with the same conduction type as a base layer  4 . The diffusion region  23  is doped with at least one of zinc, beryllium, carbon, magnesium, and manganese. 
     The diffusion region  23  is formed by selectively doping ions to a portion at which the base electrode  11  is formed on the prevention layer  5  using a mask or the like or by heat-treating a substance containing diffused ions such as ZnO so as to diffuse Zn. Thus, the contact resistance between the base electrode  11  and the base layer  4  becomes small. 
     Although the present invention has been shown and described with respect to the best modes of embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.