Abstract:
Disclosed is a semiconductor device including a heterojunction bipolar transistor in which the front surface of a base layer and the surface of an emitter-base junction are covered with a high-resistivity layer of compound semiconductor containing at least one constituent element common to an emitter layer and the base layer.

Description:
This application is a divisional application of application Ser. No. 07/233,010, filed Aug. 17,1988. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a bipolar transistor employing a heterojunction, and a method of manufacturing the same. 
     A bipolar transistor utilizing a heterojunction such as of GaAs/AlGaAs can raise the impurity concentration of a base layer without incurring degradation in the current gain. As a result, a lower base resistance can be realized with the base layer thinner than in conventional transistors, and a high-speed operation becomes possible. 
     FIG. 3 shows a schematic sectional view of a mesa type heterojunction bipolar transistor in a prior art. Usually, such a device is fabricated in the following way: An n +  -type GaAs collector contact layer 32, an n-type GaAs collector layer 33, a p +  -type GaAs base layer 34, an n-type AlGaAs emitter layer 35 and an n +  -type GaAs emitter contact layer 36 are successively stacked and grown on a semi-insulating GaAs substrate 31 by MBE (molecular-beam epitaxy) or the like. Thereafter, the surfaces of the base layer 34 and the collector contact layer 32 are exposed by mesa etching, and electrodes 37, 38 and 39 respectively corresponding to an emitter, a base and a collector are formed. Then, the device is finished up. 
     For the high speed operation of the heterojunction bipolar transistor, it is very important to further lower the external base resistance and to reduce the base-collector capacitance. To these ends, it is effective to microfabricate the device and to shorten the interelectrode distances thereof. The microfabrication of the device, however, brings about the problem that the D.C. current gain of the device lowers with reduction in the emitter size thereof (the emitter size effect), as discussed in, for example, Japanese Journal of Applied Physics, Vol. 24, No. 8 (1985), pp. L596-L598. When an SiN film formed by sputtering is employed as a protective film, the emitter size effect mitigates as compared with those in the cases of other protective films (SiO 2  films produced by sputtering and CVD (chemical vapor deposition)). In practical use, however, the emitter size effect needs to be relieved still more. 
     SUMMARY OF THE INVENTION 
     The prior art stated above has had the problem that the lowering of the current gain attendant upon the microfabrication of the device (the emitter size effect) cannot be sufficiently relieved. 
     An object of the present invention is to remarkably relieve the emitter size effect. 
     The object can be accomplished in such a way that the surface of a base layer and the surface of an emitter-base junction are covered with a high-resistivity layer, especially a high-resistivity epitaxial layer, made of a compound semiconductor which is of the same sort as the compound semiconductors of emitter and base layers, that is, which contains at least one constituent element common to the emitter and base layers. 
     More concretely, in a compound-semiconductor bipolar transistor having a structure in which a collector layer, a base layer and an emitter layer are successively stacked on a semiconductor substrate; a semiconductor device according to the present invention is so constructed that said base layer includes an inner base region on which said emitter layer is placed in an identical plan shape, and an outer base region which is other than said inner base region, that the periphery of a p-n junction region defined between said inner base region and said emitter layer, and a part of a front surface of said outer base region as surrounds said p-n junction are covered with a compound-semiconductor high-resistivity layer which contains at least one element contained in both said emitter layer and said base layer as a constituent element thereof, that a base contact layer which is made of a compound semiconductor having the same conductivity type as that of said base layer is formed on a part of said outer base region not overlaid with said compound-semiconductor high-resistivity layer, and that a base electrode is formed on said base contact layer. 
     It has been revealed that, by covering the surface of the base and the surface of the emitter-base junction with the compound-semiconductor high-resistivity layer, the D.C. current gain (h FE ) of the transistor becomes greater than in the case of the SiN protective film, so the emitter size effect can be relieved still more. This is interpreted as follows: Since the compound-semiconductor high-resistivity layer contains the constituent element common to both the base layer and the emitter layer as described above, the lattice constant thereof is substantially equal to those of the base layer and emitter layer. Accordingly, the surface state density of the high-resistivity layer at the interfaces thereof with the base layer and emitter layer become much lower than in the case of the SiN film having heretofore been used. As a result, recombination currents in the base surface and the emitter-base junction surface decrease remarkably. 
     In addition, by providing the base electrode through the compound-semiconductor high-resistivity layer made of, for example, the high-resistivity epitaxial layer, the parasitic capacitances between the emitter and the base and between the collector and the emitter can be decreased. Thus, a high-speed operation can be performed. 
     Moreover, according to the semiconductor device of the present invention, the base contact layer and the compound-semiconductor high-resistivity layer can be formed so as to be substantially even with the emitter layer (or an emitter contact layer if formed), and a planar structure implemented with an Si device can be realized so as to facilitate integration. Further, the formation of the base contact layer can lower, not only the contact resistance of the base region relative to the base electrode, but also the D.C. resistance between the base electrode and the base region, so that the resistance of the base can be lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(a)-1(g) are processing sectional views of a semiconductor device in Embodiment 1 of the present invention; 
     FIG. 2 is a sectional view of a semiconductor device in Embodiment 2 of the present invention; and 
     FIG. 3 is a sectional view of a prior-art semiconductor device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1: This embodiment will be described with reference to processing sectional views shown in FIGS. 1(a)-1(g). 
     On an n +  -type GaAs substrate 11, and using MBE or the like, there are successively stacked and grown an n-type GaAs collector layer 12 having a thickness of about 4000-5000 Å, a p +  -type GaAs base layer 13 having a thickness of about 500-1000 Å, an n-type AlGaAs emitter layer 14 having a thickness of about 1000 Å, and an n +  -type GaAs emitter contact layer 15 having a thickness of about 1000 Å. Thereafter, using conventional CVD and lithographic techniques, an SiN film mask 16 which corresponds to the shape of an emitter pattern is formed on the emitter contact layer 15 (FIG. 1(a)). 
     Subsequently, an emitter region which is made up of the n +  -type GaAs emitter contact layer 15 and the n-type AlGaAs layer 14 is formed using the SiN film mask 16 and by dry etching with CF 4  or the like (FIG. 1(b)). 
     At the next step, any of proton, boron and oxygen is ion-implanted using the SiN film mask 16, whereby the part 112 of the collector layer 12 underlying the outer base region of the base layer 13 is rendered insulating (FIG. 1(c)). 
     Next, an undoped GaAs layer 17 is grown on the external base layer 13 by MOCVD (organometallic CVD) or the like, thereby to bury the emitter layer and the emitter contact layer 15 (FIG. 1(d)). Here, even the emitter contact layer 15 is buried for flattening the front surface of the device. To the accomplishment of that relief of the emitter size effect which is the original object of the present invention, it suffices to bury only the emitter layer 14. In addition, when the side walls of the emitter layer 14 and emitter contact layer 15 are made vertical or inverted-mesa-like by the etching illustrated in FIG. 1(b), the front surface of the device is perfectly flattened by the burying growth based on the aforementioned MOCVD. 
     Subsequently, the SiN film mask 16 is formed with a side-wall insulator film 18 of SiO 2  (FIG. 1(e)), and using them as a mask, an acceptor impurity such as beryllium (Be) is implanted, whereupon it is activated by predetermined annealing so as to form a base contact layer 113 (FIG. 1(f)). 
     Next, ohmic electrodes 19 and 110 made of AuGe/Au are respectively formed on the emitter contact layer 15 and the n +  -type GaAs substrate 11 to serve as a collector contact layer, and an ohmic electrode 111 made of Au/Zn/Au is formed on the base layer 13, by evaporation and a lift-off method. Then, the heterojunction bipolar transistor of the present invention is finished up (FIG. 1(g)). 
     When the D.C. current gain of this transistor was measured, a value nearly equal to that in the case of a large area was obtained even in case of a small emitter size. It has accordingly been found that the emitter size effect is sufficiently relieved. 
     Besides, the heterojunction bipolar transistor of this embodiment has its feature in that, as shown in FIG. 1(g), the front surface of the base and the surface of the emitter-base junction are covered with the high-resistivity epitaxial layer 17 made of the same compound semiconductor as that of the base. 
     Embodiment 2: In Embodiment 1, the collector is led out from the rear surface of the n +  -type GaAs substrate, whereas in Embodiment 2, a collector is led out from the same ride as that of an emitter or a base. Now, this embodiment will be described with reference to FIG. 2. On a semi-insulating GaAs substrate 21 and in the same manner as in Embodiment 1, there are successively grown an n +  -type GaAs collector contact layer 22, an n-type GaAs collector layer 23, a p +  -type GaAs base layer 24, an n-type AlGaAs emitter layer 25 and an n +  -type GaAs emitter contact layer 26 which are the constituent portions of a transistor. Thereafter, an emitter electrode 211 and a base electrode 212 are formed via the steps of FIGS. 1(a)-1(g) illustrated in Embodiment 1. Here, a collector electrode 214 is formed after an n +  -type collector lead-out portion 213 has been formed by etching a high-resistivity epitaxial layer 27 as well as the base layer 24 and ion-implanting a donor impurity such as Si. Numeral 29 in FIG. 2 indicates the part of the collector layer rendered insulating, and numeral 210 a base contact layer. 
     With this transistor, the same effect as in Embodiment 1 is attained. 
     Moreover, in this embodiment, the emitter electrode 211, base electrode 212 and collector electrode 214 are formed on the front surface of the device, so that the versatility of circuit designs is high. 
     In a case where the isolation of the high-resistivity epitaxial layer 17 or 27 in Embodiment 1 or 2 stated above is insufficient, the SiO 2  film 18 or an SiO 2  film 28 is removed after the formation of the emitter and base electrodes (FIG. 1(g) or FIG. 2), and the ion implantation of proton, boron, oxygen or the like is carried out using the electrodes 19, 111 or 211, 212 as a mask, whereby the effect of the isolation of the high-resistivity epitaxial layer 17 or 27 can be more enhanced. 
     Besides, although the epitaxial layer (a single crystal) has been employed as the high-resistivity semiconductor layer in Embodiments 1 and 2 stated above, it may well be replaced with an amorphous layer or a polycrystalline layer. In this case, the insulation between the emitter and the base is enhanced. 
     According to the present invention, recombinations in the surface of a base-emitter junction can be remarkably reduced, so that the emitter size effect can be removed. In addition, an emitter-base parasitic capacitance can be lowered, and the implementation of a planar structure is possible. Accordingly, a heterojunction bipolar transistor capable of higher-speed operation can be realized owing to the microfabrication of the device. 
     Besides, since an emitter-base distance is shortened by utilizing a side-wall insulator film, the resistance of a base can be lowered with a favorable controllability. Further, since an acceptor impurity is implanted into an undoped GaAs layer, a base lead-out portion which is stable and which is of lower resistance can be formed. Owing to such lowering in the base resistance, a heterojunction bipolar transistor having an excellent high-frequency response has been realized.