Heterojunction bipolar transistor

Abstract: In a first heterojunction bipolar transistor (HBT) of the present invention, base layers (105,106) and collector layers (103,104) are respectively divided into a plurality of layers and one of the base layers (105,106) provided closer to the collector layer region (103,104) is set lower in impurity concentration than the other thereof provided closer to an emitter layer (107), thus solving a problem that thermal histories during epitaxial growth or during processes cause a set impurity distribution to be destroyed due to diffusion and thus a heterojunction is shifted from a p-n junction. Since minority carriers in the base can smoothly flow toward the collector, there can be realized an excellent HBT having a very high current gain and a very high cut-off frequency. In a second HBT of the invention, a base region (204, 205) comprises a first base layer (205) of a low concentration having the same energy band gap as an emitter region (206) and to be changed to a complete depletion layer in a thermally balanced state and a graded second base layer (204) of a high concentration, and the first and second base layers form a heterojunction, thereby realizing an excellent HBT having a high speed performance which can exhibit a sufficient grading effect while preventing deterioration of an emitter-base voltage withstanding characteristic. Further, in a third HBT of the invention, since an emitter region (306,307) comprises a first emitter layer (307) and a second emitter layer (306) of a lower concentration made of the same material as the first emitter layer while a base region (304,305) comprises a first base layer (305) having the same energy band gap as the emitter region (306,307) and a second base layer (304) narrower in an energy band gap than the first base layer (305), an excellent HBT having a high operational speed can be realized.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heterojunction bipolar transistors and more particularly, to a heterojunction bipolar transistor having an improved base and collector structure. 2. Description of the Related Art A heterojunction bipolar transistor (hereinafter referred to as HBT) having an emitter region made of semiconductor material that is larger in band gap than a base region, has been widely studied as a microwave transistor or a high-speed logical circuit transistor, since the heterojunction transistor is more excellent in high frequency and switching characteristics than a homojunction bipolar transistor. The HBT using GaAs or AlGaAs as the semiconductor material, in particular, has been increasingly considered to offer a very promising prospect as a superhigh speed device, since the transistor is larger in carrier mobility than a transistor using Si. However. because such compound semiconductor as GaAs or AlGaAs is more expensive and brittler than Si, the HBT using such material has been considered to be rather unsuitable in the current circumstances for its mass production when compared with the transistor using Si. In order to overcome such circumstances, there have been made attempts to introducing the heterojunction into the Si bipolar transistor to attain its higher performances. And there has been recently reported a so-called silicon-based HBT having a wide gap emitter of Si 1-x C x (0.0< x ≦ 1.0) grown on Si or having a narrow gap base of Si 1-x Ge x (0.0< x≦1.0, hereinafter x is assumed to be in the same range) grown on Si, which has been made on an experimental basis. In particular, the latter Si 1-x Ge x /Si HBT, in which a base layer is made of material smaller in band gap than Si and thus its turn-on voltage is small, requires less power consumption than the prior art Si bipolar transistor. In addition, since Si 1-x Ge x /Si HBT uses a base layer made of mixed crystal Si 1-x Ge x , a so-called graded base structure wherein an electric field for acceleration of carriers is provided in the base can be employed therefor and thus the transistor can be operated at a speed faster than that of the prior art Si bipolar transistor. In this way, the Si 1-x Ge x /Si HBT, when compared with the prior art homojunction silicon bipolar transistor, requires less power consumption and produces a higher operational speed, but has a problem which follows, because of its so-called double heterostructure, i.e., having the other heterojunction also in the collector side. Fig. 11 shows a diagrammatical cross-sectional view of an epitaxial wafer used in a prior art Si 1-x Ge x /Si series HBT. The epitaxial wafer is obtained usually for instance by a molecular beam epitaxy (MBE) technique, a chemical vapor deposition (CVD) technique based on a limited reaction processing (LRP). The HBT is prepared with use of such a wafer. That is, the wafer is made by sequentially epitaxially growing an n + type Si layer 21 of 200nm thickness and having a doping concentration of 1x10 20 cm -3 as a collector contact layer, an n - type Si layer 22 of 500nm thickness and having a doping concentration of 5x10 16 cm -3 as a collector layer, a p + type Si 0.9 Ge 0.1 layer 23 of 50nm thickness and having a doping concentration of 1x10 19 cm -3 as a base layer, an n type Si layer 24 of 150nm thickness and having a doping concentration of 1x10 17 cm -3 as an emitter layer, and an n + type Si layer 25 of 100nm thickness and having a doping concentration of 1x10 20 cm -3 as an emitter cap layer all formed on a p - type Si substrate 20. The impurity doping concentrations for the respective epitaxial layers are set to become uniform. The thicknesses of the respective layers and dopant types illustrated in Fig. 11 refer to so-called design values set prior to the execution of the epitaxial growth. Shown in Fig. 12 is an energy band profile within a device when it is assumed that the dopants are activated as carriers in a condition where the thicknesses of the respective layers of a wafer and the impurity distributions are the same as design values. In the drawing, electron energy is expressed as positive. As will be seen from the drawing, an energy band discontinuity at a heterojunction interface between the Si 0.9 Ge 0.1 and Si layers is about 0.1eV, 80% of which, i.e. 0.08eV, appears as a band discontinuity on the side of a valence band and the remainder, i.e. 0.02eV, that is very small, appears as a band discontinuity on the side of a conduction band. It is known that such tendency is not changed and the band discontinuity value of the conduction band side is not changed and substantially constant even when a content of Ge to Si is increased to a fairly high level. Accordingly, in the case of an npn type bipolar transistor, the band discontinuity of the conduction band side is very small even when the heterojunction interface between the Si 0.9 Ge 0.1 and Si layers is abruptly changed, so that electrons can smoothly flow from an emitter to a collector while holes can be effectively trapped within a base layer by means of a potential barrier caused by the large band discontinuity of the valence band side. Thus, it will be appreciated that, so long as at least this material series is employed, the use of the abrupter heterojunction enables the transistor to have a higher performance. In the course of actually fabricating a transistor with use of the aforementioned epitaxial wafer, the wafer experiences various thermal histories including epitaxial growth itself. That is, heating the wafer to a high temperature causes a doping profile so far set to be inevitably changed due to thermal diffusion. In particular, the base layer is as thin as at most 100nm and very high in doping concentration, the thermal diffusion of the dopant out of the base layer becomes a problem. Fig. 13 shows an impurity profile in a wafer of a resultant transistor completed after the wafer of Fig. 11 experiences various thermal histories, in which a design impurity profile is represented by solid lines and the final impurity profile is by broken lines and two-dot chain lines. It will be seen from Fig. 13 that, since the dopant of the base layer diffuses into the emitter and collector sides and p type and n type dopants are effectively changed as compensated for, the heterojunction and the p-n junction are shifted from their set positions. Shown in Fig. 14 is a model form of energy band profile corresponding to the effective doping profile of the final transistor shown in Fig. 13. In Fig. 14, electron energy is set to be positive as in Fig. 12. It will be seen from the drawing that, since the collector is usually set to be lower in impurity concentration than the emitter for the purpose of making small the capacitance of the base/collector junction, the effective impurity type in the vicinity of the base/collector junction in the collector is p type due to the diffusion of the p type dopant from the base layer thereto, which results in that the heterojunction is formed in the p type region and a potential barrier to electrons is formed in the conduction band. For this reason, so long as such an epitaxial wafer film structure as in the prior art is employed, not only it is impossible to obtain a previously set doping profile for the completed final transistor but also the potential barrier formed on the conduction band side prevents the flow of electrons from the base to the collector. This results in that the electron transportation efficiency is remarkably reduced and thus the current amplification factor and the operational speed are reduced, which becomes a serious problem in the transistor fabrication. This problem is not limited to the aforementioned Si 1-x Ge x /Si HBT and holds true even for an HBT of an npn type and of a double heterostructure using such semiconductor material having the band discontinuity of the heterojunction appearing mainly on the valence band side as InP/GaInAsP compound semiconductor material and when employing such an epitaxial wafer film structure as in the prior art. Further, even when an HBT of a pnp type and of a double heterostructure is to be prepared using such materials having the band discontinuity appearing mainly on the conduction band side as AlGaAs and GaAs, this problem becomes serious. In this way, the HBT of the double heterostructure using such semiconductor material that the band discontinuity of the heterojunction appears mainly on the valence band side has had a problem that, since the heterojunction and the p-n junction, in particular, at the base/collector junction are shifted by the thermal histories. It is difficult to realize a high current gain and a high speed operation inherent in an HBT. In an HBT, it is common practice to make thin a base region for the purpose of obtaining a high speed operation. However, making thin the base region, at the same time, causes a base sheet resistance to be increased. To avoid this, it is desirable to set the base region at a high impurity concentration. Electronics Letters, vol. 17, no. 8, April, 1981 pages 301 - 302 discloses a n - GaAs/p - GaAs/p + GaAs/nAlGaAs structure, i.e., a structure in which a base has a two-layered structure and a base-collector junction is a homojunction. In this structure, the p - GaAs layer having a low concentration is provided in contact with the collector region. IEEE Transactions on Electron Devices, vol. ED-34, no. 2, February 1987, pages 185-197 discloses a hetero bipolar transistor having an abrupt double heterojunction. In view of the above circumstances, it is an object of the present invention to provide a layer structure in an epitaxial wafer for use in fabricating an HBT of an npn type which has a base/collector junction of an abrupt heterojunction and uses a semiconductor material causing band discontinuity of the heterojunction to appear mainly on the side of a valence band, which layer structure can prevent formation of a potential barrier to electrons in the vicinity of a base/collector junction of a conduction band to thereby obtain a smooth conduction band profile allowing quick flow of electrons. Another object of the present invention is to provide a layer structure in an epitaxial wafer for use in fabricating an HBT of a pnp type which has a base/collector junction of an abrupt heterojunction and uses a semiconductor material causing band discontinuity of the heterojunction to appear mainly on the side of a conduction band, which layer structure can prevent formation of a potential barrier to positive holes in the vicinity of a base/collector junction of a conduction band to thereby obtain a smooth valence band profile allowing quick flow of positive holes. Thus, as set out in claim 1, the present invention provides a heterojunction bipolar transistor comprising a first semiconductor layer of a first conductivity type, a second semiconductor layer of a second conductivity type, a third semiconductor layer of the second conductivity type, a fourth semiconductor layer of the first conductivity type and fifth semiconductor layer of the first conductivity type, said first to fifth layers being sequentially formed as laminated, wherein the forbidden band widths of said first, fourth and fifth semiconductor layers are larger than the forbidden band widths of said second and third semiconductor layers, an heterojunction between the third and fourth semiconductor layers being formed so that the forbidden band widths abruptly vary within a range of several atomic layers, the first semiconductor layer functioning as an emitter, the second and third semiconductor layers functioning as a base, and the fourth and fifth semiconductor layers functioning as a collector, said transistor being characterised by satisfying the following relationship: W 2 ≥ W 3 and N 2 ≥ N 3 and N 4 > N 5 where, said second semiconductor layer has an impurity doping concentration N 2 and a thickness W 2 , said third semiconductor layer has an impurity doping concentration N 3 and a thickness W 3 , said fourth semiconductor layer has an impurity doping concentration N 4 and a thickness W 4 and said fifth semiconductor layer has an impurity doping concentration N 5 . Specific embodiments of the present invention are disclosed in the dependent claims. In other words, in the HBT of the present invention, one of the sublayers of the base layer on the side of the collector layer is lower, or equal to, in impurity concentration than the other on the side of the emitter layer, the base sublayer having the lower impurity concentration is smaller, or equal to, in thickness than the other having the higher impurity concentration, while one of the sublayers of the collector layer on the side of the base layer is set higher in impurity concentration than the other. As a result, even when the wafer is subjected to thermal histories during processes in fabricating the transistor where dopants of the higher concentration base sublayer are thermally diffused toward the collector layer side, so long as the diffusion stops in the lower concentration base sublayer, any shift between a p-n junction and a heterojunction will not occur. In addition, even when dopants of the higher concentration base sublayer are thermally diffused into the collector layer, the collector layer of the relatively high concentration compensates for the dopant type, so that the formation of a potential barrier to minority carriers in the base layer can be prevented and thus carriers entering from the emitter layer into the base layer can move quickly toward the collector layer. In this way, in an embodiment of the HBT of the present invention, since each of the base and collector layers is divided into a plurality of sublayers and one of the sublayers of the base layer disposed on the side of the collector layer is set lower in impurity concentration than the other disposed on the side of the emitter layer, even when the dopants of the higher-concentration base sublayer are diffused toward the collector layer during the fabricating processes of the transistor, a positional shift between the heterojunction and p-n junction can be prevented since the diffusion of the dopants of the higher-concentration base sublayer stops in the lower-impurity-concentration sublayer. Further, one of the sublayers of the collector layer disposed on the side of the base layer is set higher in impurity concentration than the other disposed on the side of the collector sublayer, even when the dopants of the higher-concentration base sublayer are diffused toward the collector layer and reach the collector region, the dopants are compensated for by the higher-concentration collector sublayer, the type of the dopants of the collector region remains the same or is changed to an i layer, thus ensuring the elimination of a possibility that a positional shift between the heterojunction and p-n junction may take place. Accordingly, when the layer structure according to the present invention is employed, a potential barrier to minority carriers can be prevented from being formed in the base layer so that carriers entering from the emitter layer into the base layer can move quickly into the collector layer, whereby there can be provided an HBT which has a very high current gain and a very high cut-off frequency f T . In an embodiment of the HBT of the present invention, the following relationship is satisfied: where, one of the sublayers of the base layer disposed closer to the collector layer is set to have an impurity doping concentration N 3 and a dielectric constant ε 3 and one of the sublayers of the collector layer disposed closer to the base layer is set to have an impurity doping concentration N 4 and a dielectric constant ε 4 and a thickness W 4 , and a built-in voltage at the base/collector junction is V bi . That is, this embodiment of the HBT of the present invention is featured in that the thicknesses and doping concentrations of the respective layers are set so that the higher concentration sublayer of the collector layer provided closer to the base layer is depleted in a thermally balanced state. As a result, dopants are not diffused from the base layer to the higher concentration collector sublayer so that, even when the impurity concentration cannot be effectively reduced, since the thicknesses and doping concentrations of the layers are set so as to previously deplete the higher concentration collector sublayer (otherwise, the capacitance of the base/collector junction becomes large due to the high-concentration p-n junction), the capacitance of the base/collector junction can be suppressed not to be very large. In this way, the HBT of this embodiment of the present invention is featured in that the thicknesses and doping concentrations of the base and collector layers contacted with a hetero-interface between the base and collector are set so as to deplete the higher concentration sublayer of the collector layer provided closer to the base layer. With such a structure, dopants are not diffused from the base layer to the higher-concentration collector sublayer so that, even when the impurity concentration cannot be effectively reduced, since the thicknesses and doping concentrations of the layers are set so as to previously deplete the higher concentration collector sublayer (otherwise, the capacitance of the base/collector junction becomes large due to the high-concentration p-n junction), the capacitance of the base/collector junction can be suppressed not to be very large. Thus, there can be provided an HBT which has a very high maximum oscillation frequency f MAX and a very small propagation delay time t Dd . BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a heterojunction bipolar transistor (HBT) in accordance with a first embodiment of the present invention; Fig. 2 shows a design impurity profile of the HBT of the first embodiment of the present invention and an impurity profile after subjected to processes; Fig. 3 shows energy band profiles of the HBT of the first embodiment in a thermally balanced state obtained through simulation; Fig. 4 is a graph showing relationships between current gain and current density obtained through simulation with respect to the HBT of the first embodiment of the present invention and a prior art HBT; Fig. 5 is a graph showing relationships between cut-off frequency and current density obtained through simulation with respect to the HBT of the first embodiment of the present invention and the prior art HBT: Fig. 6 is a cross-sectional view of an HBT in accordance with a second embodiment of the present invention; Fig. 7 is a cross-sectional view of an HBT in accordance with a third embodiment of the present invention; Fig. 8 is a cross-sectional view of an HBT in accordance with a fourth embodiment of the present invention; Fig. 9 is a cross-sectional view of an HBT in accordance with a fifth embodiment of the present invention; Fig. 10 is a cross-sectional view of an HBT in accordance with a sixth embodiment of the present invention; Fig. 11 is a cross-sectional view of epitaxial layers of a prior art HBT; Fig. 12 shows energy band profiles of a prior art HBT of a film structure when an impurity profile according to design values was attained; Fig. 13 shows a design impurity profile of the prior art HBT and an impurity profile after subjected processes; Fig. 14 shows energy band profiles of the prior art HBT in a thermally balanced state obtained through simulation; DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be detailed in connection with its embodiments. Embodiment 1: Referring first to Fig. 1, there is shown a cross-sectional view of an HBT using a Si 1-x Ge x material in accordance with a first embodiment of the present invention, in which the types of dopants are ones set prior to epitaxial growth. Prior to the fabrication of this transistor, it is first necessary to sequentially epitaxially grow semiconductor layers on a p - type Si substrate 101. The epitaxial growth is carried out by, for instance, a molecular beam epitaxy (MBE) technique, a chemical vapor deposition technique based on a limited reaction processing (LRP). Explanation will next be made as to the detailed fabrication conditions according to the sequential fabrication steps. First epitaxially grown on the p - type Si substrate 101 is an Sb-doped Si layer 102 of an n + type having an impurity concentration of 1x10 20 cm -3 and a thickness of 200nm as a collector contact layer. Then epitaxially grown on the layer 102 are an Sb-doped Si layer 103 of an n - type having an impurity concentration of 5x10 16 cm -3 and a thickness of 450nm as a first collector layer and subsequently an Sb-doped Si layer 104 of an n type having an impurity concentration of 4x10 17 cm -3 and a thickness of 50nm as a second collector layer sequentially. Next epitaxially grown on the layer 104 are a B-doped Si 0.9 Ge 0.1 layer 105 of a p - type having an impurity concentration of 5x10 17 cm -3 and a thickness of 30nm as a first base layer and then a B-doped Si 0.9 Ge 0.1 layer 106 of a p + type having an impurity concentration of 1x10 19 cm -3 and a thickness of 470nm as a second base layer sequentially. Epitaxially grown on the layer 106 are an Sb-doped Si layer 107 of an n type having an impurity concentration of 5x10 17 cm -3 and a thickness of 150nm as an emitter layer and subsequently an Sb-doped Si layer 108 of an n + type having an impurity concentration of 1x10 20 cm -3 and a thickness of 100nm as a emitter cap layer sequentially. A wafer having the layers thus epitaxially grown is subjected to an etching process until a base electrode formation region reaches the p + type Si 0.9 Ge 0.1 layer 106 and at the same time, a collector electrode formation region is subjected to an etching process until it reaches the n + type Si layer 102 as the collector contact layer. After that, the resultant wafer is subjected to a CVD process to deposit a silicon oxide film 112 on the entire surface of the wafer and then to a selective etching process to remove those parts of the silicon oxide film corresponding to the contact areas with an emitter, a base and a collector. Finally, an emitter electrode 109, a base electrode 110 and a collector electrode 111 all as aluminum layers are formed, at which stage a transistor is completed. Fig. 2 shows impurity profiles for a transistor having the same structure as the thus-fabricated HBT in its depth direction, in which a design impurity profile set according to design values is denoted by a solid line, an Sb impurity profile varied under the influence of thermal histories of the processes is denoted by a chain-dotted line, and a B impurity profile varied under the influence of the thermal histories of the processes is denoted by a broken line. The impurities of the high-concentration base layer are diffused into the collector region. However, the provision of the low-concentration base layer causes the concentration of impurities diffused into the collector to be reduced and that part of the collector layer of a relatively high concentration close to the base causes the diffused impurities to be compensated for and thus the type of dopants of the collector becomes n or i. Shown in Fig. 3 are energy band profiles for the transistor in its thermally balanced state obtained through simulation, which profiles correspond to the impurity profiles of Fig. 2. It will be appreciated from the drawing that, unlike the prior art (Fig. 14), a potential barrier is not formed in the base layer on its collector layer side and thus electrons in the base can smoothly be moved toward the collector. Fig. 4 is a graph showing relationships between current gain and current density obtained through simulation with respect to the HBT of the present embodiment having such impurity profiles as shown in Fig. 3 and the prior art HBT having such impurity profiles as shown in Fig. 13 for their comparison. It will be seen from the drawing that the HBT of the present embodiment exhibits a current gain higher than that of the prior art one in all the current density zone. Shown in Fig. 5 is a graph showing relationships between cut-off frequency and current density obtained through simulation with respect to the HBT of the present embodiment having such impurity profiles as shown in Fig. 3 and the prior art HBT having such impurity profiles as shown in Fig. 13 for their comparison. It will be seen from the drawing that the HBT of the present embodiment exhibits a cut-off frequency higher than that of the prior art one in a current density zone higher than 10 2 A/cm 2 . As a result, when the layer structure of the present invention is employed, there can be provided an HBT which is excellent not only in DC characteristics but also in high frequency characteristics. Embodiment 2: Fig. 6 shows an HBT in accordance with a second embodiment of the present invention. In this case, the present embodiment is substantially the same as the first embodiment, except that the double base layers 105 and 106 in the first embodiment are replaced by a p + type Si 0.8 Ge 0.2 single-layer 113. Embodiment 3: Fig. 7 shows an HBT in accordance with a third embodiment of the present invention. In this case, the present embodiment is substantially the same as the first embodiment, except that the double base layers 105 and 106 in the first embodiment are replaced by a p + type Si 1-x Ge x layer (0 ≦ x ≦ 0.2) 117 and a p - type Si 1···x Ge x layer (0.2 ≦ x ≦ 0.3) 116. In addition, the composition ratio x in the base is set so as to gradually increase from its base side to the collector side. In other words, an electric field for acceleration of electrons is provided so that electrons entered into the base from the emitter can quickly move to the collector side within the base. Embodiment 4: Fig. 8 shows an HBT in accordance with a fourth embodiment of the present invention. In this case, the present embodiment is substantially the same as the first embodiment, except that a base is of a 2-layer structure comprising a p ++ type Si layer 119 and a p - type Si 0.8 Ge 0.2 layer 118. However, since the p ++ type Si layer 119 is set to have a very high doping concentration, the layer 119 is substantially equal in energy band gap to the Si 0.8 Ge 0.2 layer 118 under the influence of the band gap reduction effect. Thus, since the entire base region has a substantially narrow gap when compared with the emitter or collector, the fourth embodiment resembles the first embodiment with regard to the band structure. Although explanation has been made in connection with only the Si 1-x Ge x /Si HBT in the foregoing embodiments, the same explanation can also hold true for an npn type transistor using InP/GaInAsP material wherein the band discontinuity of a heterojunction appears mainly on the side of a valence band and a pnp type transistor using AlGaAs/GaAs material wherein the band discontinuity of a heterojunction appears mainly on the side of a conduction band. Embodiment 5 : Explanation will next be made as to an npn type HBT using InP/GalnAsP material as a fifth embodiment of the present invention. Shown in Fig. 9 is a cross-sectional view of such an HBT. The transistor having a double heterostructure comprises an n + type InP layer 131 as a collector contact layer, an n - type InP layer 132 as a first collector layer, an n type InP layer 133 as a second collector layer, a p - type Ga 0.47 In 0.53 As layer 134 as a first base layer, a p + type Ga 0.47 In 0.53 As layer 135 as a second base layer, an n type InP layer 136 as an emitter layer, and an n + type InP layer 137 as an emitter cap layer, these layers being all sequentially formed on a semi-insulating InP substrate 130, the semiconductor ratio of the base region being set so as to provide lattice matching to the semiconductor substrate. In an InP/Ga 0.47 In 0.53 As heterojunction, since the amount of band discontinuity on the side of a conduction band is about 0.06eV while the amount of band discontinuity on the side of a valence band is about 0.43eV, that is, since most of the difference in forbidden band width appears on the side of the valence band, electrons can smoothly flow from the emitter to the collector. Though the base region has been made of Ga 0.47 In 0.53 As in the present embodiment, the similar effect can be obtained, in place of the above material, by using Ga 1-x In x As 1-y P y whose composition ratio is selected to provide lattice matching to InP. Explanation has been made In connection with only the npn type transistor in the foregoing embodiments, the present invention can be similarly applied even to a pnp type transistor. Embodiment 6: A pnp type HBT using AlGsAs/GaAs series material will be explained as a sixth embodiment of the present invention. Referring to Fig. 10, there is shown a cross-sectional view of the transistor which has a double heterostructure. More in detail, the transistor comprises a p + type GaAs layer 151 as a collector contact layer, a p type transition layer Al x Ga 1-x As, a p - type Al 0.3 Ga 0.7 As layer 152 as a first collector layer, a p type Al 0.3 Ga 0.7 As layer 153 as a second collector layer, an n - type GaAs layer 154 as a first base layer, an n + type GaAs layer 155 as a second base layer, a p type transition layer Al x Ga 1-x As, a p type Al 0.3 Ga 0.7 As layer 156 as an emitter layer, and a p + type GaAs layer 157 as an emitter cap layer, these layers being all sequentially formed on a semiinsulating GaAs substrate 150, the semiconductor ratio of the base region being set so as to provide lattice matching to the semiconductor substrate. In an Al 0.3 Ga 0.7 As/GaAs heterojunction, since the amount of band discontinuity on the side of a valence band is about 0.14eV while the amount of band discontinuity on the side of a conduction band is about 0.24eV, that is, since most of the difference in forbidden band width appears on the side of the conduction band, holes can smoothly flow from the emitter to the collector. Though the collector region has been made of Al 0.3 Ga 0.7 As in the present embodiment, the similar effect can be obtained, in place of the above material, by using Al x Ga 1-x As having an arbitrary composition ratio x. In addition, the present invention may be modified and carried out in various ways within the scope of the invention as defined by the appended claims.

Claims: A heterojunction bipolar transistor comprising a first semiconductor layer (107) of a first conductivity type, a second semiconductor layer (106) of a second conductivity type, a third semiconductor layer (105) of the second conductivity type, a fourth semiconductor layer (104) of the first conductivity type and fifth semiconductor layer (103) of the first conductivity type, said first to fifth layers being sequentially formed as laminated, wherein the forbidden band widths of said first, fourth and fifth semiconductor layers (107, 104, 103) are larger than the forbidden band widths of said second and third semiconductor layers (106, 105), an heterojunction between the third and fourth semiconductor layers (105, 104) being formed so that the forbidden band widths abruptly vary within a range of several atomic layers, the first semiconductor layer (107) functioning as an emitter, the second and third semiconductor layers (106, 105) functioning as a base, and the fourth and fifth semiconductor layers (104, 103) functioning as a collector, said transistor being characterised by satisfying the following relationship: W 2 ≥ W 3 and N 2 ≥ N 3 and N 4 > N 5 where, said second semiconductor layer has an impurity doping concentration N 2 and a thickness W 2 , said third semiconductor layer (105) has an impurity doping concentration N 3 and a thickness W 3 , said fourth semiconductor layer (104) has an impurity doping concentration N 4 and a thickness W 4 and said fifth semiconductor layer (103) has an impurity doping concentration N 5 . A heterojunction bipolar transistor according to claim 1, further characterised in that the relationship between said third and fourth semiconductor layers which follows in satisfied: where, said third semiconductor layer (105) has the impurity doping concentration N 3 and a dielectric constant ε 3 , said fourth semiconductor layer (104) has the impurity doping concentration N 4 , a dielectric constant ε 4 , and the thickness W 4 , and a built-in voltage between said third and fourth semiconductor layers (105, 104) is V bi . A heterojunction bipolar transistor according to claim 1, further characterised in that said first, fourth and fifth semiconductor layers (107, 104, 103) are made of Si and said second and third semiconductor layers (106, 105) are made of Si 1-x Ge x , where 0.0< x ≤1.0, x being a composition ratio. A heterojunction bipolar transistor according to claim 3, further characterised in that said second and third semiconductor layers (106, 105) are made of Si 1-x Ge x having an identical composition ratio, said second semiconductor layer having an impurity doping concentration equal to the impurity doping concentration of said third semiconductor layer, whereby said second and third semiconductor layers form a single layer. A heterojunction bipolar transistor according to claim 3, further characterised in that said second and third semiconductor layers (106, 105) are made of Si 1-x Ge x having an identical composition ratio, said second semiconductor layer having a higher impurity doping concentration than said third semiconductor layer. A heterojunction bipolar transistor according to claim 1, further characterised in that said first, fourth and fifth semiconductor layers (107, 104, 103) are made of Si, and said second and third semiconductor layers (106, 105) are respectively made of Si 1-x Ge x , x being a composition ratio, said second and third semiconductor layers forming together a graded structure of Si 1-x Ge x , where 0.0 ≤ x ≤ 1.0, the composition ratio x increasing towards the fourth semiconductor layer. A heterojunction bipolar transistor according to claim 1, further characterised in that said first, fourth and fifth semiconductor layers (107, 104, 103) are made of InP, said second and third semiconductor layers (106, 105) are made of GaInAs, and said second semiconductor layer (106) has a higher impurity doping concentration than said third semiconductor layer (105). A heterojunction bipolar transistor according to claim 1, further characterised in that said first, fourth and fifth semiconductor layers (107, 104, 103) are made of AlGaAs, said second and third semiconductor layer (106, 105) are made of GaAs, and said second semiconductor layer (106) has a higher impurity doping concentration than said third semiconductor layer (105).

Inventors: ENDO TAKAHIKO, KATOH RIICHI, ENDO, TAKAHIKO, KATOH, RIICHI, ENDO, TAKAHIKO, C/O INTELLECTUAL PROPERTY DIVISION, KATOH, RIICHI, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L29
IPC codes: H01L29