Patent Publication Number: US-2007096150-A1

Title: Heterojunction bipolar transistor

Description:
BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      The present invention relates to a heterojunction bipolar transistor, and particularly to an epitaxial substrate which makes up a heterojunction bipolar transistor.  
      (2) Description of the Related Art  
      The heterojunction bipolar transistor (below abbreviated as HBT) can perform a lower distortion amplification operation and a simpler power supply operation in comparison to a field-effective transistor, and in recent years has become a key device in mobile communications and optical communications systems.  
      In order to obtain high-current gain in a homojunction bipolar transistor, the efficiency for the carrier injection from the emitter layer to the base layer must be increased, and the donor concentration of the emitter layer must be increased to greater than the accepter concentration of the base layer. Thus impurities with a high degree of concentration cannot be doped into the base layer. On the other hand, in an HBT, since a band discontinuity (Δ Ev) is generated in the top part of the valence band by utilizing a material with a band gap energy larger than the base layer for the emitter layer, a structure for suppressing an inflow of holes in the base layer into the emitter layer can be realized (see for example Japanese Laid-Open Patent Application No. 2004-71669). Thus, in an HBT, a high current gain can be maintained and limitations related to doping concentration, as in a homojunction bipolar transistor, are eliminated. Accordingly, since a high concentration impurity can be doped into the base layer, base resistance can be kept low even if the thickness of the base layer is thinned, and therefore high frequency properties can be improved by decreasing the input resistance.  
      A sectional view of a conventional GaAs-type HBT is illustrated in  FIG. 1 .  
      In this HBT, a subcollector layer  502  made of n + -GaAs (thickness=6000 Å, concentration of n-type impurity=5×10 18  cm −3 ), a collector layer  503  made of n − -GaAs (thickness=6000 Å, concentration of n-type impurity=5×10 16  cm −3 ), a base layer  504  made of p + -GaAs (thickness=1000 Å, concentration of p-type impurity=4×10 19  cm −3 ), an emitter layer  505  made of n-InGaP matching the lattice constant of GaAs (thickness=300 Å concentration of n-type impurity=3×10 17  cm −3 ), an emitter layer  506  made of n-GaAs (thickness=500 Å, concentration of n-type impurity=3×10 18  cm −3 ), an emitter layer  507  made of n + -GaAs (thickness=500 Å, concentration of n-type impurity=5×10 18  cm −3 ), a grading layer  508  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=changes from 0.5×10 19  cm −3  to 1×10 19  cm −3 ), and a cap layer  509  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=1×10 19  cm −3 ) are stacked sequentially on a substrate  501  made of semi-insulating GaAs and form an epitaxial layer as an HBT structure. A collector electrode  510 , a base electrode  511  and an emitter electrode  512  are formed during the manufacturing process of the transistor on the subcollector layer  502 , the base layer  504  and the cap layer  509  respectively. Note that an HBT in which the emitter layer  505  is made of InGaP was exemplified as a conventional HBT, however the emitter layer  505  is also sometimes made of AlGaAs, which has a larger band gap energy than GaAs.  
     SUMMARY OF THE INVENTION  
      In the HBT shown as a conventional example in  FIG. 1 , InGaP is used in the emitter layer  505 . InGaP has a higher band gap energy compared to GaAs, which makes up the base layer  504 . Accordingly, an example energy band diagram in the heterojunction (the energy band diagram of the A-A′ line in  FIG. 1 ) is shown in  FIG. 2 . From  FIG. 2  it is shown that when an emitter layer is made of a material that has a higher band gap energy than a material which makes up the base layer, not only will ΔEv generate, a spike-shaped band discontinuity (ΔEc) in the bottom part of a conduction band will also occur at the hetero interface of the base layer and the emitter layer. In other words, it is shown that an ΔEv of approximately 0.3 eV, and an ΔEc of approximately 0.2 eV generates in a GaAs/InGaP heterojunction. Here, in order to improve the current gain as above, it is preferable that an ΔEv which suppresses the reverse influx of holes in the base layer into the emitter layer is as large as possible. Whereas in order to reduce the offset voltage, which is one of the electrical characteristics of the transistor, it is preferable that the spike-shaped ΔEc, which functions as a barrier to electron injection from the emitter layer into the base layer, be made as small as possible or omitted.  
      Thus, the present invention takes as its first object providing a heterojunction bipolar transistor which can omit band discontinuities (ΔEc) in the bottom part of the conduction band in the heterojunction of the base-emitter, in consideration of the problems above.  
      A second object is to provide a heterojunction bipolar resistor which can increase band discontinuities (ΔEv) in the top part of the valence band in the heterojunction of the base-emitter.  
      In order to solve the problems above, the heterojunction bipolar transistor in the present invention includes a substrate made of semi-insulating GaAs; and an epitaxial layer which lattice-matches the substrate, including: a base layer made of GaPSb; and an emitter layer made of a semiconductor material that has the same electron affinity as the GaPSb and a band gap energy larger than GaPSb. Here, the emitter layer may be made of InGaP. Also, the composition of the GaPSb which makes up the base layer may be GaP x Sb 1-x , where 0.30≦X≦0.35, and the emitter layer may be made of one of AlGaAs, AlGaInP and AlGaPSb.  
      With this configuration, GaPSb, which matches lattices with GaAs, is utilized as a semiconductor material which makes up the base layer instead of utilizing GaAs in the same way as a conventional GaAs HBT which is made up of GaAs. Accordingly, a spike-shaped discontinuity does not generate in the conduction band (ΔEc) of the base-emitter hetero interface and discontinuities in the valence band (ΔEv) increase. As a result, since electrons arrive in the base layer from the emitter layer without being affected by the ΔEc, and the reverse influx of holes from the base layer into the emitter layer is suppressed, an HBT with a small offset voltage and a high current gain can be realized.  
      Also, the epitaxial layer may further include a collector layer made of GaPSb.  
      With this configuration, the collector layer is made of GaPSb and the base collector interface is a homojunction made from GaPSb/GaPSb which include some V group elements. As a result, in the manufacturing process, exchanges of raw material sources which are easily mixable do not occur compared to when the collector layer is made of GaAs and the base collector interface is a homojunction made from GaPSb/GaAs. Therefore, it becomes possible to make an abrupt base collector interface without mixed elements, and form a satisfactory pn junction and a base-collector interface with few electron hole recombinations in the interface.  
      According to the present invention, since band discontinuities (ΔEc) in the bottom part of the conduction band in the base-emitter hetero interface are eliminated, an HBT can be realized which is not influenced by the ΔEc while operating. In other words, an HBT with a small offset voltage can be achieved.  
      Also, band discontinuities in the top part of the valence band increase at the base-emitter hetero interface and the current gain improves compared to a conventional HBT in which the base layer is made of GaAs. In addition, the temperature dependence of the current gain decreases.  
      Also, since the base collector interface is a homojunction not made from GaPSb/GaAs, but instead from GaPSb/GaPSb which includes some V group elements, it becomes possible to compose an abrupt base collector interface without mixed elements, and as a result, electron hole recombinations in the interface can be reduced.  
     FURTHER INFORMATION ABOUT TECHNICAL BACKGROUND TO THIS APPLICATION  
      The disclosure of Japanese Patent Application No. 2005-318896 filed on Nov. 1, 2005 including specification, drawings and claims is incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the Drawings:  
       FIG. 1  is a sectional view of a conventional HBT.  
       FIG. 2  is an energy band diagram for the heterojunction in the conventional HBT (an energy band diagram for the A-A′ line in  FIG. 1 ).  
       FIG. 3  is a sectional view of the HBT in the first embodiment of the present invention.  
       FIG. 4  is an energy band diagram for the heterojunction of the HBT in the first embodiment (an energy band diagram for the A-A′ line in  FIG. 3 ).  
       FIG. 5  is a sectional view for the HBT in the second embodiment of the present invention.  
       FIG. 6  is an energy band diagram for the heterojunction of the HBT in the second embodiment (an energy band diagram for the A-A′ line in  FIG. 5 ). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Below, a Heterojunction Bipolar Transistor (HBT) in the embodiments of the present invention is described with reference to diagrams.  
     First Embodiment  
       FIG. 3  is a sectional view of the HBT in the present embodiment.  
      In this HBT, a subcollector layer  102  made of n + -GaAs (thickness=6000 Å, concentration of n-type impurity=5×10 18  cm −3 ), a collector layer  103  made of n-GaAs (thickness=6000 Å, concentration of n-type impurity=5×10 16  cm −3 ), a base layer  104  made of p + -GaPSb in which carbon is doped (thickness=1000 Å, concentration of p-type impurity=4×10 19  cm −3 ), an emitter layer  105  made of n-InGaP which matches the lattice constant of GaAs (thickness=300 Å, concentration of n-type impurity=3×10 17  cm −3 ), an emitter layer  106  made of n-GaAs (thickness—500 Å, concentration of n-type impurity=3×10 18  cm −3 ) an emitter layer  107  made of n + -GaAs (thickness=500 Å, concentration of n-type impurity=5×10 18  cm −3 ), a grading layer  108  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=changes from 0.5×10 19  cm −3  to 1×10 19  cm −3 ), and a cap layer  109  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=1×10 19  cm −3 ) are stacked sequentially on a substrate  101  made of semi-insulating GaAs by utilizing epitaxial crystal growth technology, and form an epitaxial layer  100  as an HBT structure. A collector electrode  110 , a base electrode  111  and an emitter electrode  112  are formed during the manufacturing process on the subcollector layer  102 , the base layer  104  and the cap layer  109  respectively. Also, the composition of GaPSb is assumed to be GaP x Sb 1-x  (0.30≦X≦0.35) so that GaPSb, which makes up the base layer  104 , reaches a lattice constant which matches the lattice of GaAs.  
      Note that the emitter layer  105  is made of InGaP. However, the present invention is not limited to InGaP, and any other materials can be used if they have the same electron affinity as GaPSb, the semiconductor material which makes up the emitter layer  105 , and a larger band gap energy than GaPSb. It goes without saying that even if the base layer  104  is made of for example AlGaAs, AlGaInP or AlGaPSb and so on, the same effect can be obtained.  
      In addition, the emitter layer  106  and the emitter layer  107  are made of GaAs. However, the emitter layer  106  and the emitter layer  107  may each be made of GaPSb.  
       FIG. 4  shows an energy band diagram for the heterojunction of the HBT in the present embodiment (an energy band diagram for the A-A′ line in  FIG. 3 ).  
      From  FIG. 4  it is shown that a band discontinuity (ΔEc) in the conduction band is eliminated in the base-emitter hetero interface, and in the valence band, band discontinuities larger than a conventional HBT (ΔEv=approximately 0.6 eV) generate.  
      According to the HBT in the present embodiment as above, the base layer  104  is made of GaPSb and the emitter layer  105  is made of InGaP, InGaP having the same electron affinity as GaPSb which makes up the base layer, and a larger band gap energy than GaPSb. As a result, band discontinuities (ΔEc) in the bottom part of the conduction band in the base-emitter hetero interface are eliminated and electrons injected from the emitter layer into the base layer can reach the base layer unaffected by ΔEc. Thus, an HBT which is unaffected by ΔEc while operating can be realized. In other words, an HBT with a small offset voltage is realizable.  
      Additionally, according to the HBT in the present embodiment, GaPSb (Eg=1.39 eV) which makes up the base layer  104  has about the same energy gap as GaAs (Eg=1.42 eV). Thus, compared to a conventional HBT with a base layer made of GaAs, ΔEc decreases and band discontinuities (ΔEv) increase in the top part of the valence band in the base-emitter hetero interface. As an effect of the large ΔEv, the reverse influx of holes in the base layer into the emitter layer is suppressed and the current gain improves. Also, with the rise in temperature, electric current, caused by the reverse influx of holes in the base layer into the emitter layer, increases and the current gain decreases, but since this kind of electric current decreases as a result of the large ΔEv, the temperature dependence of the current gain decreases compared to when the base layer is made of GaAs.  
     Second Embodiment  
       FIG. 5  is a sectional view of the HBT in the present embodiment.  
      In this HBT, a subcollector layer  302  made of n + -GaPSb (thickness=6000 Å, concentration of n-type impurity=5×10 18  cm −3 ), a collector layer  303  made of n-GaPSb (thickness=6000 Å, concentration of n-type impurity=5×10 16  cm −3 ), a base layer  304  made of p + -GaPSb in which carbon is doped (thickness=1000 Å, concentration of p-type impurity=4×10 19  cm −3 ), an emitter layer  305  made of n-InGaP matching the lattice constant of GaAs (thickness=300 Å, concentration of n-type impurity=3×10 17  cm −3 ), an emitter layer  306  made of n-GaAs (thickness=500 Å, concentration of n-type impurity=3×10 18  cm −3 ), an emitter layer  307  made of n + -GaAs (thickness=500 Å/concentration of n-type impurity=5×10 18  cm −3 ), a grading layer  308  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=changes from 0.5×10 19  cm −3  to 1×10 19  cm −3 ), and a cap layer  309  made of n + -InGaAs (thickness=500 Å, concentration of n-type impurity=1×10 19  cm −3 ) are sequentially stacked on a substrate  301  made of semi-insulating GaAs by utilizing epitaxial crystal growth technology, and form an epitaxial layer  300  as an HBT structure. A collector electrode  310 , a base electrode  311  and an emitter electrode  312  are formed during the manufacturing process on a subcollector layer  302 , a base layer  304  and a cap layer  309  respectively. Also, the composition of GaPSb is assumed to be GaP x Sb 1-x  (0.30≦X≦0.35) so that GaPSb, which makes up the base layer  304 , reaches a lattice constant which matches the lattice constant of GaAs.  
      Note that the emitter layer  305  is made of InGaP. However, the present invention is not limited to InGaP, and any other materials can be used if they have the same electron affinity as GaPSb, the semiconductor material which makes up the base layer  304 , and a larger band gap energy than GaPSb. It goes without saying that the same effect is obtained when the emitter layer  305  is made of for example AlGaAs, AlGaInP or AlGaPSb and so on.  
      Also, the emitter layer  306  and the emitter layer  307  are made of GaAs. However, the emitter layer  306  and the emitter layer  307  may each be made of GaPSb.  
       FIG. 6  shows an energy band diagram for the heterojunction of the HBT in the present embodiment (an energy band diagram for the A-A′ line in  FIG. 5 ).  
      From  FIG. 6  it is shown that a band discontinuity (ΔEc) is eliminated in the base-emitter hetero interface, and that in the valence band, band discontinuities larger than a conventional HBT (ΔEv=approximately 0.6 eV) generate.  
      According to the HBT in the present embodiment as above, the base layer  304  is made of GaPSb, and the emitter layer  305  is made of InGaP which has the same electron affinity as GaPSb, which makes up the base layer, and has a larger band gap energy than GaPSb. As a result, band discontinuities (ΔEc) in the bottom part of the conduction band in the base-emitter hetero interface can be eliminated and electrons injected into the base layer from the emitter layer can reach the base layer unaffected by ΔEc, therefore an HBT which is unaffected by ΔEc while operating can be realized. In other words, offset voltage makes a small HBT realizable.  
      According to the HBT in the present embodiment, GaPSb (Eg=1.39 eV), which makes up the base layer  304 , has about the same energy gap as GaAs (Eg=1.42 eV). Accordingly, compared to an HBT in which the base layer is made of GaAs, ΔEc decreases and band discontinuities (ΔEv) increase in the top part of the valence band in the base-emitter hetero interface. As a result, the reverse influx of holes in the base layer into the emitter layer is suppressed by a large ΔEv, and the current gain improves. Additionally, with the rise in temperature, the electric current, which is caused by the reverse influx of holes in the base layer into the emitter, increases and the current gain decreases. Since this kind of electric current decreases as an effect of the increased ΔEv, the temperature dependence of the current gain decreases compared to when the base layer is made of GaAs.  
      Also, according to the HBT in the present embodiment, the collector layer  303  utilizes GaPSb, and the base collector interface is a homojunction not made from GaPSb/GaAs, but instead from GaPSb/GaPSb which includes some V group elements. As a result, in the manufacturing process, exchanges of raw material sources which are easily mixable do not occur compared to when the collector layer  303  is made of GaAs. Accordingly, it is possible to form an abrupt base collector interface without mixed elements, and thus a base collector interface with few electron hole recombinations in the interface can be produced since a satisfactory pn junction can be made compared to when the collector layer is made of GaAs.  
      Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.  
      For example, the thickness of the semiconductor layer included in the epitaxial layer and the carrier concentration are examples and are not limited to these examples.  
      Also, in the embodiments, the dopant of the base layer is carbon; however the present invention is not limited to carbon as long as the dopant makes the base layer a p-type layer.  
     INDUSTRIAL APPLICABILITY  
      The present invention can be utilized for a heterojunction bipolar transistor and especially for a mobile communications or an optical communications system.