Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2000-03-10
2003-08-26
Thomas, Tom (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S198000, C257S201000, C438S235000, C438S309000, C438S312000
Reexamination Certificate
active
06611008
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor that is a semiconductor device of a III-V compound, a semiconductor device employing the same and a heterojunction bipolar transistor fabricating method.
Conventionally, as an AlGaAs/GaAs heterojunction bipolar transistor (HBT), there has been a structure as shown in FIG.
8
. In
FIG. 8
, there are shown an Au/Ge/Ni emitter ohmic electrode
1
, an n
+
-type GaAs cap layer
2
(100 nm in thickness, donor concentration n=5×10
18
cm
−3
(n representing the donor concentration hereinafter)) and an n-type Al
1−y
Ga
y
As crystal mixture ratio graded cap layer
3
(20 nm in thickness, n=5×10
17
cm
−3
, y=0.35→0.0 (y=0.35 on the substrate side and y=0 on the surface side)). There are also shown an n-type Al
0.35
Ga
0.65
As ballast layer
4
(200 nm in thickness, n=5×10
16
cm
−3
), an n-type Al
0.3
Ga
0.7
As emitter layer
5
(10 nm in thickness, n=5×10
17
cm
−3
), a p
+
-type GaAs base layer
6
(80 nm in thickness, acceptor concentration p=2×10
19
cm
−3
(p representing the acceptor concentration hereinafter)) and a Ti/Pt/Au base ohmic electrode
7
. There are further shown an n-type GaAs collector layer (700 nm in thickness, n=2×10
16 cm
−3
), an n
+
-type GaAs sub-collector layer
9
(500 nm in thickness, n=5×10
18
cm
−3
), an Au/Ge/Ni collector ohmic electrode
10
and a semi-insulating GaAs substrate
11
.
In connection with the AlGaAs/GaAs HBT having the aforementioned construction, it is well known that integrating the ballast layer having a crystal mixture ratio x of 0.15≦x≦0.4 increases the temperature coefficient of the ballast resistance and is effective for the uniformity of current and the stability of temperature through the restraint of thermal runaway within a wide range of temperature when the crystal mixture ratio x is adjusted(prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-349847). The HBT having an AlGaAs ballast layer as described above has an emitter layer made of only AlGaAs that has a doping concentration and an Al crystal mixture ratio different from those of the ballast layer.
However, the aforementioned conventional AlGaAs/GaAs HBT has had the following problems. That is, during operation at a high temperature and a high current, the AlGaAs/GaAs HBT having the aforementioned AlGaAs ballast layer causes a serious problem that the resistance of the ballast layer is disadvantageously reduced due to a high junction temperature.
The reduction in ballast resistance due to the increase in junction temperature often reduces the effect of the ballast resistance as follows. In detail,
FIG. 9
shows a graph of collector current density Jc to base-to-emitter voltage Vbe characteristics (Jc-Vbe characteristics). According to these Jc-Vbe characteristics, the junction temperature increases much when a collector-to-emitter voltage Vce is high, and this causes a curve of a negative slope. This fact indicates the instability of the aforementioned HBT and causes a non-uniformity in terms of current. The above-mentioned negative slope is ascribed to a modulation of conductivity (reduction in resistance) of the ballast resistance layer due to hole injection.
FIG. 10
schematically shows the band structure of the aforementioned HBT. The band offset energy &Dgr;Ev of the valence band between AlGaAs and GaAs is about 36% of a difference &Dgr;Eg between their band gap energies, and the remaining 64% is the band offset energy &Dgr;Ec of the conduction band between them. As it is shown in
FIG. 11
, the band offset energy &Dgr;Ev is defined as an energy barrier in the valence band generated at the interface of two different semiconductor materials. When one material has electron affinity &khgr;
1
and band gap energy Eg
1
and the other material has electron affinity &khgr;
2
and band gap energy Eg
2
, the energy barrier &Dgr;Ev is expressed by:
&Dgr;
Ev=
(&khgr;
2
+Eg
2
)−(&khgr;
1
+Eg
1
).
The band offset energy &Dgr;Ev(x) with respect to AlGaAs can be expressed by &Dgr;Ev(x)=0.449x(eV) (0<x<0.45). Therefore, the band offset energy &Dgr;Ev in the valence band with respect to typical AlGaAs/GaAs of a crystal mixture ratio x=0.3 becomes about 135 meV (=0.449×0.3×10
3
meV). As a result, hole injection from the base layer to the emitter layer occurs, as a consequence of which the current gain h
FE
of the AlGaAs/GaAs HBT is significantly reduced at high temperature. The temperature dependency of hole injection at a constant collector current is given by Jp~exp(−&Dgr;Ev/kT). In this case, Jp represents the Hall current density, k represents the Boltzmann's constant and T represents the absolute temperature. Each hole injected from the base layer into the emitter layer diffuses toward the ballast layer, and the movement of electrons from the GaAs cap layer to the ballast layer occurs in order to satisfy space charge neutralization condition. Under the conditions of high current and high temperature, the hole density of the ballast layer is equivalent to the dope concentration of the ballast layer. As a result, the aforementioned electron movement causes a reduction in the resistance of the ballast layer.
FIG. 10
schematically shows this phenomenon.
The hole density in the ballast layer at high current and high temperature can be calculated from the aforementioned temperature dependency of the current gain h
FE
of the HBT and the mobility of an electron and a hole as follows:
p≈N
D
&mgr;
e
/(
h
FE
(
T
)&mgr;
h
)
where p represents the hole density in the ballast layer, N
D
represents the doping concentration of the donor in the ballast layer, h
FE
(T) represents the temperature-dependent current gain, and &mgr;
e
and &mgr;
h
are the mobility of an electron and a hole respectively. According to the above expression, assuming that the doping concentration N
D
of the donor in the ballast layer is N
D
=5×10
16
cm
−3
and the current gain h
FE
is h
FE
=30, then the hole density to be injected into the ballast layer is 2.2×10
16
cm
−3
in the typical AlGaAs/GaAs HBT where the electron mobility and hole mobility are 4000 cm
2
/Vsec and 300 cm
2
/Vsec, respectively. This hole density is comparable to the doping concentration N
D
, and this indicates that a considerable degree of conductivity modulation (reduction in resistance) of the ballast layer occurs. This conductivity modulation exhibits a similar characteristic on an AlGaAs ballast layer having a crystal mixture ratio x in the range of 0.15≦x≦0.4 but also an AlGaAs ballast layer in the range of 0≦x≦0.45. For a given ballast resistance, the smaller the crystal mixture ratio x becomes, the greater the electron mobility becomes, so that the donor concentration in the ballast layer becomes low. Therefore, the hole injection becomes a more serious problem. In contrast to this, the ballast layer generally comes to have a high density when the crystal mixture ratio x exceeds 0.45, and the hole injection comes to exert scarce influence. However, the ballast layer originally has a poor performance when the crystal mixture ratio x is not smaller than 0.45.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an HBT capable of restraining the conductivity modulation of the ballast layer that is the cause of a deterioration in temperature characteristics by preventing hole injection from the base layer into the emitter layer.
In order to achieve the above-mentioned object, the present invention provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer made of AlGaAs, wherein the emitter layer is comprised of a single layer or a multiplicity of layers, and at least one of which is comprised of a materia
Ishimaru Yoshiteru
Twynam John Kevin
Birch & Stewart Kolasch & Birch, LLP
Kang Donghee
Sharp Kabushiki Kaisha
Thomas Tom
LandOfFree
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