Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
1998-11-13
2002-06-04
Lee, Eddie (Department: 2812)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S201000, C257S615000, C257S587000, C257S592000
Reexamination Certificate
active
06399971
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device of a hetero-junction bipolar transistor structure, and a method for fabricating the same.
Recently optical communications systems and mobile communication systems which have high efficiency are required. To make these systems highly efficient semiconductor devices are essential. Hetero-junction bipolar transistors (hereinafter called “HBTs”), which are known as high-speed devices, are one of such devices whose efficiency improvement is prospective.
A structure of a conventional HBT will be explained with reference to FIG.
11
.
A collector contact layer
102
formed of an n
+
-InGaAs layer is formed on a semi-insulating InP substrate
100
. A collector layer
104
of an i-InGaAs layer is formed on the collector contact layer
102
. A base layer
106
of a p
+
-InGaAs layer is formed on the collector layer
104
. An emitter layer
108
of an n-InP layer is formed on the base layer
106
. An emitter contact layer
110
of an n
+
-InGaAs layer is formed on the emitter layer
108
. An emitter electrode
112
of WSi film is formed on the emitter contact layer
110
. The emitter contact layer
110
and the emitter layer
108
are processed in a mesa-shape, and a base electrode
116
is formed on an exposed part of the base layer
106
. The base layer
106
and the collector layer
104
are processed in a mesa-shape, and a collector electrode
118
is formed on an exposed part of the collector contact layer
102
. Thus, an InP/InGaAs-based HBT is formed.
To make the HBT-ICs more speedy, a higher maximum oscillation frequency f
max
is necessary. A maximum oscillation frequency f
max
is expressed by
f
max
=(f
T
/(8&pgr;×R
B
×C
BC
))
wherein a maximum cut-off frequency is represented by f
T
, a base resistance is represented by R
B
, and a base-collector capacitance is represented by C
BC
. A maximum oscillation frequency f
max
is proportional to a reciprocal of a square root of a base resistance R
B
((1/(R
B
))), and for a higher maximum oscillation frequency f
max
, it is necessary to obtain a lower base resistance R
B
.
In GaAs-based HBTs, recently carbon (C) is dominantly used as a dopant for the bases from the viewpoint of ensured reliability, etc., and doping techniques for higher concentrations of above 1×10
20
cm
−3
have been developed.
On the other hands, in InP/InGaAs-based HBTs, actually doping techniques using carbon as a dopant for the base have not been sufficiently established. The base layer cannot be heavily doped with carbon, and this will be because carbon is not dissociated from hydrogen in forming InGaAs layer to be the base layer to be taken in the films in the form of CH, and the carbon does not function as an acceptor (hydrogen passivation). This phenomenon is conspicuous especially in MOCVD method using hydrogen as a carrier gas and a hydrogen content gas as a source gas.
As a result, InP/InGaAs-based HBTs have very high maximum cut-off frequencies f
T
but cannot have sufficiently maximum oscillation frequencies f
max
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a structure of a semiconductor device which enables an InP/InGaAs-based HBT to have a lower base resistance, and a method for fabricating the same.
The above-described object can be achieved by a semiconductor device comprising: a collector layer; a base layer of a carbon-doped Ga
x
In
1−x
As
y
Sb
1−y
layer having one surface connected to the collector layer; an emitter layer connected the other surface of the base layer; a base contact layer of a carbon-doped GaAsSb layer electrically connected to the base layer; and a base electrode formed on the base contact layer. The semiconductor device of such structure can have a much reduced base resistance R
B
, whereby InP/GaInAsSb-based HBTs including InP/InGaAs-based HBTs can have higher maximum oscillation frequency f
max
. Because of the carbon-doped semiconductor layer the semiconductor device can have higher reliability.
In the above-described semiconductor device, it is preferable that the base contact layer is formed on said one surface or the other surface of the base layer.
In the above-described semiconductor device, it is possible that the base contact layer is formed on a surface of the collector layer connected to the base layer and has a side surface connected to a side surface of the base layer.
In the above-described semiconductor device, it is possible that the base contact layer is formed on a surface of the emitter layer connected to the base layer and has a side surface connected to a side surface of the base layer.
In the above-described semiconductor device, it is possible that the device further comprises a surface passivation layer for protecting the base contact layer formed on the surface of the base contact layer with the base electrode formed on. Because of the surface passivation layer covering the surface of the base contact layer, surface recombination on the base contact layer can be restrained, whereby dependence of current gains on sizes can be restrained, and the semiconductor device can have higher reliability.
In the above-described semiconductor device, it is possible that the base contact layer is formed of a carbon-doped GaInAsSb layer in place of said carbon-doped GaAsSb layer.
In the above-described semiconductor device, it is possible that an As composition y of the Ga
x
In
1−x
As
y
Sb
1−y
is 1, so that the base layer is formed of a InGaAs layer.
In the above-described semiconductor device, it is preferable that an In composition x of the Ga
x
In
1−x
As
y
Sb
1−y
is 0, so that the base layer is formed of a GaAsSb layer.
In the above-described semiconductor device, it is preferable that a dopant concentration of the base contact layer is not less than 1×10
20
cm
−3
.
The above-described object can be also achieved by a method for fabricating a semiconductor device comprising the steps of: forming a first semiconductor layer on a semiconductor substrate; forming a base layer of a carbon-doped Ga
x
In
1−x
As
y
Sb
1−y
layer on the first semiconductor layer; forming a second semiconductor layer on the base layer; patterning the second semiconductor layer in a mesa-shape; forming a base contact layer on the base layer exposed by patterning the second semiconductor layer; and forming a base electrode on the base contact layer. By fabricating the above-described semiconductor device fabricating method, the semiconductor device can have a much reduced base resistance R
B
, whereby InP/GaInAsSb-based HBTs including InP/InGaAs-based HBTs can have higher maximum oscillation frequency f
max
. Because of the carbon-doped semiconductor layer the semiconductor device can have higher reliability.
In the above-described method for fabricating a semiconductor device, it is preferable that the method further comprises, after the step of patterning the second semiconductor layer, a step of removing the base contact layer in a exposed region which is exposed by patterning the second semiconductor layer, wherein in the step of forming the base contact layer, the base contact layer having a side surface connected to the base layer is formed on the first semiconductor layer exposed by removing the base layer.
In the above-described method for fabricating a semiconductor device, it is preferable that in the step of forming the base layer, the base layer of an InGaAs layer which corresponds to the Ga
x
In
1−x
As
y
Sb
1−y
layer whose As composition y is 1, or a GaAsSb layer which corresponds to the Ga
x
In
1−x
As
y
Sb
1−y
layer whose In composition X is 0 is formed.
In the above-described method for fabricating a semiconductor device, it is preferable that in the step of forming the base contact layer, the base contact layer is formed of a material which lattice-matches with a material forming the base layer. The base contact layer is formed of a material which lattice-matches with a materi
Imanishi Kenji
Shigematsu Hisao
Tanaka Hitoshi
Baumeister Bradley William
Fujitsu Limited
Lee Eddie
LandOfFree
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