Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of...
Reexamination Certificate
2002-08-29
2004-01-20
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
C438S312000, C438S341000, C438S350000, C438S933000
Reexamination Certificate
active
06680234
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and a semiconductor manufacturing method. More particularly, the present invention relates to a semiconductor device and a semiconductor manufacturing method, which can minimize a drop in a current gain at a time of an increase in a collector current density and also improve a variation in a collector current.
2. Description of the Related Art
As a conventional technique, for example, there is a technique disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 7-147287). That is, this is a SiGe base bipolar transistor, in which each of a base layer and a collector layer are constituted by a single crystal silicon layer containing germanium (Element Symbol: Ge), and an occurrence of a parasitic energy barrier is protected in a base-collector junction region, and a drop in a cut-off frequency is suppressed.
FIG. 1
is a device section view describing a conventional SiGe base bipolar transistor. In
FIG. 1
, a symbol
201
denotes a P-type silicon substrate. A symbol
202
denotes an N
+
-type buried layer. A symbol
203
A denotes a first collector layer. A symbol
203
B denotes a second collector layer. A symbol
205
denotes an insulation separation oxide film. A symbol
206
denotes a diffusion layer to pull out a collector. A symbol
207
denotes an oxide film. A symbol
208
denotes a P-type poly-crystal silicon layer to pull out an outer base. A symbol
209
denotes a silicon nitride film. A symbol
210
denotes a P-type SiGe base layer (silicon germanium base layer). A symbol
211
denotes a silicon nitride film. A symbol
212
denotes a poly-crystal silicon film to pull out an emitter. And, a symbol
213
denotes an emitter layer.
In the conventional SiGe base bipolar transistor, the N
+
-type buried layer
202
having a high concentration and the first collector layer
203
A having a first Ge concentration distribution and the second collector layer
203
B having a second Ge concentration distribution are formed on the P-type silicon substrate
201
, as shown in FIG.
1
. The P-type SiGe base layer
210
resulting from a selectively epitaxial growth and the emitter layer
213
constituted by an N-type diffusion layer are formed on the first collector layer
203
A.
FIG. 2
is a graph showing the profile of a Ge percentage content (germanium percent content) and an impurity concentration with respect to a depth of the conventional SiGe base bipolar transistor. In the conventional SiGe base bipolar transistor, the first collector layer
203
A having the first Ge concentration distribution and the second collector layer
203
B having the second Ge concentration distribution (germanium concentration distribution) are formed in the N
+
-type buried layer
202
having the high concentration in which N-type impurity is doped at about 10
20
cm
−3
, as shown in FIG.
2
. Then, the P-type SiGe base layer
210
in which P-type impurity is doped at about 5×10
18
cm
−3
and the poly-crystal silicon film
212
to pull out an emitter in which the N-type impurity is doped at about 2×10
20
cm
−3
are formed on the first collector layer
203
A. The emitter layer
213
is formed by using the impurity thermal diffusion from the poly-crystal silicon film
212
to pull out an emitter in which the N-type impurity is doped.
With regard to the Ge concentration distribution of the P-type SiGe base layer
210
, in the P-type SiGe base layer
210
, an emitter region side has a low distribution, and a collector region side has a high distribution. That is, the emitter region side has a Ge percentage content of 0%, and the collector region side has a slant concentration distribution having a percentage content of 10%.
FIG. 3
shows an energy band structure at this time.
FIG. 3
is a graph showing an energy band at the slant Ge profile (slant germanium profile). As shown in
FIG. 3
, in the case of the conventional SiGe base bipolar transistor, in the P-type crystal SiGe base layer
210
, an inclination can be set for a conductive band side of an energy band, correspondingly to a Ge composition ratio (germanium composition ratio). Thus, an electron implanted from an emitter is accelerated in the P-type SiGe base layer
210
by an electrical field caused by the slant energy band structure. Hence, it is possible to reduce a base transit time of the electron and accordingly improve a cut-off frequency f
T
.
However, in the case of the conventional SiGe base bipolar transistor, in order to make the cut-off frequency f
T
higher by using the slant Ge profile, it is necessary to reduce a thickness of the P-type SiGe base layer
210
. For example, it is noted that the thickness of the P-type SiGe base layer
210
is about 90 nm and the cut-off frequency f
T
=20 GHz. In order to set the cut-off frequency f
T
to 60 GHz, the thickness of the P-type SiGe base layer
210
must be reduced to about 30 nm. This brings about the following problems.
As the first problem, a constant region is narrow because of a drop in a current gain h
FE
in a high current region when the slant Ge profile is used, although an analog circuit especially requires that a current gain h
FE
(=collector current/base current) is constant in a wide current region of a collector current.
The reason is as follows. In the profile that the impurity concentration in the base is constant and the Ge concentration is slant such as the slant Ge profile, a collector current density Jc is represented by the following equation (1):
J
c
=
q
D
n
n
i
2
NA
W
b
Δ
E
g
G
e
(
g
r
a
d
e
)
k
T
exp
[
Δ
E
g
G
e
(
0
)
k
T
]
equation
(
1
)
&Dgr;
EgGe(0): Contraction Amount of Band Gap by Ge at Tip of Depletion Layer between Emitter And Base
&Dgr;
EgGe(grade): Ge Slant Amount in Neutral Base
Dn: Diffusion Constant of Electron in Base
NA: Base Impurity Concentration
Wb: Base Width
ni: Intrinsic Carrier Density
q: Charge
In the case of the slant Ge profile of the conventional SiGe base bipolar transistor, when the thickness of the P-type SiGe base layer
210
is reduced, this reduction causes the Ge inclinations (germanium inclinations) to be sharp at the positions of the formations of the emitter-base junction and the depletion layer between the emitter and the base. This results in a large reduction in a contraction amount
&Dgr;
E
g,Ge
(0) of a band gap when the depletion layer between the emitter and the base is contracted as a voltage between the emitter and the base is made higher. Thus, the collector current is dropped in accordance with the equation (1), and the current gain h
FE
is largely dropped.
The second problem is the occurrence of a large variation in a collector current flowing when a certain voltage is applied between the base and the emitter. The variation in the collector current has great influence on a circuit operation when a transistor is used as a constant current source, for example, such as a case of an ECL circuit.
The reason of the variation in this collector current is as follows. The emitter layer
213
is formed by using the impurity thermal diffusion from the poly-crystal silicon film
212
to pull out an emitter in which the N-type impurity is doped. The reduction in the thickness of the P-type SiGe base layer
210
causes the Ge inclination (germanium inclination) to be sharp at the position where the emitter-base junction is formed. As a result, the slight variation at the junction position leads to the difference of the contraction amount
&Dgr;E
g,Ge
(0) in the band gap. That is, the collector current is proportional to the contraction amount
&Dgr
Foley & Lardner
Lebentritt Michael S.
NEC Electronics Corporation
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