Semiconductor device

Details Semiconductor device Semiconductor device

2002-02-08

2004-04-06

Lee, Eddie (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Heterojunction device

Having graded composition

C257S197000, C257S200000, C257S592000

Reexamination Certificate

active

06717188

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device that functions as a heterojunction bipolar transistor, and in particular to measures for suppressing variations in properties such as the current amplification factor.
In recent years, progress in miniaturization and self-aligning technology has led to smaller and faster bipolar transistors formed using a silicon substrate. Ordinary bipolar transistors are so-called homojunction bipolar transistors, which use a silicon substrate and a monocrystalline silicon layer epitaxially grown on the silicon substrate.
On the other hand, heterojunction bipolar transistors (hereinafter, referred to as “HBT”) are being actively researched and developed for the purpose of further increasing operating speeds. In particular recently, there has been a strong push for the development of HBTs in which a SiGe layer, which is a mixed crystal of silicon and germanium, is epitaxially grown on a silicon substrate, and the SiGe layer is taken as a base layer (hereinafter referred to as “SiGe-HBT”).
FIG. 8
is a cross-sectional view of a conventional SiGe-HBT. As shown in the drawing, conventional SiGe-HBTs are formed using a Si substrate
101
and a Si epitaxial layer
102
epitaxially grown on the Si substrate
101
. They also include an N
+
buried layer
110
, which is provided spanning a portion of the Si substrate
101
and a portion of the Si epitaxial layer
102
, and an N
+
collector lead layer
111
, which is provided by introducing a relatively high concentration of N-type impurities into a portion of the Si epitaxial layer
102
. The portion of the Si epitaxial layer
102
other than the N
+
collector lead layer
111
is an N
−
collector diffusion layer
112
, which includes a low concentration of N-type impurities. Furthermore, LOCOS isolations
116
separating the Si epitaxial layer
102
into the bipolar transistor formation regions, and deep trench isolations
117
, which extend downward from the LOCOS isolations
116
and into the Si substrate
101
, are also provided. However, in the bipolar transistor formation region there is no deep trench isolation
117
provided below the LOCOS isolation
116
separating the N
+
collector lead layer
111
and the N
−
collector diffusion layer
112
.
Furthermore, a SiGe film
108
, which is made of a SiGe mixed crystal semiconductor layer and includes P-type impurities, and a Si film
109
, serving as the cap layer, are formed by epitaxial growth on the N
−
collector diffusion layer
112
of the Si epitaxial layer
102
. The SiGe-HBT also includes a P
+
base polysilicon film
114
, which is formed on the region spanning from the lateral surfaces of the SiGe film
108
and the Si film
109
to the upper surface of the Si film
109
and includes a high concentration of P-type impurities, and an N
+
emitter polysilicon film
113
including a high concentration of N-type impurities, which is provided over an opening formed in the P
+
base polysilicon film
114
including a high concentration of P-type impurities. However, the P
+
base polysilicon film
114
and the N
+
emitter polysilicon film
113
are electrically separated from each other by an insulating film.
Here, the SiGe film
108
and the Si film
109
are epitaxially grown using MBE, UHV-CVD, or LP-CVD. The region of the Si film
109
directly below the N
+
emitter polysilicon film
113
is doped with N-type impurities (phosphorus or arsenic, for example) that have been diffused from the N
+
emitter polysilicon film
113
by RTA. That is, the N
+
region of the Si film
109
functions as the emitter region of the NPN bipolar transistor, the P
+
region of the SiGe film
108
functions as the base region of the NPN bipolar transistor, and the N
−
collector diffusion layer
112
, the N
+
buried layer
110
, and the N
+
collector lead layer
111
function as the collector region of the NPN bipolar transistor.
In the process for manufacturing the semiconductor device, after the SiGe film
108
is epitaxially grown on the Si epitaxial layer
102
, the Si film
109
is then successively grown epitaxially on the SiGe film
108
. The Si film
109
is necessary for preventing Ge contamination of the manufacturing line primarily during the further process steps after the epitaxial growth of SiGe, however, it is possible to form an emitter-base junction (hereinafter, called the “EB junction”) at a desired depth and position in the Si film
109
by adequately selecting the thermal processing conditions for diffusing the N-type impurities in accordance with the film thickness of the Si film
109
and the concentration of the N-type impurities in the emitter polysilicon film
113
.
Conventional SiGe-HBTs formed in this way have the advantage over homojunction bipolar transistors made from only a Si layer that impurities do not have to be doped to a high concentration in the emitter region to achieve a large emitter injection efficiency, and they can be expected to have a high current amplification factor (h
FE
).
FIG. 9
is an energy band diagram for comparing the band structures of Si/SiGe heterojunction bipolar transistor (SiGe-HBT) with graded composition and a Si homojunction bipolar transistor (Si-BT). In the SiGe-HBT, the height of the barrier with respect to holes injected into the emitter region from the base region can be made larger than the height of the barrier with respect to electrons injected from the emitter region into the base region. For this reason, the emitter injection efficiency does not drop even if the impurity concentration of the emitter region is lowered and the impurity concentration of the base region is raised.
Put differently, with SiGe-HBTs the narrow band gap properties of SiGe can be used to achieve a higher current amplification factor than in Si-BTs, even if the emitter region is not doped to a high concentration.
Si has a band gap of approximately 1.1 eV, and Ge has a band gap of approximately 0.7 eV. When a SiGe film includes 10 to 15% Ge, the band gap is between that of Si and Ge, at about 1.0 eV. Thus, monotonically increasing the Ge content in the SiGe film
108
from the emitter side to the collector side (graded composition) creates a graded structure in which the energy band gap Eg becomes continually smaller from the emitter side toward to the collector side, as shown by the solid line in FIG.
9
. For this reason, an internal electric field E, as expressed by the following equation (1):
E
(
eV
)=(1.1−1.0)/
qW
  (1)
(q: charge amount, W: base width) occurs in the base layer, and the minority carriers that are injected from the emitter into the base can be accelerated by the electric field E. Therefore, higher operation speeds than conventional Si-BTs, in which the minority carriers transit through the base region only by diffusion, can be easily achieved.
However, the above-described conventional SiGe-HBTs have the following problems.
FIG. 10
is a diagram showing the impurity concentration distribution and the change in the Ge content in the depth direction in the cross section taken along the line X—X shown in FIG.
8
. As shown in
FIG. 10
, the SiGe film
108
is divided into an undoped SiGe buffer layer
108
x
, and a SiGe graded composition layer
108
y
into which P-type impurities have been introduced at a high concentration, and which has a continuously changing band gap. A P-type impurity diffusion region
132
, serving as the base region, is formed in the upper part the SiGe film
108
, and an N-type impurity diffusion region
131
, which serves as the emitter region, is formed spanning from the Si film
109
into a portion of the SiGe film
108
. That is, the P-type impurity diffusion region
132
and the N-type impurity diffusion region
131
overlap. The reason for this is that during thermal processing for forming the emitter region, N-type impurities from the emitter polysilicon film
113
reach into not only the Si film
109

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