Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2002-11-20
2003-12-23
Zarabian, Amir (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S019000, C257S183000, C257S197000
Reexamination Certificate
active
06667489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a heterojunction. The present invention is useful particularly for a bipolar transistor having an SiGeC layer formed by epitaxial growth.
2. Description of the Related Art
A heterojunction bipolar transistor (HBT) made up of single-crystal SiGeC and single-crystal Si is disclosed in JP-A No. 68479/2001. As
FIG. 24
hereof shows in section, this HBT consists of a collector, a base, and an emitter. The collector comprises a layer
101
of n-type single-crystal Si and a layer
102
of n-type single-crystal SiGeC. The base is a layer
103
of p-type single-crystal SiGeC. The emitter comprises a layer
104
of n-type single-crystal SiGeC and a layer
105
of n-type single-crystal Si.
The conventional HBT has the profiles of compositional ratios of Ge and C and concentration of Boron (B) which are distributed as shown in
FIG. 25
hereof. It is to be noted that the compositional ratios of Ge and C increase in the collector in going from layer
101
(layer of n-type single-crystal Si) to layer
102
(layer of n-type single-crystal SiGeC), gradually decrease in the base in going from layer
102
to layer
104
, and further decreases in the emitter in going from layer
104
(layer of n-type single-crystal SiGeC) to layer
105
(layer of n-type single-crystal Si).
The HBT constructed as shown in
FIG. 25
has an energy band gap structure (the lower end of the conduction band and the upper end of the valence band) as shown in FIG.
26
.
FIG. 26A
shows the band structure for a small injection of electrons from the emitter, and
FIG. 26B
shows the band structure for a large injection of electrons from the emitter. It is to be noted that the energy of the conduction band in the base
103
(layer of p-type single-crystal SiGeC) decreases in going from the emitter to the collector according as the compositional ratios of Ge and C change.
In the conduction band, there is no energy barrier due to band gap at the interface between layer
101
(layer of n-type single-crystal Si) and layer
102
(layer of n-type single-crystal SiGeC layer) forming the collector. Consequently, electrons that are injected from the emitter travel through the base with an acceleration due to an electric field created by the inclined conduction band. See FIG.
26
A.
An example of HBT having a heterojunction of single-crystal SiGe and single-crystal SiC is disclosed in JP-A No. 77425/2000. As
FIG. 27
shows in section, this HBT is constructed of a collector (consisting of a layer
106
of n-type single-crystal Si and a layer
107
of n-type single-crystal SiC), a base (which is a layer
108
of p-type single-crystal SiGe), and an emitter (consisting of a layer
109
of n-type single-crystal SiC and a layer
110
of n-type single-crystal Si).
SUMMARY OF THE INVENTION
It would be desirable to have an HBT which has a heterojunction between a layer of single-crystal Si, a layer of single-crystal SiGe, and a layer of single-crystal SiGeC, capable of high-speed operation even with a large injection of electrons from the emitter.
The present invention is intended to address the following problems encountered in the conventional technologies.
In the conventional bipolar transistor having the base formed from single-crystal SiGeC, the collector consists of a layer
101
of single-crystal Si and a layer
102
(
FIG. 24
) of n-type single-crystal SiGeC placed directly thereon which has a smaller bandgap than single-crystal Si. This structure induces the following phenomenon. As electrons injected from the emitter increase, electrons from the base diffuse into the depletion layer adjacent to the collector at the base-collector junction, thereby canceling out space charges due to n-type impurity ions. This substantially expands the neutral base. The consequence is that an energy barrier appears in the conduction band at the depletion layer of the base-collector junction. This energy barrier impedes the diffusion of electrons injected from the emitter, which in turn deteriorates the HBT's high-speed performance.
FIG. 26
is an example of the energy bandgap of the HBT constructed as shown in FIG.
24
.
FIG. 26A
is a band structure for a small injection of electrons from the emitter, and
FIG. 26B
is a band structure for a large injection of electrons from the emitter. It is clearly understood that the neutral base expands as the injection of electrons from the emitter increases.
One possible way to prevent the operating speed of HBT from being decreased by the energy barrier despite a large injection of electrons from the emitter is to make thicker the layer
102
(
FIG. 24
) of n-type single-crystal SiGeC. However, this is not practical because improving the crystallinity requires decreasing the growth temperature. Since the growth rate of the SiGeC layer exponentially decreases in inverse proportion to the growth temperature, a thick layer of single-crystal SiGeC takes a very long time to grow. This leads to a low throughput and a high cost in the manufacture of SiGeC HBT.
Another example of the conventional HBT is shown in FIG.
27
. It has a base layer
108
of single-crystal SiGe and a layer
106
in the collector of single-crystal Si, with a layer
107
of n-type single-crystal SiC interposed between them. In this HBT, the collector has a larger bandgap than the base regardless of the injection of electrons from the emitter. This creates an energy barrier in the conduction band at the base-collector junction. This in turn poses a problem of impeding the diffusion of electrons. As a result, high-speed performance deteriorates.
The essential features of the present invention are summarized below with reference to
FIGS. 1 and 6
.
FIG. 1
is a sectional view showing the laminate construction of the main region of the HBT of the present invention.
FIG. 6A
is a schematic diagram showing the energy band structure of a preferred HBT of the present invention in the normal operating state.
FIG. 6B
is a schematic diagram showing the energy band structure of a preferred HBT of the present invention which manifests itself when the neutral base extends to the collector. These figures show the lower end of the conduction band and the upper end of the valence band. The references of numerals are as follows:
16
: emitter,
9
: base,
7
and
3
: collector (
3
denoting the region of the semiconductor substrate)
The present invention provides an HBT having the base and collector layers such that no energy barrier appears in the conduction band at the depletion layer of the base-collector junction. In addition, no energy step occurs in the HBT of the present invention in the neutral base when the injection of electrons from the emitter is large.
The HBT of the present invention is achieved by forming the base and collector from single-crystal SiGe as the main material for the HBT in which single-crystal SiGeC exists at the heterojunction. Basically, the selection of the material is made such that the energy gap (Eg) of the base is larger than the Eg of the collector when the injection of electrons from the emitter is large. The above-mentioned single-crystal SiGeC may be used for the base and collector regions. Single-crystal SiGeC is preferably selected for the base or collector.
In the HBT of the present invention, the base preferably comprises a layer of single-crystal SiGe or single-crystal SiGeC, and the collector preferably comprises an SiGe layer or an SiGeC layer or a laminate of SiGeC layer and SiGe layer (which will be denoted by SiGeC/SiGe). The following table summarizes the preferred selections of the materials.
Base
Collector
1
SiGe
—
SiGeC
SiGeC/SiGe
2
SiGeC
SiGe
SiGeC
SiGeC/SiGe
The emitter preferably comprises a single-crystal Si layer, a single-crystal SiGe layer, a laminate of single-crystal SiC layer and single-crystal SiGe layer (denoted by SiGe/SiC hereinafter), or a laminate of single-crystal SiGeC layer and single-crystal SiGe layer (denoted by SiGe/SiGeC hereinafter).
The present invent
Oda Katsuya
Suzumura Isao
Washio Katsuyoshi
Hitachi , Ltd.
Lewis Monica
Zarabian Amir
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