Active solid-state devices (e.g. – transistors – solid-state diode – Bipolar transistor structure
Reexamination Certificate
2002-04-19
2003-03-04
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Bipolar transistor structure
C257S592000, C257S616000, C257S655000
Reexamination Certificate
active
06528862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a bipolar transistor with a box-type germanium profile that lies outside of the emitter-base depletion region.
2. Description of the Related Art
A bipolar transistor is a three-terminal device that can, when properly biased, controllably vary the magnitude of the current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The charge carriers, which form the current, flow from the emitter terminal to the collector terminal, while variations in the voltage on the base terminal cause the magnitude of the current to vary.
FIG. 1
shows a cross-sectional diagram that illustrates a portion of a prior-art bipolar transistor
100
. As shown in
FIG. 1
, transistor
100
includes an n- epitaxial layer
110
that functions as the collector, shallow trench isolation (STI) regions
112
that are formed in layer
110
, and an epitaxial layer
114
that is formed on layer
110
and STI regions
112
.
Epitaxial layer
114
includes a p-type doped region
116
that has a lightly-doped region
116
A, and a heavily-doped region
116
B that is formed below lightly-doped region
116
A. Lightly-doped region
116
A has a top surface and a bottom region, while heavily-doped region
116
B has a top region and a bottom surface. The bottom region of lightly-doped region
116
A and the top region of heavily-doped region
116
B form a transition region where the dopant concentration increases from the concentration of lightly-doped region
116
A to the concentration of heavily-doped region
116
B.
Epitaxial layer
114
also includes an n-type doped region
118
that is formed in p-type doped region
116
. When unbiased, the physical junction between p-type doped region
116
and n-type doped region
118
forms a depletion region
120
where a junction region of doped region
116
A is free of holes and a junction region of doped region
118
is free of electrons. When doped region
116
and doped region
118
are forward biased, a smaller depletion region
120
A is formed. (A larger depletion region is formed when the doped region
116
and doped region
118
are reverse biased.)
As further shown in
FIG. 1
, epitaxial layer
114
includes a silicon germanium (SiGe) layer
114
A with a top surface that is positioned to lie within the smaller forward biased depletion region
120
A. In addition, the peripheral regions of SiGe layer
114
A have a p+ dopant concentration. Epitaxial layer
114
further includes a cap layer
114
B that is formed on SiGe layer
114
A to have substantially no germanium.
Transistor
100
also includes an n+ polysilicon (poly) region
122
that is formed on cap layer
114
B, and an oxide layer
124
that contacts cap layer
114
B and poly region
122
. Doped region
118
and poly region
122
function as the emitter of transistor
100
, while doped region
116
functions as the base.
Transistor
100
additionally includes a nitride layer
126
that contacts oxide layer
124
and poly emitter
122
, and a metalization plug
128
that is formed on poly emitter
122
. In addition, a base contact
130
is formed on SiGe layer
114
A to contact the peripheral p+ region of SiGe layer
114
A.
FIG. 2
shows a graph that illustrates a vertical view of epitaxial layer
114
under the central portion of poly region
122
. As shown in
FIG. 2
, n-type doped region
118
of epitaxial layer
114
extends down from the top surface of epitaxial layer
114
to a depth D
1
. Doped region
118
has an n+ dopant concentration at the surface that decreases to substantially zero at depth D
1
.
In addition, p-type doped region
116
of epitaxial layer
114
extends down from depth D
1
to a depth D
2
. (Epitaxial layer
114
also includes n-type dopants that outdiffused from the collector that extend from a depth D
3
to the bottom surface.) Doped region
116
includes lightly-doped region
116
A that lies adjacent to doped region
118
, and heavily-doped region
116
B that is formed under lightly-doped region
116
A.
In addition, as further shown in
FIG. 2
, SiGe layer
114
A has a substantially uniform concentration of germanium (approximately 0.15 mole fraction) throughout layer
114
A. A SiGe layer with a substantially constant concentration of germanium is known as having a box-type germanium profile.
A box-type germanium profile has a down ramp
132
that defines a thin top surface layer
134
of SiGe layer
114
A that has a decreasing germanium concentration. (The germanium concentration can not go to zero instantly.) Thus, down ramp
132
is substantially linear with a very large slope.
One important measure of a transistor is the amount of base to collector current amplification, known, as beta, provided by the transistor. The germanium in epitaxial layer
114
increases the beta when compared to a standard silicon base layer that has no germanium. This is because SiGe layer
114
A has a narrower band gap than the silicon of doped region
118
. The difference in band gaps enhances the efficiency of carrier injection from doped region
118
into doped region
116
.
To obtain the increased beta, surface layer
134
of SiGe layer
114
A must lie within the smaller depletion region
120
A when transistor
100
is forward biased. When surface layer
134
of SiGe layer
114
A does not lie within depletion region
120
A when forward biased, there is substantially no increase in the beta of transistor
100
over what can be obtained with a comparable transistor that does not utilize a silicon germanium base. Thus, if cap layer
114
B is too thick and surface layer
134
of SiGe layer
114
A does not lie within junction depletion region
120
A when forward biased, SiGe layer
114
A provides no appreciable increase in beta.
Another important measure of a bipolar transistor is the time required for a minority carrier to pass through the base region, known as the base transit time. This is an important measure for gigahertz frequency devices because the base transit time is one of the major components of the total transit time which, in turn, limits the maximum frequency of the signal.
One component of the base transit time is the boron concentration in doped region
116
. As noted above, doped region
116
(the base) has a lightly doped region
116
A adjacent to doped region
118
(the emitter) and a heavily doped region
116
B adjacent to lightly doped region
116
A.
As a result, the boron concentration decreases from a peak value at a depth D
4
in heavily doped base region
116
B when moving towards doped region
118
(the emitter). This decrease in boron concentration sets up a retarding field that slows electrons injected by the emitter. This results in increased base transit time and, as a result, a decreased fT peak.
One approach to reducing the base transit time is to utilize a SiGe layer with a triangular-type germanium profile.
FIG. 3
shows a graph that illustrates a transistor
300
with a triangular-type germanium profile. Transistor
300
is similar to transistor
100
and, as a result, utilizes the same reference numerals to designate the structures that are common to both transistors.
As shown in
FIG. 3
, transistor
300
differs from transistor
100
in that transistor
300
has a triangular-type germanium profile with a down ramp
310
that extends from the top surface of SiGe layer
314
A at depth D
1
to a point at depth D
5
that corresponds with the location of the peak germanium concentration. As a result, the concentration of germanium linearly increases from zero at the top surface of SiGe layer
314
A to approximately 0.15 mole fraction at depth D
5
near the bottom surface of SiGe layer
314
A with a slope that is much less than the slope of down ramp
132
.
In operation, the decreasing-towards-the-emitter slope of the triangular-type germanium profile sets up a uniform field across SiGe layer
314
A that is opposite to the retarding field set
National Semiconductor Corporation
Pickering Mark C.
Prenty Mark V.
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