Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2002-02-04
2004-07-06
Thomas, Tom (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S593000, C257S197000, C257S616000
Reexamination Certificate
active
06759674
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
In a silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer.
The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.
The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germanium devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.
The advantages of high speed and high frequency response discussed above require the realization of a thin highly doped base layer in the NPN SiGe HBT. For example, boron is commonly utilized to provide P-type doping of the base in an NPN silicon-germanium HBT. However, boron has a tendency to diffuse in the base. In other words, the boron profile in the base has a tendency to widen, thus undesirably widening the base. Boron diffusion is further accelerated during subsequent thermal processing steps that occur in the fabrication of the NPN SiGe HBT. The increased boron diffusion can severely degrade the high frequency performance of the NPN SiGe HBT. Thus, suppression of boron diffusion presents a major challenge in the fabrication of a NPN SiGe HBT.
One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. To effectively arrest the diffusion of boron, a heavy carbon doping level is required. For example, a concentration greater than approximately 0.1 atomic percent of carbon can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Due to the high carbon concentration, the impact on the lattice is such that the periodicity of the lattice is altered to compensate total strain. Since the in-plane strain is key to band-gap narrowing in SiGe, the addition of carbon doping counters this benefit from which increased NPN performance is derived. Thus, although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of increasing the band gap in the base and consequently diminishing the performance of the NPN SiGe HBT.
Graph
100
in
FIG. 1
shows exemplary boron, carbon, and germanium profiles in a base in an NPN SiGe HBT. Graph
100
includes concentration level axis
102
plotted against depth axis
104
. Concentration level axis
102
shows relative concentration levels of boron, carbon and germanium. Depth axis
104
shows increasing depth into the base, starting at the top surface of the base, i.e. at the transition from emitter to base in the NPN SiGe HBT. The top surface of the base in the NPN SiGe HBT corresponds to “0” on depth axis
104
.
Graph
100
also includes boron profile
106
, which shows the concentration of boron in the base, plotted against depth, i.e. distance into the base. Boron profile
106
includes peak boron concentration level
108
, which occurs at depth
114
. Graph
100
further includes carbon profile
112
, which shows the concentration of carbon in the base, plotted against depth. The concentration of carbon in carbon profile
112
increases abruptly from 0.0 to a constant level at depth
114
, and remains at a constant level from depth
114
to depth
122
. At depth
122
, the carbon concentration level decreases abruptly to 0.0.
Graph
100
further includes germanium profile
116
, which shows the concentration of germanium in the base of the present exemplary NPN SiGe HBT, plotted against depth. Germanium profile
116
begins at 0.0 concentration level at depth
110
and ramps up, i.e. increases linearly, to depth
118
. Germanium profile
116
maintains a constant concentration level from depth
118
to depth
120
. At depth
120
, germanium profile
116
ramps down, i.e. decreases linearly, to 0.0 concentration level at depth
122
. Thus, a concentration of carbon is added in the base of the NPN SiGe HBT at depth
114
, which corresponds to peak boron concentration level
108
.
Graph
200
in
FIG. 2
shows an exemplary band gap curve in the base in the present exemplary NPN SiGe HBT. Graph
200
shows band gap curve
202
, which shows the change in band gap caused by carbon profile
112
and germanium profile
116
in
FIG. 1
in the base in the present exemplary NPN SiGe HBT. Graph
200
includes change in band gap axis
208
plotted against depth axis
204
. It is noted that “0” on change in band gap axis
208
refers to the band gap of a reference base comprising only silicon, i.e. a silicon-only base. It is also noted that an upward move on band gap curve
202
indicates a decrease in the band gap of the base of the present exemplary NPN SiGe HBT relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve
202
indicates an increase in the band gap of the base relative to the band gap of a silicon-only base.
Depth axis
204
corresponds to depth axis
104
in FIG.
1
. In particular, depths
210
,
214
, and
222
, respectively, correspond to depths
110
,
114
, and
122
in FIG.
1
. At depth
210
, band gap curve
202
begins to decrease at a linear rate. As is known in the art, an increase in the concentration of germanium in a base of an NPN SiGe HBT results in a decrease in band gap. Thus, band gap curve
202
decreases from depth
210
to just prior to depth
214
as the result of a ramp up in concentration of germanium. At depth
214
, the band gap increases abruptly from band gap level
212
to band gap level
216
. This step increase in band gap corresponds to the addition of carbon in the base at depth
114
in FIG.
1
. As such, the addition of carbon in the base of an NPN SiGe HBT results in an undesirable increase in the band gap of the base. This increase in band gap creates an electric field in the NPN SiGe HBT that opposes current flow, and thus results in a decrease in the speed that the NPN SiGe HBT can achieve.
Thus, there is a need in the art to provide a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.
SUMMARY OF THE INVENTION
The present invention is directed to a band gap compensated HBT. The present invention overcomes the need in the art for a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.
According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of a first material at a first depth, where the concentration of the first material impedes the diffusion of a base dopant. For example, the first material can be carbon and the base dopant can be boron. The first material also causes a change in band gap at the first depth in the base. For example, the first material may cause an increase in band gap at the first depth in the base.
According to this exemplary embodiment, the base of the heterojunction bipolar transistor further comprises a concentration of a second material, where the concentration of the second material increases at the first depth so as to counteract the change in the band gap. For example, the second material may be germanium. The concentration of the second material, for example, may increase at the first depth by an amount required to cause a decrease in the band gap to be substantially equal to the increase in band gap caused by the concentration of the first material.
REFERENCES:
patent: 6316795 (2001-11-
Racanelli Marco
Schuegraf Klaus F.
U'Ren Greg D.
Farjami & Farjami LLP
Newport Fab LLC
Nguyen Joseph
Thomas Tom
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