Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2000-04-03
2001-11-13
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S077000, C257S198000, C438S312000, C438S694000
Reexamination Certificate
active
06316795
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method and apparatus for increasing the gain and speed of Silicon-Germanium heterojunction bipolar transistors. More specifically, the invention provides a means for using a carbon-doped silicon emitter to trap silicon interstitial atoms, to reduce transient enhanced diffusion of boron.
BACKGROUND OF THE INVENTION
Heterojunction bipolar transistor integrated circuits are used in a wide variety of applications including satellite telecommunication systems, high-speed analog/digital converters, wireless communications circuits, radar systems, and Microwave Monolithic Integrated Circuits (MMICs). Silicon-Germanium heterojunction bipolar transistors exhibit significantly enhanced performance relative to silicon homojunction transistors. Silicon-Germanium heterojunction bipolar transistors offer a lower barrier for electron injection into the base as compared to homojunction devices.
A problem common to npn Silicon-Germanium devices is the out-diffusion of boron from the Silicon-Germanium component. This out-diffusion can result from a number of factors including transient enhanced diffusion resulting from an annealing step. The boron out-diffusion into the silicon region is undesirable, in part, because it results in the formation of a parasitic conduction band barrier thereby reducing the transistor's gain and speed.
In an attempt to overcome the problems associated with boron out-diffusion, undoped SiGe spacer layers have been deposited on either side of the boron-doped SiGe base layer. This previous technology is shown schematically in
FIG. 1
a
. The boron-rich region
100
is flanked by two SiGe spacer layers
102
, the spacer layers
102
are included in anticipation of boron diffusion. Such an approach results in a base layer thickness
104
that may be twice as thick as the base layer provided for in the present invention.
An alternate approach, according to the literature, for limiting boron diffusion has been to dope the boron doped SiGe base layer with carbon
110
. This approach, while generally effective in preventing boron out-diffusion, has serious limitations. First the carbon concentration necessary to mitigate the effects of boron diffusion may necessarily exceed 1×10
19
cm
−3
with as much as 50% of the carbon located interstitially. This level of interstitial carbon can reduce the mobility, and hence the speed, of SiGe HBTs by as much as a factor of 10. This reduction results, in part, from the fact that SiGe has a higher hole mobility than SiGe doped with carbon, which is due to the lack of carbon-related complexes associated with interstitial or otherwise non-substitutional carbon. Additionally, traps associated with non-substitutional carbon can affect the position of the Fermi level in the base, which also adversely affect device performance. None of the existing methods allows for the concurrent enhancement of base mobilities and for substantial elimination of boron out-diffusion.
SUMMARY OF THE INVENTION
The embodiments described herein improve the performance of SiGe HBT devices by allowing for the concurrent enhancement of base mobilities and substantial elimination of boron out-diffusion.
One embodiment of the invention relates to a heterojunction bipolar transistor having a carbon-containing layer in the emitter. Wherein the transistor comprises a first semiconductor layer of a first conductivity type, grown on a semiconductor substrate, a second semiconductor layer of a second conductivity type grown on the first semiconductor layer, and a third semiconductor layer of the first conductivity type on the second semiconductor layer wherein the third semiconductor layer is comprised of three substantially planer regions. Further, the first and third planer regions are optionally of substantially of the same composition; and the second planer region contains doped silicon and carbon.
In another embodiment of the invention a heterojunction bipolar transistor has a first semi-conducting layer that is comprised of silicon doped with approximately 5×10
16
atoms of antimony per cm
−3
, and the second semi-conducting layer is comprised of SiGe
0.4
and doped with approximately 1×10
20
atoms of boron per cm
−3
. The third semi-conducting layer is comprised of three substantially regions, and the first planer region of the third semi-conducting layer is doped with approximately 2×10
18
atoms of antimony per cm
−3
and the second planer region is comprised of Si
0.999
C
0.001
doped with approximately 2×10
18
atoms of antimony per cm
−3
and the third planer region of the third semi-conducting layer is doped approximately 2×10
18
atoms of antimony per cm
−3
.
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patent: 5708281 (1998-01-01), Morishita
patent: 5859447 (1999-01-01), Yang et al.
patent: 05090278-A (1993-04-01), None
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Sturm et al., JEDM, Tech. Dig. p. 249, 1996.
J.W. Erickson, J. Sheng,Y. Gao, H. Pham, and I.L. Singer, “Carbon Overcoat Composition and Structure Analysis by Secondary Ion Mass Spectrometry”, J. Vac. Sci. Technol. A 16(3) May/Jun. 1998.
H.J. Osten, B. Heinemann, D. Knoll, G. Lippert, and H. Rucker, “Effects of carbon on boron diffuxion in SiGe: Principles and impact on bipolar devices”, J. Vac. Sci. Tech. B, vol. 16, No. 3, p. 1750 (1998).
Flynn Nathan
Forde Remmon R.
HRL Laboratories LLC
Tope-Mckay & Associates
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