Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
1999-06-01
2001-07-03
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S197000, C257S026000, C257S592000, C257S616000
Reexamination Certificate
active
06255674
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority of German Patent Application No. 198 24 110.0 filed May 29, 1998, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The invention relates to a silicon-germanium hereto bipolar transistor, having a silicon emitter, a silicon germanium base, and a silicon collector.
To this point, only diode oscillators have been significant in generating high-frequency power. In contrast, transistors have been considered unsuitable for such oscillators because of the relatively high base path resistance (S. Luryi, “Ultrafast operation of heterostructures [sic] bipolar transistors resulting from coherent base transport of minority carriers,” Proc. ISDRS '93, Charlottesville, 1993, p. 59).
FIG. 1
schematically illustrates the function of a pin velocity-modulated diode, by way of the electrical field distribution E in the p emitter
1
, the i drift zone
2
and the n collector
3
, as a typical example of a diode oscillator. The surface charge of the drifting electrons divides the drift space into a region of higher field intensity in front of the drifting electrons, and a region of lower field intensity behind the electrons. With a constant emitter-collector potential, the field intensities increase steadily on both the emitter and collector sides during the electron drift. Because the field intensities decrease inside the emitter
1
and the collector
3
, this increase means a discharge of electrons in the n region and a discharge of defect electrons in the p region. This influence current can be measured in the external electric circuit during the electron drift. According to
FIG. 2
, analogous considerations also apply to the bipolar transistor if the desired phase shift of the collector current is effected by the transit time in the base-collector space-charge zone. The influence current then flows in the base-collector circuit. The relatively high base resistance (typically>50&OHgr;) is present in this circuit, however, so this configuration is unsuitable for a damping reduction of an external resonating circuit.
To this point, silicon-germanium bipolar transistors have become known from DE 42 41 609, DE 43 01 333 and DE 196 17 030; they include only a base having a uniform doping and germanium concentration, or a simple variation in the doping or germanium concentration, which results in a high base resistance and therefore the described disadvantage.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a bipolar Si/Si/Ge heterotransistor structure in which the finite transit time of the charge carrier is utilized to generate a negative initial resistance in the base, and the in-phase components of the influence current are minimized in the emitter-collector circuit.
The above object generally is achieved according to the present invention by a silicon-germanium heterobipolar transistor having a silicon emitter, a silicon-germanium base and a silicon collector, wherein starting from the emitter, the base includes a change in the Ge content in the form of a step-wise increase, and a step-wise, but decreasing change in the doping concentration. Advantageous embodiments and modifications of the invention are disclosed and discussed.
The silicon-germanium bipolar transistor of the invention includes a change in the Ge content, which starts from the emitter and increases step-wise in the region of the base, alone or with a likewise step-wise, but opposing, change in the doping concentration, with a step height that causes step heights to occur which, seen in terms of energy, are larger than those of the optical phonon energy of the semiconductor material.
A step-wise increase in the Ge content is especially advantageous in connection with an opposing, likewise stepped change in the doping concentration.
Typically, the base has three to seven steps with a total thickness of 50 to 300 nm. The germanium content is preferably between 6 and 30%, and the doping in the case of p doping has a concentration between 8×10
19
and 5×10
18
cm
−3
.
A particular advantage of the invention is that the diffusion in the base of a transistor structure, which is normally non-directional, is directed through the graduation in the germanium concentration or the doping concentration, including the combination of the two, in the base.
REFERENCES:
patent: 5006912 (1991-04-01), Smith et al.
patent: 5047365 (1991-09-01), Kawanaka et al.
patent: 5177025 (1993-01-01), Turner et al.
patent: 5304816 (1994-04-01), Grinberg et al.
patent: 5329144 (1994-07-01), Luryi
patent: 5422502 (1995-06-01), Kovacic
patent: 5721438 (1998-02-01), Tang et al.
patent: 06 00 643 A2 (1993-11-01), None
patent: 06 21 642 A2 (1994-04-01), None
Patton et al., “75-GHz fT SiGe-Base Heterojunction Bipolar Transistors,” IEEE Elec. Dev. Lett., vol. 11, No. 4, Apr. 1990.
Jorke Helmut
Luy Johann-Friedrich
Daimler-Chrysler AG
Jackson, Jr. Jerome
Kunitz Norman N.
Venable
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