Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
2000-09-11
2004-05-18
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S011000, C257S012000, C257S015000, C257S024000, C257S183000, C257S191000, C257S200000
Reexamination Certificate
active
06737684
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device having a heterojunction portion such as a bipolar transistor, a diode, or an I
2
L.
Because of its excellent RF characteristics, a bipolar transistor has conventionally been used as an active device operable in the microwave/milliwave bands. In particular, most vigorous research and development has been directed to a heterojunction bipolar transistor (HBT) using a III-V compound semiconductor such as GaAs. In recent years, attention has been focused on a HBT using a SiGe material, which is a IV-IV compound material that can be fabricated on a low-cost silicon substrate.
The following are the two representative types of structures for implementing higher-speed SiGe HBTs. One of the two types is a HBT reported in Document 1 (L. Harame et al., “Optimization of SiGe HBT Technology for High Speed Analog and Mixed-Signal Applications,” IEDM Tech. Dig. 1993, p.71), which comprises a Si collector layer, a SiGe base layer, and a Si emitter layer. In the SiGe base layer, a Ge composition ratio is increased gradually from a region in contact with the Si emitter layer toward a region in contact with the Si collector layer to provide a graded composition base layer. The other of the two types is a HBT reported in Document 2 (A. Schuppen et al., “Enhanced SiGe Heterojunction Bipolar Transistors with 160 GHz-fmax,” IEDM Tech. Dig. 1995, p.743.), which comprises a Si collector layer, a SiGe base layer, and a Si emitter layer. The SiGe base layer has an extremely reduced thickness, an increased Ge composition ratio, and an increased doping concentration to provide a uniform composition base layer.
FIG. 18
is a band diagram of the former heterojunction bipolar transistor having the graded composition base layer. As can be seen from the band state shown in the drawing, an electric field induced by the graded composition causes carriers injected into the SiGe base layer to drift in the SiGe base layer toward the collector layer. Since the traveling of the carriers caused by the drift electric field is at a higher speed than the traveling thereof caused by diffusion, a base transit time is reduced and excellent RF characteristics are obtained.
FIG. 19
is a band diagram of the latter heterojunction bipolar transistor having the uniform composition base structure. As can be seen from the band state shown in the drawing, the base layer is extremely thinned to reduce the base transit time and provide excellent RF characteristics. In this case, the thinning of the base layer incurs the risk of increasing the base resistance, so that the base layer is doped with a high-concentration impurity to lower the base resistance. In addition, SiGe having a high Ge composition ratio is used in the base layer to prevent reverse injection of carriers from the base layer doped with the high-concentration impurity into the emitter, so that a heterojunction barrier formed between the SiGe base layer and the Si emitter layer is increased. In this case also, excellent RF characteristics are obtained. In particular, the doping concentration in the base layer is increased to reduce the base resistance and thereby increase a maximum oscillation frequency.
In a conventional Si LSI using a bipolar transistor, on the other hand, it has frequently been performed to compose a diode by using a PN junction portion between the base and collector of the NPN bipolar transistor and use the diode as an element of a logic circuit. This is because the NPN bipolar transistor has a structure suitable for the formation of a large number of built-in diodes since the PN junction portion between the base and collector has a high breakdown voltage (with respect to a reverse bias) and the N-type collector layer is used as a common region in the substrate.
However, such a PN junction diode has the drawback that it is unsuitable for use in a device operating at a high speed due to minority carriers accumulated in each of the P-type and N-type regions thereof. Specifically, electrons as minority carriers are accumulated in the P-type base layer of the NPN bipolar transistor layer, while holes as minority carriers are accumulated in the N-type collector layer thereof. In a typical high-speed bipolar transistor, the P-type base layer is formed extremely thin to reduce the base transit time so that the accumulation of the electrons in the P-type base layer presents substantially no problem even in the PN junction diode composed by using a part of the bipolar transistor. However, the N-type collector layer is formed to have a sufficient thickness in the range of 0.5 to 1 &mgr;m in order to retain a high breakdown voltage, so that numerous holes are accumulated therein, which eventually limits the speed of the PN junction diode.
As a method of increasing the operating speed by suppressing the accumulation of minority carriers in such a collector region, there has been known one reported in Document 3 (M. Ugajin et al., “The base-collector heterojunction effect in SiGe-base bipolar transistors,” Solid-State Electron., vol.34, pp.593, 1991), in which a SiGe/Si heterojunction is provided in the base/collector junction to impart a wider band gap to the collector layer. By thus imparting the wider band gap to the collector layer, a heterojunction barrier is formed in the base/collector junction portion to suppress injection of holes from the base layer to the collector layer, thereby reducing the amount of holes accumulated in the collector and increasing the operating speed of the diode.
There has also been known a method disclosed in Document 4 (M. Karlsteen et al., “Improved switch time of I2L at low power consumption by using a SiGe heterojunction bipolar transistor,” Solid-State Electron., vol.38, pp.1401, 1995), in which a heterojunction is provided in a base/collector junction portion in an I
2
L (Integrated Injection Logic) circuit into which a plurality of bipolar transistors have been integrated, thereby suppressing the accumulation of minority carriers and increasing the operating speed.
However, the aforesaid bipolar transistor and the diode using the bipolar transistor have the following disadvantages.
In the conventional HBT using the graded composition base shown in
FIG. 18
, it is required to greatly vary the Ge composition ratio in order to increase the intensity of the drift electric field induced by the graded composition. In short, it is required to minimize the Ge composition ratio in a region of the base layer in contact with the emitter layer and maximize the Ge composition ratio in a region of the base layer in contact with the collector layer. To satisfy the requirement, the region of the base layer in contact with the emitter layer normally has a pure Si composition without containing Ge, so that the base/emitter PN junction forms a silicon/silicon homojunction. In increasing the maximum oscillation frequency fmax of the HBT, it is effective to reduce the. base resistance as represented by the following equation (1). If a base doping concentration is increased to reduce the base resistance, however, an increased number of holes are naturally injected from the base layer into the emitter layer.
In the case where the emitter/base junction forms a homojunction or where the emitter/base junction forms a heterojunction but has a nearly pure Si composition at the end of the base, the quantity of carriers reversely injected into the emitter is increased because the base layer has no heterojunction barrier at all or, if any, an extremely low heterojunction barrier, so that the current amplification factor &bgr; is not increased.
f
max
=
f
T
8
π
·
R
B
·
C
BC
(
1
)
f
r
: current gain cutoff frequency
R
B
: base resistance
C
BC
: base/collector junction capacitance
The fact that the current amplification factor &bgr; is not increased can also be derived from the relationship represented by the following equation (2), which is established among the current amplification factor &bgr;, the band discontinuity value &Dgr;Ev of a val
Harafuji Kenji
Takagi Takeshi
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