Active solid-state devices (e.g. – transistors – solid-state diode – Bipolar transistor structure – With means to increase current gain or operating frequency
Reexamination Certificate
2002-04-23
2004-01-06
Wille, Douglas A. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Bipolar transistor structure
With means to increase current gain or operating frequency
C257S197000, C257S616000
Reexamination Certificate
active
06674150
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor which includes a base layer containing a SiGeC layer with a small degree of lattice strain.
Conventionally, heterojunction bipolar transistors (HBT's) have a heterojunction barrier formed on the boundary between the energy bands of the two semiconductor materials differing in band gap from each other at a junction between the emitter, the base and the collector for the purpose of improving carrier accumulation and a current amplification ratio. Such HBT's have come to be used as an active device in a microwave and millimeter wave bands by making use of their high frequency characteristic. Above all, HBT's using a semiconductor of a Group III-V compound such as GaAs have been studied and developed most energetically; however, in recent years, HBT's using SiGe material, which is a Group IV—IV compound and can be formed on a silicon substrate are being drawn attention (SiGe-HBT's). These SiGe-HBT's are being drawn attention also because the base layer made of a SiGe layer with a narrow band gap makes them operable at a lower voltage than Si-BJT's.
SiGe-HBT's proposed so far to achieve speedups are classified into two typical types: the one with a SiGe base layer having a graded composition where the Ge content is gradually increased in the direction from the emitter layer side to the collector layer side (Reference 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), and the other with a base layer having a high Ge content and a high concentration of impurity doping so as to make the base layer extremely thin (Reference Document 2) (A. Schuppen et al., “Enhanced SiGe Heterojunction Bipolar Transistors with 160 GHz-fmax,” IEDM Tech. Dig. 1995, p.743.).
In the former transistors provided with the base layer having a graded composition, the graded composition develops an electric field, which facilitates carriers injected in the base layer to drift the base layer. The drifting of the carriers due to the drift electric field is higher in speed than carrier diffusion, so that the time required to transit the base layer (base transit time) is accelerated to provide a high frequency transistor.
On the other hand, the latter heterojunction bipolar transistors have a base layer composed of SiGe having a uniform composition with a high Ge content and having a narrow band gap. The base layer is doped with a high-concentration impurity for carriers in order to decrease its thickness while suppressing a punch through between the emitter and the collector, thereby to accelerate the base transit time. In this case, the base layer having a narrower band gap than the emitter layer reduces the built-in potential of the PN junction between the emitter and the base, thereby achieving a large collector current and a high frequency characteristic at a low voltage.
However, these prior art heterojunction bipolar transistors have the following inconveniences.
First, in the heterojunction bipolar transistors having the graded composition base structure, the gradient of the composition must be large enough to have a large drift electric field. In other words, of the base layer, the region in contact with the emitter layer must have a small Ge content, and the region in contact with the collector layer must have a large Ge content. For this, the region of the base layer that is in contact with the emitter layer is generally made from Si only, without Ge. Since the PN junction between the base and the emitter in this case is a homogeneous junction between silicon and silicon, low-voltage operation cannot be expected. In addition, further acceleration of the base transit time for the improvement of the high frequency characteristic requires to further increase the Ge content in the region of the base layer that is in contact with the collector layer; however, when the Ge content is too large, the difference in lattice constant between Si and Ge (lattice mismatch) in the SiGe layer formed on the Si substrate causes dislocations in the base layer, deteriorating the reliability. This indicates that there are limits on an increase in the Ge content. According to Reference Document 3 (S. R. Stiffler et. al., “The thermal stability of SiGe films deposited by ultrahigh-vacuum chemical vapor deposition,” J. Appl. Phys., 70 (3), pp. 1416-1420, 1991.), the upper limit for a practical Ge content in the base layer of a heterojunction bipolar transistor is around 10%. Therefore, under the present circumstances, it is difficult to increase the gradient of the composition of the base layer in order to provide a transistor with a higher frequency or a lower voltage.
On the other hand, the heterojunction bipolar transistors with the uniform composition base structure also have the issue of the critical thickness of the base layer, which causes dislocations due to the above-described lattice constant difference. In reality, the SiGe-HBT having a high Ge content shown in Reference Document 2 suppresses the occurrence of dislocations by not using a process requiring a high temperature treatment during fabricating. Therefore, a silicon process requiring a high temperature treatment cannot be applied, making it impossible to realize a mixed device like a BiCMOS device or an integrated circuit. As a result, there are limits on achieving lower-voltage operation by further reducing the built-in potential.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a heterojunction bipolar transistor which can operate at a low voltage and a high speed while maintaining high reliability by providing a means for reducing the amount of lattice strain in the base layer even when there is a large difference between the average band gap of the collector layer and the emitter layer, and the band gap of the base layer.
The heterojunction bipolar transistor of the present invention comprises a first semiconductor layer made from semiconductor material containing Si as a component; a second semiconductor layer made from semiconductor material containing Si, Ge and C as components, having a band gap narrower than the first semiconductor layer and consisting of a top layer, a center layer and a bottom layer; a third semiconductor layer made from Si as a component, and having a band gap wider than the second semiconductor layer stuched in this order onto a substrate; and a heterojunction barrier provided between the first semiconductor layer and the second semiconductor layer, and further comprises: a collector layer formed in the first semiconductor layer and containing a first conductive impurity; a base layer formed in the second semiconductor layer and containing a second conductive impurity; and an emitter layer formed in the third semiconductor layer and containing a first conductive impurity, the second semiconductor layer having an average lattice strain of 1.0% or less.
By controlling the Ge and C contents in the second semiconductor layer represented by, for example, Si
1-x-y
Ge
x
C
y
where x is the Ge content and y is the C content, it becomes possible to realize low-voltage operation due to a reduction in the built-in potential of the PN junction between the emitter and the base, and to improve operation speed due to the graded composition base structure. In that case, unlike the SiGe layer epitaxially grown on the Si layer, there is no strict upper limit for the Ge content to prevent lattice defect resulting from lattice mismatch. In other words, in the second semiconductor layer including Si, Ge and C as its components, the difference in band gap between the second semiconductor layer and the first and third semiconductor layers can be enlarged, while the average lattice strain, which results from the lattice mismatch with the first and third semiconductor layers that are made from Si and other materials, is restricted to 1.0% or less. As a result, highly reliable and functional hetero
Kanzawa Yoshihiko
Takagi Takeshi
Toyoda Kenji
Yuki Koichiro
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Wille Douglas A.
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