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Journal ArticleDOI

Modelling of base transit time in Si/Si1?y?zGeyCz/Si HBTs and composition profile design issue for its minimization

01 Nov 2003-Semiconductor Science and Technology (IOP Publishing)-Vol. 18, Iss: 11, pp 907-913
TL;DR: In this paper, the authors developed a generalized model for the calculation of base transit time τb for such HBTs and showed that τb becomes smallest for triangular material (Ge and C) profiles as compared to box type and trapezoidal profiles.
Abstract: From a practical point of view, incorporation of a small amount of C in the base of Si/SiGe/Si HBT eliminates the problems of B outdiffusion and of lattice mismatch. We have developed in this paper a generalized model for the calculation of base transit time τb for such HBTs. In our model, we have considered the effect of emitter–base and collector–base depletion widths and the variation of saturation-drift velocity in the base–collector depletion region with molar Ge content. It is observed that τb becomes smallest for triangular material (Ge and C) profiles as compared to box type and trapezoidal profiles. In addition the triangular Ge profile with C codoping does not affect film stability and device performance. Our calculation shows that for the same value of strain between Si and SiGe and between Si and SiGeC, the latter device yields the smaller value of τb.
Citations
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Journal ArticleDOI
TL;DR: In this article, the threshold voltage and sub-threshold slope of the strained-Si channel n-MOSFETs were determined, taking into account quantum-mechanical effects, and the effect of bandgap narrowing due to heavy channel doping and surface roughness at the Si/SiGe heterointerface for ultra thin channels.
Abstract: In this paper, the threshold voltage and subthreshold slope of strained-Si channel n-MOSFETs are determined, taking into account quantum-mechanical effects, and the effect of bandgap narrowing due to heavy channel doping and the effect of surface roughness at the Si/SiGe heterointerface for ultra thin channels. Quantum-mechanical effects have been incorporated by considering three components, (1) the modified subband energy of 2D inversion layer charges at the silicon dioxide–silicon interface, (2) the increased effective oxide thickness and (3) the altered value of ground state energy due to surface roughness. The analytical results of threshold voltage and threshold voltage difference are presented with reference to unstrained-Si channel for strained-Si MOSFETs by employing poly-Si gate and titanium nitride gate, the work function of which can be varied over a wide range. In addition, we have predicted the dependence of threshold voltage on different values of oxide thickness, channel doping concentration, and on the molar content, x, of Ge in the Si1−xGex virtual substrate. When compared with the theoretical data of Nayfeh et al our analytical results agree more closely with our experimental results and also with measured and simulated data of threshold voltage for a wide range of devices available in the literature. Furthermore, we have calculated the subthreshold slope of strained-Si channel MOSFETs for different amounts of Ge in the SiGe layer.

18 citations

Journal ArticleDOI
TL;DR: This work considers the problem of determining the doping profile that minimizes base transit time in a (homojunction) bipolar junction transistor, and shows that a similar approach can be used to maximize the cutoff frequency, taking into account junction capacitances and forward transit time.
Abstract: We first consider the problem of determining the doping profile that minimizes base transit time in a (homojunction) bipolar junction transistor. We show that this problem can be formulated as a geometric program, a special type of optimization problem that can be transformed to a convex optimization problem, and therefore solved (globally) very efficiently. We then consider several extensions to the basic problem, such as accounting for velocity saturation, and adding constraints on doping gradient, current gain, base resistance, and breakdown voltage. We show that a similar approach can be used to maximize the cutoff frequency, taking into account junction capacitances and forward transit time. Finally, we show that the method extends to the case of heterojunction bipolar junction transistors, in which the doping profile, as well as the profile of the secondary semiconductor, are to be jointly optimized.

16 citations


Cites methods from "Modelling of base transit time in S..."

  • ...The method also extends to the problem of minimizing base transit time in heterojunction bipolar transistors (HBTs), as studied in [3], [4], [17], [18]....

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Journal ArticleDOI
TL;DR: A temperature dependent model for the threshold voltage Vt and subthreshold slope S of strained-Si channel MOSFETs is presented and Vt is found to be sensitive to strain while S is weakly dependent on strain.

12 citations

References
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Journal ArticleDOI
Jan W. Slotboom1, H.C. de Graaff1
TL;DR: In this paper, the authors used optical absorption measurements on uniformly doped silicon samples to determine the bandgap in silicon and used the bipolar transistor itself as the vehicle for measuring the band gap in the base.
Abstract: Theory predicts appreciable bandgap narrowing in silicon for impurity concentrations greater than about 1017 cm−3. This effect influences strongly the electrical behaviour of silicon devices, particularly the minority carrier charge storage and the minority carrier current flow in heavily doped regions. The few experimental data known are from optical absorption measurements on uniformly doped silicon samples. New experiments in order to determine the bandgap in silicon are described here. The bipolar transistor itself is used as the vehicle for measuring the bandgap in the base. Results giving the bandgap narrowing (ΔVg0) as a function of the impurity concentration (N) in the base (in the range of 4.1015–2.5 1019 cm−3) are discussed. The experimental values of ΔVg0 as a function of N can be fitted by: δV g0 = V 1 ln N N 0 + ln 2 N N 0 +C where V1, N0 and C are constants. It is also shown how the effective intrinsic carrier concentration (nie) is related with the bandgap narrowing (ΔVg0).

716 citations

Journal ArticleDOI
TL;DR: In this article, the fabrication of silicon heterojunction bipolar transistors which have a record unity-current-gain cutoff frequency (f/sub T/) of 75 GHz for a collector-base bias of 1 V, an intrinsic base sheet resistance (R/sub bi/) of 17 k Omega / Square Operator, and an emitter width of 0.9 mu m is discussed.
Abstract: The fabrication of silicon heterojunction bipolar transistors which have a record unity-current-gain cutoff frequency (f/sub T/) of 75 GHz for a collector-base bias of 1 V, an intrinsic base sheet resistance (R/sub bi/) of 17 k Omega / Square Operator , and an emitter width of 0.9 mu m is discussed. This performance level, which represents an increase by almost a factor of 2 in the speed of a Si bipolar transistor, was achieved in a poly-emitter bipolar process by using SiGe for the base material. The germanium was graded in the 45-nm base to create a drift field of approximately 20 kV/cm, resulting in an intrinsic transit time of only 1.9 ps. >

444 citations

Journal ArticleDOI
Subramanian S. Iyer1, Gary L. Patton1, J.M.C. Stork1, Bernard S. Meyerson1, David L. Harame1 
TL;DR: In this article, the authors discuss the growth and properties of pseudomorphic Si/sub 1-x/Ge/sub x/ structures and then focus on their applications, especially the Si-sub 1 -x/ge/sub X/-base heterojunction bipolar transistor (HBT).
Abstract: Advanced epitaxial growth techniques permit the use of pseudomorphic Si/sub 1-x/Ge/sub x/ alloys in silicon technology. The smaller bandgap of these alloys allows for a variety of novel band-engineered structures that promise to enhance silicon-based technology significantly. The authors discuss the growth and properties of pseudomorphic Si/sub 1-x/Ge/sub x/ structures and then focus on their applications, especially the Si/sub 1-x/Ge/sub x/-base heterojunction bipolar transistor (HBT). They show that HBTs in the Si/sub 1-x/Ge/sub x/ system allow for the decoupling of current gain and intrinsic base resistance. Such devices can be made by using a variety of techniques, including molecular-beam epitaxy and chemical vapor deposition. The authors describe the evolution of fabrication schemes for such HBTs and describe the DC and AC results obtained. They show that optimally designed HBTs coupled with advanced bipolar structures can provide performance leverage. >

428 citations

Journal ArticleDOI
TL;DR: In this paper, the Moll and Ross integral relations for the current flow through the base region of a bipolar transistor and for the base transit time were generalized to the case of a heterostructure bipolar transistor with a nonuniform energy gap.
Abstract: The two integral relations by Moll and Ross for the current flow through the base region of a bipolar transistor, and for the base transit time, are generalized to the case of a heterostructure bipolar transistor with a nonuniform energy gap in the base region.

247 citations

Journal ArticleDOI
TL;DR: In this paper, a performance tradeoff associated with the use of an intrinsic spacer layer to reduce parasitic leakage at low temperatures and the consequent base resistance degradation due to enhanced carrier freeze-out is identified.
Abstract: The DC design considerations associated with optimizing epitaxial Si- and SiGe-base bipolar transistors for the 77-K environment are examined in detail. Transistors and circuits were fabricated using four different vertical profiles, three with a graded-bandgap SiGe base, and one with a Si base for comparison. All four epitaxial-base profiles yield transistors with DC properties suitable for high-speed logic applications in the 77-K environment. The differences between the low-temperature DC characteristics of Si and SiGe transistors are highlighted both theoretically and experimentally. A performance tradeoff associated with the use of an intrinsic spacer layer to reduce parasitic leakage at low temperatures and the consequent base resistance degradation due to enhanced carrier freeze-out is identified. Evidence that a collector-base heterojunction barrier effect severely degrades the current drive and transconductance of SiGe-base transistors operating at low temperatures is provided. >

115 citations