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.

ABHIJIT BISWAS

Radio Physics and Electronics

P K BASU

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

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

Semiconductor Science and Technology

We present a temperature dependent model for the threshold voltage V t and subthreshold slope S of strained-Si channel MOSFETs and validate it with reported experimental data for a wide range of temperature, channel doping concentration, oxide thickness and strain value. Such model includes the effect of lattice strain on material, temperature dependent effective mass of carriers, interface-trapped charge density and bandgap narrowing due to heavy channel doping. Also considered are polydepletion effects, carrier localization effect in the ultra-thin channel and quantum-mechanical effects. Our investigation reveals that the threshold voltage reduces linearly with increasing temperature whereas the subthreshold slope increases. In addition Vt is found to be sensitive to strain while S is weakly dependent on strain. Moreover, the channel doping concentration influences both Vt and S, and also the rate of change of V t with temperature. Furthermore, S decreases for a lightly doped channel particularly at lower temperatures. © 2014 Elsevier Ltd. All rights.

Microelectronics Reliability

Temperature dependent model for threshold voltage and subthreshold slope of strained-Si channel MOSFETs with a polysilicon gate

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 Si 1-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. © 2008 IOP Publishing Ltd.

Modelling of threshold voltage and subthreshold slope of strained-Si MOSFETs including quantum effects

We have presented a generalized method of calculating the Early voltage VA of a Si/Si1-x-yGexCy/Si heterojunction bipolar transistor (HBT) and successfully explained the published measured values of VA with and without incorporation of C. The investigation was carried out on the dependence of VA of an N-p-N HBT on different compositions, namely Ge and C, incorporated into the base and on their various distribution profiles such as box, trapezoidal and triangular, for an exponential base doping. It is found that the triangular compositional profile yields the largest value of VA as compared to box-type and trapezoidal profiles for fixed amounts of individual composition in the base. The highest grading, which corresponds to a triangular profile, generates the highest value of effective intrinsic-carrier concentration local to the CB junction causing very little change in minority carrier concentration gradient in the base with VBC. The small change in minority carrier concentration is responsible for the increased value of VA. Furthermore, a large value of VA is obtained for rising and falling exponential base doping profiles relative to the box doping profile with the same amount of base doping. The value of VA is found to increase with the incorporation of small amounts of C in the SiGe base due to its ability to prevent boron out-diffusion as reported for a fabricated device. Further improvement in the value of VA is accounted for due to the sharp fall-off of the C profile at the collector-base junction as observed from the SIMS profile of a fabricated device. For the same amount of strain between Si and SiGe, and between Si and SiGeC, the latter yields a larger value of VA.

Estimated effect of germanium and carbon on the Early voltage of a Si1-x-yGexCy heterojunction bipolar transistor

Incorporation of C in the SiGe base of a heterojunction bipolar transistor (HBT) prevents outdiffusion of B across the junctions, improves film stability by reducing strain, and leads to better process integration. This paper investigates theoretically how the values of figure of merit such as current gain (β), cut-off frequency (fT) related inversely to the base transit time (τb), and the Early voltage (VA) change with a small amount (&lt;1%) of C incorporation in the base. For this purpose, we have derived generalized expressions for these quantities using a box-type, trapezoidal and triangular material (Ge and C) profile and an exponential doping profile. The variation of saturation-drift velocity with Ge mole fraction has been used to calculate τb. A comparison between the calculated values, keeping the strain the same in both SiGe and SiGeC bases, indicates that all values of figure of merit are enhanced with C incorporation. The value of VA increases in SiGeC HBTs with small amounts of C because of prevention of B outdiffusion across the collector-base junction and sharp fall off of the C profile at the same junction. © 2001 Elsevier Science Ltd. All rights reserved.

Journal	Semiconductor Science and Technology
Publisher	-
ISSN	0268-1242
Open Access	No