IBM/Analog collaboration seeks to corner SiGe market

1998

Hole mobilities in relaxed and strained undoped SiGe layers have been calculated with a one-dimensional self-consistent bipolar Monte Carlo simulation code. We have adopted a novel bandstructure model that incorporates strain effects in the alloy valence band. Both alloying and strain enhance the hole mobility compared with bulk Si and we find that alloy scattering is the dominant scattering mechanism. An alloy potential of 1.4 eV was obtained by matching our Monte Carlo data on drift mobilities to experimental Hall mobility measurements. Uncertainties in this value arise from scatter in the experimental data and a lack of detailed knowledge of the Hall factor.

Section: Introductionmentioning

confidence: 99%

Calculation of hole mobilities in relaxed and strained SiGe by Monte Carlo simulation

1998

“…The ability to make SiGe heterostructures has enabled applications based on techniques from bandstructure engineering in III-V compound semiconductors. The best studied of SiGe devices is the Si/SiGe/Si heterojunction bipolar transistor (HBT) due to the demonstration of unitygain cut-off frequencies exceeding 100 GHz [2]. The maximum frequency of oscillation f max depends on the hole mobility in the base layer via the base sheet resistance.…”

Section: Introductionmentioning

confidence: 99%

Modelling of hole mobilities in heavily doped strained SiGe

1998

Low-field hole mobilities have been calculated for heavily relaxed and strained doped SiGe alloy layers with Ge contents varying from 0 to 50%, using a novel semi-analytical bandstructure model which incorporates the effects of strain on the valence band of the alloy. We obtain poor results compared with experiment for mobilities in heavily doped Si, and attribute this to (i) a failure of the Born approximation at low carrier energies and (ii) the omission of additional effects associated with heavy doping and high carrier concentrations. For the strained doped and intrinsic alloy we observe that both the in-plane and out-of-plane hole drift mobilities increase with increasing Ge content relative to those for Si. These enhancements are due mainly to the effects of strain, and to a lesser extent due to alloying with Ge, but are offset by the presence of alloy scattering. Our results are sensitive to the details of the models used for scattering by ionized impurities; however, the large uncertainties and scatter of the experimental data preclude accurate estimates of the alloy potential. We find that an alloy potential of 2.0 eV gives an in-plane mobility consistent with experimental data for intrinsic material. Our calculations for heavily doped layers are affected by uncertainties in the value of the alloy potential and highlight a need for a better quantitative understanding of the scattering processes which are important in heavily doped alloy layers.

“…As well as the excellent prospects offered by HBTs generally for high-speed operation, especially in digital and microwave applications, Si/SiGe structures also have the potential for integration into existing Si processes [3]. Since the first Si/SiGe HBTs emerged in 1987, devices have been fabricated which have demonstrated these advantages practically [4]; Patton et al produced an Si/SiGe device with a current gain of 135 and a then-world-record cutoff frequency f T of 75 GHz [5], while Gruhle et al observed an f T of 91 GHz in a later device [6]. These improvements in f T arise from the greater design freedom permitted by the presence of the heterojunction, which offers higher current gains which can then be traded for optimization of other transistor characteristics; for example, lower emitter doping and the emitter-base heterojunction both contribute to a reduction in the emitter delay time, while lower base transit times have been achieved by using narrower bases.…”

Section: Introductionmentioning

confidence: 99%

Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

1998

A one-dimensional self-consistent bipolar Monte Carlo simulation code has been used to model carrier mobilities in strained doped SiGe and the base-collector region of Si/SiGe/Si and SiC/Si heterojunction bipolar transistors (HBTs) with wide collectors, to study the variation of the cutoff frequency f T with collector current density J C . Our results show that while the presence of strain enhances the electron mobility, the scattering from alloy disorder and from ionized impurities reduces the electron mobility so much that it is less than that of Si at the same doping level, leading to larger base transit times τ B and hence poorer f T performance for large J C for an Si/SiGe/Si HBT than for an SiC/Si HBT. At high values of J C , we demonstrate the formation of a parasitic electron barrier at the base-collector interface which causes a sharp increase in τ B and hence a dramatic reduction in f T . Based on a comparison of the height of this parasitic barrier with estimates from an analytical model, we suggest a physical mechanism for base pushout after barrier formation that differs somewhat from that given for the analytical model.