Conductivity of silicon

Even though silicon is metallic in appearance, it is not generally classified as a metal. The electrical conductivity of silicon is so much less than that of ordinary metals it is called a semiconductor. Silicon is an example of a network solid (see Figure 20-1)—it has the same atomic arrangement that occurs in diamond. Each silicon atom is surrounded by, and covalently bonded to, four other silicon atoms. Thus, the silicon crystal can be regarded as one giant molecule. 

Figure 4.22 Schematic diagram of a field effect transistor. The silicon-silicon dioxide system exhibits good semiconductor characteristics for use in FETs. The free charge carrier concentration, and hence the conductivity, of silicon can be increased by doping with impurities such as boron. This results in p-type silicon, the p describing the presence of excess positive mobile charges present. Silicon can also be doped with other impurities to form n-type silicon with an excess of negative mobile charges.

C.J. Glassbrenner and G.A. Slack. Thermal Conductivity of Silicon and Germanimn from 3 °K to the Melting Point Physical Review A 134 (1963), 1058-1069. 

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of silicon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Silicon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

Russell, L.M., Donaldson, K.Y., Hasselman, D.P.H., Corbin, N.D. Petrovic, J.J. and Rhodes, J.F. Effect of vapor-liquid-solid and vapor-solid silicon carbide whiskers on the effective thermal diffusivity/conductivity of silicon nitride matrix composites , J. Am. Ceram. Soc., 74[4] (1991) 874-877. 

If silicon technology is involved all thermal sensors suffer from the high thermal conductivity of silicon, which dramatically decrease their sensitivity [12]. However, by use of micromachining and integrated silicon technology a powerful thermal biosensor can be realized. Using a thermopile integrated on a thin micromachined silicon membrane reduces thermal loss due to the substrate and so excellent performance can be accomplished [13]. 

The small conductivity of silicon can be enhanced at normal temperatures if the silicon crystal is doped with certain other elements. For example, when a small fraction of silicon atoms is replaced by arsenic atoms, each having one more valence electron than silicon, extra electrons become available for conduction, as shown in Fig. 16.32(a). This produces an n-type semiconductor, a substance whose conductivity is increased by doping it with atoms having more valence electrons than the atoms in the host crystal. These extra electrons lie close in energy to the conduction bands and can easily be excited into these levels, where they can conduct an electric current [see Fig. 16.33(a)). 

In this section, we compare BTE and MD predictions of bulk thermal conductivity of silicon to the available experimental data [74] over a temperature range of 500K to lOOOK (Fig. 4). The BTE predictions are obtained from the full phonon dispersion model described in section 2.3. This full phonon dispersion model involves an adjustable parameter, the Gruneisen constant (y), which is set... 

We next present the BTE and MD predictions of thermal conductivities of silicon thin films. The thermal properties of silicon thin films are of paramount importance to the transistor industry. Silicon-on-insulator (SOI) and strained silicon transistors are composed of silicon thin films. In both cases, the thin silicon film is deposited on top of poor thermally conducting materials, and the thermal energy generated by the Joule effect has to be removed along the silicon film plane. A thorough understanding of the thermal properties of thin silicon films is essential for the accurate prediction of the thermal response of these transistors. The dimensions of the silicon thin film in... 

SOI and strained silicon transistors are comparable to or smaller than the phonon s mean free path (which, for silicon, has been estimated as 300 nm at 300K) [53], In this limit, the film surfaces alter the phonon dispersion relations [76], and the phonon-surface scattering may become the predominant scattering mechanism [3, 53], Since phonons are the main carriers of thermal energy in silicon, these effects alter the thermal conductivity, which differs from that of bulk silicon [10, 36, 77], Measurements of the thermal conductivities of silicon films of thicknesses down to 74 nm found a reduction of 50% with respect to the bulk value at 300K [53], This reduction depends on the temperature and the thickness of the film [3, 53],... 

Volz, S.G. and G. Chen, Molecular Dynamics Simulation of Thermal Conductivity of Silicon Nanowires. Applied Physics Letters, 1999. 75(14) p. 2056-2058. 

Materials of which compositions are isotopically controlled are expected to show improved physical and nuclear properties which can not be attained by usual combinations of elements[l]. Natural silicon is composed of three stable isotopes such as 2 Si, "Si and "Si. If the purified Si is obtained, high thermal conductivity of silicon and its compounds is achievable because of suppressing isotope scattering against phonon conduction. 

Y.W. Lee, et al., Study on the mechanical properties and thermal conductivity of silicon carbide-, zirconia- and magnesia aluminate-based simulated inert matrix nuclear fuel materials after cyclic thermal shock../. Nucl. Mater. 319, 15-23 (2003). 

Briefly, three points of porous SiC-based catalytic support properties can be emphasized (i) SiC shows very good mechanical properties which gives resistance to erosion and attrition, in addition to a high thermal stability (ii) SiC has a higher thermal conductivity compared with the more conventional supports which could prevent the metal sintering (iii) SiC is particularly inactive with respect to chemical reagents such as acids or bases. Therefore, the active phase can be easily reprocessed after simple acidic or basic treatments. Among refractory materials, the thermal conductivity of silicon carbide, SiC (500 W m-1K-1 for crystalline state, at room temperature) is close to that of metals such as Ag or Cu (400-500 Wm K-1). 

What effect will the presence of phosphorus impurity have on the conductivity of silicon at any given temperature ... 

Fortuitously, the good electrical conductivity of silicon prevents these charges from building up to dangerous levels, which explains why the reported pattern placement errors due to such hypothesized deflections have been smaller than measurement capability (20 nm). Another method for minimizing charging... 

We also can enhance the conductivity of silicon by doping the crystal with an element such as boron, which has only three valence electrons, one less than silicon. Because boron has one less electron than is required to form the bonds with the surrounding silicon atoms, an electron vacancy, o hole, is created, as shown in Fig. 10.29(b). As an electron fills this hole, it leaves a new hole, and this process can be repeated. Thus the hole advances through the crystal in a direction opposite to movement of the elec-tronsjumping to fill the hole. Another way of thinking about this phenomenon is that in pure silicon each atom has four valence electrons and the low-energy molecular orbitals are exactly filled. Replacing silicon atoms with boron atoms leaves vacancies in these... 

What is the thermal conductivity of silicon nanowires, n-alkane single molecules, carbon nanotubes, or thin films How does the conductivity depend on the nanowiie dimension, nanotube chirality, molecular length and temperature, or the film thickness and disorder More profoundly, what are the mechanisms of heat transfer at the nanoscale, in constrictions, at low tanperatures Recent experiments and theoretical studies have dononstrated that the thermal conductivity of nanolevel systems significantly differ from their macroscale analogs [1]. In macroscopic-continuum objects, heat flows diffusively, obeying the Fourier s law (1808) of heat conduction, J = -KVT, J is the current, K is the thermal conductivity and VT is the temperature gradient across the structure. It is however obvious that at small scales, when the phonon mean free path is of the order of the device dimension, distinct transport mechanisms dominate the dynamics. In this context, one would like to understand the violation of the Fourier s... 

N. Hirosaki, Y. Okamoto, M. Ando, F. Munakata and Y. Akimune, Effect of Grain Growth on the Thermal Conductivity of Silicon Nitride , Journal of the Ceramic Society of Japan, 104, 1996, 50 54. 

---