Silicon electron/hole mobility

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

From Table 2.3, which lists typical // values, it can be seen that the hole mobility in conjugated polymers is lower than that in organic crystals and amorphous silicon, but much larger than that in undoped poly(N-vinyl carbazole). Therefore, conjugated polymers have potential for applications in conducting opto-electronic and photonic devices. In principle, this also applies to liquid-crystal systems that can exhibit enhanced molecular order due to their self-organizing ability, as has been pointed out in a progress report [42]. 

Silicon has the valence electronic structure 3s 3p. The partially filled p orbitals might lead one to suppose that silicon has a partially filled valence band and would therefore be an electrical conductor. Because silicon is covalently bonded, the two 3s electrons and the two 3p electrons occupy sp hybrid orbitals. This results in a solid with two electron energy bands, each with four closely spaced sublevels, one for each electron in the valence shell of Si. The four electrons occupy and fill the valence band at 0 K and are therefore nonconducting. However, at temperatures above 0 K, a few electrons can be thermally promoted from the valence band into the conduction band there, they become conductors of electricity. When an electron leaves the valence band, it leaves behind a positive hole that is also mobile, thus producing an electron-hole pair. Both the electron and the hole are charge carriers in a semiconductor. Semiconductors such as Si and Ge are called intrinsic semiconductors their behavior is a result of the bandgap and band structure of the pure material. 

Figure 18.18 For silicon, dependence of room-temperature electron and hole mobilities (logarithmic scale) on dopant concentration (logarithmic scale).

Figure 18.19 Temperature dependence of (a) electron and (fe) hole mobilities for silicon that has been doped with various donor and acceptor concentrations. Both sets of axes are scaled logarithmically.

The hole concentration is known to be 2.0 X 10 m". Using the electron and hole mobilities for silicon in Table 18.3, compute the electron concentration. 

Figure 6.13 Electron and hole mobilities in Sii. Gex alloys. Solid lines and figures refer to electron properties while dashed lines and points represent hole behaviors. Figure redrawn with permission from Van der Walle, C.B. SiGe heterojunctions and band offsets, in Kasper, Erich, and Lyutovich, Klara, editors. Properties of Silicon Germanium and SiGe.Carbon. London INSPEC, 2000, p. 149. Copyright 2000, The Institution of Engineering Technology.

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