The Basic Properties of Silicon

Pure crystalline silicon is a brittle material with a gray metallic appearance. Its mechanical properties, such as Knoop hardness (950-1150 kg mm-2), Young s modulus (190 GPa for (111), 170 GPa for (110), 130 GPa for (100)), torsion modulus (4050 kg mm-2) and compression breaking strength (5000 kg cm-2) vary slightly with crystal orientation. Silicon has a low thermal expansion coefficient (2.33x 1(T6 K-1) and a high thermal conductivity (148 W K-1m-1). Crystalline silicon melts at 1413 °C (1686 K). 

The atomic weight of silicon is 28.086 (4.6638xlO-23 g per atom). Its density of 2.328 gem-3 corresponds to roughly 5 X1022 atoms cm-3. Silicon has the same crystal structure as diamond (face-centered cubic, fee) with a lattice constant of 0.543095 nm. 

The electronic properties of silicon are essential in the understanding of silicon as an electrode material in an electrochemical cell. As in the case of electrolytes, where we have to consider different charged particles with different mobilities, two kinds of charge carriers - electrons and holes - are present in a semiconductor. The energy gap between the conduction band (CB) and the valence band (VB) in silicon is 1.11 eV at RT, which limits the upper operation temperature for silicon devices to about 200 °C. The band gap is indirect this means the transfer of an electron from the top of the VB to the bottom of the CB changes its energy and its momentum. 

The figure on the inner front cover of this book can be used to convert between doping density, carrier mobility and resistivity p for p- or n-type doped silicon substrates. One of the major contaminants in silicon is oxygen. Its concentration depends on the crystal growth method. It is low in FZ material and high (about 1018 cm-3) in Czochralski (CZ) material. 

A piece of silicon immersed in an electrolyte behaves similarly to a Schottky diode, a metal-semiconductor contact, as discussed in Chapter 3. Under reverse 

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