4.10 Sspd_chapter2.2.1.and 2.2.1.solid state equivalent of vacuum

2.2.1.solid state equivalent of all the vacuum tube devices.

2.2.2. crystalline structure of silicon

Silicon is a Group IV element with 4 electrons in its outermost orbit. Each Silicon atom is co-valently bond to 4 neighbouring Si atoms. In the process the octave is completed for all the Si stoms except those on the surface of the specimen. The surface has dangling bonds which need to be passivated. The crystalline structure of Silicon is called Diamond Lattice Structure.

Silicon has a lattice constant of ‘a’=5.43A° at 300K. It is two interpenetrating FCC(Face-Center-Cube) lattice with one sub-lattice displaced with respect to the other by quarter of the distance along the cross diagonal of the cube.

The whole diamond lattice cube is divided into 8 sub-cubes.Two sub-cubes in the upper half and two sub-cubes in the lower half contain the tetrahedral structure. The four sub-cubes are so arranged that the top view gives seamless continuity of co-valent sharing of the electrons with four neighbouring atoms as shown in Figure 2.2.4.

Tetrahedral structure shown in Figure 2.2.3. is repeadted throughout the lattice space as a consequence there are:

4 body atoms = 4 × 1 = 4 atoms

6 face atoms = 6 × ½ = 3 atoms.

8 corner atoms = 8 ×1/8 = 1 atoms

Hence there are 4+3+1= 8 atoms per unit cell.

Therefore atom number density = 8 / a ³ =8/(5.43A°) ³ = 5×10 ²² atoms/cc.

Generally all crystalline solid material have a packing density of this order. This is the reason whuy in Copper each atom being univalent contributes 1 electron. Hence conducting electron concentration ‘n’ ~ 10 ²² electrons/cc .

Weight Density = atom density×weight per atom.

Weight per atom = Weight of 1 gram-mole/Avogadro Number=A/A _vog