Introduction

Semiconductors are a special class of materials with properties in between those of insulators and conductors.

They typically have narrow bandgaps of < 2 eV

Semiconductors are generally covalently bonded substances.

The elements Si and Ge are intrinsic semiconductors.

Si has a band gap of 1.1 eV while that for Ge is 0.55 eV.

Modern synthetic semiconductors include the III-V compounds such as GaAs, GaP, InSb, InAs and InP. These have a range of different band gaps

Carbon, silicon, germanium and tin are atoms in ascending order of atomic number from column IV A of the period table. Each is characterised by having four valence electrons in its outermost shell of electrons, and requires a further four to make up the full complement of the shell. All can solidify to form elemental, covalently bonded crystals where the four valence electrons of one atom are shared between its four nearest neighbours so that every atom effectively gains eight electrons in its valence shell. A group IV atom and its four nearest neighbours from a tetrahedron as shown in Figure 1.

 

tetrahedronFigure 1: Schematic diagram to show the orientation of covalently bonded group 4 atoms. A tetrahedron is formed by the nearest neighbours, with the principal atom located at its centre.


Taking a larger scale perspective of the arrangement of the atoms, or crystal lattice , it is found that they organise themselves into two interpenetrating face centred cubic (fcc) sub-lattices, one displaced from the other by 1/4(a 0 , a 0 , a 0 ) along a diagonal of the unit cell . a 0 is called the lattice constant or lattice parameter and is a measure of the size of the unit cell, often expressed in Angstrom (A) units (1A=0.1nm or 1x10 -10 m). It is determined by techniques such as X-ray diffractometry. Figure 2 shows a complete unit cell for a group 4 crystal covalently bonded with the diamond structure.

 

Rotating Lattice

Figure 2: Unit cell of a crystal such as silicon or germanium


This structure is of course difficult to visualise and draw, hence it is usually represented by an equivalent 2-D, "square" arrangement shown in figure 3.

Rotating Lattice

Figure 3: 2-D representation of a covalently bonded crystal at 0K, eg Si. Note that the heavy lines between adjacent atoms depict the covalent bonds which contain TWO electrons and are all completely filled.


In the example shown, it is assumed that the solid, Si say, is both crystallographically perfect and pure. At 0 K, all the covalent bonds are complete and there are no free charge carriers moving around randomly through the lattice; the crystal is an insulator.

Before commenting further on the elemental "semiconductors", it is worth mentioning another group of technologically important solids which possess semiconducting properties to varying degrees, namely the III-V compounds . These are formed when equal numbers of group III and group V elements combine with the same basic arrangement as the group IV elemental solids. The difference lies in the fact that whereas the elemental solids contain only one type of atom such that every atom in the (perfect) lattice is bonded to four identical nearest neighbour atoms, in the III-V compounds a group III atom is bonded to four group V nearest neighbours, and a group V atom is bonded to four group III nearest neighbours. The two interpenetrating fcc sub-lattices now contain either all group III atoms or all group V atoms. Figures 4 and 5 show the 3-D and 2-D representations, respectively, for these materials.

Rotating  GaAs Lattice

Figure 4 : Diagram to show the 3D unit cell of a III-V semiconductor compound (eg. GaAs, gallium arsenide) with the zinc blende lattice.

Static GaAs

Figure 5 : 2-D representation of a III-V semiconductor. Note the way in which the group III and V alternate through the lattice on their individual fcc sub-lattices.

Although the Silicon Devices and Technology 3 course will concentrate on Si, it is important to remember that the electronic band structure of Si (and Ge) makes it unsuitable for certain applications; a prime example is light emitting devices. Most semiconductor lasers and LEDs are made from the III-V materials; it is NOT possible to get efficient light emission from Si. (There is now a great deal of interest world-wide in the II-VI compounds because it has been shown that blue and green LEDs and laser diodes can made from them, but that's yet another story!).




 

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