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The transport of charges through solids does not only depend on the properties of the electron, but also on the arrangement of the atoms in the solid. Since this is quite important for semiconductors, this is behaviour is further investigated in this post.

Semiconductors have an electrical conductivity between metals and insulators. The conductivity can be varied by changing temperature, optical excitation, and impurity content. One of the most important characteristic of semiconductors which distinguish them between metals and isolators is its energy band gap. The energy band gap determines the wavelength of light that can be absorbed or emitted. Since the band gap is very different between the different semiconductors, a wide range of wavelengths can be absorbed or emitted (GaP 2.3eV and GaAs 1.43eV). Furthermore, the electrical and optical properties of semiconductors can be strongly affected by impurities, which can be precisely added, which is called doping.

Semiconductors can be found in the group IV in the periodic table. Those elements are called elemental semiconductors. Whereas compound semiconductors can be found in group III and V. Ge was widely used in the early days of semiconductor. Nowadays, Silicon is used the most (integrated circuits, rectifiers, transistors) and compounds are used for high speed devices and the absorption or emission of light (light emitting diode is made from GaN).

Periodic Structure

The periodicity in a crystal is defined in terms of a symmetric array of points in space. If the solid is completely periodical (lattice vector at one point is the same at another point in the solid), then the atomic arrangement is crystalline. If it is not periodical at all, it is amorphous. And when it consists of a lot of small areas/grains which are crystalline itself, it is called polycrystalline. Metals are normally polycrystalline. It is important to know that a metal that is crystalline is more ductile/soft than a metal that is polycrystalline, since the atoms can easier be shifted. The smaller the grains in a polycrystalline metal, the harder it gets. Steel is harder than iron because iron has large grains, whereas the other elements in steel lead to smaller grains, which make steel hard.

To come back to the topic: Semiconductors are crystalline, which also means they are anisotropic (Their behaviour in one direction is completely different in another direction. In example, graphite is crystalline, and we think it very week, since we write with it. But actually it is very strong in one direction and very weak in the other one. One sheet of graphite is called graphene, which is a very strong material.) and can be described with a unit cell (smallest regular structure in a crystal). This unit cell can afterwards be used to determine mechanical properties, but also electrical properties (allowed energies).

Wavers

Production

The starting material of a waver is sand, SiO_2. Afterwards, Si gets extracted so that it has a purity of 1ppb.

Since the Si is still in polycrystalline form, a process called liquid- encapsulated Czochralski (LEC) growth method is used to produce a signle-crystal material named ingot. This ingot is afterwards sawed into wavers which are about 775 um thick. Afterwards, the waver is lapped and ground on both sides to achieve a flat surface. The final waver costs several hundred dollars. ⇒ From sand, a lot of money was created.

The wafers are cut along the lattice. But the notch on the waver is along the 110 direction.

Epitaxial Growth

Epitaxial growth is called the technique (VPE, MOCVD, MBE) of growing an oriented single crystal layer on a substrate wafer. ⇒ This means the new layer on the waver has the same crystal type as the wafer and therefore the layer properties are clear.

Questions

• Si ingots are grown in 100 directions and then a notch is ground on one side of the cylinder to delineate 110 direction.(page 35) Why is the chip produced in 110 direction (is it mechanical stronger when one saws it in 110 direction?)
• Is the direction also important for the electrical properties? Can the current density be higher in 100 direction than in 110?
• Why is it important to know that a semiconductor is normally in the diamond and zync blend structure?
• Is it correct that silicon has a fcc structure?
• Could you explain again the lattice constant a and why it is important (heteroepitaxy when one wants to create a different crystal on the waver page 38)
• I can not just answer the questions on page 47. Can we go through the questions?