Semiconductor Materials Example

According to their ability to conduct electrical current, materials are divided into three categories: Conductors, Insulators, and Semiconductors. The most prominent example of conductors are metals, such as copper Cu, aluminum Al, and silver Ag; and of the insulators polymers and glass. The third class will be discussed below: semiconductors.

The electrical properties of semiconductors are among those of insulators and conductors.Silicon Si and Germanium Ge are well known examples of semiconductors that are frequently used in the manufacture of a variety of electronic devices.. The electrical properties of semiconductors can be changed by several orders of magnitude, adding controlled amounts of foreign atoms to the materials.

Semiconductors behave as insulators at low temperatures, but if this is increased, they behave as conductors. This duality of conductivity is due to the fact that the valence electrons of the material are loosely bound to their respective nuclei. atomic, but not enough, so that the rise in temperature will allow them to leave the atom to circulate through the atomic lattice of the material. As soon as an electron leaves an atom, it leaves a hole in its place that can be filled by another electron that was circulating in the lattice.

This is the case of the previously mentioned chemical elements, Silicon Si and Germanium Ge, which have four valence electrons at their last level. It should be noted that, to add energy to the semiconductor material, in addition to heat transfer, light can be applied.

To better understand the behavior of semiconductor materials, the Theory of Bands will be used.

Band Theory

 The concept of Valencia band, which is the accumulated energy possessed by valence electrons.

In addition, this theory handles the definition of Conduction Band, as the energy together that electrons have to withdraw from their atoms. The electrons that are in the conduction band can circulate through the material if there is an electrical voltage that drives them between two points.

Based on the two Bands, the cases of Conductor, Insulator and Semiconductor will be studied to have a perspective for the latter.

For a Conductor, the Energy of the Valencia Band is greater than that of the electrons of the Conduction Band. In such a way that the bands overlap and many Valencia electrons are placed on the Conduction very easily, and therefore, with the option of circulating in the middle.

For an Insulator, on the other hand, the Energy of the Conduction Band is much greater than the Energy of the Valencia Band. Here there is a gap between the Valencia Band and the Conduction Band, so that the Valencia electrons cannot access the Conduction Band which will be empty. That is why the insulator does not conduct. Only at high temperatures can these materials be conductive.

In the case of Semiconductors, the Conduction Band is still greater than the Valencia Band, but the gap between the two is considerably smaller, so that with an energetic increase, the Valencia electrons jump to the Conduction Band and can circulate through the medium. When an electron jumps from the Valencia Band to the Conduction Band, it leaves an egg in the Valencia Band that is also considered a carrier of electric current.

In semiconductors, two types of electric current carriers are distinguished: negatively charged electrons, and holes, positively charged.

Types of Semiconductors 

There are two classes of Semiconductors according to their purity. Semiconductor Materials in their pure state are known as Intrinsic Semiconductors; and there are Extrinsic Semiconductors, which are pure but contaminated with impurities in minute proportions, like one particle in every million.

This contamination process is called Doping, which in turn manifests itself in two types.

The first type of Doping is Type N, in which the material is contaminated with valence 5 atoms, such as Phosphorus P, Arsenic As, or Antimony Sb. By involving the fifth valence electron in the structure of tetravalent atoms, is forced to wander through the semiconductor material, without finding a stable site where Be placed. The set of these errant electrons is called Majority Electrons.

The second type of Doping is Type P, in which the semiconductor material is contaminated with atoms of valence 3, such as Boron B, Gallium Ga, or Indium In. If this atom is introduced into the material, a hole remains where an electron should go. The hole moves easily through the structure of the material, as if it were a carrier of positive charge. In this case, the holes are Majority Carriers.

Semiconductor Application: Diode

The diode is an electronic component that consists of the union of two extrinsic semiconductor crystals, one of type N and the other type P. By joining them, part of the excess N-type electrons pass to the P-type crystal, and part of the P-type holes pass to the N-type crystal. A strip called the Transition Zone is created at the junction, which has an electric field that behaves like a barrier that opposes the passage of more electrons from Zone N to Zone P and of holes from Zone P to Zone N.

When a Diode is connected to a battery, two different cases occur: Forward Bias and Reverse Bias.

In Direct Polarization, the positive pole is connected to crystal P and the negative pole to crystal N. This makes the transition zone much narrower, breaking the barrier and allowing the free passage of the current. In this condition, the Diode is Conductive.

In Reverse Polarization, the positive pole connects to crystal N and the negative pole to crystal P. This makes the transition zone much wider, reinforcing the barrier that prevents the passage of current. In this case, the Diode is Insulator.

The applications of the Diode are multiple. However, the most popular application is the one that uses it as a Rectifier. A Rectifier is a system capable of converting a sinusoidal alternating input signal into another that has the same sense, to later convert alternating current into direct current. Before rectifying the current, a transformer is used that reduces the value of the voltage.

Examples of Semiconductor Materials

By the group in which they are present in the Periodic Table, these are some examples of semiconductor elements:

Group IIIA: Boron B, Aluminum Al, Gallium Ga, Indium In.

Group VAT: Silicon Si, Germanium Ge.

Group VA: Phosphorus P, Arsenic As, Antimony Sb.

Group VIA: Sulfur S, Selenium Se, Tellurium Te.