Semiconductors basics

- what semiconductors are and the way in which current is able to flow in them

Semiconductors and semiconductor technology forms the basis of most of the electronics industry these days. Transistors, diodes, integrated circuits and many more devices all have semiconductor technology in common. As a result of the enormous degree of flexibility that semiconductor technology provides, it has enabled electronics to take over many areas of daily life, that fifty years ago could not have been conceived.

The first step in looking at semiconductor technology is to see what types of material there are in terms of their electrical properties. Basically there are three types:

  • Conductors


  • Non-conductors


  • Semiconductors

In order to understand what semiconductors are and why they are a distinct group on their own, it is necessary to look first at why some materials conduct electricity (conductors) and why others do not (non-conductors).

Conductors and Non-Conductors

An electric current occurs when there is a flow of electrons in a certain direction. As electrons have a negative charge, their movement means that charge is flowing from one point to another and this is what an electric current is.

To enable the current to flow the electrons must be able to move freely within the material. In some materials electrons are moving freely around the lattice, although the number of electrons and the available spaces for them balances out so the material itself does not carry a charge. In these materials the electrons are moving freely but randomly. By placing a potential difference across the conductor the electrons can be made to drift in one direction and this constitutes an electric current. Many materials are able to conduct electricity, but metals form the most common examples.

Unlike metals, there are many other materials in which all the electrons are firmly bound to their parent molecules and they are not free to move. Accordingly when a potential is placed across the substance very few electrons will be able to move and very little or no current will flow. These substances are called non-conductors or insulators. They include most plastics, ceramics and many naturally occurring substances like wood.

Semiconductors

Semiconductors do not fall into either the conductor or non-conductor categories. Instead they fall in between. A variety of materials fall into this category, and they include silicon, germanium, gallium arsenide, and a variety of other substances.

In its pure state silicon is an insulator with no free electrons in the crystal lattice. However to understand how it acts as a semiconductor first look at the atomic structure of silicon in its pure state. Each molecule in the crystal lattice consists of a nucleus with three rings or orbits containing electrons, and each electron has a negative charge. The nucleus consists of neutrons that are neutral and have no charge, and protons that have a positive charge. In the atom there are the same number of protons and electrons so the whole atom has no overall charge.

The electrons in the silicon, as in any other element are arranged in rings with strict numbers of electrons in each orbit. The first ring can only contain two, and the second has eight. The third and outer ring of the silicon has four. The electrons in the outer shell are shared with those from adjacent atoms to make up a crystal lattice. When this happens there are no free electrons in the lattice, making silicon a good insulator. A similar picture can be seen for germanium. It has two electrons in the inner most orbit, eight in the next, 18 in the third, and four in the outer one. Again it shares its electrons with those from adjacent atoms to make a crystal lattice without any free electrons.

Impurities

In order to make silicon or any other semiconductor into a partially conducting material it is necessary to add a very small amount of impurity into the material. This considerably changes the properties.

If traces of impurities of materials having five electrons in the outer ring of their atoms are added they enter the crystal lattice sharing electrons with the silicon. However as they have one extra electron in the outer ring, one electron becomes free to move around the lattice. This enables a current to flow if a potential is applied across the material. As this type of material has a surplus of electrons in the lattice it is known as an N-type semiconductor. Typical impurities that are often used to create N-type semiconductors are phosphorous and arsenic.

It is also possible to place elements with only three electrons in their outer shell into the crystal lattice. When this happens the silicon wants to share its four electrons with another atom with four atoms. However as the impurity only has three, there is a space or a hole for another electron. As this type of material has electrons missing it is known as P-type material. Typical impurities used for P-type material are boron, and aluminium.

Holes

It is easy to see how electrons can move around the lattice and carry a current. However it is not quite so obvious for holes. This happens when an electron from a complete orbit moves to fill a hole, leaving a hole where it came from. Another electron from another orbit can then move in to fill the new hole and so forth. The movement of the holes in one direction corresponds to a movement of electrons in the other, hence an electric current.

From this it can be seen that either electrons or holes can carry charge or an electric current. As a result, they are known as charge carriers, holes being the charge carriers for a P-type semiconductor and electrons for an N-type semiconductor.

Summary

The principle behind semiconductors can look fairly straightforward. However it took many years before many of its properties could be exploited, and many more before they could be refined. Nowadays, many of the processes used with semiconductors have been highly optimised and the components like integrated circuits are highly sophisticated. However they rely on the fact that different areas of the semiconductor can be doped to make P-type and N-type semiconductors.

List of common semiconductor terms

  • Charge carrier   - Charge carrier is a free a free (mobile, unbound) particle carrying an electric charge, e.g. an electron or a hole.

  • Conductor   - A material in which electrons can move freely and electricity can flow.

  • Electron   - A sub-atomic particle carrying a negative charge.

  • Hole   - The absence of a valence electron in a semiconductor crystal. The motion of a hole is equivalent to motion of a positive charge, i.e. opposite to the motion of an electron.

  • Insulator   - A material in which there are no free electrons available to carry electricity.

  • Majority carrier   - Current carriers, either free electrons or holes that are in excess i.e. in the majority in a specific area of a semiconductor material. Electrons are the majority carriers in N-type semiconductor, and holes in a P-type area.

  • Minority carrier   - Current carriers, either free electrons or holes that are in the minority in a specific area of a semiconductor material

  • N-type   - An area of a semiconductor in which there is an excess of electrons.

  • P-type   - An area of a semiconductor in which there is an excess of holes.

  • Semiconductor   - A material, that is neither an insulator nor a full conductor that has an intermediate level of electrical conductivity and in which conduction takes place by means of holes and electrons.



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