Conductors, Insulators and Semiconductors
All materials can be classified according to to their electrical properties and grouped into either
- Conductors
- Insulators
- Semiconductors
All materials are made up of atoms, These atoms contribute to the electical properties a material exhibit to conduct electrical current.
The property that determines the conductivity of an atom is its Valence Shell (outermost shell of electrons). All of the inner shells are collectively refered to as the "core". The core also includes the nucleus (which contains the protons and neutrons of an atom).
Carbon, which is often used as the material resistors are made out of, has 4 celectrons in the valence shell and two electrons in the inner shell (there are only two shells in a Carbon atom). The nucleus of a Carbon aton consists out of 6 protons and 6 neutrons and therefore has a positive charge of +6 in the nucleus of the atom. Since the valence shell only has 4 electrons, the net charge for the core of the atom is +4 (+6 for the nucleus and -2 for the two electrons in the inner shell).
Conductors
A conductor is a material that can easily conduct electrical current. The best conductors are single-element materials such as copper, silver, gold and aluminium, all of which have only one valence electron very loosely bound to the atom. These valence electrons require the least amount of energy for the electron to break away from their atom and become a free electron.
Hence a "conductive material" is one which has many free electrons that, when moving in the same direction, make up the electric current.
Insulators
A material hat does not conduct electrical current under normal conditions is classified as an Insulator. Valence electrons in insulators are very tightly bound to the atoms and therfore have very few free electrons. The best insulators are materials made out of compounds of material instead of single-element material.
Semiconductors
A semiconductor is a material that in its pure intrinsic state is neither a good conductor nor a good insulator. Semiconductors can be made out of either single-element material such as Silicon, Germanium and Carbon. Compound semiconductors can be made out of Gallium Aresenide are also commonly used.
Semiconductors are specifically atoms that have four valence electrons in the valence shell of the atom.
Energy Bands
Just as all shells in an atom, the valence shell represents a band of energy levels and all electrons are confined to their band represented by their shell. The valence shell, being the outermost shell, provides the ability to electrons in the shell with enough energy to break out of the shell (and atom as a whole) to become a free electron.
This free electron, when leaving the atom, exists in what is known as the Conduction Band.
In an Insulator, the Conduction band, has a large energy gap difference between itself and the Valence Band of the atom. Therefore a large amount of energy is required to push an electron from the Valence band to Conduction band.
In a Semiconductor, there is a smaller gap between the Valence band of the atom and the Conduction band. The energy gap can also be controlled by specific means to increase or decrease the energy gap in a Semiconductor, thus making it harder or easier to conduct electrical current.
In a Conductor, there is no gap between the Conduction Band and the Valence Band, in fact, they overlap each other. Therefore, in a conductive material there is always a large number of free electrons.
Semiconducting Atom vs Conductor Atom
Why is Silicon a semiconductor and copper a conductor?
The core of a Silicon atom has a net charge of +4 (14 protons - 10 electrons) and the core of a Copper atom has a net charge of +1 (29 protons - 28 electrons).
The valence electron in the Copper atom has an force of '+1' trying to attract it closer to the nucleus while the Silicon atom has a force of "+4" tring to attract its valence electrons to the nucleus.
Therfore, there is 4 times as much force trying to attract the valence electrons in a Silicon atom than there is force trying to attract the valence electron of the Copper atom.
Finally, the valence electron in the Copper atom is in the 4th shell while the Silicon atom's valence electrons are in its 3rd shell.
Therefore, the valence electron in the Copper has more energy and requires less energy to break free from the valence shell since the Copper atom's valence shell is further away from the nucleus (thus requireing less energy to break free).
Even though the Silicon atom has more capacity to carry electrical current, it requires a much larger force for electrons to move and break free therefore is a a weaker conductor than Copper.
