Conduction in doped semi-conductors.
Coductors
Imagine a wire not forming part of a circuit. It is a conductor.
In a conductor there are millions and millions of electrons throughout the structure. The outermost ones are only very loosely held on their atom. These electrons freely exchange positions from atom to atom but there is no overall movement of charge in the wire and the overall charge is neutral. For every electron there is a proton.
In the diagram the black blobs represent the atoms and the red dots are the “free” electrons that are loosely held. These electrons change places with each other continuously but there is no overall flow of charge. Some of the electrons will move to the right and some to the left, some up, some down.
If a potential difference is applied across the ends of the wire then the electrons still continue to change places but overall there is a drift of electrons towards the positive terminal. Some electrons still move to the left and some to the right, some up, some down, but overall now there is a general drift in one direction.
Insulators
In a pure insulator there are no free electrons. Almost every electron forms part of a covalent bond and is therefore not free to move around the crystal lattice structure. Obviously there are a few outermost electrons that cannot be involved in bonds because there is no neighbouring atom with which to bond. Relatively speaking these are few in number. This is a very stable arrangement and the electrons do not move easily. If there are no free charge carriers then it is an electrical insulator.
The green hoops illustrate covalent bonds. Note that all outermost electrons are involved in these bonds. There are no free charge carriers.
Let us now look at an illustration of pure silicon. It follows the above pattern. There are FOUR electrons in the outermost shell and so FOUR electrons are available to bond. They are ALL required to form a perfect lattice structure.
When “growing” these crystal structures it is possible to introduce impurities. This process is known as “doping”. If small amounts of another element with FIVE outermost electrons is introduced in the lattice then four of the electrons are used in the bonding process and ONE electron is left over. Only about one atom per million is needed to turn pure silicon into an effective semiconductor.
This “extra” electron is free to move around the structure. Remember only about one impurity atom per million is needed.
Important note – The overall charge of a doped lattice is NEUTRAL. Although there is an “extra” electron there is a proton in the nucleus to balance it so the overall charge is neutral.
This is known as n-type semiconductor because there are “free” negative electrons as the charge carriers.
What happens if you use an impurity with only THREE outermost electrons?
In this case the lattice forms normally but there is one electron “missing” from the structure. Again the overall charge is neutral because there is also one less proton in the nucleus of the doped atom.
This “missing electron” has been given the name of a “positive hole”. This is an unfortunate name really because it is NOT positive. It is called positive because it “attracts” negative electrons because an electron is needed to complete the structure. It is just a place (a hole) where an available electron would happily go to complete the structure. This means that a neighbouring electron could move into that “hole” but it would leave behind another “hole”. This movement of holes (caused by movement of electrons) behaves just like having a free electron. These “holes” are free charge carriers.
This is known as p-type semiconductor because it is positive holes that carry the charge.