In a regular periodic crystal lattice, electrons as the carriers of electrical current are not allowed to move around freely. Instead, they have to obey certain rules enforced by quantum mechanics.
As a consequence, electrons have to occupy so-called “energy bands” which are separated from each other by small or large “band gaps”.
What is Conduction Band and Valance Band
|Energy Band analogy|
In the language of solid state physics, these two storey's are called “valence band” and “conduction band”, respectively.
In the same way, electrons in a solid crystal have to obey the quantum-mechanical “exclusion principle” formulated by the famous physicist Wolfgang Pauli.
Now that the blueprints of our semiconductor building and the basic rules for its inhabitants have been defined, let us start to occupy this building with people.
Further inhabitants can only occupy the first floor, where they again have sufficient room to move about. As a consequence, the overall cargo stream through the building will again start to increase, reach a maximum, and eventually come to an end when also the first floor is fully occupied.
A. What is a Conductor?
Crystals in which energy bands are only partially filled will belong to the group of electrical conductors, since their electrons can move more or less freely through the crystal lattice.
If, on the other hand, all energy bands are fully occupied or completely empty, no electrical current can pass through the crystal at all and we are dealing with an electrical insulator.
Which situation will be encountered for a given solid crystal depends on how many electrons per atom are available to occupy the energy bands of the crystal.
For example, almost all metals are very good electrical conductors due to a half-filled conduction band, whereas metal oxides very often are good insulators with a completely filled valence band and an empty conduction band.
B. What is a Semi-Conductor?
Semiconductors are solids which are able to pass an electrical current much better than insulators, but at the same time not as efficiently as an electrical conductor. Obviously, semiconductors are solids in which for one reason or the other a few of the many tiles on the ground floor remain empty or a few of the conduction band tiles are occupied, or both.
C. What is an Insulator?
Insulator, where all tiles in the ground floor are occupied by exactly one inhabitant and all tiles in the first floor are empty. Thus, no charge transport can occur.
Doping and the Band Theory
In the analogue of our semiconductor building, doping can be achieved by adding special tiles with the following properties.
|Holes and Electrons in an Atom|
|Valance Band with Holes|
Therefore, doping of a semiconductor crystal with acceptor atoms is referred to as “p-type” doping (“p” as in positive).
2. Donor Ion and Electron Theory
The second possibility to induce controlled electrical conduction in an insulator is the doping with donor impurities.
|Conduction Band with Extra Electrons|
|Recombination in a Diode|