Generation is the creation of a free electron in the conduction band due to energy being absorbed in the lattice of atoms. With an applied energy, some of the electrons in the lattice manage to break free and will leave behind a broken covalent bond (or a hole). The hole is a positively charged particle occupying the Valence band; like electrons in the conduction band, these holes are free to move within the valence band. Recombination occurs when an electron meets a hole; it re-forms the covalent bond and releases energy. An electron will move from the Conduction band back into the Valence band. Just Remember:
- Generation breaks covalent bonds and increases conductivity – requires energy.
- Recombination forms covalent bonds and decreases conductivity – releases energy.
And that in a semiconductor:
- Mobile conduction electrons with negative charge occupy the conduction band.
- Mobile holes with positive charge occupy the valence band.
In an intrinsic (pure) group IV semiconductor, the density of electrons in the conduction band is equal to the density of holes in the valence band. This density is given by the equation:
- is the density of electrons in an intrinsic semiconductor
- is a constant that will be usually provided – it is specific to different materials
- is the energy gap within the atom (see here for more info)
- is the Boltzmann constant –
- is the temperature in Kelvin
On the whole:
- will increase as the temperature increases
- is small for semiconductors with a large energy gap ()
So we’ve just briefly touched on electrons and holes and how heat energy is used to move electrons (thus creating holes). We will now look at how semiconductors can be doped in order to alter the number of electrons and holes and thus change the electrical conductivity.
Atoms in group IV of the periodic table are usually used for doping, along with atoms from group III and group V. Since atoms in group IV have 4 electrons on their outer shell, in their regular lattice they combine to form perfectly filled outer shells. When doped with group III atoms, the outer shells will have an electron missing (therefore creating a hole) and when doped with group V atoms, there will be an extra electron.
- Since an electron has a negative charge, group III doping will form a positive material since there will be electrons missing. Such as Silicon and Phosphorus.
- Likewise group V doping will form a negative charge since there is an excess of electrons. Such as Silicon and Boron.
A positive material is a P-Type material (i.e. a material with extra holes) and a negative material is an N-Type material (a material with extra electrons).
After this doping process, the semiconductor is referred to as an Extrinsic Semiconductor. Just remember:
- N-Type (positive): increased number of mobile electrons in the conduction band.
- P-Type (negative): increased number of holes in the valence band.
In the next tutorial we will have an in-depth look at these 2 materials. We will look at the electrons in the conduction band and the holes in the valence band and why they are there.