In the previous tutorial, we briefly went through the process of doping group IV atoms in order to create N-Type and P-Type materials. We will now go on to discuss this in more detail and look at where these extra electrons or holes will be within the atom.
N-Type doping is the introduction of impurity atoms from group V of the periodic table into a pure material from group IV.
The extra electrons occupy a donor level just below the Conduction Band at low temperatures.
If there is a density of n-type dopant atoms, the density of electrons in the Conduction band is approximately at room temperature. It is important to note that room temperature will give the atoms enough energy for the electrons to be excited from the donor levels into the conduction band.
Once an electron has been excited into the conduction band it will leave behind a positive charge – this is NOT a hole since the covalent bond has not been broken. This positive charge will sit on the donor atom (in this case the Phosphorus atom) and is completely immobile; this means the semiconductor remains electrically neutral.
The Group V atom with a Group IV crystal lattice is known as donor atoms.
P-Type doping is the introduction of impurity atoms from group III of the periodic table into a pure material from group IV.
The Boron atoms create acceptor levels just above the valence band and at low temperatures these are unoccupied.
If there is a density of p-type dopant atoms, the density of holes in the Valence band is approximately at room temperature. Again remember that room temperature will give the atoms enough energy for the electrons to be excited from the valence band into the acceptor levels.
The movement of electrons to the Acceptor levels will leave the Boron atoms negatively charged and again the semiconductor will remain electrically neutral. This negative charge on the Boron atom is completely immobile.
The Group III atom with a Group IV crystal lattice is known as acceptor atoms.
Electrons & Holes in Semiconductors
As mentioned before, the electrical conductivity of a material is based upon the direct movement of electrons though a structure. The core electrons cannot take part in this because they are so closely bound to the nucleus: it is only the electrons in the Valence Band and Conduction Band.
It should be noted that electrons in semiconductors are excited from the valence band into the conduction band with an applied heat: this is called Thermal Excitation. Since this happens at anything above 0 Kelvin, we won’t go into much detail about it, just know that applied heat can increase the conductivity of a material and as mentioned before, at room temperature (290 Kelvin) these materials will have a certain conductivity. Of course heat is not the only way of making a material more conductive: it’s possible by absorbing light energy or by doping as we have already discussed.
The average electron can be calculated by using:
- is the average electron energy
- is the Boltzmann constant –
- is the temperature in Kelvin
We will now move onto Carrier Densities in Semiconductors and the Law of Mass Action. Next tutorial.