In a pure silicon semiconductor the excitation of electrons, that is allowing electrons jump out of covalent bonds will require an energy of 1.1 eV [1]. This is why a silicon crystal is a poor conductor at room temperature.

In order to increase the electric conductivity of silicon crystals, you have to increase the amount of free charged particles. This can be best achieved by doping the semiconductor crystal, so that a part of the tetravalent Si atoms in the crystal [2] are replaced by for instance trivalent boron atoms (B). As a result of this, the three valence electrons of the B-atom will be incorporated into the tetrahedral crystal lattice of the silicon, but are able to create only three complete covalent bonds with the four connecting Si-atoms. One of the bonds created by the B-atom with one of the four Si-atoms will be deficient in electrons, which is provided by the Si-atom, since the B-atom was able to participate in the build-up of the tetrahedral system of covalent bonds in the silicon crystal with three outer-shell electrons only. That is, there will be one bond, where only one electron is found and beside it there is a shortage of electrons, i.e. a hole. Thus the number of holes appearing in a doped crystal will correspond to the number of atoms creating the impurity.

Since it is not absolutely necessary for the electrons to break away from the bond with the atom in order to move, it is sufficient for conductivity if the electron jumps over from one covalent bond into the hole in another in the neighbourhood, thus a boron-doped crystal can be made better conductor than a pure silicon crystal by adding less external energy. That is, the energy needed to excite a doped semiconductor crystal is reduced as a result of doping.

This type of contaminated semiconductor is called p-type (positive type) doping semiconductor, the impurities acceptor (receiving) atoms.

Beside the thermally excited self-electrons of the silicon crystal and the consequently generated self-holes the holes of the contaminating atom will also participate in conduction.

Nse < Nsh + Nph

(Nph being the number of holes in the fourth, electron
deficient bond of the p-type impurity boron atom)

In a p-doped semiconductor the majority charge carriers are the holes (while in this case the minority charge carriers being the self-excited electrons of the silicon).

Please note that the doped crystal has no surplus charges. The electron deficiency is measured against the needs of forming covalent bonds, but all in all the number of protons in the atomic nucleuses in the crystal as a whole equals the number of electrons in it.

In a p-type doped semiconductor conduction will be implemented by the self-holes left over after the excited electrons quitting the covalent bond, through the holes ‘brought in’ by the contaminating B-atom, and by the electrons exiting from the covalent bond. Conductivity certainly is the result of a flow of electrons in all cases.

While a semiconductor crystal is exposed only to a thermal effect and light, the actual movement of the electrons and the apparent movement of holes in the crystal is chaotic.

Electric conduction requires the semiconductor crystal to be connected to an external voltage source. While electrons involved in the electric conductivity move around in the crystal in a delocalised manner, holes fixed in covalent bonds carry out their apparent movements through electrons jumping over from the adjacent bonds. In a covalent bond electrons jump from one bound state to another, and at the end as a result of their movements it looks as if the holes would have been also involved in conduction as real charged particles moving in opposite direction with the electrons.

However, the holes as apparent charge carriers are in fact immovable. For instance, a hole is left in the place of an electron jumping to the left, that is to the right from the new place of the electron. This is why holes are considered to be charge carriers moving opposite to the electrons.

Upon the impact of an external electrical field the resulting motion of the charged particles involved in conduction will become opposite to each other and
parallel with the force lines of the external electrical field.

Conductivity of a doped semiconductor may increase by 2-3 orders of magnitude.

Key statements

1. Conductivity of the silicon crystal is improved by the addition of atomic impurities.

2. When a part of the silicon atoms is replaced by trivalent boron atoms, the number of wholes in the crystal will be increased.

3. Electrons jumping over into the holes can move around with a greater degree of freedom even in the case of smaller energy level than they could do in the silicon crystal lattice, hence the conductivity of the crystal will be substantially improved.

4. Thermal motion of the freely wandering electrons and holes is chaotic, but their resultant motion will become organised, opposite to each other and parallel with the field lines of the external electrical field upon the impact of the external electrical field.

5. Electrons and holes involved in conductivity will create the current flow jointly.

[1] 1 eV = 1,6 × 10-19 J energy
[2] Every 104-107th silicon atoms are replaced with impurities atoms.
3. LeifiPhysik: Eigenleitung im Siliziumkristall
4. University of Cambridge: Intrinsic and Extrinsic Semiconductors