In a pure silicon semiconductor 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 pentavalent phosphorus atoms (P).

As a result of this, the four valence electrons of the P-atom will be incorporated into the tetrahedral crystal lattice of the silicon, forming four covalent bonds with the four connecting Si-atoms. The fifth outer shell electron of the phosphorus atom stays in connected to the P-atom in a state without a bond. However, this fifth electron is extremely easy to tear out from the attraction of the phosphorus atom, that is upon the impact of a lesser external energy the fifth electron of the P-atom would work as a free charge carrier. Thus a P doped crystal can be made better conductor than a pure silicon crystal by adding less external energy.

This type of contaminated semiconductor is called n-type (negative type) doping semiconductor, the impurities donor (donating) atoms.

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

Nse + Nne > Nsh
 

(Nne being the number of the fifth electrons in the n-type impurity phosphorus atoms)

In this case the majority charge carriers are the electrons (while in this case the minority charge carriers being the holes left over in the place of the self-excited electrons in the covalent bond).

Please note that the doped crystal has no surplus charges. The electron surplus 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 the semiconductor conduction will be implemented by the self-holes and self-electrons, and by the electrons of the fifth valence electrons of the impurity atoms. 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. 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 doping, the addition of impurities.

2. If a part of the silicon atoms is replaced by pentavalent phosphorus atoms, the number of the freely moving electrons in the crystal will be increased.

3. The fifth outer shell electrons would break free from the phosphorus atom due to relatively small amount of energy, and are able to move around in the lattice freely. This way conductivity of the crystal is improved substantially.

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