At a low temperature, near 0 K (-273 oC) semiconductor materials behave like insulators. Another typical property of semiconductors is that their resistance is dependent on the temperature. Their resistance [1] declines exponentially with the temperature. The higher their temperature, the better they conduct electricity. (It is the other way round with metals.)

The reason why the resistance of a semiconductor material decreases as an impact of temperature (or light) is the excitation of the electrons in them. A condition for excitation is that the electrons be given such an amount of energy which is sufficient to tear them away from the covalent bond. It follows from this that the energy of the exiting electron will be higher than that it had in the covalent bond. In the place of the exiting electron a bond short of an electron will be left over. The electron deficient spot of the covalent bond is called a hole. The hole is assigned with the energy of the quitting electron when it was still in the covalent bond, and with the absolute value of its charge.

The consequence of the crystal excitation is, thus, the emergence of a volatile electron which may move around among the atomic sites in the crystal lattice with a relative degree of freedom and an energy level greater than before, as well as the creation of a hole, the energy of which equals the energy of the said electron while it was still in the bond. The process is called electron-hole pair generation.

Recombination would counterweigh the generation process in a thermal equilibrium which happens when electrons and holes find each other. In other words, recombination in this sense means the phenomenon when the electron falls in place in the hole, the electron-hole pair ceases to exist while the energy absorbed by the electron at the time of generation will be emanated to its environment in the form of electromagnetic waves.

That is, the density of electrons and holes at a given temperature is steady and depends on the properties of the pure semiconductor. Excitation caused by thermal motion is called thermal excitation.

The excitation of the electrons provides an answer to the conductive properties of semiconductors. An interesting feature of semiconductor conductivity is that electricity is generated and determined by delocalised electrons and ‘agile’ holes jointly. [2]  Upon increasing the temperature, a pure semiconductor would conduct electric current more and more, because more and more free charge carriers (electrons and holes) are created in it by the thermal excitation. [3]

In a pure silicon crystal, the number of the electrons quitting the covalent bond (Nse number of self-electrons) and the number of holes left in place of them (Nsh number of self-holes) is identical.

Nse = Nsh

Conductivity created by self-electrons and self-holes is called self-conduction or intrinsic conduction, and semiconductors thus produced self semiconductors (structural or intrinsic semiconductors). In semiconductors the thermal energy of the crystal is able to cover the energy needed for self-conduction.

In most semiconductors the thermally delocalised – agile, mobile – charge carriers (electrons and holes) are present in a very low number, less by orders of magnitude than in metal conductors. For instance, in a silicon crystal 1.5×1010 thermally excited electrons can be found in every cm3 at a temperature of 298 K (25 oC), and 1.1×106 in gallium-arsenide (GaAs), while in metals the conductive electron density is typically at around 1028/cm3. In other words, free charge carriers appear in the semiconductor crystal as a result of the excitation, which in turn causes reduced resistance and increased conductivity upon the impact of exposure to light or heat. Circuit elements made of such pure semiconductor materials include for instance thermistors and photoelectrical resistance cells, which vary their electric resistance upon the impact of temperature or light.

Holes are considered equivalent but oppositely charged charge carriers compared to electrons. Due to their unlike charges, electrons and holes migrate the opposite direction upon the impact of external electrical fields. (As long as no external electrical field exists, the movements of the charged particles is chaotic.)

It is worth to take note of the fact that while holes can move only in a system of covalent bonds – since a hole is nothing else but the place of an electron broken out of the covalent bond –, conductive electrons are free to move among atomic sites in the crystal lattice as long as they have the appropriate energy.

Since the number of excited electrons in a pure silicon semiconductor corresponds to the number of the holes, there are free charge carriers, but no surplus charges.

Key statements

1. If the crystal is exposed to more energy than that typical for the crystal, an amount of electrons proportional with the energy added will be excited and quit the covalent bonds.

2. Each excited electron leaves a hole behind, the energy level of which corresponds to the energy of the electron in the bond, with a relative charge of +1. The process is called electron-hole pair generation.

3. The hole is also considered to be a charge carrier.

4. The process, when the electron falls into the hole is called the recombination. During recombination the electron will emit energy. In certain types of semiconductors this happened in the form of photons, others would warm up the crystal lattice with the emanated energy.

5. Movements of the hole and of the electron jumping over from the adjacent covalent bond into the hole are considered to happen in  opposite directions.