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Why Does Conductivity Increase With Temperature In Semiconductors?
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Electrical conductivity increases in semiconductors with increasing temperature. As you increase the temperature, electrons from the valence band are able to jump to the conduction band, creating free movement between the two bands, thus, increasing the conductivity.
Electrical Conductivity (EC) measures the ability of a material to transmit an electrical current over a certain distance, usually measured in Siemens (S) per distance (usually m-meter). It is a Conductivity meter that is used to read the electrical charge and measure the conductance.
When it comes to conductance, you probably already know that a substance that conducts electricity is called a conductor, and one that doesn’t is known as an insulator. For example, copper is a good conductor, and rubber is a good insulator. Semiconductors are somewhere between the two, and the most common semiconductor used today, is silicon.
To determine the electrical properties of a substance, we measure the resistivity (electrical resistivity). As an example, conductors have low resistance and therefore can conduct electricity very easily, compared to insulators that have high resistance, and therefore it is very difficult for electricity to pass through.
A semiconductor that contains almost no impurities (“a chemical substance inside a confined chemical phase”) will conduct little to no electricity. However, when elements are added to semiconductors like temperature, electricity can pass through them more easily.
The Relationship Between Conductivity & Temperature In Semiconductors
In conductors, conductivity is usually limited by electron-photon scattering. This means, the higher the temperature, the more electron collisions with thermal photons take place. This activity reduces the “mean free path” of electrons, increasing the resistivity of the conductor. However, this is not entirely the case with semiconductors.
Two things affect the conductivity in semiconductors when temperature increases:
- The mobility of mobile charge carriers (electrons).
- The amount/concentration of carriers available.
In a semiconductor, mobility and carrier concentration are both temperature-dependent.
When the temperature is increased in a semiconductor, both the electrons and the atoms gain more energy. When the electrons gain more energy, the atoms vibrate more, increasing the scattering of electrons. This occurs in both regular metal conductors and semiconductors.
However, conductivity generally only increases with temperature in semiconductors. In metal conductors, increasing the temperature usually results in a decrease in conductivity, or it increases the resistivity.
What Are Valence & Conduction Bands?
Semiconductors are also classified by their bands: a fully occupied (with electrons) valence band, and an unoccupied conduction band.
A valence band contains an electron orbital where electrons can jump off and move to the conduction band when they become excited*. So, to put it simply, a valence band is the outermost electron orbital of an atom that electrons can occupy.
A conduction band contains the band of electron orbitals that electrons jump onto from the valence band. When the electrons become excited and jump onto the orbital, they are then free to move inside the material. It is this movement of electrons that creates an electrical current.
*The term “excited” in this case, refers to the state of an atom, ion, or molecule with an electron that has a higher energy level than normal.
How Does Temperature Affect Valence & Covalent Bands?
As the temperature is increased in semiconductors, more electrons become excited, jumping to the conduction band from the valence band. Because of the holes (empty states) created in the valence band, any available electrons can occupy the spaces, increasing the conductivity.
For example, at 0-degrees Kelvin (K), the valence band inside the semiconductor is completely filled, and the conduction band is empty. When energy is applied, the electrons can move freely to the conduction band. Usually, a semiconductor is a poor conductor, but when the temperature is increased, the gap between the two bands becomes less, allowing electrons to move from the valence band to the conduction band very easily.
So, to put that a bit more simply, when temperature increases, the gap between the bands decreases allowing free movement of electrons, therefore increasing the conductivity. Hence, when temperature increases, more electrons have the energy to cross between the bands.
So, what about applying low temperatures?
Even if a voltage is applied at lower temperatures, the electrons will not gain enough energy to move to the conduction band, therefore no movement of electrons takes place, so the electrical conductivity in the semiconductor is near or at absolute zero (0 K). When semiconductors are at absolute zero, they make perfect insulators for everyday use – like your mobile phone.
Electrical conductivity increases in semiconductors with increasing temperature, because, as temperature increases, the number of electrons from the valence band are able to jump to the conduction band. It is the increase of temperature that excites the electrons and shortens the band gap allowing a “free movement” of electrons between the two bands, consequently increasing the conductivity.
Conductivity & Temperature
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