Semiconductors
In general, semiconductors are inorganic or organic materials that can control their conduction depending on chemical structure, temperature, illumination, and the presence of dopants. The name semiconductor comes from the fact that these materials have electrical conductivity between a metal, like copper, gold, etc., and an insulator, like glass. They have an energy gap of less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between the valence band and conduction band where electron states are forbidden. In contrast to conductors, semiconductors’ electrons must obtain energy (e.g., from ionizing radiation) to cross the band gap and reach the conduction band. The properties of semiconductors are determined by the energy gap between valence and conduction bands.
Characteristics of Semiconductors
Semiconductors have a conductivity level between that of a conductor and an insulator, with their conductivity controlled by doping, temperature, and applied electric fields, and are used extensively in electronic devices.
Some of the key characteristics of semiconductors include:
- Variable conductivity: Semiconductors can be made to conduct electricity under certain conditions, such as when exposed to light or heat. They can also be made to act as insulators under different conditions.
- Bandgap: Semiconductors have a bandgap, which is the energy required to move an electron from the valence band to the conduction band. The size of the bandgap determines the energy required for the semiconductor to become a conductor.
- Doping: Semiconductors can be doped with impurities to modify their electrical properties. Doping introduces additional electrons or “holes” into the material, which can increase or decrease its conductivity.
- Temperature dependence: The electrical conductivity of semiconductors is highly dependent on temperature. As the temperature increases, the conductivity of the material generally increases as well.
- Light sensitivity: Some semiconductors are sensitive to light and can be used in applications such as photovoltaic cells, light sensors, and LEDs.
- Minority carriers: In semiconductors, electrons and holes are known as minority carriers. These carriers can be manipulated and controlled to produce desired electrical properties in the material.
Here is a table with 3 intrinsic semiconductors and 2 p-type and n-type semiconductors, along with 4 key properties:
| Semiconductor | Type | Band Gap (eV) | Electron Mobility (cm²/Vs) | Hole Mobility (cm²/Vs) | Thermal Conductivity (W/mK) |
|---|---|---|---|---|---|
| Silicon (Si) | Intrinsic | 1.12 | 1500 | 450 | 150 |
| Germanium (Ge) | Intrinsic | 0.67 | 3900 | 1900 | 60 |
| Gallium Arsenide (GaAs) | Intrinsic | 1.43 | 8500 | 400 | 46 |
| Boron-doped Silicon (p-Si) | p-type | 1.12 | 1500 | 1800 | 150 |
| Phosphorus-doped Silicon (n-Si) | n-type | 1.12 | 1500 | 4500 | 150 |
| Aluminum-doped Gallium Arsenide (p-GaAs) | p-type | 1.43 | 8500 | 200 | 46 |
| Silicon-doped Gallium Arsenide (n-GaAs) | n-type | 1.43 | 8500 | 800 | 46 |
Types of Semiconductors
Semiconductors can be classified into two basic types based on their electronic properties:
- Intrinsic Semiconductors: These are pure semiconductors that are made up of a single element (e.g., Silicon, Germanium) and have no intentional doping with impurities. Intrinsic semiconductors have a specific number of electrons in their valence band and conduction band. They conduct electricity when they are heated, and some electrons gain sufficient energy to break free from their bonds and become free electrons in the conduction band.
- Extrinsic Semiconductors: These are impure semiconductors that are intentionally doped with impurities to change their electronic properties. Extrinsic semiconductors can be further classified into two types:
- p-type semiconductors: In p-type semiconductors, impurity atoms such as boron are introduced into the semiconductor material. These impurities have fewer valence electrons than the semiconductor material, which results in “holes” (absence of electrons) being created in the valence band. These holes can conduct current like positive charge carriers, which gives the material its p-type designation.
- n-type semiconductors: In n-type semiconductors, impurity atoms such as phosphorus are introduced into the semiconductor material. These impurities have more valence electrons than the semiconductor material, which creates excess electrons in the conduction band. These excess electrons can conduct current like negative charge carriers, which gives the material its n-type designation.
