Semiconductors
Semiconductivity refers to the ability of certain materials to conduct electricity, but only under certain conditions. Semiconductors have electrical conductivity between that of conductors (materials that allow electrical current to flow easily) and insulators (materials that do not allow electrical current to flow easily).
In semiconductors, the valence band (highest band of electrons that are completely filled) is separated from the conduction band (lowest band of electrons that are partially filled) by a bandgap. This means that a certain amount of energy is required to move an electron from the valence band to the conduction band, and the amount of energy required is related to the width of the bandgap.
At room temperature, some electrons in the valence band of semiconductors can acquire enough thermal energy to jump the bandgap and move into the conduction band, creating free electrons that can move freely through the material. However, the number of free electrons is relatively low compared to conductors, which means that semiconductors have lower electrical conductivity.
The electrical conductivity of semiconductors can be increased by doping, which involves intentionally adding impurities to the semiconductor material to alter its electronic properties. Doping can create excess electrons (n-type doping) or holes (p-type doping) in the semiconductor, which can improve its conductivity and make it useful in various electronic applications.
Semiconductors are widely used in electronic devices such as transistors, diodes, solar cells, and integrated circuits. They also have applications in optoelectronics, quantum computing, and other advanced technologies.
Characteristics of Semiconductors
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.
Theory of Semiconductivity
The theory of semiconductivity is based on the behavior of electrons and holes in a crystalline lattice structure. This theory is known as the electronic band structure.
The electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called band gaps or forbidden bands).
Semiconductors have a valence band, which is the highest energy band that is completely filled with electrons, and a conduction band, which is the next higher energy band that is empty or only partially filled with electrons. The energy gap between the valence and conduction bands is called the band gap.
At absolute zero temperature, all of the electrons in a semiconductor are in the valence band and there are no free electrons in the conduction band. However, at room temperature or higher, some electrons in the valence band can be excited by thermal energy or by an external energy source, such as light or an electric field, and jump into the conduction band, leaving behind a hole in the valence band.
The movement of these free electrons and holes in the crystal lattice structure of the semiconductor can be described by the laws of quantum mechanics. The behavior of these charge carriers is influenced by factors such as the crystal structure, the doping concentration and type, the temperature, and the presence of impurities or defects in the crystal lattice.
Intrinsic semiconductors have a perfectly balanced number of free electrons and holes, and their conductivity is determined by the intrinsic concentration of free electrons and holes, which increases exponentially with temperature. Extrinsic semiconductors, which are doped with impurities, have a much higher concentration of free electrons or holes, which dramatically increases their conductivity and makes them useful for electronic devices.
