How do semiconductors work
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.
Electrical Conductivity of Semicondutors
There are several ways in which an electron in a semiconductor material can be excited into the conduction band, where it is free to move and carry an electric current. Some of the most common ways include:
- Thermal energy: At higher temperatures, some of the electrons in the valence band can gain enough thermal energy to break free and move into the conduction band. This is known as thermal excitation and is a common way in which electrical conductivity increases with temperature.
- Electromagnetic radiation: Electromagnetic radiation, such as light or other forms of radiation, can also excite electrons into the conduction band. When photons of energy equal to or greater than the bandgap energy of the semiconductor are absorbed by the material, electrons can be excited into the conduction band, creating electron-hole pairs.
- Doping: When a semiconductor material is doped with impurities, it creates excess electrons or holes in the material, which can contribute to electrical conductivity. In an n-type semiconductor, for example, doping with a donor atom such as phosphorus creates excess electrons in the material that can move into the conduction band and contribute to electrical conductivity.
- Electric field: An external electric field can also excite electrons into the conduction band. When a voltage is applied across a semiconductor material, it can create an electric field that accelerates electrons and holes in opposite directions, causing some of the electrons to move into the conduction band.
Overall, the excitation of electrons into the conduction band is a fundamental process in the operation of semiconductor devices and is essential for the flow of electric current through the material.
p-n Junction
When a semiconductor is doped with impurities, it creates excess electrons (n-type doping) or holes (p-type doping) in the material, which can carry electrical charge. These excess electrons or holes can move around the material, allowing for the flow of electric current.
When two differently doped regions of a semiconductor material are brought together, a p-n junction is formed. At the p-n junction, the excess electrons from the n-type region and the holes from the p-type region diffuse across the junction and combine, creating a region that is depleted of charge carriers called the depletion region.
The p–n junction possesses a useful property for modern semiconductor electronics. A p-doped semiconductor is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them can become depleted of charge carriers, and hence non-conductive, depending on the relative voltages of the two semiconductor regions. By manipulating this non-conductive layer, p–n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction.
Bias is the application of a voltage relative to a p–n junction region:
- Forward bias. When a voltage is applied across the p-n junction in the forward bias direction (i.e., the positive terminal is connected to the p-type region and the negative terminal to the n-type region), the depletion region becomes narrower and allows the flow of current through the material.
- Reverse bias. In the reverse bias direction (i.e., the positive terminal is connected to the n-type region and the negative terminal to the p-type region), the depletion region becomes wider, preventing the flow of current through the material. However, if the reverse voltage is increased to a certain threshold value, the material can undergo a process called avalanche breakdown, in which the depletion region suddenly collapses and allows a large amount of current to flow through the material.
The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.
Bipolar Junction Transistor
A bipolar junction transistor (BJT) is a three-terminal electronic device that can amplify or switch electronic signals. It is made by joining three layers of semiconductor material together: an n-type layer, a p-type layer, and another n-type layer (for an NPN transistor) or a p-type layer (for a PNP transistor).
The three regions of the BJT are called the emitter, base, and collector. The base is located between the emitter and the collector and is made very thin to allow for easy flow of charge carriers from the emitter to the collector.
The BJT operates by controlling the flow of charge carriers (electrons or holes) from the emitter to the collector using a small current at the base. When a small current is applied to the base, it changes the voltage across the base-emitter junction, which allows a larger current to flow from the emitter to the collector.
BJTs can be used as amplifiers or switches, depending on how they are configured in a circuit. In an amplifier circuit, a small input signal is applied to the base, and the BJT amplifies it to a larger output signal at the collector. In a switch circuit, the BJT is either fully on or fully off, depending on the voltage applied to the base.
BJTs are widely used in electronic applications, such as audio amplifiers, radio receivers, and digital logic circuits. They are preferred in low-voltage, low-power applications, where their high current gain and fast switching speeds make them a popular choice.
Materials for Semiconductors
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 Semiconductors
The theory of semiconductors 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.
