A semiconductor is a substance that possesses an electrical conductivity between that of an insulator (for example, plastic) and a conductor (for example, iron). The resistivity of these materials decreases as their temperature increases; metals act in the opposite manner. Their conducting properties can be changed in useful manners by placing impurities (the process is called doping) into materials’ crystal structures. When two varied doped sections are in the same crystal structure, a semiconductor junction is made. The characteristics of charge carriers, like electrons, electron holes, and ions, at the junction are the foundation of transistors, diodes, and most electronic components.
Some important examples of semiconductor materials are germanium, silicon, gallium arsenide, and element, existing close to the “metalloid staircase” section of the periodic table. Silicon is the most used semiconductor, and gallium arsenide is the second most widely used semiconductor and is employed in solar cells, laser diodes, microwave-frequency ICs, etc. Chip shortage is mainly due to the lack of industrially processed silicon.
Semiconductor devices display a wide range of properties, such as showing variable resistivity, passing electric current more smoothly in one orientation than the other, and possessing sensitivity to heat or light. As the electrical characteristics of a semiconductor can be altered by doping and by applying light or electrical fields, semiconductor devices can be used for energy conversion, switching, and amplification. The two types of semiconductors are intrinsic and extrinsic semiconductors (they are classified as per doping nature). The silicon’s conductivity is extended by adding a tiny amount of pentavalent (arsenic, antimony, or phosphorous) or trivalent (indium, boron, or gallium) atoms. This method is called doping, and the created semiconductors are called extrinsic or doped semiconductors. Apart from the method of doping, the semiconductor conductivity can be enhanced by increasing the temperature. This is different to a metal’s behaviour, in which conductivity ascends with the rise in temperature.
The current understanding of a semiconductor’s properties depends on the quantum mechanics to describe the motion of charge carriers inside a crystal lattice. The process of doping drastically increases the count of charge carriers linked to the crystal. If a doped semiconductor has free holes, it is known as “p-type”, and when it has free electrons, it is called n-type. The materials employed in electronic tools are doped under precise parameters to regulate the regions and concentration of n and p-type dopants. One single semiconductor crystal can possess many n and p-type regions; the junctions between these sections are responsible for the useful electronic properties. By using a hot-point probe, we can easily analyse whether a semiconductor material is n or p-type.
Doping a semiconductor can be achieved in many methods, including injecting impurities into superhot semiconductors, striking semiconductors with dopant atoms, and heating semiconducting materials in a region with dopant atoms. There are two forms of dopants that can be imposed into a semiconductor. This results in two forms of extrinsic semiconductors.
- n-type: Pentavalent impurities (for example, P and As) are added, which places an extra valence electron that needs relatively less energy to conduct current. Adding pentavalent impurity puts a new energy range (known as donor levels) close to the conduction band in the energy band figure.
- P-type: Trivalent impurities (for example, In and B) are injected, which places an extra hole that needs relatively less energy for conduction. Adding trivalent impurity puts a new energy range (known as acceptor levels) close to the valence band in the energy band figure.
An intrinsic semiconductor, also known as an undoped semiconductor (i-type semiconductor), is a pure semiconductor without any traceable dopant species present. The count of charge carriers is thus determined by the characteristics of the substance itself instead of the count of impurities. In these semiconductors, the number of holes and excited electrons is equal: p = n. This could be the scenario even after doping the material, though only if it is injected with both acceptors and donors equally. In this scenario, p = n still stands, and the semiconductor material stays intrinsic, even though it is doped. The physics of semiconductors shows that some conductors are both extrinsic and intrinsic, but only if p (electron acceptor vacant/dopant holes that behaves as positive charges) is equal to n (electron donor excited/dopant electrons).
Important Applications of Semiconductors
Half-Wave Rectifier: It produces a direct current (DC). But, it possesses intermittent intensity at half a cycle, and it vanishes at the remaining half.
Full-Wave Rectifier: It transforms alternating current (AC) to direct current (DC). It is utilised in solar cells and many battery-powered applications, as it is only able to produce continuous currents.
Varactor Diode: This electronic component is used to adjust the frequency of an electric circuit’s resonance. Also, it is applied to generate a phase-locked loop (a control system that produces an output whose phase is connected to an input signal’s phase). It is used in cell phones, radio, television, and different transmitters.
Photodiode: In this device, silicon absorbs the photon energy of the incident light to produce additional pairs of gaps and electrons, causing a drastic change in the intensity of the current. It is used digital cameras, solar cells, street lighting lamps, smartphone light sensors as well as automatic doors.
LED (Light Emitting Diode): It is characterised by small size, quick response, and much better lifespan. The light spreads uniformly and is utilised as an alternative to tungsten and fluorescent lamps.