Semiconductor materials play an important role in modern science and technology. Common semiconductors are mainly silicon, germanium, gallium arsenide and cadmium sulfide.
Silicon is the most commonly used semiconductor material mainly because of its abundant resources, low cost and suitability for mass production. Silicon semiconductors play a key role in microelectronics, such as in computer chips and memories.
Germanium is another element semiconductor, which is similar to silicon but has some different characteristics. Although germanium has higher electrical conductivity than silicon, it is not as widely used as silicon, mainly because germanium is relatively expensive.
Gallium arsenide is a composite semiconductor composed of gallium and arsenic. It is particularly valuable in high frequency and optoelectronic devices. For example, gallium arsenide is used to make radio frequency integrated circuits, microwave amplifiers and high-performance solar cells.
Cadmium sulfide is also a composite semiconductor, which is composed of cadmium and sulfur. It is mainly used in photoelectric applications, such as solar cells and photodetectors.
These semiconductor materials are the core of modern electronic and optoelectronic devices. Different semiconductors are more popular in specific applications because of their specific properties.
Definition and Properties of Semiconductor
Introduction of conductivity
Semiconductor is a material between conductor and insulator, and its conductivity changes with temperature. When the temperature rises, the conductivity of semiconductors increases, which is contrary to the behavior of ordinary metal conductors. The conductivity of semiconductors comes from the electrons and holes inside them. Electrons are negatively charged, while holes can be regarded as positively charged. The increase of conductivity is due to the fact that more valence band electrons gain enough energy to transition to the conduction band with the increase of temperature, thus increasing the number of conductive electrons.
Band theory
In solid state physics, energy band theory is the basis of explaining the conductivity of semiconductors, conductors and insulators. It holds that electrons do not move freely in solids, but are confined to specific energy levels or bands. The gap between these energy bands is called band gap. For semiconductors, the band gap is small, and electrons can transition from valence band to conduction band under the action of certain external energy (such as heat energy), which leads to conductivity.
Intrinsic Semiconductor and Doped Semiconductor
Intrinsic semiconductors are pure semiconductors and do not contain any doped impurities. At room temperature, its conductivity mainly depends on the electron-hole pairs produced by thermal excitation. Doping semiconductor is to add trace impurity elements on the basis of semiconductor to change its conductivity. According to the types of impurities, doping can be divided into n-type and p-type. n-type semiconductor is to add pentavalent elements, such as phosphorus, arsenic, etc., so that it has an extra electron and increases the electron concentration. In p-type semiconductors, trivalent elements, such as boron and aluminum, are added to generate holes and increase the concentration of holes.
Elemental semiconductor
Silicon (Si)
Physical properties
Silicon is the most commonly used semiconductor material. It is a tetravalent element with a face-centered cubic lattice structure. Silicon has a melting point of 1414 ° C and a density of 2.33 g/cm. Its band gap is 1.12 eV, and it exhibits certain conductivity at room temperature, which makes it semiconductor industry has a wide range of applications.
The conductivity of silicon crystal can be controlled by external factors such as temperature and illumination. At room temperature, the resistivity is about 10 ^ 3 Ω · cm, but with the increase of temperature, the resistivity will gradually decrease. In addition, the thermal expansion coefficient of silicon is about 2.6 x 10-6/C, which is very important to ensure the stability of the chip in high temperature environment.
Main Applications
Silicon is the preferred material for making integrated circuits and solar cells. Due to its superior electrical properties, excellent thermal stability and relatively low cost, silicon occupies a dominant position in microelectronics industry. The most common applications are in CPU, memory, sensors and other devices. In the field of solar energy, polysilicon and monocrystalline silicon solar cells have a large market share because of their high conversion efficiency and cost-effectiveness.
Germanium (Ge)
Physical properties
Germanium is a kind of tetravalent semiconductor similar to silicon. Its lattice structure is rhombic and its band gap is 0.66 eV, which is slightly smaller than that of silicon, which makes germanium have better conductivity at low temperature. Germanium has a melting point of 938.3 ° C and a density of 5.32 g/cm. Its resistivity is 0.46 Ω · cm at room temperature, which is much lower than that of silicon, but its conductivity at high temperature is not as good as that of silicon.
Main Applications
Although the price of germanium is higher than that of silicon, it is indispensable in some specific applications, such as cryogenic detectors and infrared detectors, because of its superior conductivity at low temperature. In addition, germanium is also used as a substitute material for silicon to manufacture high-frequency electronic devices and optoelectronic devices. In the past, with the development of germanium-based silicon-germanium heterostructure technology, its application in high-performance computing and data storage has also been promoted.
Composite semiconductor
Gallium arsenide (GaAs)
Physical properties
Gallium arsenide is a kind of composite semiconductor, and its constituent elements are arsenic and gallium. The band gap of this semiconductor is 1.43 eV, which is larger than that of silicon and germanium, which makes it particularly attractive for high frequency and optoelectronic applications. Gallium arsenide has a density of 5.32 g/cm and a melting point of 1238 C. In addition, compared with silicon, GaAs has higher electron mobility, which makes it perform well in high frequency applications.
Main Applications
Gallium arsenide is widely used in high frequency and optoelectronic devices. It is an ideal material for producing RF integrated circuits, microwave frequency amplifiers and switches. In addition, due to its large band gap, it is also commonly used to make red and green LEDs. In the photovoltaic field, GaAs solar cells are appreciated for their high energy conversion efficiency and long life. However, compared with silicon, the price of GaAs is relatively high, which limits its use in some commercial applications.
Cadmium sulfide (CdS)
Physical properties
Cadmium sulfide is a kind of II-VI composite semiconductor, which is composed of cadmium and sulfur. It has a straight band gap, about 2.42 eV, which makes it very useful in the optoelectronic field. Cadmium sulfide has a density of 4.82 g/cm and a melting point of 1750 C. It has good chemical stability and excellent optical properties, especially for visible and ultraviolet light.
Main Applications
Cadmium sulfide is mainly used in optoelectronic fields, such as solar cells, photodetectors and photodiodes. It is also used as part of thin film transistors, which play a role in some display technologies. Because of its high sensitivity to ultraviolet rays, it is also used to make ultraviolet detectors. Although cadmium sulfide has many interesting applications, it contains toxic cadmium, which causes environmental and health concerns, so it needs to be used and disposed of with great care.
Organic semiconductor
Conductive polymer
Physical properties
Conducting polymer is a kind of organic polymer material with conductivity. Compared with traditional inorganic semiconductors such as silicon and gallium arsenide, conductive polymers usually have lower conductivity, but their advantages lie in processability, mechanical flexibility and customizable electronic properties. Common conductive polymers include polypyrrole, polybenzothiophene and polystyrene.
Main Applications
Conductive polymers are widely used in flexible electronics, sensors, organic photodiodes (OLED) and organic solar cells. Because of their processability and flexibility, they are often used to make curved or flexible electronic devices, such as flexible display screens or wearable sensors. In addition, conductive polymers also show great potential in biomedical fields, such as electrical stimulation therapy and biology sensor and other applications.
Small organic molecule
Physical properties
Small organic molecules are organic compounds with low molecular weight, which have adjustable electronic properties and good optical properties. Compared with conductive polymers, small organic molecules usually have better charge transport performance and thermal stability. Some common organic small molecule semiconductor materials include fullerenes, phthalocyanines and dibenzothiophenes.
Main Applications
Organic small molecules are widely used in organic field effect transistors, organic photodiodes and organic solar cells. Because of their good charge transport performance and adjustable optical properties, they are considered as ideal materials for manufacturing high performance and low cost organic electronic devices. In addition, small organic molecules are also commonly used in chemical sensors and biosensors, especially in applications requiring high sensitivity and fast response.
Organic field effect transistor
Physical properties
Organic field effect transistors (OFETs) are a kind of field effect transistors using organic semiconductor materials as the main channel. Compared with traditional silicon-based field effect transistors, OFETs usually have lower carrier mobility, but their manufacturing process is simple and cost cheap.
Main Applications
OFETs are widely used in flexible electronics, low-cost sensors and organic display technology. Because of its simple manufacturing process and low cost, OFETs are considered as the key components of the next generation of printable and flexible electronics products. In addition, OFETs are also used in biomedical sensors, especially in applications requiring high integration and real-time detection.
