Semiconductor ceramics are a type of ceramic material with semiconductor properties. Their basic principle is based on the crystal structure inside the ceramics, the electron conduction mechanism, and the influence of impurities and defects. The following is a detailed introduction to its basic principles and applications for you:

Basic principle

Crystal structure and band theory

The crystal structure of semiconductor ceramics is usually formed by the combination of metal ions, oxygen ions, etc. through ionic or covalent bonds. In an ideal crystal structure, atoms are arranged in a certain pattern to form a lattice.

According to the band theory, electrons in a crystal have different energy states, forming a series of bands. Among them, the valence band is a low-energy band filled with electrons, and the conduction band is a high-energy band where electrons can move freely. In insulators, there is a large bandgap width (Eg > 3eV) between the valence band and the conduction band, making it difficult for electrons to transition from the valence band to the conduction band. Therefore, the electrical conductivity is very poor. In semiconductor ceramics, the bandgap width is relatively narrow (generally 0.1-3eV). Under certain conditions (such as temperature rise, light exposure, application of an electric field, etc.), electrons in the valence band can obtain sufficient energy to transition to the conduction band, thereby generating conductive carriers (electrons and holes), and endowing the material with a certain degree of electrical conductivity

2. The influence of impurities and defects

Impurity doping: Introducing specific impurity atoms into semiconductor ceramics is an important means to change their electrical properties. For example, when aluminum (Al) atoms are doped in zinc oxide (ZnO) ceramics, the Al atoms replace some of the positions of the Zn atoms. Because the number of valence electrons in Al atoms is less than that in Zn atoms, holes will be generated in the crystal. These holes can act as conductive carriers, thereby enhancing the electrical conductivity of the material. This doping method is called acceptor doping. Conversely, if the number of valence electrons of the doped impurity atoms is greater than that of the matrix atoms, such as doping gallium (Ga) in ZnO, additional electrons will be introduced, increasing the electron carrier concentration, which is called donor doping. By controlling the type and concentration of impurities, the conductivity type (N-type or P-type) and electrical conductivity of semiconductor ceramics can be precisely regulated.
Defect formation: In semiconductor ceramics, there are also various crystal defects, such as vacancies (vacancies formed by the absence of atoms), interstitial atoms (atoms occupying lattice interstitial positions), etc. These defects will introduce additional energy levels in the band gap, affecting the electron transition and electrical conductivity. For instance, oxygen vacancies are common defects in some oxide-semiconductor ceramics. They can act as donors to provide electrons, increase the concentration of electron carriers, and thereby alter the electrical properties of the material.

3. Grain boundary effect
Semiconductor ceramics are usually polycrystalline materials, composed of many tiny grains, with grain boundaries existing between the grains. The atomic arrangement at the grain boundaries is irregular, and there are many defects and impurity accumulations. Grain boundaries have a significant impact on the transport of electrons and can act as potential barriers to prevent the movement of electrons. When electrons approach grain boundaries, due to the traps and potential barriers at the grain boundaries, electrons need to overcome a certain amount of energy to pass through, which causes changes in the electron concentration near the grain boundaries and subsequently affects the electrical properties of the entire material. By controlling the composition, structure and properties of grain boundaries (such as by adding grain boundary modifiers, etc.), the height and width of grain boundary barriers can be adjusted, thereby regulating the electrical conductivity, dielectric properties, etc. of semiconductor ceramics.

Applications of Semiconductor Ceramics
Sensitive element

◦ Thermistor: It is made by taking advantage of the property that the resistance of semiconductor ceramics changes with temperature. For instance, the commonly used materials for negative temperature coefficient (NTC) thermistors include oxide semiconductor ceramics of manganese (Mn), cobalt (Co), nickel (Ni), etc. When the temperature rises, the carrier concentration inside the ceramic increases and the resistance decreases. This characteristic makes it widely used in fields such as temperature measurement, temperature control and temperature compensation, for instance, in the temperature sensors of automotive engines and the temperature detection components of air conditioners.

◦ Varistor: A varistor with zinc oxide (ZnO) as its main component is a typical semiconductor ceramic varistor element. Under normal voltage, its resistance is very high and it hardly conducts electricity. When the voltage exceeds a certain threshold, the resistance drops sharply and the current passing through increases rapidly, thereby playing an overvoltage protection role. It is commonly used at the power input end of electronic devices and for lightning protection in power systems, etc., to prevent equipment damage caused by momentary overvoltage.

◦ Gas-sensitive resistors: Some semiconductor ceramics have adsorption and reaction properties for specific gases, thereby altering their electrical performance. For instance, tin dioxide (SnO₂) ceramics are sensitive to reducing gases such as carbon monoxide and hydrogen. When these gases are present in the environment, they adsorb on the ceramic surface and undergo chemical reactions, causing changes in the electron concentration on the ceramic surface and variations in resistance. By detecting these resistance changes, the detection and monitoring of these gases can be achieved. It is widely applied in fields such as industrial waste gas emission monitoring and household gas leakage alarms.

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2. Capacitor

• Multilayer ceramic capacitors (MLCCS) : Some semiconductor ceramics have a relatively high dielectric constant, such as barium titanate (BaTiO₃) -based ceramics. By fabricating ceramic materials into a multi-layer structure with metal electrodes sandwiched in the middle, the capacitance value can be greatly increased. MLCC is small in size, has a large capacitance and a low equivalent series resistance. It is widely used in various electronic devices, such as mobile phones, computers and tablet computers, for circuit functions like filtering, coupling and bypassing, ensuring the stable operation of the circuit.

3. Semiconductor ceramic heating elements
Some semiconductor ceramics can generate heat under the action of an electric field and have good electrothermal conversion performance. For instance, silicon carbide (SiC) ceramic heating elements have a moderate resistivity, good high-temperature resistance and oxidation resistance. In heating equipment such as industrial electric furnaces, ceramic kilns, and household electric heaters, electrical energy is converted into thermal energy through the current passing through the semiconductor ceramic heating element to achieve the heating function.
4. Application of Semiconductor Ceramic Sensors in Biomedicine
In the field of biomedical detection, biosensors can be fabricated by taking advantage of the gas-sensitive or pressure-sensitive properties of semiconductor ceramics. For example, specific components in the exhaled gas of living organisms are detected based on the gas-sensitive characteristics of semiconductor ceramics for disease diagnosis. The concentration changes of certain markers in exhaled gas (such as specific volatile organic compounds) may be related to diseases. Detecting the concentrations of these markers through semiconductor ceramic gas-sensitive sensors provides a basis for the early diagnosis of diseases. In addition, by taking advantage of the pressure-sensitive properties of semiconductor ceramics, sensors can be fabricated to detect pressure changes within living organisms. For instance, in cardiovascular system monitoring, they can monitor pressure changes within blood vessels and assist in the diagnosis of cardiovascular diseases.