Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors.
The main characteristics of thermistors are: ① High sensitivity, with a resistance temperature coefficient 10 to 100 times greater than that of metals; ② Wide operating temperature range: room temperature devices are suitable for -55℃ to 315℃, high temperature devices are suitable for temperatures above 315℃ (currently reaching up to 2000℃), and low temperature devices are suitable for -273℃ to 55℃; ③ Small size, capable of measuring the temperature of gaps, cavities, and blood vessels in living organisms that other thermometers cannot measure; ④ Convenient to use, with resistance values  selectable arbitrarily between 0.1 and 100kΩ; ⑤ Easy to process into complex shapes, allowing for mass production; ⑥ Good stability and strong overload capacity.

Due to the unique properties of semiconductor thermistors, they can be used not only as measuring elements (such as measuring temperature, flow rate, and liquid level), but also as control elements (such as thermal switches and current limiters) and circuit compensation elements. Thermistors are widely used in various fields such as household appliances, power industry, communications, military science, and aerospace, with extremely broad development prospects.

First, PTC thermistors. PTC (Positive Temperature Coefficient) refers to the phenomenon or material where the resistance increases sharply at a certain temperature, exhibiting a positive temperature coefficient. It can be specifically used as a constant temperature sensor. This material is a sintered body with BaTiO3, SrTiO3, or PbTiO3 as the main components, incorporating trace amounts of oxides such as Nb, Ta, Bi, Sb, Y, and La to control atomic valence and achieve semiconductivity. This semiconductorized BaTiO3 material is often simply referred to as semiconductor ceramic. Simultaneously, oxides of Mn, Fe, Cu, and Cr are added to increase its positive temperature coefficient of resistance, along with other additives. It is formed using general ceramic processes and sintered at high temperatures to semiconduct platinum titanate and its solid solution, thus obtaining a thermistor material with positive characteristics. Its temperature coefficient and Curie temperature vary with the composition and sintering conditions (especially the cooling temperature).

Barium titanate crystals have a perovskite structure and are ferroelectric materials; pure barium titanate is an insulating material. Adding trace amounts of rare earth elements to barium titanate and subjecting it to appropriate heat treatment causes a sharp increase in resistivity by several orders of magnitude near the Curie temperature, producing the PTC effect. This effect is related to the ferroelectricity of BaTiO3 crystals and the phase transition of the material near the Curie temperature. Barium titanate semiconductor ceramics are polycrystalline materials with grain boundaries between the grains. When this semiconductor ceramic reaches a certain temperature or voltage, the grain boundaries change, resulting in a rapid change in resistance.

The PTC effect of barium titanate semiconductor ceramics originates from the grain boundaries (intergranular boundaries). For conductive electrons, the grain boundaries act as a potential barrier. At low temperatures, due to the internal electric field of barium titanate, electrons easily overcome the barrier, resulting in low resistance. When the temperature rises to near the Curie point (i.e., the critical temperature), the internal electric field is disrupted, preventing conductive electrons from overcoming the barrier. This is equivalent to an increase in the potential barrier, leading to a sudden increase in resistance and the PTC effect. The physical models for the PTC effect in barium titanate semiconductor ceramics include the Heyward surface barrier model, the barium vacancy model by Daniels et al., and the superposition barrier model. These models offer reasonable explanations for the PTC effect from different perspectives.

PTC thermistors appeared in 1950, followed by PTC thermistors using barium titanate as the primary material in 1954. PTC thermistors are used industrially for temperature measurement and control, as well as for temperature detection and regulation in certain automotive parts. They are also widely used in civilian equipment, such as controlling the water temperature of instantaneous water heaters, air conditioners, and cold storage facilities, and for gas analysis and anemometers, utilizing their own heating properties.

Besides functioning as a heating element, the PTC thermistor also acts as a switch, combining the functions of a sensitive element, heater, and switch; hence, it is called a "thermal switch." When current flows through a component, it causes a temperature rise, i.e., the temperature of the heating element increases. When the Curie point temperature is exceeded, the resistance increases, thus limiting the increase in current. The decrease in current then leads to a decrease in the component temperature. The decrease in resistance, in turn, increases the circuit current, causing the component temperature to rise again, creating a cyclical process. Therefore, it has the function of maintaining the temperature within a specific range and also acts as a switch. This temperature-resistance characteristic is used to create heating sources, such as heaters, soldering irons, dryers, and air conditioners. It can also provide overheat protection for electrical appliances.

Second, NTC thermistors. NTC (Negative Temperature Coefficient) refers to the phenomenon and material of a thermistor whose resistance decreases exponentially with increasing temperature, exhibiting a negative temperature coefficient. This material is a semiconductor ceramic made by thoroughly mixing, molding, and sintering two or more metal oxides such as manganese, copper, silicon, cobalt, iron, nickel, and zinc. It can be used to make thermistors with a negative temperature coefficient (NTC). Its resistivity and material constant vary with the proportion of material components, sintering atmosphere, sintering temperature, and structural state. Currently, non-oxide NTC thermistor materials, such as silicon carbide, tin selenide, and tantalum nitride, have also emerged.

The development of NTC thermistors has undergone a long process. In 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has a negative temperature coefficient and successfully applied it to temperature compensation circuits in aerospace instruments. Subsequently, due to the continuous development of transistor technology, thermistor research made significant progress. In 1960, the NTC thermistor was developed and is widely used in temperature measurement, temperature control, and temperature compensation.
Its measurement range is generally -10 to +300℃, but can also be -200 to +10℃.

Thermistor thermometers can achieve an accuracy of 0.1℃, with a sensing time as short as 10 seconds. It is suitable not only for grain storage temperature measuring instruments but also for temperature measurement in food storage, medicine and health, scientific farming, oceans, deep wells, high altitudes, and glaciers.