Barium titanate is based and doped with other polycrystalline ceramic materials, which has low resistance and semiconducting characteristics. This is achieved by purposefully doping a chemically expensive material as a lattice element of the crystal: a portion of the barium ion or titanate ion in the lattice is replaced by a higher valence ion, thus obtaining a certain number of conductive free electrons.
For the PTC thermistor effect, that is, the reason for the step increase in resistance value, is that the material structure is composed of many small crystallites, forming a barrier at the interface of the grain, the so-called grain boundary (grain boundary), preventing electrons from crossing the boundary into the adjacent region, thus producing high resistance. This effect is counteracted when the temperature is low; The high dielectric constant and spontaneous polarization strength on the grain boundary hinder the formation of the barrier at low temperatures and allow electrons to flow freely. At high temperatures, the dielectric constant and polarization strength are greatly reduced, resulting in a large increase in barrier and resistance, showing a strong PTC effect.
PTC thermistors are sensitive components with early development, many types and mature development. PTC thermistors are composed of semiconductor ceramic materials and use the principle that temperature-induced resistance changes. If the concentrations of electrons and holes are n and p, and the mobility is μn and μp, respectively, the conductance of the semiconductor is:
σ=q（nμn+pμp）
Because n, p, μn, and μp are all functions of temperature T, conductance is a function of temperature, so the temperature can be deduced from measuring conductance, and a resistance-temperature characteristic curve can be made. This is how semiconductor thermistors work.
