PROPERTIES AND USES OF THERMISTORS 



179 



such as uranium oiide are mixed semiconductors; electronic semiconductors 

 include most oxides such as MnsOs, FejOs, NiO, carbides such as silicon 

 carbide, and elements such as boron, silicon, germanium and tellurium. 

 In ionic and mixed conductors, ions are transported through the solid. 

 This changes the density of carriers in various regions, and thus changes 

 the conductivity. Because this is undesirable, they are rarely used in mak- 

 ing thermistors, and hence we will concentrate our interest on electronic 

 semiconductors. 



The theoretical and experimental physicists have established that there 

 are two types of electronic semiconductors which can be called N and P 

 type, depending upon whether the carriers are negative electrons or are 

 equivalent to positive "holes" in the filled energy band. In N type, the 



ACCEPTOR 

 M PURITIES 



INTRINSIC 



Fig. 6. — Schematic energy level diagrams illustrating intrinsic, N and P types of semi- 

 conductors. 



carriers are deflected by a magnetic field as negatively charged particles 

 would be and conversely for P type. The direction of deflections is ascer- 

 tained by measurement of the sign of the Hall effect. The direction of the 

 thermoelectric effect also fixes the sign of the carriers. By determining 

 the resistivity, Hall coefficient and therm.oelectric power of a particular 

 specimen at a particular temperature it is possible to determine the density 

 of carriers, whether they are negative or positive, and their mobility or mean 

 free path. The mobility is the mean drift velocity in a field of one volt per 

 centimeter. 



The existence of these classifications is explained by the theoretical physi- 

 cist^ . 3 , 4 j^ terms of the diagrams in Fig. 6. In an intrinsic semiconductor 

 at low temperatures the valence electrons completely fill all the allowable 

 energy states. According to the exclusion principle only one electron can 

 occupy a particular energy state in any system. In semiconductors and 



