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BELL SYSTEM TECHNICAL JOURNAL 



per centigrade degree. 6) The maximum permissible power that can be 

 dissipated consistent with good stability and long life, for continuous opera- 

 tion, and for surges. This can be computed from the dissipation constant 

 and the maximum permissible temperature rise. This and the resistance- 

 temperature relation determine the maximum decrease in resistance. 



Properties of Semiconductors 



As most thermistors are made of semiconductors it is important to discuss 

 the properties of the latter. A semiconductor may be defined as a substance 



io« 



10* 



2 

 I 



O . 

 I 10' 

 ill 

 o 

 z 

 ,< 



y,n-2 



KT' 



-100 



100 200 



TEMPERATURE °C 



300 



400 



Fig. 2. — Logarithm of specific resistance versus temperature for three thermistor ma- 

 terials as compared with platinum. 



whose electrical conductivity at or near room temperature is much less than 

 that of typical metals but much greater than that of typical insulators. 

 While no sharp boundaries exist between these classes of conductors, one 

 might say that semiconductors have specific resistances at room tempera- 

 ture from 0.1 to 10* ohm centimeters. Semiconductors usually have high h 

 negative temperature coefKicients of resistance. As the temperature is 

 increased from O^C. to 300°C., the resistance may decrease by a factor of a 

 thousand. Over this same temperature range the resistance of a typical 

 metal such as platinum will increase by a factor of two. Figure 2 shows 

 how the logarithm of the specific resistance, p, varies with temperature, T, 

 in degrees centigrade for three typical semiconductors and for platinum. 



