PROPERTIES AND USES OF THERMISTORS 



177 



resistivity obeys equation (1). As the concentration of a particular im- 

 purity increases, the conductivity increases and the impurity conductivity 

 predominates to higher temperatures. Some impurities are much more 

 effective in increasing the conductivity than others. One hundred parts 

 per million of some impurities may increase the conductivity of pure silicon 

 at room temperature by a factor of 10^ Other impurities may be present 



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310 



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o 



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TEMPERATURE °K 



Fig. 4. — Logarithm of the conductivity of various specimens of silicon as a function 

 of inverse absolute temperature. The conductivity increases with the amount of im- 

 purity. 



in 10,000 parts per million and have a small effect on the conductivity. 

 Two samples may contain the same concentration of an impurity and still 

 differ greatly in their low temperature conductivity; if the impurity is in 

 solid solution, i.e., atomically dispersed, the effect is great; if the impurity 

 is segregated in atomically large particles, the effect is small. Since heat 

 treatments affect the dispersion of impurities in solids, the conductivity of 

 semiconductors may frequently be altered radically by heat treatment. 

 Some other semiconductors are not greatly affected by heat treatment. 



