178 



BELL SYSTEM TECHNICAL JOURNAL 



The impurity need not even be a foreign element; in the case of oxides or 

 sulphides, it can be an excess or a deficiency of oxygen or sulphur from the 

 exact stoichiometric relation. This excess or deficiency can be brought 

 about by heat treatment. Figure 5 shows how the conductivity depends 

 on temperature for a number of samples of cuprous oxide, CU2O, heat 



ID' 



1.0- 

 o 



^io-« 



I- 

 y 



8io- 



tCT^°i 



xlQ- 



temperature: °k 



Fig. 5. — Logarithm of the conductivity of various specimens of cuprous oxide as a 

 function of inverse absolute temperature. The conductivity increases with the amount 

 of excess oxygen above the stoichiometric value in CuoO. Data from reference 1. 



treated in such a way as to result in varying amounts of excess oxygen from 

 zero to about one per cent.' The greater the amount of excess o.xygen the 

 greater is the conductivity in the low temperature range. At high tem- 

 peratures, all samples have about the same conductivity. 



Semiconductors can be classified on the basis of the carriers of the current 

 into ionic, electronic, and mixed conductors. Chlorides such as NaCl and 

 some sulphides are ionic semiconductors; other sulphides and a few oxides 



