It is, nevertheless, a high tribute to the skill and care ofmany experimenters that, in spite of this, semiconductors had been recognizedas a distinct class of substances and their main properties appreciated longbefore a comprehensive theory was available to account for them.
These effects are now known to be due to oxide films oractual gaps separating the individual crystals but led to the metals titaniumand zirconium once being listed as semiconductors.
It must be admitted, however, that no infallible criterionwas available till the quantum theory of solids gave an understanding of thereasons for the various properties observed.
4、A review of this early work has beengiven by K. Lark-Horowitz together with a very extensive bibliographycontaining over 350 references. Earlier reviews by B' Guddenalso deal extensively with this phase and discuss in some detail the problemsof identifying semiconductors.
5、The latter, which is generally known asthe carrier mobility, and which we shall define more precisely later generallytends to decrease as the temperature is raised, especially at the highertemperatures, and this accounts for the decrease in conductivity of metals withincreasing temperature.
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