Selenium is a semiconductor used in photocopying machines. what type of semiconductor would be formed if a small amount of gallium impurity is added to pure selenium?

Out of the following given choices;

(A) substitutional type

(B) p-type

(C) n-type

(D) None of these

The answer is B. The gallium (a pentavalent dopant) is an impurity that will donate 3 valence electrons to the selenium semiconductor, therefore leaving one excess hole. This is because selenium requires 4 valence electrons to form covalent bonds with neighboring atoms. The ‘holes’ will, therefore, more than the electrons in the semiconductor, and will be the majority carriers of charge in the semiconductor. Doping is used to create small circuits in the selenium semiconductor.