Work Step by Step
To serve as a donor impurity in silicon or Ge, an element should have 5 valence shell electrons. Of these, 4 will be occupied making the usual bonds that Si/Ge would make, while the extra serves as a mobile negative charge carrier. To serve as an acceptor impurity in silicon or Ge, an element should have 3 valence shell electrons. This lack of an electron compared to Si/Ge produces a “hole”, which acts as a mobile positive charge carrier.