Which of the following is a trivalent doping element?

The correct answer is C, Boron. Boron has three electrons in its outermost shell, so it is a trivalent element and is used as a p-type dopant. When a trivalent atom such as boron, aluminium, gallium or indium is added to silicon or germanium, it can form only three covalent bonds with its four neighbours, so one bond is left incomplete. That empty place is called a hole, it behaves like a positive charge carrier, and the doped crystal becomes a p-type semiconductor in which holes are the majority carriers. Adding a pentavalent atom instead leaves a free electron and gives an n-type semiconductor. Option A is wrong because arsenic has five valence electrons and is pentavalent. Option B is wrong because antimony is also pentavalent and is used for n-type doping. Option D is wrong because phosphorus has five valence electrons and donates an electron to the crystal. Exam tip: trivalent dopants such as boron, aluminium, gallium and indium give p-type material, while pentavalent ones such as phosphorus, arsenic and antimony give n-type.