(a) Silicon has four electrons in its outer shell \left(3 s^{2} 3 p^{2}\right), but gallium has three \left(4 s^{2} 4 p^{1}\right). With this electron deficiency, silicon doped with gallium should be a p-type semiconductor. (b) Now silicon is being doped with antimony, which has an outer shell 5 s^{2} 5 p^{3}. There is an extra electron, making this an n-type semiconductor.