Why is carbon not a semiconductor given that it is in the fourth group of the periodic table?
Carbon is a non-metal, and non-metals are poor conductors of electricity because the bond structure is a "close-packed arrangement." Silicon and germanium which are also in Group IVA are semi-conductors and are classified as metalloids. Metalloids show properties of both metals and non-metals. The have low electrical conductivity compared to metals but conduct more than non-metals. This property makes them useful in small scale electronics.
It's a general rule that elements within the same group have similar properties, however some periodic trends show more of a diagonal pattern. Fluorine, near the upper right of the periodic table, is the least metallic element and Francium, on the bottom left, is the most metallic. This is because trends related to electron configuration are influenced not only by valance electrons but also by shielding by inner electrons which increases going down in a group. The metalloids fall along a diagonal stair-step separating metals from non-metals. The image below shows the metalloids in yellow.