Certain substances like germanium (Ge), silicon (Si), selenium (Se), carbon (C) etc. are neither good conductors like copper nor perfect insulator like mica. In other words, resistivity of these materials lie between conductors and insulators. These types of substances are classified as semiconductors. The materials whose electrical conductivity lies between those of conductors and insulators and have negative temperature coefficient of resistance are called as semiconductors. The resistivity of semiconductor lie roughly in the range of 10-4 ohm to 0.5 ohm at room temperature.
There are many alloys and elements hanimg resistivity in that range, but they cannot be regarded as semiconductors. In fact, semiconducto3rs have a number of other unexpected properties which distinguish them from conductors, insulators and resistance materials. Semiconductors are so named in because they fall between insulators and and conductors on the conductivity scale. While there is no sharp dividing line, they typically have a resistivity of the order of 10-4 ohm to 10 ohm. The dilectric constant is in the range of 5 to 50.