In Electrical Engineering the term insulator refers to any material that doesn’t conduct an electric current under normal conditions. While the term conductor refers to materials that easily conduct electrical current. Besides conductors and insulators, there is another class of materials whose ability to conduct electrical current between conductors and insulators.
Semiconductors themselves are generally poor conductors of electricity. However, the conductivity of semiconductors can be increased by adding impurities. The process of controlled addition of impurities to semiconductors is known as doping.
The below video by MIT OpenCourseWare illustrates the Doping process in detail: