Difference Between Metals, Semi Conductors and Insulators
Bulk materials, as well as atoms and molecules, are made up of positive and negative charges. These charges appear in equal amounts, and therefore matter normally is electrically neutral—that is, it has no excess positive or negative charge. However, there are numerous mechanisms by which matter can gain an excess positive or negative charge. The simplest and most familiar case is the ionization of atoms or molecules. This process is possible because only a low energy is required to remove an electron from or add an electron to the molecule, forming a positive or negative ion. Similarly, electrons can easily be added to or removed from bulk matter to give it a net charge.
Each atom in a metal gives up one or more electrons, thereby forming a pool of free electrons that move randomly within the metal. When under the influence of an externally applied force—an electric field—these electrons drift in an orderly fashion from one end of the metal to the other, and the metal provides little resistance to the flow of charge. Thus metal wires are a key component of an electric circuit through which current flows.
Semiconductors also allow current to flow, and semiconductor devices, such as diodes, have properties that enable currents to flow in one direction more easily than in the opposite direction. This and other semiconductor properties give rise to a host of applications in electronics.
An insulator, or dielectric material, has virtually no freely moving electrons. In a dielectric material, an external electric field cannot cause charges to drift through the material as in metals, but there can be a small displacement of the electrons in one direction with respect to the positive nuclei of the molecules. This relative displacement, which is called polarization of the material, is important in many electrical phenomena.