:PROPERTIES: :ID: dd39c4e7-850b-4923-ba1e-225ce565371d :mtime: 20231027035327 :ctime: 20231027035326 :END: #+title: transistor #+filetags: :public:project: * Semi-conductor Since there are many free electrons in a metal, it is difficult to control its electrical properties • Consequently, what we need is a material with a low free electron density, i.e., a semiconductor, e.g., Silicon • By carefully controlling the free electron density we can create a whole range of electronic devices * Doping ** N-type doping n-type silicon (Group IV) is doped with arsenic (Group V) that has an additional electron that is not involved in the bonds to the neighbouring Si atoms The additional electron needs only a little energy to move into the conduction band. This electron is free to move around the lattice Owing to its negative charge carriers (free electrons), the resulting semiconductor is known as n-type Arsenic is known as a donor since it donates an electron ** P-type doping p-type silicon (Group IV) is doped with boron (B, Group III) The B atom has only 3 valence electrons, it accepts an extra electron from one of the adjacent Si atoms to complete its covalent bonds This leaves a hole (i.e., absence of a valence electron) in the lattice This hole is free to move in the lattice – actually it is the electrons that do the shifting, but the result is that the hole is shuffled from atom to atom Owing to its positive charge carriers (free holes), the resulting semiconductor is known as p-type B is known as an acceptor