What is a semiconductor?


A semiconductor is a material that behaves in between a conductor and an insulator. At ambient temperature, it conducts electricity more easily than an insulator, but less readily than a conductor.  At very low temperatures, pure or intrinsic semiconductors behave like insulators. At higher temperatures though or under light, intrinsic semiconductors can become conductive.  The addition of impurities to a pure semiconductor can also increase  its conductivity.


Examples of semiconductors include chemical elements and compounds such as silicon, germanium, and gallium arsenide. The conductivity of a semiconductor increases with temperature, light, or the addition of impurities because these increase the number of conductive valence electrons of the semiconductor. Valence or outer electrons are the carriers of the electrical current.


In an intrinsic semiconductor such as silicon, the valence electrons of an atom are paired and shared with other atoms, making covalent bonds that hold the crystal together. Under such circumstances, these valence electrons are not free to move around as electrical current. Temperature or light excites the valence electrons out of these bonds, freeing them to conduct current. The vacant positions left behind by the freed electrons, also known as holes, can move around as well, contributing to the flow of electricity. The energy needed to excite the electron and hole is known as the energy gap.


Doping is the process of adding impurities to an intrinsic semiconductor to increase its ability to conduct electricity. The difference in the number of valence electrons between the doping material, or dopant, and host semiconductor results in negative (n-type) or positive (p-type) carriers of electricity. The dopant is known as an acceptor atom if it 'accepts' an electron from the semiconductor atom.  It is known as a donor atom if it 'donates' an electron to the semiconductor atom.


For example, a silicon atom has four valence electrons, two of which are required to form a covalent bond. In n- type silicon, donor atoms such as phosphorus (P), with five valence electrons, replace some silicon and provide extra negative electrons. In p-type silicon, acceptor atoms with three valence electrons such as aluminum (Al) lead to an absence of an electron, or hole, which acts like a  positive electron. The extra electrons or holes conduct electricity.


When a p-type semiconductor region is placed adjacent to an n-type region, they form a diode, and the region of contact is called a p-n junction. A diode is a two-terminal device that conducts current in only one direction. Combinations of such junctions are used to make transistors and other semiconductor devices whose electrical behavior can be controlled by the appropriate electrical stimuli.  The result of combining many transistors and other active components along with passive ones on a single chip of silicon is the integrated circuit, a complex electronic device designed to perform certain functions depending on the controlling signals.   


See Also:  p-n JunctionDiodeBipolar Transistor;  MOSFETJFETIC Manufacturing




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