Semiconductor

Almost every electronics devices (mostly active components) ranging from transistor to ICs are all made of semiconductor. Semiconductor is one of the most important Electronics engineering material ever, which revolutionized  the modern world of Electronics. semiconductor devices have found wide application because of their compactness,reliability, power efficiency and low cost . They are also used in power devices ,optical sensors , light emitters(LED) and even solid state lasers.
        Resistivity of metals like iron and copper is 0.1 x  10^-6 Ω·m and  1.7 x   10^-8 Ω·m . The resistivity of  insulators like glass is 9 x 10^-11 Ω·m , whereas the resistivity of semiconductor substance  are between 10^-4   to 0.5  Ω·m . Thus those substance which have resistivity lower than insulators and higher than conductors are the semiconductors. 

Pure silicon crystal
(source : www.images-of-elements.com)

      Anyway, Resistivity is not the only factor which decide whether the substance is a semiconductor or not . There are other factors(properties) too which make a semiconductor crystal unique and quite useful in the field of electronics . like ...

• The resistivity of the semiconductor decreases with temperature an vice versa (semiconductor materials behaves as insulator at absolute zero (zero Fahrenheit))
• The second and the most important property is that if  suitable impurity are added (doped) to a semiconductor block like arsenic,gallium , phosphorous the current conducting property increases , giving rise to a new improved semiconductor block with high conductivity  "Extrinsic semiconductor "  

Extrinsic semiconductor are mainly of two kinds "the P-type" and the "the N-type" . The P-type semiconductor are created by doping impurities with valency 3 (trivalent) like boron,aluminium etc and N- type semiconductors are created by doping impurities with valency 5 (pentavalent) impurity like arsenic and phosphorous .
                       like in a silicon crystal with valency 4 when a trivalent impurity (impurity for p-type semiconductor) is doped  the 3 outer electron (valence electron) of silicon crystal form a covalent bond with the 3 electron of the trivalent impurity . But in the fourth covalent bond only silicon atom contributes one valence electron and there is a deficiency of electron which is called a hole .Holes are positively charged and are the prime charge carrier in P-type semiconductor.
                               Whereas when a pentavalent(valency 5) impurity are doped the four atoms of silicon forms a covalent bond with the 4 valence electron of impurity leaving behind one free electron . These free  Electrons are the prime charge carrier of N-type semiconductor.


N-type semiconductor is more preferred then P-type as the mobility (drift velocity to applied electric field across material) of electrons ( prime charge carrier of N-type) is greater than that of holes (charge carrier of P-type).