Semiconductor A Semiconductor is a substance which has resistivity in between conductors and insulators, eg:Germanium silicon, selemium,carban,etc. * It is interesting to note it is not just the resistivity alone that decides whetnes a substance is semiconductor or not.for eg, it is just possible to prepare an alloy whose resistivity falls withim the range of Semiconductor but the alloycannot be regarded as a Semiconductor. Infect, Semiconductor have a mo. Of peculiar properties which distinguish them from conductors, insulators and resistance materials. Properties of Semi conductors * The resistively of a semiconductors is les than an insulater but more than a conductors. * Semi conductors have negative temperature coefficient of resistance i.e. the resistance of a semiconductors decrease with the increase in temperature and vice-versa. When a suitable metallic impurity leg arsenic, gallium, etc is added to a semiconductor, its current conducting properties change appreciably. Classification of semiconductors Semiconductors are classified as:(i) intrinsic(pure) Semiconductors (ii) Extrinsic (impure) Semiconductors An intrinsic Semiconductors A pure semiconductor is called an intrinsic semiconductor. Two of the most important semiconductors are silicon and germanium and both belong to the IV column of the periodic table.Ts means that these have 4-valence elecmns in the outer orbit (tetravalent).The crystalline structure of both silicon and germanium is the tetrahedral pattern as snoum below Thus, each of the 4values electrons of an atom are shared by 4 neighboring atoms to form 4 covalent as shown above. At absolute zero temperature i.e. at ok,all the covalent bonds are complete. Hence no free electron is available for the conduction of current. At room temperature the valence electron acquires energy greater than the energy binding it to the nucleus. So the valence electron leaves the atom and as a result, a covalent bond is broken. The electron which leaves the atoms is called free electron and the vacancy created in the covalent bond due to release of electron is called a hole (i.e. deficiency of an electron).the hole is equivalent to a the charge i.e. This hole can be filled by an electron from the neighboring covalent bond where another hole is created. This process of movement of valence electron or free electron from one bond to another bond continues and hence holes also move throughout the semiconductor as shown below:- Si Si Si Si Si Si Si Si Si Silicon crystal with one covalent bond broken Thus,in a semiconductor,these are 2 kinds of charge carries i.e.free electron (-e) and hole (te) and the semiconductor in which no.of free electron per (ne) unit volume is equal to the no of holes per unit volume (Nh) is called an intrinsic semiconductor. i.e. In case of intrinsic semiconductor, ne = nh =ni No.of electron /m3 intrinsic carries no. of holes/m3 And In case of extrinsic semiconductors, ne = nh (ii) Extrinsic Semiconductors (Impure)(Doped Semicond) Intrinsic (pure) semiconductor has thermally generated current carries (holes and electron).These current carries are small in no. at room temperature and hence the conductivity of an intrinsic semiconductor is low. If temperature of intrinsic conductor is increased considerably to have more and more change carries to increase its conductivity, it may get damaged. So to increase the conductivity of an intrinsic semiconductor, some suitable impurities can be added in it which increases the conductivity of the semiconductor. This process of adding impurities in the intrinsic semiconductor is called Doping and the semiconductor obtained after adding impurities is called Doped/Extrinsic/Impure semiconductor. Type of extrinsic semiconductors. Depending upon the type of impurity added in the intrinsic semiconductorl, extrinsic semiconductor is classified into 2 categorys:1.p-type semiconductor 2.n-typesemiconductor 1.p-type semiconductorWhen a small amount of trivalent impurity having 3 valence electron like gallium(Ga),aluminium(Al),Boom(B),etc is added to a pure/intrinsic semiconductor. The addition o trivalent impurity provides a large no.of holes in the semiconductor. Such impurity which produce ptype semiconductor are known as accepts impurities because the holes created can accept the electron. Explainationof formation of p-type semiconductor consider a pure germanium crystal. When a small amount of trivalent impurity like gallium is added to a germanium crystal,there exists a large no.of holes in the crystal. This is because gallium is trivalent i.e.its atom has 3 valence electrons.Each atom of gallium fits into germanium crystal but now only 3 covalent bonds can formed. In the 4th covalent covalent bond, only germanium atom contributes one valence electrons which gallium has no valence electron to contribute as all its there valence electron bond are already engaged in the covalent bonds with the neighboring germanium atoms. In other words, fourth bond is incomplete; being short of are electron, this missing electron is called a hole. For each gallium atom added, one hole is created. A small amount gallium provides millions of hole. This can be further understood by the following fig: Ge Ge Ge Trivalent impurity atom Ge Ge Ge Hole Ge Ge Ge Germanium crystal forming a p-type semiconductor by adding Gallium (Ga) a trivalent impurity. The crystal of this type has an excess of holes or poritive charge carries and hence known as p-type semiconductor or positive type semiconductor. Thus, majority charge carries in p-type semiconductors are holes and the minority charge carriers are electrons which are thermally generated.
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