Submitted By :G.Kavi Chandra Class: XII
Roll No : 19 Kendriya Vidyalaya No.2 Uppal
Certificate This is to certify that G.Kavi Chandra student of Class XII, Kendriya Vidyalaya No.2 Uppal, has completed the project titled SemiConductors during the academic year 2014-2015 and submitted satisfactory report, as compiled in the following pages, under my supervision. ‘‘
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I would like to express my special thanks of gratitude to my teacher Mr N.V.N.G.K Rao who gave me the golden opportunity to do this wonderful project on the topic SemiConductors, which also helped me in doing a lot of Research and i came to know about so many new things I am really thankful to them. Secondly i would also like to thank my parents and friends who helped me a lot in finalizing this project within the limited time frame.
Semiconductors :- Most of the solids can be placed in one of the two classes: Metals and insulators. insulators . Metals are those through which electric charge can easily flow, while insulators are those through which electric charge is difficult to flow. This distinction distinction between the metals and the insulators can be explained on the basis of the number of free electrons in them. Metals have a large number number of free electrons electrons which act as charge carriers, while insulators have practically no free electrons. There are however, certain solids whose electrical conductivity is intermediate between metals and insulators. They are called ‘Semiconductors’. Carbon, silicon and germanium are examples of semi-conductors. In semiconductors the outer most electrons are neither so rigidly bound with the atom as in an insulator, nor so loosely bound as in metal. At absolute absolute zero a semiconductor becomes an ideal insulator.
Theory and Definition Semiconductors are the materials whose electrical conductivity lies in between metals and insulator. The
energy band structure of the semiconductors is similar to the insulators but in their case, the size of the forbidden energy gap is much smaller than that of the insulator. In this class of crystals, the forbidden gap is of the order of about 1ev, and the two energy bands are distinctly separate with no overlapping. At absolute o0, no electron electron has any energy even even to jump the forbidden gap and reach the conduction band. Therefore the substance substance is an insulator. insulator. But when we heat the crystal and thus provide some energy to the atoms and their electrons, it becomes an easy matter for some electrons to jump the small ( 1 ev) energy gap and go to conduction band. Thus at higher temperatures, temperature s, the crystal becomes a conductors. conductors. This is the specific property of the crystal which is known as a semiconductor.
At 0K, all semiconductors are insulators. The valence band at absolute zero is completely filled and there are no free electrons in conduction band. At room temperature temperature the electrons jump to the conduction band due to the thermal energy. When the temperature increases, a large number number of electrons cross over the forbidden gap and jump from valence to conduction band. Hence conductivity of semiconductor increases with temperature. temperature.
Pur e semi co condu ndu ctors ar ar e ca call l ed i nt r i ns nsii c se semi -
In a pure semiconductor, semiconduct or, each atom behaves as if there are 8 electrons in its valence shell and therefore the entire material behaves as an insulator at low temperatures. conductors.
A semiconductor semiconductor atom needs needs energy of the order order of 1.1ev to shake off the valence valence electron. This energy becomes becomes available available to it even at room temperature. temperature. Due to thermal thermal agitation agitation of crystal structure, electrons from a few covalent bonds bonds come out. The bond from which electron is freed, freed, a vacancy vacancy is created there. The vacancy in the covalent bond is called a hole.
This hole can be filled by some other electron in a covalent bond. As an electron from covalent bond moves to fill the hole, the hole is created in the covalent bond from which the electron has moved. Since the direction of movement of the hole is opposite to that of the negative electron, a hole behaves as a positive positive charge charge carrier. Thus, at room temperature, a pure semiconductor will have electrons and holes wandering wandering in random random directions. directions. These electrons electrons and holes are called intrinsic carriers. As the crystal is neutral, neutral, the number number of free electrons will will be equal to the number of holes. In an intrinsic semiconductor, if ne denotes the electron number density in conduction band, nh the hole number density in valence band and ni the number density or concentration of charge carriers, then ne = nh = ni
As the conductivity of intrinsic semi-conductors is poor, so intrinsic semi-conductors are of little practical importance. The conductivity conductivity of pure semi-conductor semi-conductor can, however be enormously increased by addition of some pentavalent or a trivalent impurity in a very small amount
(about 1 to 106 parts of the semi-conductor). semi-conduc tor). The process of adding an impurity to a pure semiconductor so as to improve its conductivity conductivity is called called doping. doping. Such semi-conduc semi-conductors tors are called extrinsic semi-conductors. semi-conductors. Extrinsic semiconductors semiconductors are of two types : i) ii)
n-type semiconductor p-type semiconductor
When an impurity atom belonging to group V of the periodic table like Arsenic is added to the pure semi-conductor, then four of the five impurity electrons form covalent bonds by sharing one electron with each of the four nearest silicon atoms, and fifth electron from each impurity atom is almost free to conduct electricity. As the pentavalent pentavalent impurity increases the number of free electrons, it is called donor impurity. The electrons so set free in the silicon silicon crystal are are called extrinsic carriers and the n-type Si-crystal is called ntype extrinsic extrinsic semiconductor. semiconductor. Therefore n-type n-type Si-crystal Si-crystal will have a large number of free electrons (majority carriers) and have a small number of holes (minority carriers). In terms of valence and conduction conduction band one can think that all such electrons create a donor energy level just below the conduction band as shown in figure. As the energy gap between donor energy level and the conduction band is very small, the electrons can easily raise themselves to conduction band even at room temperature. Hence, the conductivity of ntype extrinsic semiconductor is markedly increased. In a doped or extrinsic semiconductor, semiconductor, the number density of the conduction band (ne) and the number density of holes in the valence band (nh) differ from that in a pure
semiconductor. If ni is the number density of electrons is conduction band, then it is proved that ne nh = ni2
If a trivalent impurity like indium is added in pure semiconductor, the impurity atom can provide only three valence electrons for covalent bond bond formation. formation. Thus a gap is left in one of the covalent bonds. The gap acts acts as a hole hole that tends tends to accept electrons. electrons. As the trivalent impurity atoms accept electrons from the silicon crystal, it is called acceptor acceptor impurity. impurity. The holes holes so created are extrinsic carriers and the p-type Si-crystal so obtained is called p-type p-type extrinsic semiconductor. semiconductor. Again, as as the pure Sicrystal also possesses a few electrons and holes, therefore, the p-type si-crystal will have a large number of holes (majority carriers) and a small number of electrons (minority carriers). It terms of valence and conduction band one can think that all such holes create an accepter energy level just above the top of the valance valance band as as shown in figure. The electrons from from valence band can raise themselves to the accepter energy level by absorbing thermal energy at room temperature and in turn create holes in the valence band. Number density of valence band holes (nh) in p-type semiconductor is approximately equal to that of the acceptor atoms (Na) and is very large as compared to the number density of conduction band electrons (ne). Thus, nh N N a > > ne
Consider a block of semiconductor of length l1 area of cross section A and having number number density of electrons electrons and holes as
ne and nh respectively. respectiv ely. Suppose that on applying a potential difference, say V, a current I flows through it as shown in figure. The electron current (Ic) and the hole current (Ih) constitute the current I flowing through the semi conductor i.e. I = Ie + Ih (i) It ne is the number density of conduction band electrons in the semiconductor and ve, the drift velocity velocity of electrons then Ie = eneAve Similarly, the hole current, Ih = enhAvh From (i) I = eneAve + enhAvh I = eA(neve + nhvh) (ii) If is the resistivity of the material of the semiconductor, then the resistance offered by the semiconductor to the flow of current is given by : R = l/A (iii) Since V = RI, from equation (ii) and (iii) we have V = RI = l/A eA (neve + nh vh) V = le(neve + nhvh) (iv) If E is the electric field field set up across the semiconductor, then: E = V/l (v) from equation (iv) and (v), we have E = e (neve + nhvh) 1/ = = e (ne ve/E + nh vh/E) On applying electric field, the drift velocity acquired by the electrons (or holes) per unit strength of electric field is called mobility of electrons (or holes). Therefore, mobility of electrons and holes is given by : e = ve/E and h = vh/E 1/ = = e(ne e + nh h) h) (vi) Also, = 1/ is is called conductivity of the material of semiconductor = e (ne e + nh h) h) (vii) The relation (vi) and (vii) show that the conductivity and resistivity of a semiconductor depend upon the electron and hole number densities and their mobilities. As ne and nh increases with rise in temperature, therefore, conductivity of semiconductor increases with rise in temperature and resistivity decreases with rise in temperature.
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