Solid State Chemistry

Transcription

1 Solid Stte Chemistry Solids re minly chrcterised by their definite shpes nd considerble mechnicl strength nd rigidity. The rigidity rises due to the bsence of the trnsltory movement of the structurl units (like toms, ions etc.) of the solid. These units remin fixed to men position bout which they my undergo vibrtion. The forces of ttrction between these units re lrge enough. Solids re generlly clssified under two heds, nmely crystlline nd morphous. In crystlline solid, the constituent toms re rrnged in definite pttern constntly repeted nd in consequence giving rise to definite geometricl configurtion of crystlline solids. The crystls often hve plne frcture nd they lso hve shrp melting points. On the other hnd the morphous solids hve no definite geometricl shpe. The morphous solids melt slowly over temperture rnge. CRYSTAL The term crystl origintes from the Greek word Krystllos tht mens cler ice, nd ws first pplied to trnsprent qurtz stone with wrong ssumption tht these stones were formed from wter by extreme cold. In the 17th century the nme crystl ws extended to other solids possessing mnifesttions of solids tht bore the reltion to the originl rock crystls of being bounded by mny flt, shiny fces, rrnged symmetriclly. A crystl grows from melt or solution or from gseous stte by the deposition of toms or ions onto its surfces by which crystls re bounded. The surfces re referred to s fces nd n edge is formed where two surfces intersect. The ngle between the normls to the two intersecting fces is clled the interfcil ngle. In crystl the toms or ions re rrnged like soldiers on prde ground in well defined columns nd rows nd this regulrity of tomic rrngements in crystl is the bsic feture of crystls. There is bsic unit in ny rel crystl nd when the bsic unit is systemticlly repeted, reproduces the whole structure. Thus the first notble feture of the regulrity of the crystl structure is the periodicity of their ptterns nd crystl is therefore periodic rry of toms nd molecules in spce. The vrious modes of rrngement of toms of molecules in spce to stisfy the need of periodicity re governed by some specific rules nd principles which led to the development of the subject of crystllogrphy. In more specific wy crystl my be defined s solid substnce hving definite geometricl shpe with flt fces nd shrp edges. 1

2 Fundmentl lws of crystllogrphy ometricl crystllogrphy is concerned with the outwrd sptil rrngement of crystl plnes nd the geometricl shpes of the crystls nd thus crystllogrphy is dependent upon the three following fundmentl lws. Lw of constncy of interfcil ngle Lw of rtionl indices, nd Lw of symmetry. Lw of constncy of interfcil ngle or Steno s lw The lw sttes tht the ngles between the corresponding fces on vrious crystls of the sme substnce re constnt. The crystls of substnces re bounded by plne surfces which re clled fces. These fces lwys intersect t n ngle, clled interfcil ngle. Interfcil ngle hs chrcteristic vlue for given crystlline solid. It is often seen tht the crystl fces re uneqully developed, leding to vrious shpes of the crystls. But the ngle of intersection of the two corresponding fces will be sme for ny crystl of the sme substnce. In Fig. 22.1, two crystls re represented two-dimensionlly. The shpes re different but hving the sme interfcil ngle. For exmple, NCl crystllises s cubes from queous solution nd s octhedrl from ure solution but interfcil Fig Interfcil ngle. ngles of ll crystls of NCl re found to be. Lw of rtionl indices In 1784 Hüy proposed the lw of rtionl indices or rtionl intercepts. This cn be understood in the following wy. For describing the geometry of crystl usully three non-co-plnr co-ordinte xes re selected rbitrrily. These re crystllogrphic xes. According to this lw, the rtio between intercepts on crystllogrphic xes for the different fces of crystl cn lwys be represented by rtionl numbers. In other words, ll fces cut given xis t distnces from the origin, which ber simple rtio to one nother. To illustrte, let us consider plne ABC in the crystl s shown below: This plne hs intercepts OA, OB nd OC long X, Y nd Z xes t distnces 2, 3b nd 4c respectively, where OL =, OM = b nd ON = c re the unit distnces chosen long the three co-ordintes. These intercepts re in the rtio of 2 : 3b : 4c, where 2, 3 nd 4 re simple integrl whole numbers nd the stndrd intercepts re, b nd c. The rtio of the intercepts in terms of the stndrd is 2 : 3 : 4. These rtios chrcterise nd represent ny plne of the crystl. The coefficients re known s Weiss indices of the plne given. If ny plne is prllel to one xis, then it will cut it t infinity. In such cses the use of Weiss indices re rther wkwrd nd hve consequently been replced by Miller indices. The Miller indices of plne re obtined by tking the reciprocls of the coefficients of, b, nd c i.e., Weiss indices X A L Z C N c b O Fig Intercepts of crystllogrphic plnes. M B Y 2

3 nd is multiplied throughout by the lest common multiple to obtin integrl vlues. Thus, the Miller indices of the plne (2 : b : 2c) will be 1 : 2 1 : 1 i.e., 1 : 2 : 1 nd this plne or fce is 2 indicted s (1 2 1) fce of the crystl. Now, for plne perpendiculr to one xis nd prllel to the other two, hving intercepts : b : c will hve indices, s: nd Weiss indices 1 : : Miller indices 1 : 0 : 0 i.e., (100) plne. If plne produces n intercept on the negtive side, sy : b : c, the Miller indices for the plne would be (1 10), the br bove one indictes the intersection of the plne on the negtive side of the xis. The symbol 1 denotes minus unity. Thus, if fce of crystl mkes intercepts OA, OB nd OC on the three xes, then the lengths of the intercepts my be expressed s OA/, OB/b nd OC/c where,, b nd c re unit distnces long three xes. The reciprocls of these lengths will be /OA, b/ob nd c/oc, nd these reciprocl intercepts re whole number or integers i.e., /OA = h; b/ob = k nd c/oc = l where h, k, l re Miller indices of the fce or plne of the crystl nd the fce is defined s (h k l) fce. The distnce between the prllel plnes in crystl is designted s d hkl. For vrious cubic lttices, these interplnr spcings re given by the formul: d hkl = h + k + l where is the length of the side of the cube nd h, k, l re the Miller indices of the plne. Some of the Miller indices in the cse of cubic lttices re shown in Fig below. X O Y Z (0,0,1)plne (0,1,0)plne (0,1,1)plne (1,1,1)plne Fig Miller indices in the cse of cubic lttices. 3

4 The lw of symmetry Another importnt property of crystls is their symmetry. Symmetry in crystls my be with respect to : plne, line or point. The lw of symmetry sttes tht: ll crystls of the sme substnce possess the sme elements of symmetry. There re three types of symmetry elements ssocited with crystl, nmely: plne of symmetry line of symmetry, nd centre of symmetry. Plne of symmetry A crystl is sid to hve plne of symmetry when it is divided by n imginry plne into two hlves in such mnner tht one hlf is the mirror imge of the other. Line of symmetry A crystl is sid to possess line of symmetry if it is possible to drw n imginry line through the centre of the crystl nd then to revolve the crystl bout this line through 360 in such wy tht the crystl ppers unchnged more thn once. The line is clled the xis of symmetry. If similr view ppers two, three, four or six times during one complete revolution of 360, the crystl is sid to possess two, three, four or six fold xes of symmetry. Centre of symmetry A crystl is sid to possess centre of symmetry, if every fce hs nother identicl fce t n equl distnce from the centre. A crystl cn hve only one centre of symmetry. These plnes, lines nd centre of symmetry of crystl re clled its elements of symmetry. The cube hs the gretest symmetry elements: (i) Nine plne of symmetries (ii) Thirteen xes of symmetries (iii) One centre of symmetry i.e., ltogether twenty-three elements of symmetry. A A B Three fold xis B A Four fold xis Two fold xis B () (b) (c) Fig Some elements of symmetry in crystls. () plnes of symmetry; (b) xes of symmetry; (c) centres of symmetry. 4

5 Crystl lttice The ide of lttice developed from the internl regulrity suggested from the externl ppernce of the crystl. A crystl lttice is highly ordered three dimensionl structure, formed by its constituent toms or molecules or ions. Thus lttice my be defined s n infinite set of points repeted regulrly through spce. A set of points tht re repeted t distnce long line form one-dimensionl lttice. When set of points occur regulrly in plne, it constitutes two dimensionl lttice. When the set of points re repeted regulrly in three dimensions, the three dimensionl lttice or spce lttice is obtined. () b b (b) (c) (d) Fig Lttice rrngement: () one dimensionl; (b, c) two dimensionl; (d) three dimensionl or spce lttice. On the bsis of the rrngement of structurl units in their crystl lttices, crystlline substnces re clssified s in Tble

6 Tble 22.1 Crystl systems nd their chrcteristics System Axil Angles No. of Exmples chrcteristics spce lttices 1. Cubic b c α β γ 3 NCl, KCl, CsCl, ZnS 2. Tetrgonl b c α β γ 2 TiO 2, SnO 2 3. Orthorhombic b c α β γ 4 KNO 3, K 2 SO 4, BSO 4, AgBr 4. Monoclinic b c α γ 2 NHCO 3, N 2 SO H 2 O, β monoclinic sulphur 5. Triclinic b c α β γ 1 CuSO 4. 5H 2 O, K 2 Cr 2 O 7, H 3 PO 3 6. Hexgonl b c α β 1 SiO 2, HgS, BN, PbI 2 γ Trigonl b c α β γ 1 Al 2 O 3, CCO 3, grphite, or Sb, NNO 3 Rhombohedrl c c Cubic Tetrgonl Orthorhombic c 120 c b b Rhombohedrl Hexgonl Monoclinic Triclinic c Fig The seven crystl systems or crystlline forms. This clssifiction is bsed on the mgnitude of the unit cell length nd the ngle of inclintion between them. The unit cell is the smllest building unit in the spce of crystl, which when repeted over nd over gin in three dimensions, forms spce lttice of the crystlline substnce. Thus the unit cell is the essentil feture of the crystl structure. All the crystl systems re mde up of 14 types of crystl lttice nd these crystl lttices re known s Brvis lttices, shown in Fig

7 Cubic (P) Cubic (l) Cubic (F) Tetrgonl (P) Tetrgonl (I) Orthorhombic (P) Orthorhombic (C) Orthorhombic (I) Orthorhombic (F) Monoclinic (P) Monoclinic (C) Triclinic (C) CUBIC CRYSTALS Trigonl (R) Trigonl & Hexgonl (C) Fig The 14 Brvis lttices. In cubic crystl, the intercepts on the three xes re equl nd ll the ngles re equl to. A cubic crystl cn be ny of the following three types: (i) Simple crystl lttice (SC) (ii) Body centered crystl lttice (BCC) (iii) Fce centered crystl lttice (FCC) 7

8 (i) Simple Crystl Lttice. In simple crystl lttice, there re lttice points t the eight corners of the unit cell. In simple cubic structure, n tom situted t ny corner of ech unit, is shred by totl of eight unit cells, thus, ech unit cell hs 1/8 shres of every corner tom. So the totl contribution of ll the eight corner toms to ech cell = = 1 tom/unit cell of SC. () Simple lttice (b) Body centered lttice (c) Fce centered lttice Fig Brvis lttices of cubic system. 1/8 tom Fig Unit cell of simple cubic crystl. (ii) Body Centered Crystl Lttice. In body centered crystl lttice, there re lttice points t the eight corners nd t the centre of the unit cell. A BC cell hs one dditionl tom t the centre besides hving one tom ech its right corners. The tom t the centre is independent of other cells, while ech of the eight toms situted t the corners is shred by totl of eight unit cells. Thus, the totl number of toms per unit cell = 1 (t the centre) + 8 1/8 (t the 8 corners) = = 2 toms/unit cell of BCC 1/8 tom 1 tom Fig Unit cell of body centered cubic. 8

9 (iii) Fce Centered Crystl Lttice. In fce-centered crystl lttice, there re lttice points t the centre of ll fces, in ddition to those t the eight corners of the unit cell. An FCC hs one tom t the centre of the ech fce besides hving one tom t ech corner. Thus, every tom situted t the centre of fce of unit cell is shred by two djoining unit cells. Since there re six fces of cube, so tht totl number of toms per unit cell. = 1/2 6 (t the centre of six fces) + 8 1/8 (t 8 corners) = = 4 toms/ unit cell of FCC 1/2 tom 1/8 tom Fig Unit cell of fce centered cubic. The perpendiculr distnce between the djcent plnes is known s the interplnr spcing nd is denoted by d hkl. d hkl = h + k + l where = side of the cube. Atomic rdius of cubic lttice Atomic rdius of unit cell my be defined s hlf the distnce between the centres of two immedite neighbours in unit cell nd is denoted by r. The distnce between the centres of two corner toms of the cube is clled length of the cube edge nd denoted by. Thus, for simple cubic (SC) cell tomic rdius r = /2 s we know tht = 2r 2r 2r nd for body centered cell (BCC) (4r) 2 = ( ) + 2 = 3 2 r = 3 / 4 1/8 tom Fig Simple cubic cell. 9

10 4r Fig Body centered cubic cell. While for fce centered cubic cell (FCC) (4r 2 ) = or 16r 2 = 2 2 Thus r = /2 2 r = / 8 4r Fig Fce centered cubic cell. Rdius rtio Rdius rtio is the rtio of the ction rdius to tht of the nion in n ionic solid. Thus, rdius rtio Rdius of ction ( r+ ) =. Rdius of nion ( r ) Solid defects Crystls re formed depending on perfectly regulr rrngement of structurl units. But this ide of crystl is non-existent. In cse of most ionic crystls, the ions re not rrnged in perfect order nd these crystls re sid to hve defects or in other words this devition my be regrded s crystl imperfections. The importnt imperfections or defects re of the following types: Stoichiometric defects: (i) Schottky defects nd (ii) Frenkel defects. Non-stoichiometric defects or impurity defects. Stoichiometric defects (i) Schottky Defects. There re certin crystls is which some of the lttice points remin unoccupied i.e., the crystls hve vcncies. On ccount of electricl neutrlity, 10

11 equl number of ctionic nd nionic vcncies would exist in the crystl lttice s shown in Fig below. This type of crystl defect is known s Schottky defect. Such defect is found in crystls of NCl. (ii) Frenkel Defects. Frenkel defects rise with those crystls in which n ion leves its norml lttice site nd occupies n interstitil site in the sme crystl. For exmple, in AgBr crystl some Ag + ions re missing from their regulr positions nd re found to be squeezed between other ions Idel Crystl Schottky defect Frenkel defect Fig Defects in crystls. Non-stoichiometric defects Besides structurl imperfections, crystl defects my lso rise s result of the presence of smll mount of impurities. These impurities in crystlline substnce drsticlly chnge their properties. For exmple, if very smll mount of CCl 2 (less thn 0.1%) is dded to NCl, the conductivity of NCl increses by times. In the mixed crystl C 2+ ions occupy the positions of N + in crystl lttice nd the position of Cl ion in the lttice remins unchnged. The insertion of C 2+ ions for N + ions leds to crete lttice vcncies nd such vcncies permit the migrtion of ions s result of which the conductivity of impure crystl increses. One of the most common exmples for impurity defect is semiconductors. Impurity defects re introduced under controlled conditions in germnium nd silicon in the production of semiconductors. In compounds hving non-stoichiometric defects, the rtio of positive nd negtive ions differs from tht indicted in their respective chemicl formul nd the blnce of the positive nd negtive chrges is mintined by extr electron or positive ions s per necessity. There is n excess of either metl or non-metl toms in non-stoichiometric compounds. Thus two cses generte (i) Metl excess nd (ii) Metl deficiency. Metl excess my occur in the following wys: () n nion my be missing from its lttice site nd n electron be present there for mintining chrge blnce. For exmple: when NCl is treted with sodium vpour, such yellow non-stoichiometric vriety is obtined. (b) n extr metl tom my be present in n interstitil lttice site nd n electron being present in some other interstitil position blnces the chrge. ZnO exhibits this type of defect when its composition becomes Zn 1+x O. The free electron present is responsible for excittion to higher energy level by bsorption of rdition of prticulr wvelengths. This phenomenon occurs when ZnO is subjected to heting nd its colour chnges from white to yellow. Metl deficiency my lso occur s follows: A positive ion my be missing from its lttice position nd doubly chrged ction mintins the chrge blnce. FeO, FeS nd NiO re the exmples of such defect. 11

12 Role of Silicon (Si) nd rmenium () in the field of semiconductors Semiconductors re the mterils tht exhibit conductivities considerbly lower thn those of the metls. Semiconductors re defined s inorgnic crystls responding to electronegtivity when excited by het or electromgnetic rdition. Si nd in pure stte re very poor conductors of electricity. However, impurity doped Si nd exhibit semiconductivity. Doping of smll mounts of group IV or V elements improves their electricl conductivity pprecibly. Both in Si nd, ech tom is covlently bonded to four neighbours such tht ll four outer electrons of ech re involved in bonding. Let us suppose n tom of group V element like P, As, Sb or Bi is introduced in plce of, or Si toms, then the four electrons of this tom will be utilized in the formtion of covlent bonds nd the remining one will be free to move nd thus induces n enhnced conductivity. Similrly, if group III element like B, Al etc, is introduced in plce of or Si, n electron vcncy in covlently bonded structure will be generted nd such type of electron vcncy is known s hole. This hole insted of remining confined to the impurity tom, migrtes throughout the structure when filled by n electron. So, electrons re ble to move in crystl hving such electron vcncies or hole nd thus the crystl exhibits electricl conductivity. Semiconductors tht exhibit conductivity due to excess electrons re clled n-type semiconductors where n stnds for negtive hole. In other cses, there re some semiconductors in which electricl conductivity is imposed due to presence of positive holes, they re known s p-type semiconductors, discussed lter on. With rise in temperture the conductivity of semiconductors increse becuse t high temperture the excess electrons or holes become more free to move nd so the conductivity increses. Highlight: It my be noted tht both type of semiconductors (n- nd p-type) re electriclly neutrl. Trnsistors (semiconductor triodes) The word trnsistor is derived from two words trnsfer nd resistor. A triod trnsistor contins two P-N junction diodes plced bck to bck. Junction trnsistors re of two types: Grown junction type nd Alloy junction type. The following Fig () nd (b) show grown P-N-P junction triode trnsistor nd n N-P-N junction trnsistor respecttively Emitter Bse Collector (Input) E P N P C (Output) (Input) E N P N C (Output) B (Bse) () A PNP trnsistor B (Bse) (b) An NPN trnsistor Fig Trnsistors. 12

13 Function of the emitter is to inject mjority chrge crriers into the bse, nd collector is to collect these crriers through the bse. Amplifiction by trnsistor (current mplifiction or gin) Conduction within PNP trnsistor is by hole movement from the emitter to the collector. In n NPN trnsistor electron moves. The collector current is lwys less thn the emitter current i.e. I c = I e I b : The current mplifiction or gin (α) of trnsistor is, collector current α = emitter current = I Ic (α < 1) e for both PNP nd NPN trnsistors. The lloy junction trnsistor is shown in the following figure (Fig ). N-type germnium E C Indium B Fig The lloy junction trnsistor consists of two beds of indium metl lloyed on opposite sides of thin slice of n N-type. Collector is lrger in size thn the emitter. Elements of bnd theory Metls in generl re chrcterised by certin distinct fetures, these re: High therml nd electricl conductnces, the conductnce decreses with increse in temperture. High melting nd boiling points in most cses. Hrdness nd, t the sme time, ductility nd mllebility. Metllic lustre Constncy Emission of electrons by some metls under the ction of het nd light. All these bove properties of metl cnnot be explined either by covlency or by n ionic model. Seprte bonding schemes hve been proposed for the metls to give n ccurte explntion of the chrcteristics of metl. But the bond theory is the most stisfctory mong ll the proposed theories. Bnd theory Bnd theory provides the most stisfctory explntion of the chrcteristics of the metllic properties in very nturl mnner. The theory is n extention of moleculr orbitl (M.O.) tretment for lrge number of toms. For homonucler ditomic species, the combintion of two tomic orbitls gives rise to two moleculr orbitls nd these re two new energy levels for the electrons in the two joint fields of the nuclei. If this ide is extended to n 13

14 ggregte of lrge number of toms (n), ech offering one orbitl for the combintion with others, so there will be totl of n new energy levels similr to n number of moleculr orbitls (M.O.s). These lrge number of energy levels form n energy bnd by spcing closely one upon nother. The vlence electrons from ll the toms will enter every level nd metl thus consists of energy bnds formed by merging of individul tomic orbitls. Let us illustrte the following cses: Let us consider Li s n exmple. The electronic configurtion of Li tom is 1s 2, 2s 1. So, two moleculr orbitls re formed by two 2s tomic orbitls from two Li toms. Similrly three or four Li toms would give rise to formtion of three or four energy levels by combintion of their 2s-tomic orbitls. Now extending this ide for n -toms, ll 2s-tomic orbitls will combine to give 2s-energy bnd with n-energy levels. Similr combintion of the p-tomic orbitls will form 2p energy bnd contining 3n-energy levels. Ech Li tom hs only one vlence electron nd the totl n-electrons from n-toms will occupy doubly the lower n/2 energy levels in the 2s-bnd. When the electrons gin therml energy or re plced in n electric field. The electrons re rised to higher unfilled energy levels. This phenomenon explins the high therml nd electricl conductivities of metls. 2p bnd E + + Combintion of 4-tomic orbitls 2s bnd Fig Combintion of n Li toms An Mg tom hs the electronic configurtion 1s 2.2s 2.2p 6.3s 2. The 3s bnd in Mg is exctly filled by the 2n electrons of the n -Mg toms. One finds tht the 3p bnd (formed by unoccupied 3p tomic orbitls of the Mg toms) overlps the 3s bnd in Mg nd there is no energy gp between the highest occupied nd lowest vcnt electronic energy levels nd Mg is n excellent conductor. 3p bnd 3s bnd Fig Merger of the 3s nd 3p bnds in Mg. 14

15 At tempertures bove the bsolute zero, few of the electrons my be promoted to energy levels higher thn the ctul occupied region in bnds. The energy distribution of these electrons follows roughly the Mxwell Boltzmnn lw. These electrons re much less in number thn the vlence electrons nd these re responsible to the het cpcity of the metl nd hence the contribution of electrons to the specific het of metls is thus smll. This conclusion is in hrmony with the Dulong s nd Petit s lw. Vlence nd conduction bnds The outermost electrons of n tom i.e., electrons residing in the outermost shell of n tom re clled vlence electrons hving the highest energy. It is these electrons which re mostly ffected when number of toms re brought very close together s during the formtion of solid. The bnd of energy occupied by the vlence electrons is clled the vlence bnd nd is the highest occupied bnd. It my be filled by electrons completely or prtilly. The next higher permitted energy bnd is clled the empty bnd or conduction bnd. In fct, it my be defined s the lowest unfilled energy bnd. In conduction bnd, electrons cn move freely nd therefore re known s conduction electrons. The gp between these two bnds, nmely vlence bnd nd conduction bnd, is known s forbidden energy gp. Bnd Energy (ev) Conduction bnd Vlence bnd Empty or prtilly filled Fully or prtilly filled Completely filled inner bnds Fig Now if vlence electron bsorbs enough energy, it jumps cross the forbidden energy gp nd enters the conduction bnd. An electron in the conduction bnd cn jump more redily to n djcent conduction bnd thn it cn jump bck to its originl position. But, however, if the conduction electron rdites too much energy, it will suddenly repper in the vlence bnd gin. It my be noted tht the covlent forces of the crystl lttice hve their source in the vlence bnd. So when n electron is rised from the vlence bnd, covlent bond is broken nd positively chrged hole is formed. This hole cn trvel to n djcent tom by tking n electron from tht tom, which involves the breking n existing covlent bond nd then re-estblishing covlent bond by filling up the hole. It hs to be kept in mind tht holes re filled by electrons which move from djcent toms without pssing through the forbidden energy gp. This is shown in Fig below. 15

16 O E Conduction bnd Vlence bnd Fig In nother wy it my be concluded tht the conditions in the conduction bnd hve nothing to do with the hole flow. It denotes very importnt distinction between the hole current nd electron current. Although holes flow with ese, they experience more opposition thn electron flow in the conduction bnd. So, to summrise: Conduction electrons re found in conduction bnd nd move freely in the conduction bnd. Holes exist in nd move in the vlence bnd. Conduction electrons move lmost twice s fst s the holes. Conductors, semiconductors nd insultors The bnd theory of metls my be extended to other non-metllic solids s well. The electricl conduction properties of different elements nd compounds cn be explined in terms of electrons hving energies in the vlence nd conduction bnds. The electrons lying in the lower energy bnds, which re normlly filled, ply no prt in the process of conduction. Conductors Conducting mterils re those in which there is the vilbility of plenty of free electrons for electric conduction. In terms of energy bnds it mens tht electricl conductors re those which hve overlpping vlence nd conduction bnds. In fct there is no physicl distinction between these two bnds, hence the vilbility of plenty number of conduction electrons. 0 E Conduction bnd Overlp Vlence bnd 16 Fig Another point to be mentioned is tht here the forbidden gp is not present for good conductors. The totl current is simply flow of electrons. 16

17 Semiconductors A semiconductor is mteril whose electricl properties lie in between those of good conductors nd insultors. For exmple germnium () nd silicon (Si). Semiconductors re neither good conductors nor good insultors, but their electricl properties lie in between. The resistivity of the semiconductors rnges from 10 5 to 10 3 ohm-cm nd decreses with increse in temperture ccording to n exponentil lw. This is the min difference between semiconductors nd good conductors. The semiconductors which conduct even in chemiclly pure stte re clled intrinsic semiconductors. In terms of energy bnds, semiconductors re the mterils which hve lmost n empty conduction bnd nd lmost filled vlence bnd with very nrrow energy gp seprting the two. The energy gp is of the order of 1 ev. 0 E Conduction bnd Smll energy gp Vlence bnd 16 Fig At 0 K, there re no electrons in the condition bnd nd the vlence bnd is filled completely. As temperture increses, the width of the forbidden gp decreses, so tht some of the electrons re liberted into the conduction bnd. So, the electricl conductivity of semiconductors increses with rise in temperture nd in this regrd they differ from the metls whose electricl conductivity decreses with rise in temperture. Semiconductors re of two types: tom G B A F D C Covlent bond E Fig () Intrinsic. Fig (b) Hole formtion in semiconductors (conduction of current). 17

18 (i) Pure or intrinsic semiconductors. In such semiconductors the energy gp between the vlence bnd nd conduction bnd is very smll (Fig ). The electrons jump cross the gp nd hence they behve s insultors t bsolute zero. But their electricl conductivity is rised with increse in temperture to brek the covlent bonds (Fig ()) to get conducting electrons (Fig (b)). Let covlent bond is broken t A with increse in temperture [Fig (b)] nd the electron hs moved leving behind hole t A where n electron jumps from B creting hole there, nd so on. Thus negtive chrge moves from G to A. i.e. positive chrge from A to G lterntely. An intrinsic semiconductor my be defined s one in which the number of conduction electrons is equl to the number of holes. (ii) Impure or extrinsic semiconductors. Semiconductor behviour my lso be imposed in certin substnces by the deliberte ddition of impurities. Such semiconductors re clled extrinsic semiconductors. Exmple, n-type nd p-type semiconductors (discussed erlier). Those intrinsic semiconductors to which some suitble impurity or doping gent or dopnt is dded in extremely smll mounts (bout 1 prt in 10 8 ) re clled extrinsic or impurity semiconductors. Usully doping gents re: l pentvlent toms (As, Sb, P) or l trivlent toms (G, In, P, B). Pentvlent doping tom is known s donor tom s it dontes one electron to the conduction bnd of pure. The trivlent tom, on the other hnd, is clled cceptor tom s it ccepts one electron from tom. Depending on the type of doping gent, extrinsic semiconductors re divided into two clsses: (i) N-type semiconductors (ii) P-type semiconductors. Sb B. excess electron Hole Fig (c) N-type semiconductor. Fig (d) P-type semiconductor. 18

19 Emitter junction Collector junction P N P 1 ma 0.99 ma Emitter Collector V e Hole flow Hole flow 0.01 ma () V c Bse (b) Fig (e) Working of PNP trnsistor. N P N 1 ma 0.99 ma Emitter Collector Electron flow V e () Electron flow 0.01 ma V c Bse (b) Fig (f) Working of n NPN trnsistor. Highlight. Emitter rrow shows the direction of flow of conventionl current. Obviously, electron flow will be in the opposite direction. Insultors Insultors re those mterils in which vlence electrons re bound very tightly to their prent toms nd thus require very lrge electricl field to remove them from the ttrction of the nuclei. In terms of energy bnds, it mens tht insultors re those mterils which hve full vlence bnd, hve n empty conduction bnd nd hve very high energy gp between these two bnds (Fig ). 0 E Empty conduction bnd Lrge energy or forbidden gp Vlence bnd 16 Fig

20 For conduction to tke plce electrons must be given sufficient mount of energy to jump from the vlence bnd to the conduction bnd. Increse in temperture cuses some electrons to move to the conduction bnd, which ccounts for the negtive resistnce temperture coefficient of insultors. The diode s rectifier When metl is heted it emits electrons. This phenomenon is known s thermionic emission. The electrons obtined in this process is known s thermions. The current obtined by flow of thermions is clled thermionic current. Thermionic tubes contining two electrodes re clled diodes. A triode hs three elctrodes. In some types of thermionic vlves, the electrons re obtined by heting metllic wire clled the filment or cthode. Electrons Filment Fig Directly heted cthode. By rectifiction, we men the conversion of n lternting voltge to direct voltge. A diode cts s perfect rectifier. A simple circuit for obtining rectifiction with help of diode is shown in Fig Input A.C. A F R Output D.C. Fig Rectifier. Photovoltic cell Photovoltic cell is self generting cell which utilises semiconductor contcts ginst metls. When light is incident on such combintion, n internl voltge is generted which cuses the current to flow through internl circuit, here no externl bttery is needed. The e.m.f. generted is proportionl to the rdint energy received in solr btteries. 20

21 Ring Light Trnsprent film Brrier lyer Selenium Iron Bse Electron flow A Fig Photovoltic cell. The most common photovoltic cells re of the brrier-lyer type, like iron-selenium cells. In the iron-selenium cell, selenium lyer is plced on n iron disc nd then extremely thin trnsprent film of gold or silver is formed on the selenium to ct s front electrode. The brrier lyer is formed by cthode sputtering the semi-trnsprent film on the selenium. A contct ring on the silver lyer cts s one electrode nd the iron bse s the other. Working When rdint flux flls on semiconductor i.e. selenium, it ejects electrons which trvel from selenium to the front silver electrode through the brrier lyer (Fig ). The flow in the opposite direction is not permitted by the brrier lyer becuse it cts s rectifier. The e.m.f. generted internlly between silver electrode nd selenium is directly proportionl to the incident flux. Appliction l The min dvntge of photovoltic cell is tht it requires no externl bttery for its opertion, i.e. it is self-generting. l The internl e.m.f. nd hence current generted by it re lrge enough to be mesured by pointer glvnometer. l Such cells re used () in devices like portble exposure meters, (b) in direct reding illumintion meters nd with low resistnce relys for on nd off opertions nd (c) in other monitoring opertions in industries. 21