Chapter 4 ELECTRONICS4.2UNDERSTANDING SEMICONDUCTORS DIODESA student is able todescribe semiconductors in terms of resistance and free electrons.describe n-tpe and p-tpe semiconductors.describe semiconductor diodes.describe the function of diodes.describe the use of diodes as recti!ers.describe the use of a capacitor to smooth out output current and output "olta#e in a recti!er circuit.4.2 SEMICONDUCTOR DIODESSemiconductorsSemiconductors are materials $hich can conduct electricit better than insulators% but not as $ell as conductors.4.2 SEMICONDUCTOR DIODESLi#ht emittin# diodesSemiconductorsEach atom of semiconductor material has four electrons in its outermost orbit that are in"ol"ed in co"alent bondin#.4.2 SEMICONDUCTOR DIODESSemiconductorsThe "ibration of the atoms causes some electrons to brea& free from the bonds% as sho$n.Each free electron lea"es behind a hole in the bondin# structure.4.2 SEMICONDUCTOR DIODESSemiconductorsThe hole beha"e as if it has a positi"e char#e.'ree electrons (ne#ati"el char#ed) and holes (positi"el char#ed) are &no$n as charge carriers.4.2 SEMICONDUCTOR DIODESSemiconductorsConduction in a semiconductor is b means of a movement of free electrons and holes in opposite direction.4.2 SEMICONDUCTOR DIODESSemiconductorsSemiconductors cannot conduct electricit as $ell as metals because the ha"e smaller nm!ers of free electrons and holes. The conducti"it of the semiconductors can be increased b a process is called *do"#n$+.4.2 SEMICONDUCTOR DIODESDoping of Semiconductors,opin# is a process of add#n$ a small amont of #m"r#t#es into the pure crstal of semiconductor (intrinsic semiconductor).Atoms of the impurities added should ha"e almost the same si-e as the atoms of the intrinsic semiconductor.4.2 SEMICONDUCTOR DIODESn-type SemiconductorsA n-tpe semiconductor is produced $hen pentavalent atoms are added to intrinsic semiconductor atoms.4.2 SEMICONDUCTOR DIODESn-type SemiconductorsEach penta"alent atom donate a free electron% because there $ill be one e.tra electron.4.2 SEMICONDUCTOR DIODESn-type SemiconductorsThe electrons are no$ the ma/orit char#e carriers in the n-tpe semiconductor since there are more free electrons than holes.4.2 SEMICONDUCTOR DIODESn-type SemiconductorsE.amples of penta"alent atoms are Arsenic% 0hosphorus and Antimon and called donor atoms.4.2 SEMICONDUCTOR DIODESp-type SemiconductorsA p-tpe semiconductor is produced $hen trivalent atoms are added to intrinsic semiconductor atoms.4.2 SEMICONDUCTOR DIODESp-type SemiconductorsOnl three of the four bonds formed b the tri"alent atoms are complete. The "acanc is a hole $ith positi"e char#e.4.2 SEMICONDUCTOR DIODESp-type SemiconductorsThe holes are no$ the ma/orit char#e carriers in the p-tpe semiconductor since there are more holes than free electrons.4.2 SEMICONDUCTOR DIODESp-type SemiconductorsE.amples of tri"alent atoms are Indium% 1oron and 2allium and called acceptor atoms.4.2 SEMICONDUCTOR DIODESComparison between p-type semiconductor and n-type semiconductor4.2 SEMICONDUCTOR DIODESp-tpe semiconductorn-tpe semiconductor0ure SemiconductorSilicon,GermaniumSilicon,Germanium,opin# substanceIndium, Boron,GalliumPhosporus,Antimony,Arsenic'unction of dopin# substanceAcceptor atom Donor atom3alenc of dopin# substancePentaalent !rialent4a/orit char#e carrier "ole #lectron4inorit char#e carrier #lectron "oleSemiconductor DiodeA semiconductor diode is a component (de"ice) that allo$s electric current to %o& #n one d#rect#on onl.4.2 SEMICONDUCTOR DIODESSemiconductor DiodeA semiconductor diode is a p-n /unction $ith t$o electrodes. 4.2 SEMICONDUCTOR DIODESp nSemiconductor Diode $orward and %eerse BiasDiode operation: a. Current fow is permitted; the diode is forward biased. b. Current fow is prohibited; the diode is reversed biased.4.2 SEMICONDUCTOR DIODES$orward BiasA forward-biased diode conducts current and drops a small "olta#e across it% lea"in# most of the batter "olta#e dropped across the lamp.4.2 SEMICONDUCTOR DIODES$orward BiasThis for$ard-bias "olta#e drop e.hibited b the diode is due to the action of the depletion re#ion formed b the 0-N /unction under the in5uence of an applied "olta#e. 4.2 SEMICONDUCTOR DIODES$orward BiasIf a for$ard-biasin# "olta#e is applied across the 0-N /unction% the depletion re#ion collapses becomin# thinner.This ta&es a certain minimum "olta#e to accomplish.4.2 SEMICONDUCTOR DIODES$orward Bias'or silicon diodes% the tpical for$ard "olta#e is 7.8 "olts% nominal. 'or #ermanium diodes% the for$ard "olta#e is onl 7.9 "olts.4.2 SEMICONDUCTOR DIODES$orward Bias'or silicon diodes% the tpical for$ard "olta#e is 7.8 "olts% nominal. 'or #ermanium diodes% the for$ard "olta#e is onl 7.9 "olts.4.2 SEMICONDUCTOR DIODES%eersed BiasIf the batter:s polarit is re"ersed% the diode becomes re"erse-biased% and dropsallof the batter:s "olta#e lea"in# none for the lamp.4.2 SEMICONDUCTOR DIODES%eersed BiasIf a re"erse-biasin# "olta#e is applied across the 0-N /unction% this depletion re#ion e.pands% further resistin# an current throu#h it.4.2 SEMICONDUCTOR DIODESDiode as a %ecti&erA diode can act as a recti!er because it can convert alternatin# current (a.c.) #nto direct current (d.c.).There are t$o tpes of recti!cation process ;( and 4 are re"erse-biased.The current 5o$ is A-1-C-,-E-'-2-=.4.2 SEMICONDUCTOR DIODESA1C,E'2=$ull-wae recti&cationIn the ne.t half ccle% diodes > and > are for$ard-biased $hile diodes < and 9 are re"erse-biased.The current 5o$ is =-2-C-,-E-'-1-A.4.2 SEMICONDUCTOR DIODESA1C,E'2=Capacitor Smoothing1oth half-$a"e and full-$a"e recti!cations do not pro"ide a stead current.Therefore a stead "olta#e cannot be maintain across the resistor.4.2 SEMICONDUCTOR DIODES=alf-$a"e recti!cation 'ull-$a"e recti!cationCapacitor Smoothing1 connectin# a capacitor parallel to the resistance % the half-$a"e and the full-$a"e recti!ed $a"eform could be partiall smoothed out.4.2 SEMICONDUCTOR DIODESSmoothin# half-$a"e recti!cation Smoothin# full-$a"e recti!cationCapacitor Smoothing4.2 SEMICONDUCTOR DIODESCapacitor SmoothingThe h#$her the capacitance "alue% the more e?ecti"e the capacitor is in producin# a *smoother+ output "olta#e.This is because the hi#her capacitance ta&es a lon$er t#me to char#e and dischar#e.4.2 SEMICONDUCTOR DIODESSmoothin# half-$a"e recti!cation Smoothin# full-$a"e recti!cation4.2 SEMICONDUCTOR DIODESCrrent %o& thro$h a ca"ac#tor'nct#on of a ca"ac#tora. Char#in# capacitor ,urin# the !rst half-ccle% the diode is for$ard-biased. Current is allo$ed to 5o$ throu#h the diode. The "alue of this current increases to a ma.imum and then decreases to -ero. This current also 5o$s throu#h both capacitor% C and the resistor% R. Thus% the capacitor is char#ed and electric ener# is stored in it.b. ,ischar#in# capacitor ,urin# the second half-ccle% the diode is re"erse-biased. Current is no$ allo$ed to 5o$ throu#h diode. There is no current 5o$in# throu#h the both capacitor% C and resistor% R. The capacitor% C dischar#es b releasin# the store ener# to maintain the "olta#e across the resistor% R.