Lecture 2. Basic Semiconductor Physics

Transcription

1 Lecture Basc Semcductr Physcs I ths lecture yu wll lear: What are semcductrs? Basc crystal structure f semcductrs Electrs ad hles semcductrs Itrsc semcductrs Extrsc semcductrs -ded ad -ded semcductrs Semcductrs the Perdc Table Atmc umber Gru IV semcductrs Each elemet gru IV has 4 electrs ts uter mst atmc shell A Slc atm has: electrs the frst atmc shell 8 electrs the secd atmc shell 4 electrs the thrd utermst atmc shell A Slc atm 1

2 Semcductrs the Perdc Table Atmc umber Gru IV semcductrs Each elemet gru IV has 4 electrs ts uter mst atmc shell The utermst electrs are called valece electrs cre The er electrs are called the cre electrs A Slc atm Cvalet Bdg Slc A Slc atm wth 4 electrs the valece shell s draw a cart way as: Tw Slc atms ca cme tgether t frm a cvalet bd by sharg tw electrs amg themselves als draw as shared electrs a cvalet bd Cvalet bdg s eergetcally favrable (.e. Slc atms lke t frm cvalet bds wth each ther)

3 A Slc Crystal Lattce (A Cart Vew) I a Slc crystal: Each Slc atm s surruded by 4 ther Slc atms Each Slc atm frms 4 cvalet bds wth the eghbrg Slc atms Actual 3D Structure f a Slc Crystal Lattce cvalet bds Each Slc atm s surruded by 4 ther Slc atms a tetrahedral cfgurat Slc atmc desty = 5x10 cm -3 3

4 Electrs ad Hles Semcductrs - I A erfect Slc crystal lattce Electrs ad Hles Semcductrs - II stvely charged hle egatvely charged free electr A Slc crystal lattce wth e brke bd It requres eergy t break a cvalet bd The requred eergy s called the badga (badga f Slc s ~1.1 ev) A brke bd results e egatvely charged free electr ad e stvely charged hle The free electr ca freely mve arud the crystal 4

5 Electrs ad Hles Semcductrs - III stvely charged hle egatvely charged free electr A Slc crystal lattce wth e brke bd A hle ca als mve thrugh the lattce!! A hle mves whe a electr frm a eghbrg bd jums ver t fll that hle Electrs ad Hles Semcductrs - III stvely charged hle egatvely charged free electr A Slc crystal lattce wth e brke bd A hle ca als mve thrugh the lattce!! A hle mves whe a electr frm a eghbrg bd jums ver t fll that hle 5

6 Defts ad tats Used ECE 3150 The wrd electr wll usually mea a free electr (ad t a electr frmg the cvalet bd r a cre electr) The electr desty s deted by: (uts: 1/cm 3 ) The hle desty s deted by: (uts: 1/cm 3 ) The charge f a electr s: The charge f a hle s: q q 19 q Culmbs Electrs ad Hles at ear Zer Temerature A erfect Slc crystal lattce at temerature T0 K There are brke bds ad electrs ad hles (.e. = = 0 ) 6

7 Electrs ad Hles at zer Temerature hles electrs A Slc crystal lattce at temerature T>0 K Thermal eergy breaks the cvalet bds ad electr-hle ars are geerated (remember t takes eergy t break a cvalet bd) The umber f electrs ad hles geerated are equal - fr every electr geerated there s als a hle geerated (.e. = ) Quest: what s the electr ad hle desty at rm temerature? Thermal Eergy Thermal eergy s tycally measured uts f KT K s the Bltzma s cstat ad equals ~1.38 x 10-3 Jules/Kelv Temerature T s measured degrees Kelv Rm temerature crresds t T = 300 K Rm temerature crresds t a KT value f 4.14 x 10-1 Jules r 5.8 mev Eergy ev = Eergy Jules Electr charge Culmbs 7

8 Geerat ad Recmbat Semcductrs - I Geerat: The breakg f a bd t geerate a electr-hle ar s called geerat Geerat rate G(T ) s a fuct f temerature Uts f G(T ) are: cm -3 -s -1 Recmbat: A electr ca als cmbe wth a hle t frm a bd. Ths rcess s called recmbat. It s the reverse f geerat. Recmbat rate R(T ) s rrtal t the rduct R T RT kt (yu eed electrs as well as hles fr recmbat t hae) Uts f R(T ) are als: cm -3 -s -1 Geerat ad Recmbat Semcductrs - II Cdt f Thermal Equlbrum: I thermal equlbrum a steady state exsts whch the rate f electr-hle geerat s equal t the rate f electr-hle recmbat, R T G T T T k G T G T k G T By cvet, the rat s wrtte as kt Therefre, thermal equlbrum, thermal equlbrum electr ad hle destes are usually deted by ad T Sce equal umber f electrs ad hles are reset thermal equlbrum, we have, T s called the trsc carrer desty. It equals the umber f electrs (r hles) reset a ure semcductr equlbrum at a gve temerature. 10 Fr Slc, cm at rm temerature (.e. at T = 300K) T 8

9 Dg Semcductrs Dg: The trduct f certa murty atms a ure semcductr t ctrl ts electrc rertes s called dg Dg s de by tw kds f murty atms: a)dr atms b)accetr atms Drs: Dr atms are used t crease the electr desty a semcductr Gru V elemets have 5 electrs ther utermst atmc shell (e mre tha gru IV atms) Gru V elemets ca act as electr drs Slc Dg by Drs Slc (-dg) stvely charged fxed dr atm As egatvely charged free electr Dg Slc wth Arsec Atms Dr atm ccetrat s deted by: d (uts: 1/cm 3 ) Each dr atm ctrbutes e free electr t the crystal Dr atm after gvg ff a electr becmes stvely charged 9

10 Dg Semcductrs Accetrs: Accetr atms are used t crease the hle desty a semcductr Gru III elemets have 3 electrs ther utermst atmc shell (e less tha gru IV atms) Gru III elemets ca act as electr Accetrs Slc Dg by Accetrs Slc (-dg) egatvely charged fxed accetr atm - B stvely charged hle Dg Slc wth Br Atms Accetr atm ccetrat s deted by: a (uts: 1/cm 3 ) Each accetr atm ctrbutes e hle t the crystal by accetg e electr frm a eghbrg bd Accetr atm after gvg ff a hle (r equvaletly, after accetg a electr) becmes egatvely charged 10

11 11 Electr-Hle Desty Ded Semcductrs Csder a -ded semcductr thermal equlbrum: Dg desty = d Use cdt f charge eutralty: Tgether wth the relat: T bta: 0 d q d d d d If, whch s usually the case fr -dg, the the abve relats smlfy: d d d -dg lets e make the electr desty much greater tha the trsc value Electr-Hle Desty Ded Semcductrs w csder a P-ded semcductr thermal equlbrum: Dg desty = a Use cdt f charge eutralty: Tgether wth the relat: T bta: 0 a q a a a a If, whch s usually the case fr P-dg, the the abve relats smlfy: a a a -dg lets e make the hle desty much greater tha the trsc value

12 Electr-Hle Desty Vs Dg Desty -ded semcductrs Wth creasg -dg the electr desty creases abve the trsc value ad the hle desty decreases belw the trsc value d Examle: Suse the cm -3 ad 17-3 d 10 cm ad cm Sce d cm -3 Electr-Hle Desty Vs Dg Desty P-ded semcductrs Wth creasg P-dg the hle desty creases abve the trsc value ad the electr desty decreases belw the trsc value Examle: a Suse ad 17-3 a 10 cm ad 10 the cm -3 Sce a 10 3 cm cm -3 1

13 Cmud Semcductrs III-V semcductrs: Elemets gru III ca be cmbed wth elemets gru V t gve cmud semcductrs (as sed t elemetal semcductrs f gru IV) *Oe ca als have II-VI semcductrs Ga atms As atms Crystal lattce f the grus III-V cmud semcductr GaAs Elemetal ad Cmud Semcductrs S P S I A Damd Lattce (S, C, Ge, etc) A Zcblede Lattce (ZS, GaS, IP, etc) 13

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