The metal-oxide-semiconductor field-effect transistor consists of two p-n junctions either side of a MOS diode which acts as the gate.

Transcription

1 Intructn The metal-xe-emcnuctr fel-effect trantr cnt f tw -n junctn ether e f a MOS e whch act a the gate. MOSFET evce make u arun 90% f the electrnc nutry ue t ther lw wer an ther mall ze (cmare wth blar trantr). They are the ba fr MOS memry tructure uch a Flah memry an MOS lgc tructure. Befre we can unertan the MOSFET we nee t lk clely at the MOS e telf 139 The MOS e The metal-xe-emcnuctr (MOS) e ha the fllwng tructure a yu wul exect, SO 2 S Ohmc cntact (grune) the vltage ale t the metal cntact an tve f the ale ba tve wth reect t the Ohmc cntact. 140

2 Ban agram E vac eφ m eχ eφ E g 2 eψ B E E E F E metal xe -tye emcnuctr t thermal equlbrum, wth n ba ale the Ferm level f the metal an emcnuctr are algne. 141 Ieal MOS e n eal MOS e efne a fllw, 1) t zer ale ba, the energy fference between the metal an the emcnuctr wrk functn zer (the flat-ban cntn), eφ m Eg eφm eφ eφm eχ + + eψ B 2 0 2) The nly charge whch ext n the ytem at any ba are the n the emcnuctr an the wth equal an te gn n the metal urface. 3) There n carrer tranrt thrugh the xe uner c-bang cntn. Fr the ure f ur cun we wll cner a -tye emcnuctr MOS e. 142

3 MOS e uner ba accumulatn Negatve ba (<0) N current flw emcnuctr Ferm level cntant. The ban ben uwar. Exce hle are nuce at the emcnuctr-xe nterface. n e ( E E F Snce E -E F ncreae at the emcnuctr urface, hle accumulate near the nterface. Th knwn a the accumulatn cae. m Semcnuctr Devce, 2/E by S. M. Sze yrght 2002 Jhn Wley & Sn. Inc. ll rght reerve. 143 MOS e uner ba eletn Small tve ba (>0) When a mall tve ba ale t the MOS e, the ban ben wnwar, an the majrty carrer are elete. n e ( E E F Th knwn a the eletn cae. c en W Semcnuctr Devce, 2/E by S. M. Sze ur reéutc e Electrónca INOE yrght Jhn Dr. Wley Per & Sn. Rale Inc. ll unter rght reerve. 144

4 MOS e uner ba nvern Large tve ba (>0) When a large tve ba ale t the MOS e, the energy ban ben wnwar, even mre that the ntrnc Ferm level at the urface cre the Ferm level. n n e ( E E F Th knwn a the nvern cae. Where the mnrty carrer cncentratn (electrn) at the urface excee the majrty carrer cncentratn (hle). Semcnuctr Devce, 2/E by S. M. Sze yrght 2002 Jhn Wley & Sn. Inc. ll rght reerve. 145 MOS e uner ba trng nvern Weak nvern begn a n a (E F -E )>0, an the mnrty carrer cncentratn ncreae exnentally. Strng nvern ccur when the mnrty carrer cncentratn equal t the ubtrate ng level, (althugh cnvenent, th a largely arbtrary crtern). Beyn th nt mt f the atnal negatve charge cnt f the charge n n a very narrw n-tye nvern layer f wth x at the xeemcnuctr nterface (x ~1-10 nm << W). The eletn wth at t maxmum (W m ) at trng nvern nce a mall ncreae n ban benng reult n a very large ncreae n n. n + c n en W m 146

5 Surface eletn regn Semcnuctr Devce, 2/E by S. M. Sze yrght 2002 Jhn Wley & Sn. Inc. ll rght reerve. 147 Electrtatc tental Ban-benng ecrbe n term f a quantty whch we wll call the electrtatc tental. Th efne a zer n the bulk an a meaure f the ntrnc Ferm level tn wth reect t the bulk ntrnc Ferm level. t the emcnuctr urface, ψ ψ Surface tental Snce the electrn an hle cncentratn are gven by, n ( E E ( E E F n e & n e F We can exre them t a a functn f the electrtatc tental, n ( ψ ψ q( ψ ψ q B ne & n e B Ptve when ban bent wnwar E -E F 148

6 ccumulatn t Invern The carrer cncentratn at the urface are therefre, n ( ψ ψ q( ψ ψ q B ne & n e B The fllwng regn f urface tental are therefre entfe, ψ ψ < <ψ < ψ B ψ ψ B ψ > ψ B ccumulatn f hle (ban bent uwar) Flat-ban cntn Deletn f hle (ban bent wnwar) Intrnc cntn (n n n ) Invern (ban bent wnwar) 149 Deletn wth We can thnk f the urface eletn regn a a ne e n + - abrut junctn where the bult-n tental relace by the urface tental, W 2ε ψ en Our earler cnvenent-but-arbtrary efntn tate that at the net f trng nvern the electrn cncentratn at the urface equal t the ubtrate murty cncentratn (n N ), n e e ( ψ ( nv) ψ eψ kt B n e B ψ 2kT ( nv) 2ψ B ln e n N Ban agram 150

7 Deletn wth When the urface trngly nverte the eletn wth at t maxmum, W m ( nv) 2ε ψ 2ε 2ψ B en en 4ε kt ln e N Wm 2 ( N n ) haractertc In the abence f any wrk functn fference, the ale ba wll aear artly acr the xe an artly acr the emcnuctr, + ψ The tental acr the xe gven by, F Semcnuctr Devce, 2/E by S. M. Sze yrght 2002 Jhn Wley & Sn. Inc. ll rght reerve. x ε x x ε x x x 152

8 - haractertc The ttal caactance f the MOS e a ere cmbnatn f the xe caactance an the emcnuctr eletn-layer caactance, ( + ) j j where, j ε W ε x When the ale ba negatve, there n eletn layer an we have accumulatn f hle at the urface f the emcnuctr. In th cae, the ttal caactance cle t that f the xe haractertc Fgure 6.7. (a) Hgh-frequency MOS - curve hwng t arxmate egment (ahe lne). Inet hw the ere cnnectn f the caactr. (b) Effect f frequency n the - curve

9 Threhl vltage When the emcnuctr urface trngly nverte, the wth f the eletn regn at t maxmum. Th ccur at a late vltage that caue the urface tental t reach t nvern value. Th knwn a the threhl vltage, T en W m + T ( nv) + ψ + ψ ( nv) 2ε en ( 2ψ ) B ψ + Beyn trng nvern, the caactance wll reman at a mnmum value, mn + ε x ( ε x ε ) Wm nce, j ψ ε W m B 155 Real S-SO 2 MOS e The wrk functn fference between the metal an the emcnuctr generally nt zer a n ur eale cae, an certanly n t fr the mt cmmnly ue metal n the metal-s-so 2 ytem φm 3. 95e φm 4. 1e e ( φ φ ) < 0 m n + -lylcn lumnum Fr all ng level f S Hw wll th affect ur ban agram? 156

10 Fgure 6.8. Wrk functn fference a a functn f backgrun murty cncentratn fr l, n+-, an + lylcn gate materal. 157 S-SO 2 ban agram E vac eφ m E eφ E F E metal -S E F E E SO 2 158

11 S-SO 2 ban agram E vac E eφ eφ m E E F -S E F E metal E SO Flat-ban vltage t thermal equlbrum, the ban are bent wnwar the emcnuctr urface negatvely charge, an the metal tvely charge. In rer t get back t the flat-ban cntn cue fr ur eal MOS e we nee t aly a negatve ba t the metal (the flat-ban vltage), FB φ m eφ eφ m eφ eφ m Equlbrum Flat-ban cntn 160

12 Tra & charge Real MOS e are al affecte by charge trae wthn the xe an the nterface Metal SO 2 Na+ K Mble nc charge, m Oxe trae charge, t 3nm S Fxe xe charge, f x x x x x x x Interface trae charge, t (rentatn eenent) 161 Oxe charge & the flat-ban vltage Semcnuctr Devce, 2/E by S. M. Sze yrght 2002 Jhn Wley & Sn. Inc. ll rght reerve. 162

13 Oxe charge & the flat-ban vltage ner a tve charge heet er unt area, wthn the xe, nucng negatve charge n bth the metal an the emcnuctr. The flat-ban cntn reache by ncreang the charge n the metal (by alyng a negatve vltage, FB ) t reuce the electrc fel trbutn at the emcnuctr urface t zer. The area uner the electrc fel rfle at th nt gve the flat-ban vltage, FB F x ε x x x 163 Oxe charge & the flat-ban vltage Fr an arbtrary ace charge trbutn wthn the xe we can ntegrate the vlume charge enty, FB xρ ( x) x Fr a real e n whch the wrk functn fference nnzer an the nterface-trae charge neglgble, the flat-ban vltage gven by, FB φ m ( + + ) f m t 164

14 Fgure Effect f a fxe xe charge an nterface tra n the - charactertc f an MOS e. 165