Semiconductor Devices. C. Hu: Modern Semiconductor Devices for Integrated Circuits Chapter 6

Transcription

1 Semiconducor Devices C. Hu: Modern Semiconducor Devices for Inegraed Circuis Chaper 6

2 For hose of you who are sudying a bachelor level and need he old course S Mikro- ja nanoelekroniikan perusee o finish your degree, we organize an exra exam in May (Thursday 12h of May 13 -> in Micronova). Mandaory regisraion for he exam via o ville.vahanissi@aalo.fi a he laes on 1s of May. The exam language is finnish. The exam will be abou he book of Hu (Modern Semiconducor Devices for Inegraed Circuis), chapers 1-6. If you like, you can sill of course replace he old course S wih his on-going course. Excursion: Sill room for 2 sudens

3 3 differen operaion regimes & 2 ineresing ransiion poins accumulaion, fla band, depleion, hreshold, inversion FLAT-BAND THRESHOLD MOS capacior C-V characerisics Oxide capaciance Depleion region capaciance

4 Inroducion MOSFET on i s on sae Surface mobiliy, V, body effec, Q inv I-V model & characerisics

5 The mos imporan semiconducor device Building block of Gb memory chips GHz microprosessors Analog circuis RF circuis Mach he following MOSFET characerisics wih heir applicaions: small size high speed low power high gain

6 The mos imporan semiconducor device Building block of Gb memory chips GHz microprosessors Analog circuis RF circuis Mach he following MOSFET characerisics wih heir applicaions: small size Gb memory chips high speed GHz microprosessors, RF circuis low power GHz microprosessors, RF circuis high gain analog circuis

7 Basic MOSFET srucure and IV characerisics Wha is desirable? Large I on, small I off

8 Two ways of represening a MOSFET

9 The superioriy of he MOSFET-srucure is based on he abiliy o fabricae very high-qualiy oxide-silicon inerfaces and very high qualiy oxides, even in he case of exremely hin layers 1.2 nm size of four SiO 2 molecules

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11 Boh NMOSFET and PMOSFET needed PMOSFET pull-up device NMOSFET pull-down device V ou =? if V in = 0 V Small power consumpion

12 NMOSFET and PMOSFET can be fabricaed on he same chip

13 I is highly desirable o have a large ransisor curren (high circui speed) Elecron or hole mobiliy in he surface inversion layer is an imporan facor deermining he MOSFET curren How o measure he surface mobiliy I ds = W Q v = WQ m inv inv nse = WC ( V -V ) m V / oxe gs ns ds = WQ L inv m ns V ds / L

14 Mobiliy is a funcion of he average of he fields a he boom and he op of he inversion charge layer, E b and E From Gauss s Law, E b = Q dep /e s V = V -Q / C E b fb C = e s Therefore, - dep - ) oxe ( V V fb s s oxe E = -( Q = E b C = e oxe s dep - Q ( V Q inv gs inv s -V ) / e / e = fb E s b - ) C e s oxe s ( V gs -V ) 1 2 ( E b E ) = = Coxe 2e V gs s Coxe 2e s ( V ( V V gs gs 6T V oxe V 0.2 V - 2V fb 0.2 V) - 2 ) s

15 Surface mobiliy is clearly lower (1/3) han bulk mobiliy Surface roughness scaering Surface roughness scaering is sronger a higher V g, higher V and smaller T oxe

16 EXAMPLE: Wha is he surface mobiliy a V gs =1 V in an N-channel MOSFET wih V =0.3 V and T oxe =2 nm? ( V V 0.2) / 6T gs oxe

17 EXAMPLE: Wha is he surface mobiliy a V gs =1 V in an N-channel MOSFET wih V =0.3 V and T oxe =2 nm? Soluion: ( V gs V 0.2) / 6T = 1.5 V / oxe cm = 1.25 MV/cm 1 MV is a megavol. From he mobiliy figure, m ns =190 cm2/vs, which is several imes smaller han he bulk mobiliy.

18 The mos obvious way o improve is o use a semiconducor having higher mobiliy han Si MESFET = Meal-Semiconducor-Field-Effec-Transisor Operaion is based on he volage dependency of he depleion region of a reverse biased Schoky-diode Depending on he gae volage, he depleion region under he gae expands or conracs The hickness of he conducive channel is modulaed The channel curren is modulaed

19 Componen is based on GaAlAs/GaAs-heerojuncions A large Eg semiconducor, GaAlAs, serves as he gae dielecric Elecrons in he channel (GaAs) are far away from he donors (AlGaAs) Impuriy scaering decreases and mobiliy increases Smooh surface, small surface scaering

20 The gae is a PN-juncion By conrolling he deph of he depleion region, also he channel deph can be conrolled Deeper depleion region, smaller conducion channel

21 Mehod A V is deermined by exrapolaing he I ds vs. V gs curve o I ds = 0 I dsa V W = Coxe( Vgs -V ) mnsv L -V gs ds Mehod B The Vg a which Ids = 0.1 µa W/L

22 MOSFET is a 4-erminal componen, also he subsrae bias has an effec on he channel Inversion layer forms a capacior boh wih he gae (oxide) and also wih he subsrae (depleion layer) Two capaciors, wo charge componens oxe e Sio 2 C = T oxe C dep e s = W d max Q inv = -C ( V -V ) C = -C oxe oxe gs dep V Cdep ( Vgs - ( V Vsb)) C oxe sb Redefine V as V ( V sb ) C dep = V 0 Vsb = V 0 Coxe V sb

23 Body effec V is a funcion of V sb. When he source-body juncion is reverse-biased, V increases. Body effec coefficien V = V 0 V sb = C dep /C oxe = 3T oxe / W dep Body effec raises V and herefore slows down circuis. How can i be reduced?

24 Rerograde doping is popular because i reduces offsae leakage and allows higher surface mobiliy

25 Channel volage V c =V s a x = 0 and V c =V d a x = L Applied source-drain-volage causes an addiional poenial (in addiion o gae-volage) This has o be aken ino accoun when calculaing he charge in he channel Q inv = C oxe (V gs V cs V 0 (V sb +V cs ) = C oxe (V gs V cs (V 0 + V sb ) V cs ) = C oxe (V gs mv cs V ) m 1 + = 1 + 3T oxe /W dmax m is called he body-effec facor or bulk-charge facor

26 Curren in he channel I ds = WQ inv v= WQ inv m ns E = WC oxe (V gs mv cs V )m ns dv cs /dx Inegraion allong he channel L V ds Idsdx = WCoxem ns ( Vgs - mvcs V ) dv cs I ds L = WC oxe m ns (V gs V mv ds /2)V ds Model for he linear region (applies before curren sauraion) I ds W = C m oxe s( Vgs -V - L m V 2 ds ) V ds

27 Wih high enough source-drain-volages he curren in he channel sauraes dids W = 0 = C m V - V - V a V = V dv L ds V dsa V = gs oxe ns gs ds ds dsa -V m In his case he inversion charge a he drain end is 0 (channel pinch-off) Area wih high resisance is formed (large volage loss) A he channel area he poenial disribuion is no significanly changed evenhough volage is increased Curren sauraes

28 V cs (x), Q inv (x) and herefore Ids are he same I ds does no change wih V ds beyond V dsa A V ds > V dsa here exiss a shor high-field pinch-off region where Q inv = 0 and across which he volage V ds -V dsa is dropped In he pinch-off region elecrons are swep down he seep poenial drop Elecron flow raes equal (same drif field and Q inv in he channel

29 Sauraion curren W Idsa = Coxemns( Vgs -V 2mL Transconducance W gmsa = Coxem - ml 2 ) ns( Vgs V Applicable in he case of long channel (L > 10mm) )

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32 MOSFET coninues