Chaper 6 MOSFET in he On-sae The MOSFET (MOS Field-Effec Transisor) is he building block of Gb memory chips, GHz microprocessors, analog, and RF circuis. Mach he following MOSFET characerisics wih heir applicaions: small size high speed low power high gain Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-1
6.1 nroducion o he MOSFET Basic MOSFET srucure and characerisics Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-2
6.1 nroducion o he MOSFET Two ways of represening a MOSFET: Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-3
nvenion of he Field-Effec Transisor Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-4
nvenion of he Field-Effec Transisor n 1935, a Briish paen was issued o Oskar Heil. A working MOSFET was no demonsraed unil 1955. Using oday s erminology, wha are 1, 2, and 6? Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-5
Today s MOSFET Technology Gae oxides as hin as 1.2 nm can be manufacured reproducibly. Large unneling curren hrough he oxide limis oxide-hickness reducion. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-6
6.2 Complemenary MOSFETs NFET PFET When g dd, he NFET is on and he PFET is off. When g 0, he PFET is on and he NFET is off. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-7
CMOS (Complemenary MOS) nverer in PFET NFET dd S D D S 0 0 A CMOS inverer is made of a PFET pull-up device and a NFET pull-down device. ou? if in 0. C: ou capaciance (of inerconnec, ec.) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-8
CMOS (Complemenary MOS) nverer dd Conac in P + 0 ou dd PFET P + N + N+ P + P + N + N-well N-well P-subsrae in ou NFET and PFET can be fabricaed on he same chip. 0 NFET N + basic layou of a CMOS inverer Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-9
6.3 Surface Mobiliies of Elecrons and Holes g dd, dd > 0 How o measure he surface mobiliy: W Q WC oxe inv v ( g WQ µ inv ) µ ns ns / L WQ inv µ ns / L Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-10
Surface mobiliy is a funcion of he average of he fiel a he boom and he op of he inversion charge layer, b and. From Gauss s Law, g gae b Q dep /ε s + φ Q / C fb s dep oxe N T oxe - - - - - - - - b W dmax N Therefore, P-body b C ε ( Q + φ ) oxe ( fb s s b C ε oxe s dep Q ( + Q inv inv s ) / ε / ε fb s b + φ ) C + ε s oxe s ( ) 1 2 ( b + ) Coxe 2ε s Coxe 2ε s ( ( + 6T + oxe + + 0.2 2 fb + 0.2 ) 2φ ) s Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-11
Universal Surface Mobiliies ( + + 0.2)/6T oxe (M/cm) Mobiliy is a funcion of,, and T oxe. (NFET) (PFET) Wha suppresses he surface mobiliy: phonon scaering coulombic scaering surface roughness scaering ( + 1.5 0.25)/6T oxe (M/cm) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-12
EXAMPLE: Wha is he surface mobiliy a 1 in an N-channel MOSFET wih 0.3 and T oxe 2 nm? Soluion: ( + + 0.2) / 6T 1.5 /12 10 1.25 M/cm 7 oxe cm 1 M is a megavol (10 6 ). From he mobiliy figure, µ ns 190 cm2/s, which is several imes smaller han he bulk mobiliy. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-13
6.4 MOSFET and he Body Effec How o Measure he of a MOSFET 50m is measured by exrapolaing he versus curve o 0. a W Coxe( ) µ ns L Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-14
6.4 MOSFET and he Body Effec C dep ε s W d max Two capaciors > wo charge componens Q C ( ) + C inv oxe dep Cdep C oxe( ( + sb)) C oxe sb Redefine as ( sb ) C dep 0 + sb 0 Coxe + α sb Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-15
6.4 MOSFET and he Body Effec body effec: is a funcion of sb body effec coefficien: α C dep /C oxe 3T oxe / W dmax When he source-body juncion is reverse-biased, he NFET increases and he PFET becomes more negaive. model daa -2-1 0 1 2 PFET 0.6 0.4 0.2-0.2-0.4-0.6 () 0 0 NFET sb () s he body effec a good hing? How can i be reduced? Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-16
Rerograde Body Doping Profiles Body Doping (cm -3 ) 10 18 10 17 10 16 0.0 0.1 0.2 0.3 0.4 Deph (µm) model daa -2-1 0 1 2 PFET 0.6 0.4 0.2-0.2-0.4-0.6 () 0 0 NFET sb () W dep does no vary wih sb. Rerograde doping is popular because i reduces off-sae leakage. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-17
Uniform Body Doping When he source/body juncion is reverse-biased, here are wo quasi-fermi levels (E fn and E fp ) which are separaed by q sb. An NMOSFET reaches hreshold of inversion when E c is close o E fn, no E fp. This requires he band-bending o be 2φ B + sb, no 2φ B. 0 + qn C a oxe 2ε s ( 2φ B + sb 2φ B ) 0 + γ ( 2φ B + sb 2φ B ) γ is he body-effec parameer. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-18
6.5 Q inv in MOSFET Channel volage c s a x 0 and c d a x L. Q inv C oxe ( cs 0 α ( sb + cs ) C oxe ( cs ( 0 +α sb ) α cs ) C oxe ( m cs ) m 1 +α 1+ 3T oxe /W dmax m is called he body-effec facor or bulk-charge facor Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-19
6.6 Basic MOSFET Model WQ inv v WQ inv µ ns WC oxe ( m cs )µ ns d cs /dx L dx WCoxeµ ns ( mcs 0 0 ) d cs L WC oxe µ ns ( m /2) W C µ oxe s ( L m 2 ) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-20
a : Drain Sauraion olage d d W 0 Coxeµ ns ( m ) L a m Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-21
a > a cs cs (a) a (e) a x x 0 L 0 L Q inv C ox ( g m cs ) Q inv (b) (f) x x 0 L 0 L µ n Q in v d cs /dx µ n Q in v d cs /dx (c) (g) a a 0 L x 0 L x E c - - E c - - - - source source (d) drain (h) a drain Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-22
Sauraion Curren and Transconducance linear region, sauraion region a W Coxeµ 2mL ns ( 2 ) ransconducance: g m d /d g msa W Coxeµ ml ns ( ) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-23
6.7.1 CMOS nverer olage Transfer Curve Regeneraion of Digial Signal PFET 2 S dd in 0 PFET (ma) dd 0.2 NFET in 2 D in 0.5 in 1.5 in D ou 0.1 NFET S in 1 in 1 0.2 dd (ma) 0 0 2 in 1.5 in 0.5-2.0-1.5-1.0-0.5 0 0.5 1.0 1.5 2.0 () in 2 in 0 dd ou () 2.0 0.5 1.5 1.5 0.1 1.0 1 1 0.5 dd 0 0.5 1.0 1.5 2.0 ou () 0 0.5 1.0 1.5 2.0 in () Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-24
6.7.2 CMOS nverer Delay dd... 1 2 3... C C dd 2 2τ d 3 1 0 τ d : propagaion delay Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-25
τ d 1 2 pull up pull down τ d R N ( pull down C and delay ( delay 1 C 2 + delay dd ap C 2 1 dd an dd 4 an ap R P 6.7.2 CMOS nverer Delay 2 dd on 2 ) + a ( pull up dd g dd ) delay) How can he speed of an inverer circui be improved? Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-26
6.7.3 CMOS Power Consumpion P dynamic dd average curren C 2 dd f P saic dd off P direc pah dd 5 0.2P a r dynamic + 2 f f 0.2C 2 dd f Toal power consumpion P 1.2C f + 2 dd dd off Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-27
Logic Gaes dd AB A B This wo-inpu NAND gae and many oher logic gaes are exensions of he inverer. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-28
6.8 elociy Sauraion v 1 µ + s sa << sa : v µ s >> sa : v µ s sa velociy sauraion has large and deleerious effec on he on of MOSFETS Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-29
6.9 MOSFET Model wih elociy Sauraion L WQ WC inv oxe 0 0 L dx WC v ( oxe [ WC µ ns ( m oxe cs µ ns ( µ nsd ) d 1+ dx m 2 m cs ) cs cs / dx / sa ) / / sa sa ] d cs Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-30
6.9 MOSFET Model wih elociy Sauraion W L C oxe µ ns ( 1+ sa L m 2 ) long - channel 1+ / sa L Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-31
6.9 MOSFET Model wih elociy Sauraion Solving a 1+ d d 0, 2( 1+ 2( ) / m ) / m sa L A simpler and more accurae a is: 1 a m + 1 sa L sa 2v µ sa s Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-32
EXAMPLE: Drain Sauraion olage Quesion: A 1.8, wha is he a of an NFET wih T oxe 3 nm, 0.25, and W dmax 45 nm for (a) L 10 µm, (b) L 1 um, (c) L 0.1 µm, and (d) L 0.05 µm? Soluion: From,, and T oxe, µ ns is 200 cm 2-1 s -1. sa 2v sa /µ es 8 104 /cm m 1 + 3T oxe /W dmax 1.2 a m + 1 sa L 1 Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-33
EXAMPLE: Drain Sauraion olage a m + 1 sa L 1 (a) L 10 µm, a (1/1.3 + 1/80) -1 1.3 (b) L 1 µm, a (1/1.3 + 1/8) -1 1.1 (c) L 0.1 µm, a (1/1.3 + 1/.8) -1 0.5 (d) L 0.05 µm, a (1/1.3 + 1/.4) -1 0.3 Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-34
a wih elociy Sauraion Subsiuing a for in equaion gives: a W 2mL C oxe 2 ( ) µ s 1+ m L sa long - channel 1+ m L sa a ery shor channel case: a W C µ oxe s 2 Wv C ( sa oxe sa ( Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-35 sa L << ) sa L) a is proporional o raher han ( ) 2, no as sensiive o L as 1/L.
Measured MOSFET (ma/µm) 0.4 0.3 0.2 0.1 L 0.15 µm 0.4 2.5 2.0 1.5 1.0 (µa/µm) 0.03 0.02 0.01 L 2.0 µm 2.5 0.7 2.0 1.5 1.0 0.0 0 1 2 2.5 () 0.0 () Wha is he main difference beween he g dependences of he long- and shor-channel lengh curves? Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-36
PMOS and NMOS Characerisics The PMOS is qualiaively similar o he NMOS, bu he curren is abou half as large. How can we design a CMOS inverer so ha is volage ransfer curve is symmeric? Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-37
6.10 Parasiic Source-Drain Resisance conac meal dielecric spacer G gae oxide S R s R d D channel CoSi 2 or TiSi 2 a0 f a0 g, a a0rs 1+ ( ) a is reduced by abou 15% in a 0.1µm MOSFET. a a0 + a (R s + R d ) N + source or drain Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-38
Definiions of Channel Lengh L drawn L g N N L, L eff, or L e L Ldrawn L Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-39
6.11 Exracion of he Series Resisance and he Effecive Channel Lengh WC L oxe drawn µ s L ( L WC ( oxe drawn ( L) )µ s ) --------( Ω) 300 200 daa inercep 1 nclude series resisance, R R d + R s, R 100 2 R L + WC oxe L ( )µ drawn s L 1 2 L drawn (µm) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-40
6.12 Source njecion elociy Limi gae E c S D N + N + - Carrier velociy is limied by he hermal velociy when hey firs ener he channel from he source. E v a WBv hx Q inv WBv hx C oxe ( ) Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-41
body effec sb 0 6.13 Chaper Summary ( ) + α for seep rerograde body doping α 3T oxe / W dmax basic model W m Coxeµ s ( ) L 2 m sb dmax 1+ 3T oxe / W 1.2 Small α and m are desirable. Therefore, small T oxe is good. Ch.7 shows ha large W dmax is no accepable. CMOS circui speed is deermined by C dd / a, and is power by C dd2 f + dd off. Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-42
6.13 Chaper Summary characerisics can be divided ino a linear region and a sauraion region. sauraes a: a a m W C 2mL oxe µ ( s 2 ) ransconducance: g msa W Coxeµ ml s ( ) Considering velociy sauraion, a m 1 + sa L 1 a long - channel 1+ m L sa a Semiconducor Devices for negraed Circuis (C. Hu) Slide 6-43