GBM8320 Dispositifs Médicaux telligents Microelectronics Part 1: Main CMOS circuits design rules Mohamad Sawan et al. Laboratoire de neurotechnologies Polystim! http://www.cours.polymtl.ca/gbm8320/! med-amine.miled@polymtl.ca! mohamad.sawan@polymtl.ca! M5031 23 January 2013 line Main CMOS circuits design rules troduction The CMOS process CMOS technology processing The MOS Transistor Basic device physics Small Signal Model Basic analog CMOS circuits verter Voltage follower Current mirrors Amplifiers and Op-Amps. GBM5320 - Dispositifs Médicaux telligents 2 1
CMOS technology for medical implants integration Low power consumption is crucial for medical implant devices! A single-chip must allow very-low-power operation while containing amplifiers, filters, ADCs, battery management system, voltage multipliers, high voltage pulse generators, programmable logic and timing control! Recent CMOS processes are suitable for pure analog integration with high operating speed! CMOS is suitable to VLSI of both high-density digital circuits (e.g. DSP, memory, etc.) and analog circuits (amplifiers, ADC, DAC, etc.)! CMOS digital circuits feature 0 static power consumption.! High performance MOS switches à CMOS technology suitable for high accuracy sample-data circuits.! GBM5320 - Dispositifs Médicaux telligents 3 Mixed signal design overview Newer CMOS technologies with smaller feature sizes (such as 180nm and 130nm) can operate at increasingly high speed (5GHz), comparable to some bipolar technologies.! CMOS technologies become mainstream technologies for mixed-signal integration due to the advantages of low cost and high integration density. Digital circuitries cost decreases by 29% each year in CMOS technology thanks to device downscaling;! To benefit from this, analog ICs have to be integrated on the same chip with the digital circuits in mixed-signal integration;! e are in SoC (System on a Chip) era, which favors CMOS technology;! System on Chip: mixed-signal integrated circuits that contains analog, memory, logic, and embedded processor.! GBM5320 - Dispositifs Médicaux telligents 4 2
Mixed signal design overview MOSFET f t frequency is continuously increasing over time.! The minimum channel length of MOS transistors dropped from 25 mm in 1960s to 60 nm in the year 2005.! Benefit of much higher complexity, smaller volume, less power consumption and higher frequency performance.! GBM5320 - Dispositifs Médicaux telligents 5 line troduction The CMOS process CMOS technology processing The MOS Transistor Basic device physics Small Signal Model Basic blocks in CMOS Analog Circuits verter Voltage follower Current mirrors Amplifiers GBM5320 - Dispositifs Médicaux telligents 6 3
CMOS technology processing CMOS technologies have penetrated application areas, which used to be the exclusive domain of bipolar or BiCMOS technology.! of seven integrated RF transceivers introduced in 2003, four are realized in a CMOS process technology.! GBM5320 - Dispositifs Médicaux telligents 7 CMOS technology processing Four terminals: gate, source, drain, body! Source Gate Drain Polysilicon Gate oxide body stack looks like a capacitor! Gate and body are conductors! SiO 2 (oxide) is a very good insulator.! Called Metal-oxide-semiconductor (MOS)! Even though gate is no longer made of metal.! S D p Bulk Si SiO 2 G B GBM5320 - Dispositifs Médicaux telligents 8 4
CMOS technology processing Lithography process similar to printing press! On each step, different materials are deposited or etched.! Typically use p-type substrate for nmos transistors! Requires n-well for body of pmos transistors.! Vss SiO 2 p substrate n well diffusion diffusion polysilicon metal1 nmos transistor pmos transistor GBM5320 - Dispositifs Médicaux telligents 9 CMOS technology processing Substrate are tied to and n-well to! Metal to lightly-doped semiconductor forms poor connection.! Use heavily doped well and substrate contacts / taps.! p substrate n well substrate tap well tap GBM5320 - Dispositifs Médicaux telligents 10 5
CMOS technology processing Transistors and wires are defined by masks.! Cross-section taken along dashed line.! substrate tap p substrate n well well tap A VSS substrate tap nmos transistor pmos transistor well tap GBM5320 - Dispositifs Médicaux telligents 11 CMOS technology processing Six masks! n-well! Polysilicon! diffusion! diffusion! Contact! Metal! n well Polysilicon Diffusion Diffusion A Contact substrate tap nmos transistor pmos transistor well tap Metal GBM5320 - Dispositifs Médicaux telligents 12 6
CMOS technology processing 0.35_m 200nm 1.25_m p substrate n well 600_m Cross section of 0.35um CMOS technology! 6.5nm GBM5320 - Dispositifs Médicaux telligents 13 line troduction The CMOS process CMOS technology processing The MOS Transistor Basic device physics Small Signal Model Basic blocks in CMOS Analog Circuits verter Voltage follower Current mirrors Amplifiers GBM5320 - Dispositifs Médicaux telligents 14 7
MOSFET Structure The NMOS transistor is on p- substrate (bulk or body).! L Two regions form S (source) and D (drain) terminals.! MOS transistor is symmetric. S has lower potential than D for NMOS.! p- substrate is connected to the most negative voltage.! L drawn is the channel length drawn in the layout! L is the effective channel length.! t ox is the gate oxide thickness (40Å in 0.18 µm and 22Å in 0.13 µm)! GBM5320 - Dispositifs Médicaux telligents 15 MOSFET Structure If V GS > 0, the electrical field will repel holes and attracts electrons.! hen V GS reaches a value called the threshold voltage (V th ), channel under the gate becomes inverted.! Depletion region!it changes from p-type to n-type semiconductor.! S V G >>0 + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - p - substrate B D Channel n-type channel exists between the source and drain that allows carriers to flow.! GBM5320 - Dispositifs Médicaux telligents 16 8
I-V characteristics! If V GS > V th the channel is inverted. Conductivity is controlled by V GS - V th.! hen V DS > 0 current flows from drain to source.! The drain current :! = dq dt VS = 0 - + V(y) y V G S >V th y+dy B y V DS > 0 dq is the channel charge in dy at a distance y from the source, and dt is the time required for this charge to cross the length dy.! GBM5320 - Dispositifs Médicaux telligents 17 I-V characteristics (Cont d)! dq = Q I dy Q I is the induced electron charge in unit area of the channel.! The gate-to-channel voltage at a distance y from the source is V GS -V(y).! Assume this voltage exceeds V th we can write:! Q I = C ox "# V GS V ( y) V th $ % C ox = ε ox = K ε ox 0 t ox t ox dt = dy v d (y) is the electron velocity at y.! v d ( y) v d ( y ) = n E ( y ), E ( y ) = - E(y) is horizontal electrical field and µ n is the average electron mobility.! d V d y VS = 0 - + V(y) y VG S>Vth y+dy B y VDS> 0 ID = dq dt = C ox dv "# V GS V ( y) V th $ % µ n dy GBM5320 - Dispositifs Médicaux telligents 18 9
I-V characteristics (Cont d)! L V dy = C 0 DS ox #$ V GS V ( y) V th % & µ n dv 0 C ox " 2L # 2 V V GS th 2 ( )V DS V DS $ % VS = 0 VG S>Vth V DS> 0 ID If V DS << 2(V GS - V th ), is proportional to V DS.! L V V GS th )V DS - + V(y) y y+dy B y GBM5320 - Dispositifs Médicaux telligents 19 I-V characteristics (Cont d)! V G S >>V th VS = 0 VD > 0 I < µ ncox ( VGS L Vth ) VDS D I = µ C ( V V ) V L D n ox GS th DS B V DS As V DS increases, increases until the drain end of the channel becomes pinch off.! Pinch off occurs when V GD <= V th the channel is not inverted near the drain (Q I =0).!! V GD V th V DS V GS V th GBM5320 - Dispositifs Médicaux telligents 20 10
I-V characteristics (Cont d)! V S = 0 V G S >>V th V GD <V th, V DS >V GS -V th I = µ C n ( V 2L V ) 2 th D ox GS Active region Triode region Pinch -off C ox " 2L # 2 V V GS th 2L V V GS th ) 2 2 ( )V DS V DS $ % V DS =V GS V th =V Dsat I = µ C ( V V ) V L D n ox GS th DS VDS =Vdsat V DS For V DS > V GS -V th stays constant by ignoring the second order effects.! GBM5320 - Dispositifs Médicaux telligents 21 line troduction The CMOS process CMOS technology processing The MOS Transistor Basic device physics Small Signal Model Basic blocks in CMOS Analog Circuits verter Voltage follower Current mirrors Amplifiers GBM5320 - Dispositifs Médicaux telligents 22 11
Channel length modulation! increases slightly with increasing V DS due to the increasing of the depletion region width X d with V DS!! C ox 2L eff VS = 0 ( V GS V th ) 2, L eff = L X d Leff VB = 0 VG S>>Vth Xd VDS >VGS-Vth ID d = µ dv n C ox V 2 DS 2L GS V th eff ( ) 2 dl eff dv DS = L eff dx d dv DS = λ GBM5320 - Dispositifs Médicaux telligents 23 Channel length modulation (cont d)! Therefore, a good approximation to the influence of V DS on is! Triode region 2L V V GS th ) 2 ( 1+ λv DS ) Active or pinch -off region ( λ = 0) + d V dv DS DS = ( λ = 0) ( 1+ λv DS ) VGS <=V th Ideal VGS creases VDS = VGS -Vth Actual 2L V V GS th ) 2 ( 1+ λv DS ) L V V GS th )V DS V DS GBM5320 - Dispositifs Médicaux telligents 24 12
! 2013-01-23 Body effect! V S > 0 V G S V DS If V SB increases, the effective threshold voltage increases.! V SB increases, the depletion region between the channel and the substrate becomes wider à Q B k.! VB = 0 ID Q B qn A x d = 2qε Si N A 2Φ F 2qε Si N A (2Φ F +V SB ) V th = V th0 + ΔV th, V th0 = V th (V SB = 0) V th = V th0 + γ ( V SB + 2Φ F 2Φ F ), γ = 2qN K ε A Si 0 C ox γ is the body effect constant! GBM5320 - Dispositifs Médicaux telligents 25 PMOS equations ' µ p C " ox = 2L 2 ( V V 2 SG thp )V SD V $ # SD %, V > V andv > V SG thp DG thp ) ( ) µ p 2L V V SG thp ) 2 ( 1+ λv SD ), V SG > V thp andv DG < V *) thp ID Triode region Active or pinch -off region V SD = V SG - V thp Actual Ideal VSG creases VSG <= Vthp VSD GBM5320 - Dispositifs Médicaux telligents 26 13
MOS symbols D B G S NMOS PMOS The B symbol is used for substrate to avoid confusion with source.! Drain in NMOS is positioned on top while the source is positioned on top for PMOS.! Symbol with B connection is used when the source and the substrate have different voltages! Symbols w/o arrow are used for digital circuit.! GBM5320 - Dispositifs Médicaux telligents 27 Device model summary Linear / triode region V DS < V GS - Vth Saturation region V DS >= V GS - Vth eak inversion V GS < Vth V GS V DS " # nv T V T I = I e 1 e D S 0 L $ % & ' V GS I e, V >> V L nv T S 0 DS T kt 0 V = 26 mv at T = 300 K T q Strong inversion V GS > Vth 2 ) V * DS I = µ C ( ) D ox + V V V GS th DS, L - 2. µ C ) -( V V ) V *., V << V L ox GS th DS DS dsat 1 I = µ C V V + λv 2 L 2 ( ) ( 1 ) D ox GS th DS GBM5320 - Dispositifs Médicaux telligents 28 14
Small-Signal Models of MOS Transistors: NMOS Triode region Active or pinch -off region VDS = VGS -Vth Actual Ideal VGS creases VGS <=V th # µ n = L V V GS thn )V DS, V GS > V thn andv GD > V thn (V DS < V GS V thn ) % $ % µ n 2L V V GS thn ) 2 ( 1+ λv DS ), V GS > V thn and V GD < V thn (V DS > V GS V thn ) &% V DS V thn = V thn0 + γ ( V SB + 2Φ F 2Φ F ), γ = 2qN K ε D Si 0 λ = C ox dx d L eff dv DS GBM5320 - Dispositifs Médicaux telligents 29 Small-Signal Models of MOS Transistors: PMOS ID Triode region Active or pinch -off region V SD = V SG - V thp Actual Ideal VSG creases VSG <= Vthp # µ p = L V V SG thp )V SD, V SG > V thp andv DG > V thp (V SD < V SG V thp ) % $ % µ p 2L V V SG thp ) 2 ( 1+ λv SD ), V SG > V thp andv DG < V thp (V SD > V SG V thp ) &% V SD V thp = V thp0 + γ ( V BS + 2Φ F 2Φ F ), γ = 2qN K ε A Si 0 C ox dx λ = d L eff dv SD GBM5320 - Dispositifs Médicaux telligents 30 15