5172197.ppt
- 1. EE105 Fall 2007 Lecture 17, Slide 1 Prof. Liu, UC Berkeley Lecture 17 OUTLINE • NMOSFET in ON state (cont’d) – Body effect – Channel-length modulation – Velocity saturation • NMOSFET in OFF state • MOSFET models • PMOSFET Reading: Finish Chapter 6 ANNOUNCEMENTS • Wed. discussion section moved (again) to 6-7PM in 293 Cory
- 2. EE105 Fall 2007 Lecture 17, Slide 2 Prof. Liu, UC Berkeley The Body Effect B SB B TH B SB B ox Si A TH ox SB B Si A ox B Si A ox B Si A B FB ox SB B Si A B FB TH V V V C qN V C V qN C qN C qN V C V qN V V 2 2 2 2 2 ) 2 ( 2 ) 2 ( 2 ) 2 ( 2 2 ) 2 ( 2 2 0 0 • VTH is increased by reverse-biasing the body-source PN junction: is the body effect parameter.
- 3. EE105 Fall 2007 Lecture 17, Slide 3 Prof. Liu, UC Berkeley Body Effect Example ox Si A B SB B TH TH C qN V V V 2 where 2 2 0
- 4. EE105 Fall 2007 Lecture 17, Slide 4 Prof. Liu, UC Berkeley Channel-Length Modulation • The pinch-off point moves toward the source as VDS increases. The length of the inversion-layer channel becomes shorter with increasing VDS. ID increases (slightly) with increasing VDS in the saturation region of operation. sat D DS TH GS ox n sat D V V V V L W C I , 2 , 1 2 1 is the channel length modulation coefficient. L L L L L IDsat 1 1 1 DSsat DS V V L
- 5. EE105 Fall 2007 Lecture 17, Slide 5 Prof. Liu, UC Berkeley and L • The effect of channel-length modulation is less for a long- channel MOSFET than for a short-channel MOSFET.
- 6. EE105 Fall 2007 Lecture 17, Slide 6 Prof. Liu, UC Berkeley Velocity Saturation • In state-of-the-art MOSFETs, the channel is very short (<0.1m); hence the lateral electric field is very high and carrier drift velocities can reach their saturation levels. – The electric field magnitude at which the carrier velocity saturates is Esat. Si in holes for cm/s 10 6 Si in s on for electr cm/s 10 8 6 6 sat v v E
- 7. EE105 Fall 2007 Lecture 17, Slide 7 Prof. Liu, UC Berkeley Impact of Velocity Saturation • Recall that • If VDS > Esat×L, the carrier velocity will saturate and hence the drain current will saturate: • ID,sat is proportional to VGS–VTH rather than (VGS – VTH)2 • ID,sat is not dependent on L • ID,sat is dependent on W ) ( ) ( y v y WQ I inv D sat TH GS ox sat inv sat D v V V WC v WQ I ,
- 8. EE105 Fall 2007 Lecture 17, Slide 8 Prof. Liu, UC Berkeley • ID,sat is proportional to VGS-VTH rather than (VGS-VTH)2 • VD,sat is smaller than VGS-VTH • Channel-length modulation is apparent (?) Short-Channel MOSFET ID-VDS P. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting, 2004.
- 9. EE105 Fall 2007 Lecture 17, Slide 9 Prof. Liu, UC Berkeley • In a short-channel MOSFET, the source & drain regions each “support” a significant fraction of the total channel depletion charge Qdep×W×L VTH is lower than for a long-channel MOSFET • As the drain voltage increases, the reverse bias on the body-drain PN junction increases, and hence the drain depletion region widens. VTH decreases with increasing drain bias. (The barrier to carrier diffusion from the source into the channel is reduced.) ID increases with increasing drain bias. Drain Induced Barrier Lowering (DIBL)
- 10. EE105 Fall 2007 Lecture 17, Slide 10 Prof. Liu, UC Berkeley NMOSFET in OFF State • We had previously assumed that there is no channel current when VGS < VTH. This is incorrect! • As VGS is reduced (toward 0 V) below VTH, the potential barrier to carrier diffusion from the source into the channel is increased. ID becomes limited by carrier diffusion into the channel, rather than by carrier drift through the channel. (This is similar to the case of a PN junction diode!) ID varies exponentially with the potential barrier height at the source, which varies directly with the channel potential.
- 11. EE105 Fall 2007 Lecture 17, Slide 11 Prof. Liu, UC Berkeley Sub-Threshold Leakage Current • Recall that, in the depletion (sub-threshold) region of operation, the channel potential is capacitively coupled to the gate potential. A change in gate voltage (VGS) results in a change in channel voltage (VCS): • Therefore, the sub-threshold current (ID,subth) decreases exponentially with linearly decreasing VGS/m m V C C C V V GS dep ox ox GS CS / log (ID) VGS ID VGS mV/dec 60 ) 10 ( ln ) (log 1 10 T GS DS mV S dV I d S “Sub-threshold swing”:
- 12. EE105 Fall 2007 Lecture 17, Slide 12 Prof. Liu, UC Berkeley Short-Channel MOSFET ID-VGS P. Bai et al. (Intel Corp.), Int’l Electron Devices Meeting, 2004.
- 13. EE105 Fall 2007 Lecture 17, Slide 13 Prof. Liu, UC Berkeley VTH Design Trade-Off • Low VTH is desirable for high ON-state current: ID,sat (VDD - VTH) 1 < < 2 • But high VTH is needed for low OFF-state current: VTH cannot be reduced aggressively. Low VTH High VTH IOFF,high VTH IOFF,low VTH VGS log ID 0
- 14. EE105 Fall 2007 Lecture 17, Slide 14 Prof. Liu, UC Berkeley MOSFET Large-Signal Models (VGS > VTH) • Depending on the value of VDS, the MOSFET can be represented with different large-signal models. sat D DS TH GS ox sat sat D sat D DS TH GS ox n sat D V V V V WC v I or V V V V L W C I , , , 2 , 1 ) ( 1 2 1 VDS << 2(VGS-VTH) ) ( 1 TH GS ox n ON V V L W C R VDS < VD,sat DS DS TH GS ox n tri D V V V V L W C I 2 ) ( , VDS > VD,sat
- 15. EE105 Fall 2007 Lecture 17, Slide 15 Prof. Liu, UC Berkeley MOSFET Transconductance, gm • Transconductance (gm) is a measure of how much the drain current changes when the gate voltage changes. • For amplifier applications, the MOSFET is usually operating in the saturation region. – For a long-channel MOSFET: – For a short-channel MOSFET: D sat D DS ox n m sat D DS TH GS ox n m I V V L W C g V V V V L W C g , , 1 2 1 GS D m V I g sat D DS ox sat m V V WC v g , 1
- 16. EE105 Fall 2007 Lecture 17, Slide 16 Prof. Liu, UC Berkeley MOSFET Small-Signal Model (Saturation Region of Operation) D D DS o I I V r 1 • The effect of channel-length modulation or DIBL (which cause ID to increase linearly with VDS) is modeled by the transistor output resistance, ro.
- 17. EE105 Fall 2007 Lecture 17, Slide 17 Prof. Liu, UC Berkeley Derivation of Small-Signal Model (Long-Channel MOSFET, Saturation Region) ds o bs mb gs m ds DS D bs BS D gs GS D d v r v g v g v V I v V I v V I i 1 sat D DS TH GS ox n D V V V V L W C I , 2 1 2 1
- 18. EE105 Fall 2007 Lecture 17, Slide 18 Prof. Liu, UC Berkeley PMOS Transistor • A p-channel MOSFET behaves similarly to an n-channel MOSFET, except the polarities for ID and VGS are reversed. • The small-signal model for a PMOSFET is the same as that for an NMOSFET. – The values of gm and ro will be different for a PMOSFET vs. an NMOSFET, since mobility & saturation velocity are different for holes vs. electrons. Circuit symbol Schematic cross-section
- 19. EE105 Fall 2007 Lecture 17, Slide 19 Prof. Liu, UC Berkeley PMOS I-V Equations • For |VDS| < |VD,sat|: • For |VDS| > |VD,sat|: sat D DS DS DS TH GS ox p tri D V V V V V V L W C I , , 1 2 ) ( sat D DS TH GS ox sat sat D sat D DS TH GS ox p sat D V V V V WC v I or V V V V L W C I , , , 2 , 1 ) ( 1 2 1 for long channel for short channel
- 20. EE105 Fall 2007 Lecture 17, Slide 20 Prof. Liu, UC Berkeley CMOS Technology • It possible to form deep n-type regions (“well”) within a p-type substrate to allow PMOSFETs and NMOSFETs to be co-fabricated on a single substrate. • This is referred to as CMOS (“Complementary MOS”) technology. Schematic cross-section of CMOS devices
- 21. EE105 Fall 2007 Lecture 17, Slide 21 Prof. Liu, UC Berkeley Comparison of BJT and MOSFET • The BJT can achieve much higher gm than a MOSFET, for a given bias current, due to its exponential I-V characteristic.