Lecture 17 ANNOUNCEMENTS Wed Wed discussion section moved again to 6 7PM in 293 Cory OUTLINE NMOSFET in ON state cont d Body effect Channel length Channel length modulation Velocity saturation NMOSFET in OFF state MOSFET models PMOSFET Reading Finish Chapter 6 EE105 Fall 2007 Lecture 17 Slide 1 Prof Liu UC Berkeley The Body Effect VTH is increased by reverse biasing the body source PN junction VTH VFB 2 B 2qN A Si 2 B VSB Cox 2qN A Si 2 B 2qN A Si 2 B 2qN A Si 2 B VSB VFB 2 B Cox Cox Cox VTH 0 2qN A Si Cox 2 B VSB 2 B VTH 0 2 B VSB 2 B is the body effect parameter EE105 Fall 2007 Lecture 17 Slide 2 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 L VDS VDSsat I Dsat 1 1 L 1 L L L L 1 W 2 I D sat nCox VGS VTH 1 VDS VD sat 2 L is the channel length modulation coefficient EE105 Fall 2007 Lecture 17 Slide 3 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 EE105 Fall 2007 Lecture 17 Slide 4 Prof Liu UC Berkeley Velocity Saturation In state of the art MOSFETs the channel is very short 0 1 m hence the lateral electric field is veryy high g and carrier drift velocities can reach their saturation levels The electric field magnitude at which the carrier velocity saturates is Esat v vsat 8 106 cm s for electrons in Si 6 6 10 cm s for holes in Si E EE105 Fall 2007 Lecture 17 Slide 5 Prof Liu UC Berkeley Impact of Velocity Saturation Recall that I D WQinv y v y If VDS Esat L the carrier velocity will saturate and hence the drain current will saturate I D sat WQinv vsat WCox VGS VTH vsat 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 EE105 Fall 2007 Lecture 17 Slide 6 Prof Liu UC Berkeley Short Channel MOSFET ID VDS P Bai et al Intel Corp Int l Electron Devices Meeting Meeting 2004 2004 ID sat is proportional to VGS VTH rather than VGS VTH 2 VD sat sat is smaller than VGS VTH Channel length modulation is apparent EE105 Fall 2007 Lecture 17 Slide 7 Prof Liu UC Berkeley Drain Induced Barrier Lowering DIBL In a short channel MOSFET the source drain regions each support a significant fraction of the total channel depletion charge Qdepp 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 increases and hence the drain depletion region widens 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 EE105 Fall 2007 Lecture 17 Slide 8 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 b li i d by limited b carrier i diffusion diff i into i the h channel h l rather h than by carrier drift through the channel This is similar to the case of a PN jjunction diode ID varies exponentially with the potential barrier height at the source which varies directly with the channel potential EE105 Fall 2007 Lecture 17 Slide 9 Prof Liu UC Berkeley Sub Threshold Leakage Current Recall that in the depletion sub threshold region of operation the channel p potential is capacitively p y coupled p to the ggate potential p A change in gate voltage VGS results in a change in channel voltage VCS VCS Cox VGS VGS m C C dep ox Therefore Therefore the sub threshold sub threshold current ID subth D b h decreases exponentially with linearly decreasing VGS m ID log ID Sub threshold swing 1 VGS EE105 Fall 2007 VGS Lecture 17 Slide 10 d log10 I DS S dVGS S mV VT ln l 10 60mV dec V d Prof Liu UC Berkeley Short Channel MOSFET ID VGS P Bai et al Intel Corp Int l Electron Devices Meeting 2004 EE105 Fall 2007 Lecture 17 Slide 11 Prof Liu UC Berkeley VTH Design Trade Off Low VTH is desirable for high ON state current 1 2 ID sat VDD VTH But high VTH is needed for low OFF OFF state state current log ID Low VTH VTH cannot be reduced aggressively High VTH IOFF low VTH IOFF high VTH 0 EE105 Fall 2007 VGS Lecture 17 Slide 12 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 g g models VDS 2 VGS VTH RON 1 nCox W VGS VTH L EE105 Fall 2007 VDS VD sat VDS VD sat V W ID tri nCox VGS VTH DS VDS L 2 1 W 2 ID sat nCox VGS VTH 1 VDS VD sat L 2 or ID sat vsatWCox VGS VTH 1 VDS VD sat Lecture 17 Slide 13 Prof Liu UC Berkeley MOSFET Transconductance gm Transconductance gm is a measure of how much the drain current changes when the gate voltage changes I D gm VGS For amplifier applications applications the MOSFET is usually operating in the saturation region For a long channel MOSFET W g m n Cox VGS VTH 1 VDS VD sat L W g m 2 nCox 1 VDS VD sat I D L For a short channel MOSFET g m vsatWCox 1 VDS VD sat EE105 Fall 2007 Lecture 17 Slide 14 Prof Liu UC Berkeley MOSFET Small Signal Model Saturation Region of Operation The effect of channel length modulation or DIBL which cause ID to increase linearly with VDS is modeled by the transistor output resistance ro V DS 1 ro I D I D EE105 Fall 2007 Lecture 17 Slide 15 Prof Liu UC Berkeley PMOS Transistor A p channel MOSFET behaves similarly to an n channel MOSFET except p the p polarities for ID and VGS are reversed Schematic cross section Circuit symbol 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 vs an NMOSFET NMOSFET since mobility saturation velocity are different for holes vs electrons EE105 Fall 2007 Lecture 17 Slide 16 Prof Liu UC Berkeley PMOS I V Equations For VDS VD sat VDS W ID tri pCox VGS VTH VDS 1 VDS VD sat L 2 For VDS VD sat 1 W 2 ID …
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