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 Chap 6 EE105 Spring 2008 Lecture 17 Slide 1 Prof Wu UC Berkeley Body Effect Example VTH VTH 0 where 2 B VSB 2 B 2qN A Si Cox Example Typical values 0 5 B 0 48V for N A 1018 cm 3 substrate doping A substrate bias of VSB 1V produce a VTH shift of 0 2V EE105 Spring 2008 Lecture 17 Slide 2 Prof Wu 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 1 1 L I Dsat 1 L L L L L VDS VDSsat W 1 2 nCox VGS VTH 1 VDS VD sat L 2 channel length modulation coefficient W 1 2 Note in Razavi I D sat nCox VGS VTH 1 VDS L 2 I D sat EE105 Spring 2008 Lecture 17 Slide 3 Prof Wu UC Berkeley and L The effect of channel length modulation is less for a long channel MOSFET than for a short channel MOSFET 1 short channel MOSFET has larger L EE105 Spring 2008 Lecture 17 Slide 4 Prof Wu UC Berkeley Velocity Saturation In state of the art MOSFETs the channel is very short 0 1 m 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 v Saturation Velocity vsat E Drift velocity v E Slope EE105 Spring 2008 vsat 8 106 cm s for electrons in Si 6 6 10 cm s for holes in Si NMOS n 250 cm 2 V s Esat 30 000 V cm 2 PMOS n 80 cm V s Esat 80 000 V cm For L 0 1 m VD sat 0 3 V for NMOS VD sat 0 8 V for PMOS Lecture 17 Slide 5 Prof Wu 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 Spring 2008 Lecture 17 Slide 6 Prof Wu UC Berkeley Short Channel MOSFET ID VDS P Bai et al Intel Corp Int l Electron Devices Meeting 2004 ID sat is proportional to VGS VTH rather than VGS VTH 2 VD sat is smaller than VGS VTH Channel length modulation is apparent EE105 Spring 2008 Lecture 17 Slide 7 Prof Wu 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 Qdep W L VTH is lower than for a long channel MOSFET DIBL Injection Barrier Source ShortChannel LongChannel qVDS Drain Drain 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 EE105 Spring 2008 Lecture 17 Slide 8 Prof Wu 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 below VTH towards 0 V 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 EE105 Spring 2008 Lecture 17 Slide 9 Prof Wu 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 VCS Cox VGS C C dep ox Cdep 1 VGS m m 1 Cox Therefore the sub threshold current ID subth decreases exponentially with linearly decreasing VGS m ID log ID Sub threshold swing 1 VTH EE105 Spring 2008 VGS VTH VGS Lecture 17 Slide 10 d log10 I DS S dV GS S mVT ln 10 60mV dec Prof Wu UC Berkeley Short Channel MOSFET ID VGS P Bai et al Intel Corp Int l Electron Devices Meeting 2004 EE105 Spring 2008 Lecture 17 Slide 11 Prof Wu 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 state current log ID Low VTH VTH cannot be reduced aggressively High VTH IOFF low VTH IOFF high VTH 0 EE105 Spring 2008 VGS Lecture 17 Slide 12 Prof Wu UC Berkeley MOSFET Large Signal Models VGS VTH Depending on the value of VDS the MOSFET can be represented with different large signal models VDS 2 VGS VTH RON 1 nCox W VGS VTH L EE105 Spring 2008 Triode Region Saturation Region 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 2 L or ID sat vsatWCox VGS VTH 1 VDS VD sat Lecture 17 Slide 13 Prof Wu 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 the MOSFET is usually operating in the saturation region For a long channel MOSFET 2I D W g m nCox VGS VTH 1 VDS VD sat L VGS VTH For a short channel MOSFET g m vsatWCox 1 VDS VD sat EE105 Spring 2008 Lecture 17 Slide 14 ID VGS VTH Prof Wu 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 Spring 2008 Lecture 17 Slide 15 Prof Wu UC Berkeley Derivation of Small Signal Model Long Channel MOSFET Saturation Region 1 W 2 I D nCox VGS VTH 1 VDS VD sat 2 L I D I D I D 1 id vgs vbs vds gmvgs gmbvbs vds ro VGS VBS VDS gmvgs gmbvbs id vgs EE105 Spring 2008 Lecture 17 Slide 16 Prof Wu UC Berkeley PMOS Transistor A p channel MOSFET behaves similarly to an n channel MOSFET except the 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 …
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