Berkeley ELENG 105 - Lecture 17 (21 pages)

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Lecture 17



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Lecture 17

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Pages:
21
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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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 Body Effect Example VTH VTH 0 2 B VSB 2 B 2qN N A Si where Cox EE105 Fall 2007 Lecture 17 Slide 3 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 4 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 5 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 6 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



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