Lecture11-MOS_Cap_Delay-6up - EE141-Fall 2010 Digital...

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EE141 1 EECS141 1 Lecture #11 EE141 EE141 -Fall 2010 Fall 2010 Digital Integrated Digital Integrated Circuits Circuits Lecture 11 Lecture 11 MOS Capacitance MOS Capacitance and Delay and Delay EE141 2 EECS141 2 Lecture #11 Announcements Announcements ± No lab Fri., Mon. ² Labs restart next week ± Midterm #1 Thurs. Oct. 7 th , 6:30-8:00pm ² Exam is open notes, book, calculators, etc. EE141 3 EECS141 3 Lecture #11 Class Material Class Material ± Last lecture ² Using the MOS model: Inverter VTC ± Today’s lecture ² MOS Capacitance ² Using the MOS Model: Delay ± Reading (3.3.2, 5.4.2) EE141 4 EECS141 4 Lecture #11 MOS Capacitance MOS Capacitance EE141 5 EECS141 5 Lecture #11 C GD C GS C SB C DB C GB MOS Capacitances MOS Capacitances = C GCS + C GSO = C GCD + C GDO = C GCB = C diff G SD B = C diff EE141 6 EECS141 6 Lecture #11 Gate Capacitance ± Capacitance (per area) from gate across the oxide is W·L·C ox , where C ox = ε ox /t ox
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EE141 7 EECS141 7 Lecture #11 Gate Capacitance Gate Capacitance ± Distribution between terminals is complex ² Capacitance is really distributed – Useful models lump it to the terminals ² Several operating regions: – Way off, off, transistor linear, transistor saturated EE141 8 EECS141 8 Lecture #11 Transistor In Cutoff Transistor In Cutoff ² When the transistor is off, no carriers in channel to form the other side of the capacitor. – Substrate acts as the other capacitor terminal – Capacitance becomes series combination of gate oxide and depletion capacitance L S D W G C OL C GB C OL C jSB C jDB j x EE141 9 EECS141 9 Lecture #11 Transistor In Cutoff Transistor In Cutoff (cont (cont ’d) d) ² When |V GS | < |V T |, total C GCB much smaller than W·L·C ox – Usually just approximate with C GCB = 0 in this region. ² (If V GS is “very” negative (for NMOS), depletion region shrinks and C GCB goes back to ~W·L·C ox ) L S D W G C OL C GB C OL C jSB C jDB j x EE141 10 EECS141 10 Lecture #11 Transistor in Linear Region Transistor in Linear Region ² Channel is formed and acts as the other terminal –C GCB drops to zero (shielded by channel) ² Model by splitting oxide cap equally between source and drain – Changing either voltage changes the channel charge L S D W G C OL C G C OL C JC C jSB C jDB L D j x
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This note was uploaded on 02/06/2011 for the course EE 141 taught by Professor Staff during the Spring '08 term at University of California, Berkeley.

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Lecture11-MOS_Cap_Delay-6up - EE141-Fall 2010 Digital...

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