Post on 22-Dec-2015
References
• Detailed Load Capacitance Calculation (Hodges,Section 6.3)
• Detailed MOS Capacitance Model (West, Section 2.3.2)
Data-Dependent Gate Capacitance
• Effective Gate Capacitance (i.e. capacitance into the gate) in a 0.35 um process
General Capacitance Model
Typo: 2/3
General Capacitance Model:1. Thin-Oxide Capacitance
a. Voltage dependent (Cgs, Cgd, Cgb)b. Voltage independent: Col
2. Junction Capacitance (Cjsb, Cjdb)
Cgs,Cgd and Cgb
Channel extends fromsource to drain
Channel almostfrom source to drain
Cg in series with Cj
C(0V)=0.5Cg
Gate:-QUnder SiO2:Q
Intuition about Junction Capacitance
• The depletion region is narrower when the diode is forward biased, therefore the junction diode capacitance is higher.
• The depletion region is wider when the diode is reverse biased, therefore the junction diode capacitance is smaller.
Fanout Gate Capacitance
• Cfanout : fanout capacitance due to the inputs of subsequent gates, CG.
Cfanout=CG1+CG2+CG3….Assumption: Each fanout is an inverter.
Worst Case Analysis Assumption
• The thin-oxide capacitance is voltage dependent.
• The worst case analysis uses CoxWL to compute its worst case value.
Cg
tox L Cg
110 nm 5 1.61 fF/μm
7.5 nm 0.35 μm 1.65 fF/μm
2.2 nm 0.1 μm 1.61 fF/μm
Cg is approximately 1.61 fF/μm for the last 25 years.Exception: the 0.18 μm process, which has a Cg of 1.0 fF/ μm.
[Worst Case Analysis]
Redefine Cg
• For 0.13 μm,– Cg (due to tox alone): 1.6 fF/μm [Hodges, p.72]
– Col(due to Cov and Cf): 0.25 fF/ μm [Hodges, p.80]– Redefine Cg [Hodges, p.259] as• Cg=CoxL+2Col
• Cg =1.6 fF/μm+ 2 0.25 fF/μm=2 fF/μm
• Cg has been constant for over 20 years
– Multipy Cg by W to obtain the total capacitance due to tox, Cov and Cf
[Worst Case Analysis]
[Worst Case Analysis]
Fanout Gate capacitance of n Inverters
• Cfanout=2fF/μm[(Wn+Wp)1+(Wn+Wp)2…(Wn+Wp)n]
[Worst Case Analysis]
Self-Capacitance Calculation
1. Eliminate capacitors not connected to the output2. Assume the transistors are either on (Saturation) or off (Cutoff). 3. CGD is negligible in either saturation or cutoff.
Calculation of Self-CapacitanceCself=CDBn+CDBP+2COL+2CO
L
CDBn=CjnWn
CDBp=CjpWp
COL=ColWCself=CjnWn+CjpWp+2Col(Wn+Wp)
Assume Cjn=Cjp
Cself=Ceff(Wn+Wp)For 0.13: Ceff=1 fF/μm [Hodges, p. 261]