BVD-Lecture-6
MOS CAPACITANCES
Dr. Arti Noor,M. Tech Division, CDAC Noida.Email : [email protected]
9-10-2009
BVD-Lecture-6
MOSFET CAPACITANCES
No. of capacitances are found as function of:• Layout geometry.• Fabrication process.
Most of them are distributed in nature and exact calculation is complex.
Thus one needs some simple model for hand calculation.
Two types of capacitances :• Device related.• Interconnect related.
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Device related parasitic capacitances:
LEFF = Lm – 2 LD(≈0.1µm)
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MOSFET CAPACITANCES (contd. )
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MOSFET CAPACITANCES (contd. )
OXIDE Capacitances :
Cox = εox / tox
1. Overlap Caps :
CGS0 = Cox W LD
CGD0 = Cox W LD
CGB0 = Cox Wov L
• Do not depend upon bias condition.• Voltage Independent.
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MOSFET CAPACITANCES (contd. )
2. Gate-channel Capacitances:
Three Cgs, Cgd, Cgb.
• Cgb : distributed and voltage dependent in real life.
• Cgs : gate-to-channel capacitance between gate and source terminal.
• Cgd : gate-to-channel capacitance between gate and drain terminal.
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Gate-channel Caps (contd. )
• These caps are bias dependent Cgs, Cgd, Cgb.
• To calculate them consider three biasing region.
1. CUT-Off Region: no inversion, S/D not connected.
Cgb = Cox W L.
Cgs = Cgd = 0.
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Gate-channel Caps (contd. )
2. Linear- Region: inversion layer exists and shields the bulk from gate electric field.
Cgb = 0.
Cgs = Cgd = = ½ Cox W L.
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Gate-channel Caps (contd. )
3. Saturation- Region: inversion is not upto drain. Source is connected to channel and shielded from bulk.
Cgb = cgd = 0.
Cgs = 2/3 Cox W L.
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Total Oxide Caps (contd. )
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Total Oxide Caps (contd. )
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Junction Capacitances
• Csb and Cdb source-substrate and drain-substrate caps.
• Exists because of depletion region embedded in the bulk.
• Voltage dependent and is function of applied terminal voltage.
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Junction Capacitances (contd.)
• Simplified picture of diffusion to calculate Caps.• Abrupt junction is assumed for simplicity.• 2,3,4 junctions are surrounded by sidewall. 1 is
facing channel and 5 is bottom jn and facing bulk.
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Junction Capacitances (contd.)
• P+ is 10 x NA thus caps associated with this are different than other junction caps.
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Junction Capacitances (contd.)
• Reverse biased N+P junction’s depletion layer width caculation.
• NA and ND doping densities and applied voltage is V.
V is External Potential
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Junction Capacitances (contd.)
• The charge stored in this area and capacitance are
The final expression for Cj is
Cj(V) = A Cj0 / ( 1- V/ Φ0 )m where
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Junction Capacitances (contd.)
• This junction capacitance depends upon biasing.• With changing bias its estimation is difficult.• Thus large signal average junction cap, known as
equivalent cap, is calculated between two known voltages.
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Junction Capacitances (contd.)
• Sidewall Junction Capacitancs.
Zero-bias Sidewall cap per unit length is
Since all sidewalls have junction depth Xj.
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Junction Capacitances (contd.)
• Again one has to calculate equivalent large signal Sidewall Junction Capacitances.
P is equal to sum of three sides.
Total junction capacitance is equal to
Cdb = A Cj0 Keq + P Cj0sw Xj Keqsw
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EXAMPLE
Calculate the total junction capacitance at drain side for NMOST.
Given :
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EXAMPLE
Assume Bulk is at zero-bias and drain voltage varies from 0.5V to 5V.
Solution :
Cdb = A Cj0 Keq + P Cj0sw Xj Keqsw
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EXAMPLE
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EXAMPLE
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Next Class Topic
MOSFET SPICE Models.(chapter-4)