mos capacitances

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MOS CAPACITANCES

Dr. Arti Noor,M. Tech Division, CDAC Noida.Email : artinoor@cdacnoida.in

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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Next Class Topic

MOSFET SPICE Models.(chapter-4)