Channel length Modulation

Channel length modulation

1. A VLSI Lecture series igi!

Channel Length
Modulation of MOSFET

By Prof. Hitesh Dholakiya
Engineering Funda

2. Outlines

« Basics of Channel Length Modulation

Channel length Modulation in MOSFET

* Derivation of drain current for channel length modulation
«* Characteristics of MOSFET with channel length modulation

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3. Basics of Channel length Modulation

« In saturation region of working with MOSFET, there is channel length
Modulation with MOSFET.

«In that channel length will change with respect to drain voltage of
MOSFET.

«< Channel length will decrease with respect to increase in drain
voltage in saturation region.

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4. Channel Length Modulation in MOSFET

Gate G
Ves > Vro

Source S Drain D

Metal
Oxide SiO2

P Type Si Substrate

Substrate

5. “1 e «+ At pinch off point of channel, channel voltage will be Vpsar

Source $ Metal o a, * Gradual channel approximation is only valid in channel,
Bias Er Berne as per that drain current n deep saturation is given by
= €
Ip(sat) re N as — Vo)?
P Type Si Substrate 27
Hn-Cox W
+ Ip(sat) = << Wes - Vr0)*
= p(sat) 2 ‘&-4D (Ves — Vro)
++ Total charge density in channel is given by + Ip(sat) = (=a) puro . (Ves - Vro)?
Qu(Y) = —CoxWes - Ve) — Vrol TL:

> Inversion layer charge at source (Y=0) end is given by did

| Vos —Vosar
Qi = 0) = —CoxlVas — Vro] AL & /Vos — Vosar
«+ To simplify this drain current equation, we will take A

+ Inversion layer charge at Drain end (Y=L) end is given by
(Channel length modulation coefficient)

Qu(Y = L) = —CoxlVes — Vos — Vro]

«+ At the edge of saturation Vps = Vpsar a1 -4 =1-W ys > —— = =1+ ps
Vos = Vosar = Ves — Vro La T
«+ Inversion layer charge at Drain end (Y=L) in saturation | *% So drain current is given by
region is given by _ Hn Cox W 2
OY =1) ~0 + Ip(sat) = 2 1 (Vas —Vro)?.(1 + AVps)
“Effective channel length in saturation region will become

L'=

— AL

6. + Drain current is given by

Un: Cox
2

w
+ Ip(sat) = [Wes — Vro)®. (1 + aVps)

Drain Current
4

withA #0
Vesz with A=0

withA +0
Vesı with A=0

Drain Voltage