Resistor
Source(S) Drain(D)
Gate(G), Changing
cross section area
MOSFET Structure
MOSFET is 4-Terminal device
with terminals Source, Drain,
Gate and Body
Source and Body are generally
connected together
The Gate potential controls the
formation of channel

Characteristics & Regions of operation
In MOSFET we have various regions of operation
By varying V-GS
By varying V-DS
Under linear distribution of
charge
Accumulation region, VG < 0
Depletion/Weak Inversion region, VG > 0
Strong Inversion region, VG >>0
Linear/Triode region, VDS < (VGS-VTH)
Saturation region, VDS > (VGS-VTH)
Under Linear region:
L
VVTHVGSWC
I DOX
D
)( 


Resistance R :
)(
C
/1 OX
VTHVGS
L
W
R 

Under Saturation region: (can act as current source)
L
VTHVGSWC
satI
OX
D
2
2)(
,



Transconductance
L
VTHVGSW
dV
dI
g
G
D
satm
)(COX
,



Transconductance:
L
WV
dV
dI
g D
G
D
linm
OX
,
C

Triode Mode

Channel Length Modulation & Pinch off
The characteristics are true as long as we have constant distribution of charges under Gate, In
practice with increased VDS, the distribution of charge under MOS-capacitor changes causing
pinch-off ,we thus have variation in actual channel length of MOSFET. Such variation in channel
length is called channel length modulation, and by this we have a variation in Drain current
)1(
2
2)(
, D
OX
D V
L
VTHVGSWC
satI 




)1(
)(COX
, D
G
D
satm V
L
VTHVGSW
dV
dI
g 




Also Transconductanc modifies to

A MOSFET is considered to be short when the channel length ‘L’ is the same order of
magnitude as the depletion-layer widths (xdD, xdS). The potential distribution in the
channel now depends upon both, transverse field Ex, due to gate bias and also on the
longitudinal field Ey, due to drain bias When the Gate channel length <<1 m, short
channel effect becomes important . This leads to many undesirable effects in MOSFET.
Five different physical phenonomena have to be considered in short-channel devices:
 Drain induced barrier lowering and Punchthrough
 Surface scattering
 Velocity saturation
 Impact ionization
 Hot electrons
Short-channel devices & Short Channel Effects

Drain induced barrier lowering and Punchthrough
The potential barrier, in small-geometry MOSFETs, is
controlled by a two-dimensional electric field vector (in other
words by both VGS and VDS).If the drain voltage is increased
the potential barrier in the channel decreases(=pn junction
band cure with more –ve slop), leading to Drain-Induced
Barrier Lowering (DIBL). Under DIBL condition electrons
can flow between the source and drain even if VGS < VTH.
Because of charge sharing the Threshold Voltagr also decreases,;Reduction in S and D junction
depth xj can reduce the VTH shift,as charge shared by drain and source gets lower.
Punchthrough
Increase in VDS makes depletion width more and more to
increase and there is occurrence of punch through once
these depletion regions of source and drain touches.It can
be decreased by having higher doping levels either in
substrate or near source and drain.

For small-geometry MOSFETs, the electrons mobility in the channel depends
on a two-dimensional electric field (Ex, Ey). The surface scattering occurs when
electrons are accelerated toward the surface by the vertical component of the
electric field Ex
Causes a reduction in the mobility
The average surface mobility is about half as much as that of the bulk mobility
Surface scattering
G
Substrate
S D

Velocity saturation
The electron velocity is related to the electric
field through the mobility:
V= μE
For higher fields the velocity does not increase
with electric field, we have degradation of
mobility because of scattering by vertical field.
This leads to earlier saturation of current.
i.e.,before VGS-VTH. Net result is reduction in
drain current .
The velocity saturation reduces the
transconductance of short-channel devices in
the saturation condiction.

Impact ionization
The presence of high longitudinal fields can accelerate
electrons that may be able of ionizing Si atoms by
impacting against them
Normally most of the e- are attracted by the drain, so it
is plausible a higher concentration of holes near the
source
If the holes concentration on the source is able to
creates a voltage drop on the source-substrate n-p
junction of about 0.6V then
e- may be injected from source to substrate
e- travel toward the drain, increasing their energy
and create new e-h pairs
e- may escape the drain fields and afect other
devices

Hot electrons
The channel Hot Electrons effect is caused by electrons flowing in the channel for large VDS
e- arriving at the Si-SiO2 interface with enough kinetic energy >3.1ev to surmount the surface
potential barrier are injected into the oxide
This may degrade permanently the C-V characteristics of a MOSFETs

Conclusion
FOR IMPACT IONIZATION
FOR PUNCH THROUGH
Short Channel Effects are governed by
complex physical phenomena and mainly
Influenced because of both vertical and
horizontal electric field components.
To meet the current requirements of
Electronic devices, the miniaturization of
devices is important. And so is Second
Order effects which otherwise degrade the
performance of devices.

Thanks U…
Muzafar Ahmad Rather
Jamia Millia Islamia, New Delhi
muzafarsirhama@gmail.com

MOSFET and Short channel effects