Presentation On Second Order Effects

and Short Channel effects

Contents

 Second order effects and short channel effects.

Second order effects

Effects due to parameters, concentration of doping, oxide

thickness, channel length variation, high electric field
intensity inside the device channel, oxide breakdown,
avalanche breakdown of the pn regions inside the MOS
transistors comes under second order effects.
Second Order Effects
 Subthreshold leakage current
 Body Effect
 Channel length modulation
 DIBL(Drain induced barrier lowering)
 Hot carrier injection
 Impact ionization
 Avalanche breakdown
 Velocity saturation (mobility degradation)
 Surface scattering
 Punch through
 Gate Induced Drain Leakage
Subthreshold leakage current
 The current between source and drain of a mosfet, when the
transistor is in the weak inversion region, i.e for gate to
source voltages below the threshold voltage is called
subthreshold leakage current.
 In lower channel devices, due to space between source and
drain is less, there is more drift current flows under the gate.
Gradually this subthreshold current will increase.
 And it is inversely proportional to channel length and
proportional to the drain voltage and temperature.
Subthreshold leakage current
Body Effect
 Channel length modulation
In mosfet, at pinch off region, drain
current depends on the more electrical field
and also depends on the depletion region
where it opposes the drain current. Since
depletion region increases, we should have
a constant current if Vds increases. But if
we comes to lower channel devices, where
electric field due to drain voltage dominates
the depletion width opposition current, so
more drain current flows. This we called as
channel length modulation effect. The
channel length modulation factor is lambda
and it is inversely proportional to channel Channel lenght modulation
length.
 DIBL(Drain induced barrier lowering)
When drain voltage is more than bulk potentials, due to its
reverse biased depletion region, it occupies some part of space in
channel. So we need little less gate voltage required to invert the
remaining channel to turn mosfet ON. So Vt is decreased.
This effect can be reduced by changing doping
concentration of either bulk or drain. And by inserting a low doped
n material before the drain.
 Hot carrier injection
This effect occurs when more drain voltage in short channel
devices. More drain voltage generates the more electic field so that
it attracts the more electrons from the source and those electrons
will reach its maximum velocity and it gets more kinetic energy.
The electron which gets more kinetic energy called as hot electron.
This hot electron collide at the edge of the drain or gate and reflect
back and damages the gate oxide which is near to the drain region.
If gate oxide damages then direct current flows from gate to
substrate. This iscalled as hot carrier injection.
This can be avioded by using increasing the gate oxide thickness
or using high k-materials as a gate oxide. And inserting a low
doped n material before the drain.
 Impact Ionization

This effect is related to hot carrier effect.

The hot electron which is reflected from
drain also breaks the covalent bonds in
depletion region. If covalent bond breaks
then one electron and one hole will be
generated. If more covalent bond breaks,
more free electrons will be collected by drain.
Therefore extra unwanted current Ids flows
from source to drain through substrate . This
is called impact ionization effect. If we able
to control or decrease the hot carrier effect,
impact ionization effect also decreases.
 Avalanche Breakdown
As the electric field in the channel is increased, avalanche breakdown occurs in the
channel at the drain. This avalanche breakdown increases the current as in a p-n
diode. There is parasitic bipolar action taking place. Holes generated by the
avalanche breakdown move from drain to source underneath the inversion layer.
This hole current forward biases the source-bulk p-n diodes so that now also
electrons are injected as minority carriers into the p-type substrate underneath the
inversion layer. These electron-hole pairs through avalanche multiplication. The
positive feedback between and the parasitic bipolar action results in breakdown at
lower drain voltage.
As if we control hot carrier effect the avalanche breakdown can be avoided.
 Velocity Saturation and mobility degradation
The electron velocity is related to the electric field through the
mobility. For higher fields the velocity doesnot increase with
electric field, we have a degradation of mobility because of
scattering by vertical field. This leads to earlier saturation of
current i.e before Vgs-Vth. Nothing but reduction in drain
current.
The velocity saturation reduces the transconductance of short
channel devices in the saturation condition.
By using k-materials as gate this can be avoided
Velocity saturation
Surface Scattering
When the carriers travel along
the channel, they are attracted to
the surface by the electric field
created by the gate voltage. As a
result, they keep crashing and
bouncing against the surface,
during their travel, following a
zigzag path. This effectively
reduces the surface mobility of the
carriers. This change in carrier
mobility impacts the current-
voltage relationship of the
transistor.
 Drain Punch Through
 If Vds is goes on increasing, due to
reverse biased pn junction at drain, it
will have a significant widths of
depletion region.
 Since the gap between the source
and drain is very less, the depletion
region of drain will touch with the
source depletion region.
 If this happen once, the drain current
between source and drain can't be
control by the gate
By changing doping concentration of
drain or bulk punch through can be
avoided
Gate Induced Drain Leakage
 This issues comes only at gate voltage less than 0(let’s take
for Nmos) and more drain voltage.
 Basically there is reverse saturation current flows between
the drain and bulk (P) and it is very minor.
 When –ve gate voltage has applied (assume due to noise),
it attracts the holes (due to accumulation) and forms the p
channel (or P+) between the source and drain. At this case,
the thinner depletion region will form between the
generated P+ region and drain (N+). So Compare to P.N+
reverse saturation current, P+.N+ reverse saturation
current is more and it is considerable leakage current. This
leakage we call it as Gate induced drain leakage
Conclusion

The second order effects complicates device operation and

degrade device performance, these effects can be eliminated
or minimized in SOI and FINFET technology by its different
construction.
References
• https://www.allaboutcircuits.com/technical-articles/mosfet-
channel-length-modulation/
• http://www.onmyphd.com/?p=mosfet.short.channel.effects
• http://www.iue.tuwien.ac.at/phd/gehring/node83.html
• http://www.iue.tuwien.ac.at/phd/stockinger/node16.html
• https://en.m.wikipedia.org/wiki/Short-channel_effect