physics of mosfet

8/12/2019 Physics of Mosfet

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Physics of power dissipation in

MOSFET devices


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Power dissipation in MOSFET depends on

MIS (Metal-Insulator-Semiconductor) structure

Surface space charge region and the threshold voltage

Depth of depletion region

Inversion layer charge & thickness


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The MIS structure

Insulating layer of thickness d is sandwiched b/w a metal plate &

semiconductor substrate

Let the semiconductor be of p type, voltage V is applied b/w metal

plate & substrate

When V=0 for ideal MIS diode, the energy difference =0

= - { } =0

In this case the insulator has infinite resistance & doesnt have

mobile charge carriers


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Contd

The fermi level in the metal lines up with the fermi level in the

semi conductor.This is called as flat-band condition


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Contd

Case II:V is negative

When V isve,the holes are attracted to and accumulate with

semiconductor surface in contact with the insulator layer.This

is called accumulation.

In the absence of current flow, the carriers are in equilibrium

Fermi level appears as a straight line

Further increasing the voltage V, the energy bands are bend

towards upward near the surface

The fermi level in semi conductor is now below than the fermi

level in metal.


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Contd

Case II:V is positive

When V is +ve, the holes are repelled away from the surface

and leave -vely charged acceptor ions

Depletion layer is created from the surface into the semi

conductor. This is called as depletion condition.

if we increase the voltage V, the bands are bend far enough at

the surface


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Contd

When V is small ,concentration of holes>>electron

concentration. At this time orginal p type is inverted to n type.

This is called as weak-inversion

When V is increased to the extent, electron density>>hole

density .At this time strong inversion occurs.

The value of V necessary to reach the on set of strong

inversion is called thershold voltage.


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Threshold Voltage Components

Set VS=0, V

DS=0

,and

V

SB=0

. Increase VGSuntil the channel is inverted. Then a

conducting channel is formed and the depletion region

thickness (depth) is maximum as is the surface potential. The value of VGSneeded to cause surface inversion

(channel creation) is the threshold voltageVT0. The 0

refers toV

SB=0

. VGSVT0, existence the channel implies possible

current flow.


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Depth of the depletion layer

If the applied gate voltage is greater than VFB, then the

semiconductor surface will be depleted of holes.

If the applied gate voltage is less than VTH, the

concentration of conduction electrons at the surface is

smaller thanNAr(x) -qNA(x)

Width of the depletion layer=Nq A

FS

d

Si

x

2


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Charge in inversion layer

The charge in the depletion region due to the

ionized atoms left behind when the holes are

repelled away by the +ve potential on the

metal

The inversion does not begin until sb

eQ s

s

A

i

Nq s )2(

22


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BODY EFFECT

In MOSFET the terminal voltages are

expressed with respect to the source terminal

and bulk, relative to the source, may be a non-

zero voltage.

VGS=VGB-VBS

Bulk is at zero potential->s surface potential

or becomes s+VBS


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The threshold voltage becomes

The increase in the VT->Bulk bias voltage VBS

is nonzero is termed as Body Effect.


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SUB THRESHOLD CURRENT

Although no current should ideally conduct before

threshold, a small percentage of electrons withenergy greater than or equal to a few KT have

sufficient energy to surmount the potential barriers.

Sub threshold drain current flows between the

source and drain of a MOSFET when the transistor isin the sub threshold region or weak inversion region.

As a result, there is a slight amount of current

conduction below VTsub threshold conduction - leads toparasitic leakage.


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SUBTHRESHOLD SWING

Subthreshold swing is the function of channel

length and the interface state density.

Inverse of the slope of the log ID,st

versus VGS

characteristic is subthreshold swing.


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SUBMICRON MOSFET

Due to Moores Law, we continually strive toshrink transistor dimensions to achieve an

increase in speed, packing density, and power

dissipation However, we cannot simply reduce the gate

length without reengineering the remainder

of the device structureID st depends onVDS.

VT independent of L,Z and VDS.


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Effects Influencing Threshold voltage

Short channel length effect

Narrow gate width effect

Reverse short channel length effect


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Short channel length effect

In short channel, a small decrease in Vtcausesleakage current to become excessive.

A higher doping concentration may be required to

compensate for the additional Vtdecrease.

When the channel is long ,the drain-substrate and

source-substrate depletion regions account only for

only for a small section of the total distance b/w drain

and source regions. The drain depletion region expands further into the

substrate ,making the turn-on voltage even smaller .


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To consider the effect of Vds various

simplification have been proposed.

Charge sharing model considers the charge

in the channel to be shared among source ,

drain and gate.


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Ideally the only electric field lines which should terminate on the inversion charge are from the

gate!

CHARGE SHARING

However, the positive charge in the source/drain have field lines which terminate on charge in

the periphery of the inversion channel!

The result is a shared charge which should not be included in the VTequation.

Therefore VTis undesirably reduced.


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This implies that as we reduce the channel length, the depletion region from the drain

essentially merges with the depletion region from the source.

DRAIN INDUCED BARRIER

LOWERING(PUNCHTHROUGH)


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A reduction in the potential barrier between S/D implies that a smaller drain and

gate voltage is required to achieve the same drain current!


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In this case, the gate may not be able to control the current from source to drain!

There are two solutions to this problem:

Scale the source/drain thicknesses appropriately to ensure a shallow junction.

Increase the channel doping via a localized implant called a HALO IMPLANT near the

source/drain junctions


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Narrow gate width effect:-

It produces an increase in threshold voltagewith decreasing gate width.

Effective depletion charge increased ratherthan decreased.

Reverse short channel effect:-

Reduction in threshold voltage as gate lengthincreases with no uniformly doped channelregions.

NARROW WIDTH EFFECT


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NARROW WIDTH EFFECT


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GATE INDUCED DRAIN LEAKAGE


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Increase of sub-threshold currents for

negative gate bias

Narrow depletion regions

Tunneling


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END OF SESSION

physics of mosfet

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