mos transistor

Digital Integrated Circuits © Prentice Hall 1995Introduction

MOS Transistor


MOS Structure

( Two Terminal MOS Structure )


Metal Oxide(Sio2) Semiconductor

MOS Structure

( Energy band diagram for Metal, Oxide(Sio2) and

Semiconductor )


MOS Structure


( Energy band diagram of Combined MOS system )

E0

E0

E0 E0

MOS Structure


MOS System under External Bias





( N MOS operating in surface inversion region )


Structure and Operation of MOSFET

( N channel Enhancement MOSFET )





( Formation of Depletion Region in N type

Enhancement type MOSFET )




Threshold voltage

( N channel depletion MOSFET )


Operation of MOSFET Transistor

a) Linear region



b) Saturation region



c) Beyond Saturation region


Characteristics of MOSFET

a) Gradual Channel Approximation



a) Gradual Channel Approximation

( V- I characteristics of nMOS Transistor )
















Depletion Region

hole diffusionelectron diffusion

p n

hole driftelectron drift

ChargeDensity

Distance

x+

-

Electrical

xField

x

PotentialV

W2-W1

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.


The Diode

n

p

p

n

B ASiO2

Al

A

B

Al

A

B

Cross-section of pn-junction in an IC process

One-dimensionalrepresentation diode symbol


Diode Current


Forward Bias

x

pn0

np0

-W1 W20

pn(W

2)

n-regionp-region

Lp

diffusion


Reverse Bias

x

pn0

np0

-W1 W20

n-regionp-region

diffusion


SPICE Parameters



nMOS Transistor - Structure


The MOS Transistor

n+n+

p-substrate

Field-Oxyde

(SiO2)

p+ stopper

Polysilicon

Gate Oxyde

DrainSource

Gate

Bulk Contact

CROSS-SECTION of NMOS Transistor


Carriers and Current

Carriers always flow from the Source to Drain

NMOS: Free electrons move from Source to Drain.

Current direction is from Drain to Source.

• PMOS: Free holes move from Source to Drain.

Current direction is from Source to Drain.


IGFET

The dimension of SiO2 layer is about 0.02 to 0.1 micron.

Gate is isolated thus Insulated-Gate FET

Due to insulation the current flowing through the gate terminal is extremely small of the order of 10^-15 A.

Drain is always kept as more positive than the source.

The current flows from the Drain to Source

P-n junctions are kept under the reverse bias conditions

Typically the Length of the device is from 1 to 10 micron.


MOS Transistor structure

Polysilicon –Heavily doped noncrystalline silicon.

Polysilicon allows the dimensions of the transistor to

be realized accurately.

Gate Oxide – Silicon dioxide.

Thickness of gate oxide – 7 to 20nm.

No d.c. through gate.

Normally, p substrate is connected to 0V in digital

circuits and to negative voltage in analog circuits.


Symmetry

The transistor is symmetric: The Drain

(which is equivalent to a BJT’s

Collector) and the Source (which is

equivalent to a BJT’s Emitter) are fully

symmetric and therefore

interchangeable


All MOS p-n Junctions

Unlike a BJT transistor, in which one of

the p-n junctions is typically forwardly

biased, and the other reversely biased,

in a MOSFET all p-n junctions must

always be kept reversely biased!


The MOSFET Channel

Under certain conditions, a thin channel can

be formed right underneath the Silicon-

Dioxide insulating layer, electrically

connecting the Drain to the Source. The

depth of the channel (and hence its

resistance) can be controlled by the Gate’s

voltage. The length of the channel (shown in

the figures above as L) and the channel’s

width W, are important design parameters.


REGION OF OPERATIONCASE-1 (No Gate Voltage)

Two diodes back to back exist in series.

One diode is formed by the pn junction

between the n+ drain region and the p-type

substrate

Second is formed by the pn junction between

the n+ source region and the p-type substrate

These diodes prevent any flow of the current.

There exist a very high resistance.




REGION OF OPERATIONCreating a channel

Apply some positive voltage on the gate terminal.

This positive voltage pushes the holes downward in the substrate region.

This causes the electrons to accumulate under the gate terminal.

At the same time the positive voltage on the gate also attracts the electrons from the n+ region to accumulate under the gate terminal.





REGION OF OPERATIONCreating a channel

When sufficient electrons are accumulated under the gate an n-region is created, connecting the drain and the source

This causes the current to flow from the drain to source

The channel is formed by inverting the substrate surface from p to n, thus induced channel is also called as the inversion layer.

The voltage between gate and source called vgs at which there are sufficient electron under the gate to form a conducting channel is called threshold voltage Vth.


MOS Channel Formation


Current-Voltage Relations

n+n+

p-substrate

D

S

G

B

VGS

xL

V(x)+–

VDS

ID

MOS transistor and its bias conditions


MOS Transistor Current direction

The source terminal of an n-channel(p-channel) transistor is defined as whichever of the two terminals has a lower(higher) voltage.

When a transistor is turned ON, current flows from the drain to source in an n-channel device and from source to drain in a p-channel transistor.

In both cases, the actual carriers travel from the source to drain.

The current directions are different because n-channel carriers are negative, whereas p-channel carriers are positive.


Threshold Voltage: Concept

n+n+

p-substrate

DSG

B

VGS

+

-

Depletion

Region

n-channel


MOS I/V

For a NMOS, a necessary

condition for the channel to exist is:

THGS VV


REGION OF OPERATIONApplying small Vds

Now we applying some small voltage between source and drain say 0.3V.

The voltage Vds causes a current to flow from drain to gate.

Now as we increase the gate voltage, more current will flow.

Increasing the gate voltage above the threshold voltage enhances the channel, hence this mode is called as enhancement mode operation.


MOSFET Current-Voltage Relationships

•The DC gate current is always zero: IG = 0

•Therefore, when a channel is created, the drain

current equals the source current: ID =IS


MOS Transistor - Symbols

pMOS Transistor nMOS Transistor


Operation – nMOS Transistor

Accumulation Mode - If Vgs < 0, then an electric field

is established across the substrate.

Depletion Mode -If 0<Vgs< Vtn, the region under gate

will be depleted of charges.

Inversion Mode – If Vgs > Vtn, the region below the

gate will be inverted.




V =0










mos transistor

Engineering