explain the MOSFET Characteristics

1. MOSFET I-V Characteristics
Contents
•Types
•Cut off region
•Linear region
•Saturation region
•Comparison
•Conclusion and References
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2. Characteristics comes under three
category
• Cut-off region
• Linear region
• Saturation region
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3. MOSFET I-V Characteristics : Cutoff Region
•When VGS < VT, there is no channel formed between the
Drain and Source and hence IDS=0 A
•This region is called the Cutoff Region
•This region of operation is when the Transistor is OFF
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4. MOSFET I-V Characteristics : Linear Region
• When VGS> VT, a channel is formed. IDS is
dependant on the VDS voltage
• When VDS= 0v, no current flows
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5. • If VGS> VT and VDS> 0, then a current will flow from the Drain to
Source (IDS).The MOSFET operates like a voltage controlled
resistor which yields a linear relationship between the applied
voltage (VDS) and the resulting current (IDS)
• For this reason, this mode of operation is called the Linear
Region.This region is also sometimes called the triode region
(we'll use the term "linear")
• VDS can increase up to a point where the current ceases to
increase linearly (saturation)
• We denote the highest voltage that VDS can reach and still yield
a linear increase in current as the saturation voltage or VDSAT
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6. • when a voltage is applied at VD,its positive charge pushes
the majority charge carriers (holes) that exist at the edge of the
depletion region further from the Drain.
• As the depletion region increases, it becomes more difficult for the
Gate voltage to induce an inversion layer. This results in the
inversion layer depth decreasing near the drain.
• As VD increases further, it eventually causes the inversion layer to
be pinched-off and prevents the current flow to increase any
further.
• This point is defined as the saturation voltage (VDSAT)
• From this, we can define the linear region as:
VGS>VT
0 < VDS < VDSATPrepared By S ARUN M.Tech MOSFET VI Characteristics

7. Prepared By S ARUN M.Tech MOSFET VI Characteristics

8. • The Drain to Source current (IDS) is given by the
expression:
• Where:
un = electron surface mobility (units in cm2
/V·s)
Cox = Unit Oxide Capacitance (units in F/cm2
)
W = width of the gate
L = length of the gate
• Remember this expression is only valid when :
VGS>VT
0 < VDS < VDSAT
( )[ ]2
02
2
DSDSTGS
oxn
DS VVVV
L
WCu
I linear
−⋅−⋅⋅⋅
⋅
=
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9. • what is linear about this equation?
• Most of the parameters are constants during evaluation.
They are sometimes lumped into single parameters
• Notice that W and L are parameters that the designers have
control over. Most of the other parameters are defined by
the fabrication process and are out of the control of the IC
designer.
( )[ ]2
02
2
DSDSTGS
oxn
DS VVVV
L
WCu
I linear
−⋅−⋅⋅⋅
⋅
=
oxn Cuk ⋅=' ( )[ ]2
02
2
'
DSDSTGSDS VVVV
L
Wk
I linear
−⋅−⋅⋅⋅=
L
W
Cuk oxn ⋅⋅= ( )[ ]2
02
2
DSDSTGSDS VVVV
k
I linear
−⋅−⋅⋅=
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10. • What is linear about this equation?
• The -VDS
2
term alters the function shape in the linear region.
As it becomes large enough to significantly decrease IDSin
this function, the transistor enters saturation and this
expression is no longer valid.
( )[ ]2
02
2
DSDSTGSDS VVVV
k
I linear
−⋅−⋅⋅=
For a fixed VGS,
then
IDS depends on
VDS
VDS
2
has a smaller effect on
IDS at low values of VDS
since it is not multiplied by
anything
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11. • Since we know what the current will not decrease as VDS
increases past VDSAT, we can usethis expression to define VDSAT
• when VDS>(VGS-VT), then IDS in this expression begins to decrease
• we can then define VDSAT = (VGS-VT)
• So now we have the formal limits on the linear region and the validity of
this expression:
•
Linear Region : VGS>VT
0 < VDS < (VGS-VT)
( )[ ]2
02
2
DSDSTGSDS VVVV
k
I linear
−⋅−⋅⋅=
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12. MOSFET I-V Characteristics : Saturation Region
• MOSFET is defined as being in saturation when:
Saturation Region : VGS> VT and VDS > (VGS-VT)
• An increase in VDS does not increase IDS because the channel is pinched-off
• However, an increase in VGS DOES increase IDSby increasing the channel
depth and hence the amount of current that can be conducted.
• Measurements on MOSFETS have shown that the dependence of IDS on VGS
tends to remain approximately constant around the peak value reached
for VDS=VDSAT
• A substitution of VDS=(VGS-VT0) yields:
( ) ( ) ( )[ ]
( )2
0
2
000
2
2
2
TGSDS
TGSTGSTGSDS
VV
k
I
VVVVVV
k
I
sat
sat
−⋅=
−−−⋅−⋅⋅=
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13. 1st order expressions for all three
regions of operation for the MOSFET
Now we have
Region Conditions IDS
Cutoff VGS< VT
Linear VGS> VT
VDS < (VGS-VT)
Saturation VGS> VT
VDS > (VGS-VT)
0=cutoffDSI
( )2
0
2
TGSDS VV
k
I sat
−⋅=
( )[ ]2
02
2
DSDSTGSDS VVVV
k
I linear
−⋅−⋅⋅=
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14. • Conclusion
In this presentation conclude that came to know
regards VI characteristics of MOSFET.
• References
 Sedra and smith, “Microelectronic circuits”,7th
Ed.,
Oxford University Press.
Thomas L.Floyd, “Electronic devices” Conventional
current version, Pearson prentice hall.
Robert L.Boylestad, “Electronic devices and circuit
theory”,