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Metal Oxide Semiconductor Field-‐Effect Transistors (MOSFETs) : Basics
ESc201A : IntroducAon to Electronics L16
rhegde Dept. of Electrical Engineering
IIT Kanpur
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2
Introduction
Classification of MOSFET
Widely used in IC circuits
§ P channel ü Enhancement type ü Depletion type
§ N channel ü Enhancement type ü Depletion type
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MOSFET
Metal Oxide Semiconductor Field Effect Transistor
An NMOSFET
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Device Structure of Enhancement-Type NMOS
L: 1 to 10 µm W: 2 to 500 µm Thickness of oxide layer: 0.02 to 0.1 µm
5
Device Structure of Enhancement-Type NMOS
6 6
Symbols
NMOSFET
(a) Circuit symbol for the n-channel enhancement-type MOSFET.
(b) Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n channel).
(c) Simplified circuit symbol to be used when the source is connected to the body or when the effect of the body on device operation is unimportant.
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Symbols
NMOSFET
PMOSFET
Mostly Used
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Physical Operation
• Creating an n channel
• Drain current controlled by vDS
• Drain current controlled by vGS
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Creating a Channel for Current Flow
Ø The enhancement-type NMOS transistor with a positive voltage applied to the gate.
Ø An n channel is induced at the top of the substrate beneath the gate.
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Drain Current Controlled by Small Voltage vDS
Ø An NMOS transistor with vGS > Vt and with a small vDS applied.
Ø The channel depth is uniform.
Ø The device acts as a resistance.
Ø The channel conductance is proportional to effective voltage.
Ø Drain current is proportional to (vGS – Vt) vDS.
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vDS is increased
Ø Operation of the enhancement NMOS transistor as vDS is increased.
Ø The induced channel acquires a tapered shape.
Ø Channel resistance increases as vDS is increased.
Ø Drain current is controlled by both of the two voltages.
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Channel Pinch- Off
• Channel is pinched off Ø Inversion layer disappeared at the drain point Ø Drain current is n’t disappeared
• Drain current is saturated and only controlled by the vGS
• Triode region and saturation region
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Drain Current Controlled by vGS
• vGS creates the channel.
• Increasing vGS will increase the conductance of the channel.
• At saturation region only the vGS controls the drain current.
• At subthreshold region, drain current has the exponential relationship with vGS
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Transfer Characteristics
VGS(V)
ID(mA)
Vt: Threshold Voltage
For VDS ≥ VGS – VT
N-MOSFET
Cut Off
‘On’
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I-V Characteristics
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Regions of Transistor ‘Operation’
• Cut off region (vGS < Vt ) – Input voltage less than threshold voltage
vGS(V)
iD(mA)
VT: Threshold Voltage
N-MOSFET
Cut Off
‘On’
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Regions of Transistor ‘Operation’
• Triode region (vGS > VT and vDS < vGS – VT) – Linear relationship between iDS and vDS reflects resistive
behaviour for small vDS
vGS(V)
iD(mA)
VT: Threshold Voltage
N-MOSFET
Cut Off
‘On’
GS T
DS GS T
v Vv v V
>
< −
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Regions of Transistor ‘Operation’ • Saturation region (vGS > VT and vDS ≥ vGS – VT)
– Transistor is ‘on’ – Drain bias is above saturation voltage – Amplifier should operate in this region
iD(mA)
VT: Threshold Voltage
N-MOSFET
Cut Off
‘On’
GS T
DS GS T
v Vv v V
>
> −
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Saturation region (vGS > VT and vDS > vGS – VT)
The current iDS begins to saturate as vDS approaches the value of (vGS − VT).
The saturation region of MOSFET Operation
MOSFET operates in saturation region when following two conditions are met:
GS T
DS GS T
v Vv v V
>
> −
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Different values of vGS (> Vt ) provides different iDSand vDS Characteristics
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The Switch Current Source MOSFET Model
MOS Device Open State Closed State
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The Switch Current Source MOSFET Model When vGS > VT and vDS > vGS – VT the amount of current provided by the source is
( )22D GS TKi v V= −
'where n n oxWK k CL
µ= = W: gate width; L: gate Length
Unit of K: A/V2
k’n: Constant related to MOSFET properties (A/V2) µn: Electron mobility in channel Cox: Capacitance per unit area of parallel plate capacitor by gate electrode and channels
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Example-1: Determine the current iDS for the circuit shown below. Assume: K= 1mA/V2 and VT = 1V
( )22D GS TKi v V= −
( )21 2 12Di = − 0.5 mADi =
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Example-2: Assume: K= 1mA/V2 and VT = 1V
What should be the minimum value of the drain to source vDS for which MOSFET will operate in saturation region. (Assume VGS is 2V)
+ _
For the MOSFET to operate in saturation:
GS T
DS GS T
v Vv v V
>
> − 1 VDSv >
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Example-3: Assume: K= 1mA/V2 and VT = 1V
What is maximum value of vGS for which MOSFET will operate in saturation region
+ _
For the MOSFET to operate in saturation:
GS T
DS GS T
v Vv v V
>
> −5 1
6 VGS
GS
vv> −
<
1 VGSv > 1 V 6 VGSv< <
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p Channel Device
• Structure of p channel device Ø The substrate is n type and the inversion layer
is p type. Ø Carrier is hole. Ø Threshold voltage is negative. Ø All the voltages and currents are opposite to
the ones of n channel device. Ø Physical operation is similar to that of n channel
device.
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Complementary MOS or CMOS
Ø The PMOS transistor is formed in n well.
Ø Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well.
Ø CMOS is the most widely used of all the analog and digital IC circuits.
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Large Signal Equivalent Circuit Model for NMOS
Large Signal Equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output resistance ro. The output resistance models the linear dependence of iD on vDS