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EDC Lesson 12: Transistor and FET
Characteristics
LessonLesson--12:12: MOSFET (enhancementMOSFET (enhancement
and depletion mode) Characteristicsand depletion mode) Characteristics
and Symbolsand Symbols
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1. Metal Oxide Semiconductor Field Effect1. Metal Oxide Semiconductor Field Effect
Transistor (MOSFET)Transistor (MOSFET)
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Types of FETsTypes of FETs
The family of FETs may be divided into :
Junction FET
Depletion Mode MOSFET Enhancement Mode MOSFET
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Remember JFET Definition
JFET is a unipolar-transistor, which acts as a
voltage controlled current device and is a device
in which current at two electrodes is controlled by
the action of an electric field at a reversed biasedp-n junction.
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MOSFET Definition
MOSFET Field effect transistor is a unipolar-
transistor, which acts as a voltage-controlled current
device and is a device in which current at two
electrodes drain and source is controlled by theaction of an electric field at another electrode gate
having in-between semiconductor and metal very a
thin metal oxide layer .
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nn--channel depletionchannel depletion
MetalMetal--OxideOxide--Semiconductor FET (MOSFET)Semiconductor FET (MOSFET)
n channel
p
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pp--channel depletionchannel depletion
MetalMetal--OxideOxide--Semiconductor FET (MOSFET)Semiconductor FET (MOSFET)
p channel
n
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Effect of Insulating SiO2 (metal-oxide)
layer
MOSFET Very high input impedance due to
SiO2 layer compared to even reverse biased
p-n junction depletion region input
impedance in JFET
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nn--channel depletion regionchannel depletion region
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nn--channel depletion region (normally ON)channel depletion region (normally ON)
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Transfer Characteristics of nTransfer Characteristics of n--channelchannel
depletion region MOSFETdepletion region MOSFET
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There is a convenient relationship between IDSand V
GS. Beyond pinch-off
Where IDSS is drain current when VGS= 0 andVGS(off) is defined as VP, that is gate-sourcevoltage that just pinches off the channel.
The pinch off voltage VP here is a +ve quantitybecause it was introduced through VDS(sat).
VGS(off) however is negative, -VP.
2
)(
1
offGS
GS
DSSDSV
VII
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pp--channel depletion MOSFETchannel depletion MOSFET
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2. Enhancement Mode MOSFETS2. Enhancement Mode MOSFETS
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nn--channel enhancement mode MOSFETchannel enhancement mode MOSFET
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nn--channel enhancement mode Normally Offchannel enhancement mode Normally Off
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Note that with a n-channel device we apply a +ve gatevoltage to allow source-drain current, with a p-channeldevice we apply a -ve gate voltage.
n-channelenhancement
modep-channel
enhancement
mode
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pp--channel enhancement MOSFETchannel enhancement MOSFET
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pp--channel enhancement MOSFET (normallychannel enhancement MOSFET (normally
OFF)OFF)
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3. Enhancement Mode MOSFETS in detail3. Enhancement Mode MOSFETS in detail
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Basic MOSFET (nBasic MOSFET (n--channel) Enhancement modechannel) Enhancement mode
WhenVGS = 0,
the n-
channel
is verythin and
channel
width
enhances
with
+ VGS
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Channel width enhancement with +VChannel width enhancement with +VGSGS
The gate electrode is placed on top of a very
thin insulating layer.
There are a pair of small n-type regions just
under the drain & source electrodes. If apply a +ve voltage to gate, will push away
the holes inside the p-type substrate and
attracts the moveable electrons in the n-type
regions under the source & drain electrodes.
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Enhancement mode MOSFET
Increasing the +ve gate voltage pushesthe p-type holes further away andenlarges the thickness of the created
channel. As a result increases the amount of
current which can go from source todrain this is why this kind of
transistor is called an enhancementmode MOSFET.
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Subthreshold region in nSubthreshold region in n--chennel enhancement modechennel enhancement mode
ID= k exp (qVGS/kBT)
where T temperature iu in
Kelvin, is Boltzman
constant
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Subthreshold region in nSubthreshold region in n--chennel enhancement modechennel enhancement mode
ID= k (VGS VT)2
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Above ThresholdAbove Threshold ON stateON state
ID= k (VGS VT)2 =
0.278 (VGS VT)2 mA
0 1 5432 876
ID
VGS
If ID (on) = 10
mA and
VGS = 8 V, then k
=10 mA/(8V
2V)2 = 0.278
mA/V2
VT = 2 V
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Above ThresholdAbove Threshold ON stateON state
ID= k (VGS VT)2 = 0.278 (VGS VT)
2 mA
If ID (on) = 10 mA and
VGS = 8 V, then k =10 mA/(8V 2V)2 = 0.278
mA/V2
VT = 2 V ID= k (VGS VT)2
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Ideal Output Characteristics of MOSFETIdeal Output Characteristics of MOSFET
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Ideal Output linear region before saturationIdeal Output linear region before saturation
Characteristics of MOSFETCharacteristics of MOSFET
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Ideal Output Saturation formula of enhancementIdeal Output Saturation formula of enhancement
mode MOSFETmode MOSFET
chang
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Transfer Characteristics in above threshold regionTransfer Characteristics in above threshold region
and saturation region in nand saturation region in n--channel enhancementchannel enhancement
modemode
7 V
6 V5 V
4 V
3V
VGS = VT = 2V
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Transfer Characteristics in Subthreshold region andTransfer Characteristics in Subthreshold region and
saturation region in nsaturation region in n--channel enhancement modechannel enhancement mode
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4. Symbols of depletion and Enhancement4. Symbols of depletion and Enhancement
Mode MOSFETSMode MOSFETS
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Symbol of depletion mode MOSFETSymbol of depletion mode MOSFET
n-channel Depletion
Mode MOSFET
p-channel Depletion
Mode MOSFET
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Symbol of enhancement mode of MOSFETSymbol of enhancement mode of MOSFET
n-channelEnhancement Mode
MOSFET p=channel Enhancement Mode
MOSFET
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SummarySummary
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We learnt
Definitions of MOSFET
n-channel and p-channel Depletion
MOSFET normally ON, Pinch off on
VGS in n-MOSFET and + VGS in p-MOSFET
n-channel and p-channel enhancement
MOSFET normally OFF, below Threshold
and above VT on + VGS in n-MOSFET and
- VGS in p-MOSFET
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End of Lesson 12End of Lesson 12