Download DMT121 – ELECTRONIC I

DMT121 – ELECTRONIC
DEVICES
CHAPTER 5
FIELD-EFFECT TRANSISTOR (FET)
-MOSFET-
JFET vs BJT
JFET
 VGS
I D  I DSS 1 
 VP
BJT



2
I C  I B
ID = IS
IC ≈ IB
IG ≈ 0 A
VBE ≈ 0.7 V
JFET vs BJT
MOSFET
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MOSFET (Metal Oxide Semiconductor
Field-Effect Transistor)
Different from JFET – no pn junction
structure.
Gate of MOSFET is insulated from the
channel by silicon dioxide (SiO2)
layer.
2 types – enhancement and
depletion.
DEPLETION-TYPE MOSFET
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P-type material is formed
from silicon substrate.
Source and Drain terminals
are connected through
metallic contacts to n-doped
region linked by n-channel.
Gate connected to metal
contact surface but insulated
from n-channel by thin SiO2
layer – no direct connection
gate and channel of MOSFET.
SiO2 is a dielectric which sets
up opposing electric fields
within the dielectric when
exposed to externally applied
field.
n-channel depletion-type MOSFET
BASIC OPERATION &
CHARACTERISTICS @ VGS=0 V
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Gate-to-Source voltage
is set to 0 V.
A voltage VDS is
applied across the
Drain-to-Source
terminals.
An attraction for
positive potential at
Drain by free electron
of n-channel – produce
current through
channel.
At VGS = 0V, ID = IDSS
BASIC OPERATION &
CHARACTERISTICS @ VGS<0 V
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If VGS is set at a negative
voltage:
Negative potential at Gate will
pressure electron towards ptype substrate and attract holes
from substrate.
Recombination between hole
and electron will occur – reduce
number of free electron in nchannel for conduction.
More negative the bias,
recombination rate is higher.
ID is reduce with increasing
negative bias of VGS.
At pinch-off voltage, VP, ID=0A
BASIC OPERATION &
CHARACTERISTICS @ VGS>0 V
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For positive value of VGS:
Positive Gate will draw additional electron from psubstrate due to reverse leakage current and
established new carrier through the collisions
between accelerating particles.
ID will increase at rapid rate – user must aware of ID
maximum current rating.
Application of positive VGS has enhance the level of
free carriers in the channel.
Region of positive gate voltage on drain or transfer
curve is called enhancement region while region
between saturation and cutoff is called depletion
region.
BASIC OPERATION &
CHARACTERISTICS
P-CHANNEL DEPLETION-TYPE
MOSFET
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
Construction is reverse of n-channel.
All voltage polarities and current direction are
reverse.
SYMBOL
ENHANCEMENT-TYPE MOSFET

Primary difference
between
depletion-type and
enhancement-type
is the absence of
channel between
Source and Drain
terminals.
N-channel enhancement-type MOSFET
BASIC OPERATION &
CHARACTERISTICS @ VGS=0 V
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VGS is set at 0 V and a voltage applied
between Drain and Source.
With VDS at positive voltage, VGS=0 V and
terminal substrate (SS) connected to
Source – exist two (2) reverse-biased pnjunction between n-doped region and psubstrate.
It is not sufficient to have a large
accumulation of carriers (electron) at
Drain and Source if a path (channel) is
fails to exist between both terminals.
ID = 0 A
BASIC OPERATION &
CHARACTERISTICS VGS>0 V
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VDS and VGS>0 V:
Positive potential at the Gate will
pressure the holes in p-sub along
the edge of SiO2 to enter deeper
p-sub.
Result in a depletion region near
SiO2.
Electron in p-sub (minority
carrier) attracted to positive Gate
and accumulate in the region near
the surface of SiO2 layer.
As VGS increase in magnitude, the
concentration of electron
increases until eventually induced
n-type region to support current
flow between Drain and Source.
The level of VGS that results in
significant increase in ID is called
threshold voltage, VT.
BASIC OPERATION &
CHARACTERISTICS VGS>VT
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VGS>VT:
The density of free carriers in the
induced channel will increase increased ID.
If increase VDS but VGS constant, ID
will saturate.
VDG and Gate will become less and
less positive with respect to Drain.
VDG=VDS-VGS
Reduction in Gate-to-Drain voltage
will reduce the attractive forces for
free carriers (electron) – reduction in
channel width.
Channel will reduce to pinch-off and a
saturation condition established.
Any further increase in VDS at fixed
value of VGS will not affect the
saturation level of ID until breakdown
conditions are encountered.
BASIC OPERATION &
CHARACTERISTICS
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Saturation level for VDS is related to applied VGS by:
VDsat = VGS – VT
VGS < VT, ID=0 A
VGS > VT, ID=k(VGS-VT)2
P-CHANNEL ENHANCEMENT-TYPE
MOSFET
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Construction is reverse of n-channel.
All voltage polarities and current direction are
reverse.
SYMBOL
D-MOSFET BIASING
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Similarities in appearance between transfer curve of
JFET and D-MOSFET.
Primary difference: D-MOSFET permit operating
points with positive value of VGS and level of ID that
exceed IDSS.
D-MOSFET BIASING
Self-Biased Configuration:
D-MOSFET BIASING
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ID=IDSS(1-VGS/VP)2
Self-biased
configuration results
in VGS=-IDRS
D-MOSFET BIASING
Voltage-Divider Bias Configuration:
D-MOSFET BIASING
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ID=IDSS(1-VGS/VP)2
Voltage-divider
configuration results
in:
VGS=VG-IDRS
Where
VG=R2xVDD/(R1+R2)
E-MOSFET BIASING
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Transfer curve for
E-MOSFET is quite
different from JFET
and D-MOSFET.
ID=0 A if VGS<VT.
VGS>VT, ID=k(VGSVT)2
k
V
I D ( on)
GS ( on)
 VT 
2
E-MOSFET BIASING
Voltage-Divider Biasing
E-MOSFET BIASING
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Voltage-divider
configuration results
in:
VGS=VG-IDRS
Where
VG=R2xVDD/(R1+R2)
VDS=VDD-ID(RS+RD)
k
V
I D ( on)
GS ( on)  VT 
2
E-MOSFET BIASING
Feedback Biasing
E-MOSFET BIASING
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IG=0 V
VD=VG
VDS=VGS
VDS=VDD-IDRD
VGS=VDD-IDRD
When ID=0 A:
VGS=VDD
When VGS=0 V:
ID=VDD/RD
E-MOSFET BIASING