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Digital Electronics Chapter 7. Field-Effect Transistors
By: FARHAD FARADJI, Ph.D.
Assistant Professor, Electrical and Computer Engineering, K. N.
Toosi University of Technology
http://wp.kntu.ac.ir/faradji/DigitalElectronics.htm
Reference:
DIGITAL INTEGRATED CIRCUITS: ANALYSIS and DESIGN, 2005, John E.
Ayers
1
K. N. Toosi University of Technology
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7.1. Introduction Field-effect transistors (FETs) have several
significant differences
compared to bipolar junction transistors.
First, they are voltage controlled rather than current
controlled. This results in low levels of standby supply current
and standby power
dissipation.
Second, they are majority carrier devices. Third, they can be
made smaller than BJTs using same fabrication
technology.
Chapter 7. Field-Effect Transistors 2 Digital Electronics
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on.
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n bbee mmaaddee ssmmaalllleer thhaan BBJJTTs using saammee
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7.1. Introduction 3 basic types of FETs are:
metal oxidesemiconductor field-effect transistor (MOSFET),
junction field-effect transistor (JFET),
metalsemiconductor field-effect transistor (MESFET).
MOSFET is very important for ICs and is emphasized in this
chapter.
Chapter 7. Field-Effect Transistors 3 Digital Electronics
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ry impoortant for IICCs aand iis eemmpphaasizzeedd iinn tthi
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7.1. Introduction 7.1.1. MOSFET
MOSFET is also known as insulated gate field-effect transistor
(IGFET). 3 terminals of this device are source, gate, and drain,
labeled S, G, and D. Sometimes, a 4th terminal is used: body or
substrate (labeled B). Voltage applied between G and S controls
current between D and S.
Chapter 7. Field-Effect Transistors 4 Digital Electronics
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ed betwween G andd S cconttrrolss ccuurreentt bbeettwwee
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7.1. Introduction 7.1.1. MOSFET
Basic operation of MOSFET:
9 If G is biased positively with respect to S, negatively
charged electrons are attracted to interface between semiconductor
and oxide.
9 This forms a conducting channel between D and S.
9 Then, if D is biased positively with respect to S, electrons
in channel will drift from S to D.
9 This results in a conventional current from D to S. 9 Current
involves only electrons. 9 It is called an n-channel MOSFET.
Chapter 7. Field-Effect Transistors 5 Digital Electronics
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to interface between or and ooxide.
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7.1. Introduction 7.1.1. MOSFET
There are also p-channel devices. In p-channel device, S and D
are
p-type regions.
Holes drift in channel. Voltages and currents have
opposite polarities compared to those in n-channel device.
Chapter 7. Field-Effect Transistors 6 Digital Electronics
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channel.
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annnneell ddeevviiccee..
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7.1. Introduction 7.1.1. MOSFET
For device shown, no conducting channel can be between D and S
unless a positive voltage is applied between G and S.
This device is normally off. These MOSFETs are called
enhancement type.
A gate bias is required to enhance a conducting channel.
Chapter 7. Field-Effect Transistors 7 Digital Electronics
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normaallly off.
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7.1. Introduction 7.1.1. MOSFET
Depletion-type devices are normally on. A G-S bias is necessary
to deplete
conducting channel.
Normally off enhancement-type MOSFETs are preferred in ICs for
low standby dissipation.
Chapter 7. Field-Effect Transistors 8 Digital Electronics
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enhancement-type prefeerrrreedd inn IICs ffoorr
dissssiippaattiioonn.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 9 Digital Electronics
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7.1.1. MOSFET
9 Some MOSFET symbols are shown. 9 Most convenient are middle
four. 9 These result in simplest and neatest
circuit diagrams.
9 They eliminate body connection and avoid use of other
arrows.
9 Inversion circle on G indicates a p-type device.
9 Broad line in channel indicates a depletion-type device.
9 We use these simplified symbols, except in situations for
which body bias is used.
9 Take a look at this link.
ms.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 10 Digital Electronics
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7.1.2. JFET
Junction field-effect transistor (JFET) takes it name from G
structure.
G involves a p-n junction. For an n-channel device, S, D,
and
channel regions are n-type.
With zero bias between G and S, there is a conducting channel
from D to S.
JFET is a depletion-type device. If a reverse bias is applied to
the G-S junction:
It widens depletion region.
It reduces channel conductivity.
nnel device, S, D, and ons arree nn-ttyyppee.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 11 Digital Electronics
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7.1.2. JFET
A sufficiently negative bias on G will pinch off channel
entirely.
JFET is a field-effect device in which G-S bias controls D-S
current.
Unlike MOSFET, no insulating oxide layer is under G.
Gate pn junction must be kept reverse biased in order to avoid a
DC gate current.
rols D S current.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 12 Digital Electronics
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7.1.2. JFET
A p-channel JFET utilizes p-type regions for S, D, and channel.
Gate region is doped n-type. Voltages and currents are reversed in
polarity compared to n-channel
device.
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D, andd cchan
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7.1. Introduction
Chapter 7. Field-Effect Transistors 13 Digital Electronics
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7.1.2. JFET
9 Enhancement-type (normally off) JFETs can be fabricated but
with some difficulty.
9 These devices must be made so that depletion region of G
junction pinches off channel at zero G-S bias.
9 This can be done, but only with precise control of channel
thickness and doping.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 14 Digital Electronics
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7.1.2. JFET
JFETs are not used in digital ICs for 2 reasons. First, JFETs
are inherently depletion-type devices.
This results in excessive standby dissipation, unless normally
off (enhancement-type) devices are fabricated.
Second, even if normally off JFETs are used, p-n junctions used
in gates are leaky compared to MOS structures used in MOSFETs.
Have a look at this link.
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7.1. Introduction
Chapter 7. Field-Effect Transistors 15 Digital Electronics
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7.1.3. MESFET
Metal-semiconductor field-effect transistor (MESFET) is similar
to JFET.
A metal-semiconductor junction is used for G structure.
It suffers from same drawbacks as JFET.
It is not used in silicon technology. MESFETs are used in
digital ICs based on compound semiconductors like
gallium arsenide direct-coupled FET logic (DCFL) circuits.
A viable MOSFET technology does not exist in materials such as
gallium arsenide and indium phosphide.
These semiconductors exhibit speed advantages over silicon.
structure.
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in ssiilliiccoonn tteecchhnnoollooggyy..
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7.2. MOSFET Threshold Voltage
Chapter 7. Field-Effect Transistors 16 Digital Electronics
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Applying a positive bias on metal gate with respect to
semiconductor will reduce hole concentration near interface.
This situation is referred to as depletion condition.
Application of a sufficiently positive bias on gate will result
in inversion.
In this case, semiconductor becomes n-type near interface.
It is possible for semiconductor to be inverted to extent that
electron concentration near interface is equal to hole
concentration in bulk of semiconductor.
This is referred to as strong inversion.
ondition.
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7.2. MOSFET Threshold Voltage
Chapter 7. Field-Effect Transistors 17 Digital Electronics
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In an n-channel MOSFET, G-S bias necessary to cause strong
inversion in channel is called threshold voltage.
Among n-channel MOSFETs: enhancement-type transistors
have positive thresholds,
depletion-type transistors have negative thresholds.
Opposite is true for p-channel devices.
ment type transistors tive thrresholds,
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7.2. MOSFET Threshold Voltage
Chapter 7. Field-Effect Transistors 18 Digital Electronics
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A body bias (applied between body and source) allows threshold
of a MOSFET to be adjusted in the circuit.
This is exploited to overcome manufacturing tolerances in
threshold voltages.
This technique is used in modern low-power, high-speed CMOS
circuits.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 19 Digital Electronics
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Substrate is often shorted to source.
VGS is G-S bias. VDS is D-S bias. ID is drain current.
MOSFET has 3 modes of operation: cutoff, linear, saturation.
rent.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 20 Digital Electronics
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Cutoff occurs if VGS is insufficiently positive to induce a
conducting channel.
Cutoff results in zero drain current. If VGS is made more
positive than
threshold voltage (VT): a conducting channel is induced an ID
can flow.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 21 Digital Electronics
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With a small VDS: MOSFET acts like a voltage-controlled
resistance. This is linear (ohmic or triode) mode of operation.
If VDS is sufficiently large: Conducting channel will pinch off
at drain end. ID saturates. This mode of operation is called
saturation.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 22 Digital Electronics
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In characteristic curves, it is customary to plot ID vs. VDS
with VGS as a parameter.
This results in a family of curves, one for each particular
value of VGS.
Cutoff: is associated with zero ID, its locus is on VDS
axis.
In linear region: ID increases approximately linearly with VDS,
its locus is to left of parabola.
Saturation: is characterized by a constant ID, its locus is to
right of parabola.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 23 Digital Electronics
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7.3.1. MOSFET Cutoff Operation
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 24 Digital Electronics
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7.3.2. MOSFET Linear Operation
VGS > VT. VDS is small enough so that channel
does not pinch off at drain end.
MOSFET acts like a voltage-controlled resistance.
RDS is controlled variable. VGS is controlling variable.
Pinch-off at D end of channel occurs when:
This condition defines boundary between linear and saturation
operation
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 25 Digital Electronics
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7.3.2. MOSFET Linear Operation
K = device transconductance parameter.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 26 Digital Electronics
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7.3.2. MOSFET Linear Operation
k is process transconductance parameter.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 27 Digital Electronics
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7.3.2. MOSFET Linear Operation
For p-channel MOSFETs, p must be used instead of n.
All voltages and currents are opposite in polarity.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 28 Digital Electronics
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7.3.3. MOSFET Saturation Operation
MOSFET acts like a voltage-controlled current source.
ID is controlled quantity. VGS is controlling quantity.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 29 Digital Electronics
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7.3.4. MOSFET Subthreshold Operation
9 Cutoff operation: n-MOSFET: VGS < VT p-MOSFET: |VGS| <
|VT| results in ID = 0 to a first approximation.
9 If VGS is close to VT, a non-negligible ID will flow. 9 This
subthreshold current is important in modern low-voltage,
low-power
CMOS and memory circuits.
ID = 0 to a first approximation.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 30 Digital Electronics
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7.3.4. MOSFET Subthreshold Operation
Saturation or linear operation is dominated by drift of majority
carriers. Subthreshold operation occurs as result of minority
carrier diffusion. Device acts as a BJT. S injects carriers into
channel region. These injected carriers diffuse length of channel.
They are collected by D. In an n-MOSFET:
electrons are injected into p-type channel region diffuse to D,
resulting in current from D to S.
Subthreshold current flows in same direction as saturated
current.
ers intoo channel rregioon.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 31 Digital Electronics
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7.3.4. MOSFET Subthreshold Operation
If VDS is several times kT/q (~ 26 mV at room temperature),
subthreshold current is independent of VDS:
Subthreshold current increases exponentially with VGS.
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teemmppeerraattuurreeppendent off VVDDSSVV :
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 32 Digital Electronics
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7.3.4. MOSFET Subthreshold Operation
Subthreshold swing is:
Room-temperature operation of MOSFETs is characterized by S =
100 mV. Subthreshold current changes by 1 decade for every 100-mV
change in VGS. Scaling of VT below about 300 mV is accompanied by
significant
subthreshold current at VGS = 0.
This is a significant issue in design of low-power CMOS
circuits.
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7.3. Long-Channel MOSFET Operation
Chapter 7. Field-Effect Transistors 33 Digital Electronics
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7.3.5. Transit Time
It takes a finite time for majority carriers to traverse channel
in a conducting MOSFET.
This delay is called transit time (tt). In a long-channel
n-channel MOSFET, electrons are drifted in channel.
Average electric field intensity in channel is
approximately:
Carriers move at a velocity of approximately:
tt increases with square of channel length:
nnel n-channel MOSFET, elecctrons aree ddrriiffted
ric field inttensiityy in chhannel is appproxxiimmaattely
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7.4. Short-Channel MOSFETs
Chapter 7. Field-Effect Transistors 34 Digital Electronics
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Aggressive scaling of MOSFETs and channel lengths has resulted
in devices that behave differently than long channel devices.
First, VT becomes a function of channel length (short-channel
effect). Second, electric field intensity in channel may be
sufficiently large so that
carriers reach their saturated velocity.
Third, effective channel length becomes a function of VDS as a
consequence of channel length modulation.
All these effects are of practical importance in design of
high-performance CMOS circuits.
their saturated velocity.
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7.4. Short-Channel MOSFETs
Chapter 7. Field-Effect Transistors 35 Digital Electronics
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7.4.1. The Short-Channel Effect
|VT| decreases with decreasing channel length.
7.4.2. Channel Length Modulation
ID in a MOSFET saturates at VDS which causes channel to pinch
off at D end.
Further increase in VDS causes pinch-off point to move into
channel, toward S.
This increases ID by ratio L/(L L). In a long-channel MOSFET,
percentage change in ID is small. Channel length modulation effect
is important in short-channel MOSFETs.
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7.4. Short-Channel MOSFETs
Chapter 7. Field-Effect Transistors 36 Digital Electronics
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7.4.2. Channel Length Modulation
ID in a MOSFET saturates at VDS which causes channel to pinch
off at D end.
Further increase in VDS causes pinch-off point to move into
channel, toward S.
For linear operation:
For saturation operation:
is the empirical channel length modulation parameter.
eratioonn:
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7.4. Short-Channel MOSFETs
Chapter 7. Field-Effect Transistors 37 Digital Electronics
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7.4.3. Velocity Saturation
At high electric-field intensities, carrier drift velocities are
no longer proportional to electric field.
Instead, there is approximately carrier velocity saturation.
Onset of ID saturation occurs at a lower VDS. Magnitude of
saturated ID is less than before.
turation occurs at a lower VVDDSVVV .
f saattuurraatteedd IIDD iiss lleessss tthhaann
bbeeffoorree..
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7.4. Short-Channel MOSFETs
Chapter 7. Field-Effect Transistors 38 Digital Electronics
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7.4.4. Transit Time
In short-channel MOSFETs, carriers may travel at close to
saturation velocity for entire channel length.
For electrons:
For holes:
Saturation velocities in silicon MOSFETs are typically 20% lower
than bulk values.
tt is directly proportional to the channel length.
: