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Field-Effect Transistors
Chapter 20
 Introduction
 An Overview of Field-Effect Transistors
 Insulated-Gate Field-Effect Transistors
 Junction-Gate Field-Effect Transistors
 FET Characteristics
 Summary of FET Characteristics
 FET Amplifiers
 Other FET Applications
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Introduction
20.1
 Field-effect transistors (FETs) are probably the
simplest form of transistor
– widely used in both analogue and digital applications
– they are characterised by a very high input resistance
and small physical size, and they can be used to form
circuits with a low power consumption
– they are widely used in very large-scale integration
– two basic forms:
 insulated gate FETs
 junction gate FETs
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An Overview of Field-Effect Transistors
20.2
 Many forms, but basic operation is the same
– a voltage on a control input produces an electric field
that affects the current between two other terminals
– when considering
amplifiers we looked
at a circuit using a
‘control device’
– a FET is a suitable
control device
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 Notation
– FETs are 3 terminal devices
 drain (d)
 source (s)
 gate(g)
– the gate is the control input
– diagram illustrates the
notation used for labelling
voltages and currents
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Insulated-Gate Field-Effect Transistors
20.3
 Such devices are sometimes called IGFETs
(insulated-gate field-effect transistors) or sometimes
MOSFETs (metal oxide semiconductor field-effect
transistors)
 Digital circuits constructed using these devices are
usually described as using MOS technology
 Here we will describe them as MOSFETs
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 Construction
– two polarities: n-channel and p-channel
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 Operation
– gate volt controls the thickness of the channel
– consider an n-channel device
 making the gate more positive attracts electrons to
the gate and makes the gate region thicker –
reducing the resistance of the channel. The channel
is said to be enhanced
 making the gate more negative repels electrons
from the gate and makes the gate region thinner –
increasing the resistance of the channel. The
channel is said to be depleted
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– the effect of varying the gate voltage
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– gates as described above are termed DepletionEnhancement MOSFETs or simply DE MOSFETs
– some MOSFETs are constructed so that in the
absence of any gate voltage there is no channel
 such devices can be operated in an enhancement mode, but
not in a depletion mode (since there is no channel to deplete)
 these are called Enhancement MOSFETs
– both forms of MOSFET are available as either
n-channel or p-channel devices
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 MOSFET
circuit symbols
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Junction-Gate Field-Effect Transistors
20.4
 Sometimes known as a JUGFET
 Here we will use another common name – the JFET
 Here the insulated gate of a MOSFET is replaced
with a reverse-biased pn junction
 Since the gate junction is always reverse-biased no
current flows into the gate and it acts as if it were
insulated
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 Construction
– two polarities: n-channel and p-channel
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 Operation
– the reverse-biased gate junction produced a depletion
layer in the region of the channel
– the gate volt controls the thickness of the depletion
layer and hence the thickness of the channel
– consider an n-channel device
 the gate will always be negative with respect to the source to
keep the junction between the gate and the channel reversebiased
 making the gate more negative increases the thickness of the
depletion layer, reducing the width of the channel – increasing
the resistance of the channel.
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– the effect of varying the gate voltage
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 JFET circuit symbols
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FET Characteristics
20.5
 While MOSFETs and JFETs operate in different
ways, their characteristics are quite similar
 Input characteristics
– in both MOSFETs and JFETs the gate is effectively
insulated from the remainder of the device
 Output characteristics
– consider n-channel devices
– usually the drain is more positive than the source
– the drain voltage affects the thickness of the channel
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 FET output characteristics
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 Transfer characteristics
– similar shape for all forms of FET – but with a different
offset
– not a linear response, but over a small region might be
considered to approximate a linear response
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 Normal operating ranges for FETs
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 When operating about its operating point we can
describe the transfer characteristic by the change in
output that is caused by a certain change in the input
– this corresponds to the slope of the earlier curves
– this quantity has units of current/voltage, which is the
reciprocal of resistance (this is conductance)
– since this quantity described the transfer
characteristics it is called the transconductance, gm
Note:
gm 
ID
VGS
gm 
ID
VGS
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 Small-signal equivalent circuit of a FET
– models the behaviour of the device for small variations
of the input about the operating point
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Summary of FET Characteristics
20.6
 FETS have three terminals: drain, source and gate
 The gate is the control input
 Two polarities of device: n-channel and p-channel
 Two main forms of FET: MOSFET and JFET
 In each case the drain current is controlled by the
voltage applied to the gate with respect to the source
 Behaviour is characterised by the transconductance
 The operating point differs between devices
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 FET circuit
symbols:
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FET Amplifiers
20.7
 A simple DE MOSFET amplifier
– RG is used to ‘bias’ the
gate at its correct operating
point (which for a
DE MOSFET is 0 V)
– C is a coupling capacitor
and is used to couple the
AC signal while preventing
externals circuits from
affecting the bias
– this is an AC-coupled amplifier
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 AC-coupled amplifier
– input resistance – equal to RG
– output resistance – approximately equal to RD
– gain – approximately –gmRD (the minus sign shows
that this is an inverting amplifier)
– C produced a low-frequency cut-off at a frequency fc
given by
fc 
1
2CR
where R is the input resistance of the amplifier (which
in this case is equal to RG)
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 Negative feedback amplifier
– reduces problems of variability
of active components
– voltage across Rs is
proportional to drain current,
which is directly proportional
to the output voltage
– this voltage is subtracted
from input voltage to gate
– hence negative feedback
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 Source follower
– similar to earlier circuit,
but output is now taken
from the source
– feedback causes the
source to follow the input
voltage
– produces a unity-gain
amplifier
– also called a source follower
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Other FET Applications
20.8
 A voltage controlled attenuator
– for small drain-to-source
voltages FETs resemble
voltage-controlled resistors
– the gate voltage VG is used
to control this resistance and
hence the gain of the potential
divider
– used, for example, in automatic
gain control in radio receivers
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 A FET as an analogue switch
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 A FET as a logical switch
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Key Points
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

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FETs are widely used in both analogue and digital circuits
They have high input resistance and small physical size
There are two basic forms of FET: MOSFETs and JFETs
MOSFETs may be divided into DE and Enhancement types
In each case the gate voltage controls the current from the
drain to the source
 The characteristics of the various forms of FET are similar
except that they require different bias voltages
 The use of coupling capacitors prevents the amplification of
DC and produced AC amplifiers
 FETs can be used to produce various forms of amplifier and
a range of other circuit applications
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