Field Effect Transistors (FETs)
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Junction Field Effect Transistor
(JFET)
A junction field effect transistor is made up of a
gate region diffused into a channel region.
When control voltage is applied to the gate, the
channel is depleted or enhanced, and the
current between the source and the drain is
thereby controlled.
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JFET construction
n-channel JFET – A junction field effect
transistor having an n-type channel between
the source and the drain.
p-channel JFET – A junction field effect
transistor having a p-type channel between the
source and the drain.
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JFET construction (continued)
Gate – One of the field effect transistor’s electrodes
(also used for thyristor devices [chapter 20])
Source – One of the field effect transistor’s
electrodes.
Drain – One of the field effect transistor’s electrodes.
Channel – A path for a signal.
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Junction field effect transistor
(JFET) types
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JFET operations
Drain Supply Voltage (+VDD) – The bias voltage
connected between the drain and the source of the
JFET, which causes current to flow.
Gate-Source Bias Voltage (VGS) – The bias voltage
applied between the gate and the source of a field
effect transistor.
Drain Current (ID) – A JFET’s source-to-drain
current.
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Relationship between a JFET’s DC
supply voltage and output current.
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Differences between
BJT and JFET
Current-Controlled Device (BJT) – A device in
which the input junction is normally forward
biased and the input current controls the output
current.
Voltage-Controlled Device (JFET) – A device
in which the input junction is normally reverse
biased and the input voltage controls the output
current.
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Unipolar Device (JFET) – A device in which only
one type of semiconductor material exists between
the output terminals and therefore the charge carriers
have only one polarity (unipolar).
Bipolar Device (BJT) – A device in which there is a
change in semiconductor material between the output
terminals (NPN or PNP between the emitter and the
collector), so the charge carriers can be one of two
polarities (bipolar).
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JFET characteristics
Drain Characteristic Curve – A plot of the drain current (ID)
versus the drain-to-source voltage (VDS).
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JFET characteristic curve
Pinch-off Voltage – The value of VDS at which
further increases in VDS will cause no further increase
in ID.
Constant-current Region – The flat portion of the
drain characteristic curve. In this region ID remains
constant despite changes in VDS.
Breakdown Voltage (VBR) – The voltage at which a
damaging value of ID will pass through the JFET.
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JFET characteristics
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JFET characteristic curve (cont.)
Drain-to-Source Current with Shorted Gate
(IDSS) – The maximum value of drain current
achieved by holding VGS at 0 volts.
Gate-to-Source Cutoff Voltage or VGS(off) –
The negative VGS bias voltage that causes ID to
drop to approximately zero.
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Transconductance
Transconductance – Also called mutual conductance, it is the
ratio of a change in output current to the initiating change in
input voltage.
∆I D
δm =
∆VGS
δ m = transconduc tan ce in Siemens(S )
∆I D = Change in drain current
∆VGS = Change in gate − source Voltage
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Voltage gain
AV = δ m x R D
Where
AV = Voltage gain
δm = Transconductance
RD = Drain Resistor
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JFET data sheet
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JFET biasing
Gate Biasing
Self Biasing
Voltage Divider Biasing
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Gate biasing
VGS = VGG
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Gate biasing (Continued)
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Self-biasing
VG = 0 Volts
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Voltage divider biasing
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JFET circuit configurations
Common-Source
Common-Gate
Common-Drain
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Common-Source (C-S) circuits
An FET configuration in which the source is grounded and
common to the input and output signal.
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Common-Gate (C-G) circuits
An FET configuration in which the gate is grounded and
common to the input and the output signal.
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Common-Drain (C-D) circuits
An FET configuration in which the drain is grounded and
common to the input and the output signal.
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Source follower
Another name used for a common drain
circuit configuration.
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JFET applications
Digital JFET Circuits
Analog JFET Circuits
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Digital JFET circuits
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Analog JFET circuit
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Testing JFETs
A transistor tester will test BJTs
and JFETs. An ohmmeter can be
used to test a JFET.
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Metal Oxide Semiconductor
Field Effect Transistor
(MOSFET)
The MOSFET has a metal gate that is insulated
from the semiconductor channel by a layer of
silicon dioxide. The input voltage at the gate
will generate an electric field which will have
the effect of changing the channel’s size.
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D-type MOSFET construction
and types
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Types of MOSFETs
Depletion-Type MOSFET – A field effect transistor
with an insulated gate that can be operated in either
the depletion or enhancement mode.
n-Channel D-Type MOSFET – A depletion type
MOSFET having an n-type channel between its
source and drain terminals.
p-Channel D-Type MOSFET – A depletion type
MOSFET having a p-type channel between its source
and drain terminals.
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Operating Modes of MOSFETs
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D-MOSFET biasing
Zero Biasing – A configuration is
which no bias voltage is applied
at all.
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D-MOSFET application
Cascode Amplifier Circuit – An amplifier circuit consisting of
a self-biased
common-source
amplifier in
series with a
voltage-divider
biased
common-gate
amplifier.
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Dual Gate D-MOSFET
A metal oxide semiconductor FET having two
separate gate electrodes.
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Enhancement-Type (E-Type)
MOSFET
Enhancement-Type MOSFET or E-MOSFET – A
field effect transistor with an insulated gate that can
only be turned ON if the channel is enhanced.
n-Channel E-Type MOSFET – An enhancement-type
MOSFET having an n-type channel between its
source and drain terminals.
p-Channel E-Type MOSFET – An enhancement-type
MOSFET having a p-type channel between its source
and drain terminals.
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E-type MOSFET construction
and type
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E-type MOSFET operation and
characteristics
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E-MOSFET biasing
Drain-Feedback Biasing – A configuration in which the gate
receives a bias voltage feedback from the drain: VGS =VDS
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Vertical-channel E-MOSFET
An enhancement type
MOSFET that, when turned
ON, forms a vertical channel
between source and drain.
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Testing MOSFET with an
ohmmeter
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End of Field Effect Transistors
(FETs)
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