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Chapter 5
FIELD-EFFECT TRANSISTORS
(FETs)
Chapter 5 FETs
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FETs is also a three terminal devices. For the FET an electric field is established by the
charges present that will control the conduction path of the output circuit without the need
for direct contact between the controlling and controlled quantities.
Differences
BJTs
FETs
(i) Current-controlled devices
(i) Voltage-controlled devices
Current I
is a direct function
Current I
is a function of the voltage
C
of the level of IB.
D
VGS applied to the input circuit.
Chapter 5 FETs
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(ii) Bipolar device (npn and pnp)
(ii) Unipolar device (n-channel and p-channel)
Conduction level is a function of
Two carriers, electrons and holes.
Conduction level is a function of
one type of carrier, either electrons
(n-channel) or holes (p-channel).
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Types of FETs :
(i)
Junction FETs (JFET) and
(ii)
Metal-Oxide-Semiconductor FETs (MOSFET)
MOSFET can be divided to two types:
(i)
Depletion type and
(ii)
Enhancement type
.
The MOSFET transistor has become one of the most important devices used in the design
and construction of integrated circuits for digital computers. Its thermal stability and other
general characteristics make it extremely popular in computer circuit design.
Chapter 5 FETs
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CONSTRUCTION and CHARACTERISTICS of JFETs
Ex: n-channel JFETs
The major part of the structure is the n-type
material that forms the channel between
the embedded layers of p-type material.
The top of the n-type channel is connected
through an ohmic contact to a terminal
referred to as the drain (D), while the lower
end of the same material is connected through
an ohmic contact to a terminal referred to as
the source (S).
The two p-type materials are connected
together and to the gate (G) terminal.
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Therefore, the drain and source are connected
to the ends of the n-type channel and the
gate to the two layers of p-type material.
Under no-bias condition, there are two p-n junctions. The result is a depletion region at
each junction. A deple