ORIENTAL INSTITUTE OF
SCIENCE & TECHNOLOGY
Submitted By :-
It is with great reverence that we
express out gratitude to our guide ,Hon’ble “Miss Abhilasha
Verma “ Department of Electronics & communication ,
Oriental institute of science & technology Bhopal(M.P.) for
her precious guidance and help in this project work . The credit
for a successful completion of this project goes to our keen
interest, timing, guidance and valuable suggestion otherwise
our endeavwould have been futile .
FET ( Field Effect Transistor)
The field-effect transistor (FET) is a transistor that uses an
electric field to control the shape and hence the conductivity of a
channel of one type of charge carrier in a semiconductor material.
FETs are unipolar transistors as they involve single-carrier-type
operation. The concept of the FET predates the bipolar junction
transistor (BJT), though it was not physically implemented until
after BJTs due to the limitations of semiconductor materials and
the relative ease of manufacturing BJTs compared to FETs at the
SOME BASIC INFORMATION
• FETs can be majority-charge-carrier devices, in
which the current is carried predominantly by
majority carriers, or minority-charge-carrier
devices, in which the current is mainly due to a
flow of minority carriers.
• The device consists of an active channel through
which charge carriers, electrons or holes, flow
from the source to the drain. Source and drain
terminal conductors are connected to the
semiconductor through ohmic contacts.
• The conductivity of the channel is a function of the potential
applied across the gate and source terminals.
The FET's three terminals are:
• Source (S), through which the carriers enter the channel.
Conventionally, current entering the channel at S is designated
Drain (D), through which the carriers leave the channel.
Conventionally, current entering the channel at D is designated
by ID. Drain-to-source voltage is VDS.
Gate (G), the terminal that modulates the channel conductivity.
By applying voltage to G, one can control ID.
ABOUT THE TERMINALS OF
• All FETs have source, drain, and gate terminals that correspond
roughly to the emitter, collector, and base of BJTs. Most FETs
have a fourth terminal called the body, base, bulk, or substrate.
• This fourth terminal serves to bias the transistor into operation;
it is rare to make non-trivial use of the body terminal in circuit
designs, but its presence is important when setting up the
physical layout of an integrated circuit.
• The size of the gate, length L in the diagram, is the distance
between source and drain. The width is the extension of the
transistor, in the direction perpendicular to the cross section in
• Typically the width is much larger than the length of the gate. A
gate length of 1 µm limits the upper frequency to about 5 GHz,
0.2 µm to about 30 GHz.
Types of Field Effect Transistors
P P +
DC Voltage Source
Operation of a JFET
Operation of Junction Field Effect Transistor (JFET)
• When neither any bias is applied to the gate (i.e. when VGS =
0) nor any voltage to the drain w.r.t. source (i.e. when VDS =
0), the depletion regions around the P-N junctions , are of
equal thickness and symmetrical.
• 2. When positive voltage is applied to the drain terminal D
w.r.t. source terminal S without connecting gate terminal G to
supply, as illustrated in fig. 9.4, the electrons (which are the
majority carriers) flow from terminal S to terminal D whereas
conventional drain current ID flows through the channel from D
to S. Due to flow of this current, there is uniform voltage drop
across the channel resistance as we move from terminal D to
terminal S. This voltage drop reverse biases the diode.
• The gate is more “negative” with respect to those points in the
channel which are nearer to D than to S. Hence, depletion layers
penetrate more deeply into the channel at points lying closer to
D than to S. Thus wedge-shaped depletion regions are formed,
as shown in figure. when Vds is applied. The size of the
depletion layer formed determines-
the width of the channel and
hence the magnitude of current ID flowing through the channel.
• The metal–oxide–semiconductor field-effect transistor
(MOSFET, MOS-FET, or MOS FET) is a transistor used for
amplifying or switching electronic signals. Although the
MOSFET is a four-terminal device with source (S), gate (G),
drain (D), and body (B) terminals.
• the body (or substrate) of the MOSFET often is connected to
the source terminal, making it a three-terminal device like other
• Because these two terminals are normally connected to each
other (short-circuited) internally, only three terminals appear in
electrical diagrams. The MOSFET is by far the most common
transistor in both digital and analog circuits, though the bipolar
junction transistor was at one time much more common.
Figure p-Channel FET circuit symbols. These are the same as the circuit symbols for n-channel devices,
except for the directions of the arrowheads.
Few important advantages of FET over
● Unipolar device i.e. operation depends on
only one type of charge carriers (h or e)
● Voltage controlled Device (gate voltage
controls drain current)
● Very high input impedance (≈109
● Source and drain are interchangeable in
● Low-frequency applications
● Low Voltage Low Current Operation is possible
● Less Noisy as Compared to BJT
● No minority carrier storage (Turn off is faster)
● Self limiting device
● Very small in size, occupies very small space in Ics
● Low voltage low current operation is possible in
● Zero temperature drift of out put is possible
Disadvantages of FET
• It has a relatively low gain-bandwidth product compared to a
BJT. The MOSFET has a drawback of being very susceptible to
overload voltages, thus requiring special handling during
• The fragile insulating layer of the MOSFET between the gate
and channel makes it vulnerable to electrostatic damage during
handling. This is not usually a problem after the device has been
installed in a properly designed circuit.
Uses of FET
• The most commonly used FET is the MOSFET. The CMOS
(complementary metal oxide semiconductor) process technology
is the basis for modern digital integrated circuits. This
processtechnology uses an arrangement where the (usually
"enhancement-mode") p-channel MOSFET and n-channel
MOSFET are connected in series such that when one is on, the
other is off.
• In FETs, electrons can flow in either direction through the
channel when operated in the linear mode. The naming
convention of drain terminal and source terminal is somewhat
arbitrary, as the devices are typically (but not always) built
symmetrically from source to drain. This makes FETs suitable
for switching analog signals between paths (multiplexing). With
this concept, one can construct a solid-state mixing board, for
• A common use of the FET is as an amplifier. For
example, due to its large input resistance and low
output resistance, it is effective as a buffer in common-
drain (source follower) configuration