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7/31/2019 Fet Research
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SEMICONDUCTOR EESB313
TABLE OF CONTENTS
INTRODUCTION .............................................................................
2
HISTORY OF FET
.............................................................................3
THEORY AND TYPES OF FETS ..........................................................5
FET APPLICATIONS
.............................................................................8
ADVANTAGES AND DISADVANTAGES OF FETS ............................10
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INTRODUCTION
Everything is achievable through technology. In this modern era that we live in,
technology mainly contribute in making our life better. Everything from as big as ship,
factories, cars, electrical appliances ,computer chips to the smallest element of circuit such
as transistors, resistors are produced better by each day. Things become smaller, bigger,
cheaper to produce but most of all it must be better than its predecessor. But the concept is
still the same.
Which is why I would like to make a research about FIELD EFFECT TRANSISTOR
or often called as FET. FET is basically a transistor, but what differentiate it from Bipolar
Junction Terminal (BJT) is that it only uses 1 terminal instead of two in BJT. So what is so
special about FET? Later in the research we will find out all about FET.
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HISTORY OF FET
The first patent for the field-effect transistor principle was filed in Canada by Austrian-
Hungarian physicist Julius Edgar Lilienfeld on October 22, 1925, but Lilienfeld published no
research articles about his devices, and they were ignored by industry. In 1934 German
physicist Dr. Oskar Heil patented another field-effect transistor. There is no direct evidence
that these devices were built, but later work in the 1990s show that one of Lilienfeld's
designs worked as described and gave substantial gain. Legal papers from the Bell Labs
patent show that William Shockley and a co-worker at Bell Labs, Gerald Pearson, had built
operational versions from Lilienfeld's patents, yet they never referenced this work in any of
their later research papers or historical articles.
The work emerged from their war-time efforts to produce extremely pure germanium
"crystal" mixer diodes, used in radar units as a frequency mixer element in microwave radar
receivers. A parallel project on germanium diodes at Purdue University succeeded in
producing the good-quality germanium semiconducting crystals that were used at Bell Labs.
Early tube-based technology did not switch fast enough for this role, leading the Bell team to
use solid state diodes instead. With this knowledge in hand they turned to the design of a
triode, but found this was not at all easy. Bardeen eventually developed a new branch of
quantum mechanics known as surface physics to account for the "odd" behavior they saw,
and Bardeen and Brattain eventually succeeded in building a working device.
After the war, William Shockley decided to attempt the building of a triode-like
semiconductor device. He secured funding and lab space, and went to work on the problem
with Brattain and John Bardeen.
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The key to the development of the transistor was the further understanding of the
process of the electron mobility in a semiconductor. It was realized that if there was some
way to control the flow of the electrons from the emitter to the collector of this newly
discovered diode, one could build an amplifier. For instance, if you placed contacts on either
side of a single type of crystal the current would not flow through it. However if a third
contact could then "inject" electrons or holes into the material, the current would flow.
To make the story short, field-effect transistors were invented by Julius Edgar
Lilienfeld in 1925 and by Oskar Heil in 1934, but practical devices were not able to be made
until 1952 (the JFET). The MOSFET, which largely superseded the JFET and had a more
profound effect on electronic development, was first proposed by Dawon Kahng in 1960.
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THEORY AND TYPES OF FETS
All FETs have a gate, drain, and source terminal that correspond roughly to the base,
collector, and emitter of BJTs. Aside from the JFET, all FETs also have a fourth terminal
called the body, base, bulk, orsubstrate. 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 diagram perpendicular to the cross section. Typically the
width is much larger than the length of the gate. A gate length of 1m limits the upper
frequency to about 5 GHz, 0.2m to about 30 GHz.
The names of the terminals refer to their functions. The gate terminal may be thought
of as controlling the opening and closing of a physical gate. This gate permits electrons to
flow through or blocks their passage by creating or eliminating a channel between the source
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and drain. Electrons flow from the source terminal towards the drain terminal if influenced by
an applied voltage. The body simply refers to the bulk of the semiconductor in which the
gate, source and drain lie. Usually the body terminal is connected to the highest or lowest
voltage within the circuit, depending on type. The body terminal and the source terminal are
sometimes connected together since the source is also sometimes connected to the highest
or lowest voltage within the circuit, however there are several uses of FETs which do not
have such a configuration, such as transmission gates and cascoded circuits.
Current Control:
The control terminal is called the gate. Remember that the base terminal of a
bipolar transistor passes a small amount of current. The gate on the FET passes
virtually no current when driven with D.C. When driving the gate with high frequency
pulsed D.C. or A.C. there may be a small amount of current flow. The transistor's
"turn on" (a.k.a. threshold) voltage varies from one FET to another but is
approximately 3.3 volts with respect to the source.
When FETs are used in the audio output section of an amplifier, the Vgs
(voltage from gate to source) is rarely higher than 3.5 volts. When FETs are used in
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switching power supplies, the Vgs is usually much higher (10 to 15 volts). When the
gate voltage is above approximately 5 volts, it becomes more efficient (which means
less voltage drop across the FET and therefore less power dissipation).
MOSFETS are commonly used because they are easier to drive in high current
applications (such as the switching power supplies found in car audio amplifiers). If a bipolar
transistor is used, a fraction of the collector/emitter current must flow through the base
junction. In high current situations where there is significant collector/emitter current, the
base current may be significant. FETs can be driven by very little current (compared to the
bipolar transistors). The only current that flows from the drive circuit is the current that flows
due to the capacitance. As you already know, when DC is applied to a capacitor, there is an
initial surge then the current flow stops. When the gate of an FET is driven with a high
frequency signal, the drive circuit essentially sees only a small value capacitor. For low to
intermediate frequencies, the drive circuit has to deliver little current. At very high
frequencies or when many FETs are being driven, the drive circuit must be able to deliver
more current.
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FET APPLICATIONS
The channel of a FET (explained below) is doped to produce either an N-type
semiconductor or a P-type semiconductor. The drain and source may be doped of opposite
type to the channel, in the case of depletion mode FETs, or doped of similar type to the
channel as in enhancement mode FETs. Field-effect transistors are also distinguished by the
method of insulation between channel and gate. Types of FETs are:
The DEPFET is a FET formed in a fully-depleted substrate and acts as a sensor,
amplifier and memory node at the same time. It can be used as an image (photon)
sensor.
The DGMOSFET is a MOSFET with dual gates.
The DNAFETis a specialized FET that acts as a biosensor, by using a gate made of
single-strand DNA molecules to detect matching DNA strands.
The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized
FET designed to provide a very fast recovery (turn-off) of the body diode.
The HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure
FET), can be made using bandgapengineering in a ternary semiconductor such as
AlGaAs. The fully depleted wide-band-gap material forms the isolation between gate
and body.
The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It has a
structure akin to a MOSFET coupled with a bipolar-like main conduction channel.
These are commonly used for the 200-3000 V drain-to-source voltage range of
operation. Power MOSFETs are still the device of choice for drain-to-source voltages
of 1 to 200 V.
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http://en.wikipedia.org/wiki/Doping_(semiconductor)http://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/w/index.php?title=DEPFET&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Dual_Gate_MOSFET&action=edit&redlink=1http://en.wikipedia.org/wiki/DNA_field-effect_transistorhttp://en.wikipedia.org/wiki/DNA_field-effect_transistorhttp://en.wikipedia.org/wiki/Biosensorhttp://en.wikipedia.org/wiki/FREDFEThttp://en.wikipedia.org/wiki/HEMThttp://en.wikipedia.org/wiki/Bandgaphttp://en.wikipedia.org/wiki/Bandgaphttp://en.wikipedia.org/wiki/AlGaAshttp://en.wikipedia.org/wiki/AlGaAshttp://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/Power_MOSFEThttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/N-type_semiconductorhttp://en.wikipedia.org/wiki/P-type_semiconductorhttp://en.wikipedia.org/w/index.php?title=DEPFET&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Dual_Gate_MOSFET&action=edit&redlink=1http://en.wikipedia.org/wiki/DNA_field-effect_transistorhttp://en.wikipedia.org/wiki/Biosensorhttp://en.wikipedia.org/wiki/FREDFEThttp://en.wikipedia.org/wiki/HEMThttp://en.wikipedia.org/wiki/Bandgaphttp://en.wikipedia.org/wiki/AlGaAshttp://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/Power_MOSFEThttp://en.wikipedia.org/wiki/Doping_(semiconductor)7/31/2019 Fet Research
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The ISFET is an Ion-Sensitive Field Effect Transistor used to measure ion
concentrations in a solution; when the ion concentration (such as pH) changes, the
current through the transistor will change accordingly.
The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to
separate the gate from the body.
The MESFET (MetalSemiconductor Field-Effect Transistor) substitutes the p-n
junction of the JFET with a Schottky barrier; used in GaAs and other III-V
semiconductor materials.
The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well
structure formed by graded doping of the active region.
The MOSFET (MetalOxideSemiconductor Field-Effect Transistor) utilizes an
insulator(typicallySiO2) between the gate and the body.
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http://en.wikipedia.org/wiki/ISFEThttp://en.wikipedia.org/wiki/ISFEThttp://en.wikipedia.org/wiki/JFEThttp://en.wikipedia.org/wiki/JFEThttp://en.wikipedia.org/wiki/MESFEThttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/Schottky_barrierhttp://en.wikipedia.org/wiki/MODFEThttp://en.wikipedia.org/wiki/Quantum_wellhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Electrical_insulationhttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/ISFEThttp://en.wikipedia.org/wiki/JFEThttp://en.wikipedia.org/wiki/MESFEThttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/P-n_junctionhttp://en.wikipedia.org/wiki/Schottky_barrierhttp://en.wikipedia.org/wiki/MODFEThttp://en.wikipedia.org/wiki/Quantum_wellhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Electrical_insulationhttp://en.wikipedia.org/wiki/Silicon_dioxide7/31/2019 Fet Research
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ADVANTAGES AND DISADVANTAGES OF FETS
FET has high input resistance (approx. in order of 100-M ohms for a JFET and 1010
to 1015 ohms for the MOSFET). Thus, the FET is a voltage-controlled device, like a tube,
and not current-controlled, like a conventional transistor. It shows a high degree of isolation
between input and output. The FET has some advantages and some disadvantages relative
to the bipolar transistor. Field-effect transistors are preferred for weak-signal work, for
example in wireless communications and broadcast receivers. They are also preferred in
circuits and systems requiring high impedance. The FET is not, in general, used for high-
power amplification, such as is required in large wireless communications and broadcast
transmitters
All field-effect transistors are unipolar rather than bipolar devices. That is, the main
current through them is comprised either of electrons through an N-type semiconductor or
holes through a P-type semiconductor while conventional transistor is a bipolar device
relying upon two types of charge carrier, electrons and holes. The FET is less noisy than a
bipolar transistor.
But on the other hand FET has disadvantages too. FET has relatively low gain-
bandwidth product compared to conventional transistors.
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