Fet Research

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    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)
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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_dioxide
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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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