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PHYSICAL ELECTRONICS

PRESENTATION NO-3

Presented by enevirathna

Registration No . 110

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Different types of FETs

Junction FET (JFET)

Metal-Oxide-Semiconductor FET (MOSFET)

Metal-Semiconductor FET (MESFET)

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Metal-Semiconductor FET (MESFET)

GaAs MESFETs are the most commonly used andimportant active devices in microwave circuits.

In fact, until the late 1980s, almost all microwaveintegrated circuits used GaAs MESFETs.

Although more complicated devices with betterperformance for some applications have been

introduced, the MESFET is still the dominant activedevice for power amplifiers and switching circuits in themicrowave spectrum.

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Simplified structure of GaAs MESFET

Source Gate

n+

Semi-insulating GaAs substrate

n+

Drain

Semi-insulating GaAs buffer

n-- epilayer

transistor is formed on semi-insulating GaAs substrate. n-doped epilayer is necessary to realize channel and is madethicker to minimize source and drain resistances.

two heavily doped n regions with low resistivity between metalsource and drain.

semi-insulating buffer for high resistivity and to prevent impurities in

substrate from diffusing into epilayer.

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Source Gate

n+

Semi-insulating GaAs substrate

n+

Drain

Semi-insulating GaAs buffer

n

-

- epilayer

If two n+ regions are biased at different potentials, lower-potential n+region acts as source for electrons, which then flow through channeland are drained by higher-potential n+ region.

Depletion region under Schottky-barrier gate is formed .

Thickness of depletion region can be varied through gate potential bychanging of channel conductivity.

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Vs = 0 V

n+ n

+

n-- epilayer

Vg +VdId

Vds

linear

region

when Vgs = 0 V and Vds is raised from zero to some low value, depletion

region under gate is relatively narrow. where current is proportional to Vds

Vds is increased further ,channel current increases; however,depleted region widens towards drain and more of voltage increase is droppedacross depletion region (charge domain) whereas less is dropped across

unsaturated region.

Id

Vds

Vs = 0 V

n+ n

+

n-- epilayer

Vg +Vd

saturation

region

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GaAs MESFET Characteristics

The operation of any FET is characterized by several regions of operation suchas linear or ohmic region, saturation region, cutoff, subthreshold region and

inverse region.

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Different types of GaAs MESFETmodels

The Shockley modelThe JFET model

The Hyperbolic tangent modelThe statz model

The cubic modelThe VTT model

Other models

Sub threshold modelSchottky barrier diode modelsGate source and gate drain capacitance models

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The JFET model

Gate

Drain

Source

n-channel JFET

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The Statz model

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I-V characteristics of JFET and Statz model

JFET statz

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Applications of MESFET

The use of GaAs rather than Si MESFETs because GaAshas high electron mobility and it is primary material forMESFETs.

High frequency devices, cellular phones, satellitereceivers, radar, microwave devices.

The higher transit frequency of the MESFET makes itparticularly of interest for microwave circuits.

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References

Course materials

Thank You.