Impatt (Microwave)

7/27/2019 Impatt (Microwave)

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avalanche transit-time devices

IMPact Avalanche and Transit Time IMPATT diode

TRApped Plasma Avalanche TriggeredTr nsit-TRAPATT diode

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IMPATT IMPact ionization Avalanche Transit-Time

used in high-frequency and microwavedevices. made with silicon carbide owing to their high

r k wn fi l .

operate at frequencies between about 3 and 100GHz or more.

A main advantage is their high power capability.

.negative resistance for dc and ac I-V 1800

Used in low power radar systems to alarms

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An IMPATT diode is reverse biased above the

breakdown voltage. The high doping levels produce a thin depletion

region. The resulting high electric field rapidly

accelerates carriers which free other carriers incollisions with the crystal lattice.

Holes are swept into the P+ region. Electrons

drift toward the N regions. The cascading effectcreates an avalanche current which increaseseven as voltage across the junction decreases.

The pulses of current lag the voltage peakacross the junction. A negative resistanceeffect in conjunction with a resonant circuitproduces oscillations at high power levels.

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The resonant circuit

is the lumped circuitequivalent of awaveguide section,

diode is mounted.

Low power RADAR

transmitters may usean IMPATT diode asa power source. They

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High potential gradient-400KV/cm

High power 100MW/cm3 Operating temperature-upto 2500C

Avalanche is not instantaneous Drift velocity depend on electric field

Time taken b the ulse to reach the cathode

depend on velocity and thickness on highlydoped n+ layer.

The ac current is approxi 180 degree out of

phase with the applied voltage this gives rise tonegative conduction and oscillation is resonantcircuit.

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The two important term of Impatt Diode are below -

Negative Resistance : A combination of delay involvedingenerating avalanche current multiplication togetherwith delay due to transit time through the drift spaceprovide necessary 180 degree between current and

vo ageImpatt diode exhibits this kind of negative resistance.

Impatt Ionization: If a free electron with suficient kineticenergy strikes a silicon atom, it can break a covalent

band and lebrated from the bond .If this kinetic energy isgained from an applied electric field, the liberation of theelectron from the bond is term as Impatt ionization.

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typical Impatt diode.

The gold alloy contact is used as it has low ohmic and thermal

resistance.

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.

The original DC field is just at the threshold ofthe allowing this situation but this thresholdvoltage is exceed only during the positive half

cycle of A.C voltage .it is a cumulative processand takes time. A 90 degree phase difference ora delay has taken place.

The holes produced in the avalanche rapidlyreach the p+ contact taking no part in theprocess but the electrons are released into n

region where they do not combine which eitherdonor or holes.

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The electron drift at their maximum velocity

across the n region and current continuous toflow in the external circuit which they are intransit.

When this current pulse actually arrives at thecathode terminal, the A.C voltage is at itsnegative peak and the second delay of 90

degree has taken place. This time dependsupon the velocity and the thickness of the highlydoped n+ layer.

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Practical consideration

S/C Si,Ge,GaAs,InP

Si-cheaper, easy to fabricate

GaAs-lower noise, high efficiencies, high

operat ng req.

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TRAPATT

A pnjunction diode, similar to the IMPATT diode, butcharacterized by the formation of a trapped space-charge plasmawithin the junction region; used in the generation and amplificationof microwave power. Derived from trapped plasma avalanche transittime diode.

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IMPATT diode mounted on coaxial cavity

a short circuit half wavelength away fromthe diode oscillations begins.

Plasma of electrons and holes is generated

due to avalanche action so that a large

potential across the junction whichopposes applied AC field.

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Total voltage is reduced

Current pulses trapped behind it. Much slower drift velocity so longer transit time so

that for a given thickness operating frequency is much

lower When current pulses does arrive at cathode, the

diode voltage is much lower than IMPATT diode,dissipation is much lower, efficiency much higher.

Operation similar to class-c, pulsed operation thanCW operation

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n+-p-p+

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Drift velocity of TRAPATT diode much less

than IMPATT diode , so Either operating frequency must be lower


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PAGE 6,7 D/TRAPATT

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Gunn diode-

Tunnel diode

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Impatt (Microwave)