TRAPATT DIODE

ANKIT KUMAR PANDEYM.TECH 3rd sem

ALLAHABAD UNIVERSITY1

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TRAPATT DIODE Derived from the Trapped Plasma Avalanche Triggered Transit

mode device.

It is a p-n junction diode characterized by the formation of atrapped space charge plasma within the junction region.

It was first reported by Prager in 1967.

It is a high efficiency microwave generator.

The TRAPATT diode is typically represented by acurrent pulse generator and the diode’s depletion-layercapacitance.

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COMPARISON WITH TEDS.

Transferred electron devices generate relativelylow-power microwave radio signals.

There are no p-n junction in TEDs so frequencyis a function of load and natural frequency ofdevice.

In TRAPATT diode there is ability to switch veryhigh currents with less than a nanosecond riseand fall times (transition times). 3

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TYPICAL PARAMETERS;

Power = 1.2 kw at 1.2 GHz

Maximum efficiency = 75% at 0.6 GHz

Frequency range (operating) = 0.5 GHz to 50GHz

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STRUCTURE;Typically silicon with N type depletion region

width=2.5 to 12.5 micro m

p+ region = 2.5 to 7.5 micro m

Diode’s diameter range=50 to 750 micro m

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PRINCIPLE OF OPERATIONA high field avalanche zone propagates through

the diode and fills the depletion layer with adense plasma of electrons and holes that becometrapped in the low field region behind the zone.

The basic operation of the oscillator is asemiconductor p-n junction diode reversedbiased to current densities well in excess of thoseencountered in normal avalanche operation.

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OPERATION;

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OPERATION; At point A electric field is uniform throughout the sample but less than

avalanche breakdown. Diode charge like a linear capacitor.

When magnitude of electric field increases above the breakdown voltage.

Then sufficient number of charge carriers is generated, the particle current (Ip)

exceeds the external current(Ie), and the electric field is depressed throughout

the depletion region, causing the voltage to decreases. B to C a dense plasma

of electron and hole is generated.

At point C to D some of the electrons and holes drift out of the ends of the

depletion layer the field is further depressed and traps the remaining plasma. 8

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A long time is required to remove the plasma as shown in graph from D to

E.

At point E the plasma is removed, but residual charge of electron in one

end of the depletion region and residual charge of holes in other ends.

At point F all the charges that was generated has been removed. The point

F to G the diode charges like capacitor.

At point G diode current goes to zero for half a period and the voltage

remains constant at Vs until the current comes back on and the cycle

repeats.

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The TRAPATT mode can operate at low frequenciessince discharge time of plasma can be considerablygreater than the nominal transit time of the diode athigh field.

The TRAPATT mode is known as transit-time mode inthe real sense that the time delay of carriers in transit(i.e. the time between injection and collection) isutilized to obtain a current phase shift favorable foroscillation.

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RF power is delivered by the diode to an external loadwhen the diode is placed in a proper circuit with theload.

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ADVANTAGES & DISADVANTAGE OF

TRAPATT DIODE

Advantages

More suitable for pulsed operation

15 to 40% efficiency is obtained

It can operate between 3 to 50GHz

Disadvantages

Noise figure is greater than 30dB, it is also very

noisy. 12

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APPLICATION

Low power Doppler radars or local oscillators

for radars.

Landing system

Radio altimeter

S-band pulsed transmitters for phase array radar

system.

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REFERENCES; MICROWAVE AMPLIFIERS AND OSCILLATORS

John W. Lunden, Jennifer E. Doyle.

Power frequency characteristics of TRAPATT diodemode- D.L. Scharfetter (23rd oct 1969).

Pozar, David M. (1993). Microwave EngineeringAddison-Wesley Publishing Company.

Microwave devices and circuits- S.Y. Liao.

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