microwave devices

1. TRAPATT DIODE
ANKIT KUMAR PANDEY
M.TECH 3rd sem
ALLAHABAD UNIVERSITY 1
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2. TRAPATT DIODE
 Derived from the Trapped Plasma Avalanche Triggered Transit
mode device.
 It is a p-n junction diode characterized by the formation of a
trapped 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 a
current pulse generator and the diode’s depletion-layer
capacitance.
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3. COMPARISON WITH TEDS.
 Transferred electron devices generate relatively
low-power microwave radio signals.
 There are no p-n junction in TEDs so frequency
is a function of load and natural frequency of
device.
 In TRAPATT diode there is ability to switch very
high currents with less than a nanosecond rise
and fall times (transition times). 3
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4. TYPICAL PARAMETERS;
 Power = 1.2 kw at 1.2 GHz
 Maximum efficiency = 75% at 0.6 GHz
 Frequency range (operating) = 0.5 GHz to 50
GHz
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5. 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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6. PRINCIPLE OF OPERATION
 A high field avalanche zone propagates through
the diode and fills the depletion layer with a
dense plasma of electrons and holes that become
trapped in the low field region behind the zone.
 The basic operation of the oscillator is a
semiconductor p-n junction diode reversed
biased to current densities well in excess of those
encountered in normal avalanche operation.
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7. OPERATION;
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8. 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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9.  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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10.  The TRAPATT mode can operate at low frequencies
since discharge time of plasma can be considerably
greater than the nominal transit time of the diode at
high field.
 The TRAPATT mode is known as transit-time mode in
the real sense that the time delay of carriers in transit
(i.e. the time between injection and collection) is
utilized to obtain a current phase shift favorable for
oscillation.
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11.  RF power is delivered by the diode to an external load
when the diode is placed in a proper circuit with the
load.
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12. 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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13. 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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14. REFERENCES;
 MICROWAVE AMPLIFIERS AND OSCILLATORS
John W. Lunden, Jennifer E. Doyle.
 Power frequency characteristics of TRAPATT diode
mode- D.L. Scharfetter (23rd oct 1969).
 Pozar, David M. (1993). Microwave Engineering
Addison-Wesley Publishing Company.
 Microwave devices and circuits- S.Y. Liao.
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