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AVALANCHE TRANSIT TIME DEVICES

KUNAL KANT SINGH RAJYA KHATOON

ECE (7th sem.)CIST, BHOPAL

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It is possible to make a microwave diode exhibit negative resistance by having a delay between voltage and current in an avalanche together with transit time through the material.

Such devices are called avalanche transit time devices.

There are three distinct modes of Avalanche Devices:-

1) IMPATT (Impact Ionization Avalanche Transit Time Device)

2) TRAPATT (Trapped Plasma Avalanche Triggered Transit Device)

3) BARITT (Barrier Injected Transit Time Device)

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It is a form of high power diode used in high frequency electronics and microwave devices.

Typically made from silicon carbides due to their high breakdown fields.

Frequency- 3 to 100 GHz High power capability Generate high level of phase noise – avalanche

process.

IMPATT DIODE :-

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WORKING:- An extremely high voltage is applied. A normal diode will breakdown, but IMPATT is

constructed such that it withstand these conditions easily.

Such a high potential causes flow of minority charge carriers across junction.

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Increased velocity of electrons and holes results additional electrons & holes by knocking out of crystal structure, called Impact ionization.

These additional C.C. Continue at junction & snowball into avalanche, but since it’s a multiplication process avalanche is not instant.

This process takes time and results in 90˚ phase shift b/w voltage and current.

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PERFORMANCE CHARACTERISTICS:- Efficiency- 30% (theoretical) ,<30% (practical) Frequency- 1 to 300 GHz Maximum output power for single diode – 4W Several diodes combined- 40W Pulse powers - 4W

ADVANTAGES:- High power capability High levels of performance Highly stable frequency 

DISADVANTAGES:- Very noisy (avalanche is noisy process) Poor tuning range than Gunn diodes

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APPLICATIONS:- Microwave oscillators (e.g. MW Generators) Negative resistance amplification Intrusion alarm network, police radar FM( frequency modulation) ****************************************************

TRAPATT DIODE It is derived from the IMPATT diode and is closely related to it. 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 capable of

operating from hundred MHz to several GHz.

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The TRAPATT diode is typically represented by a current pulse generator and the diode’s depletion-layer capacitance.

WORKING:- Depletion region filled with dense plasma of electrons and holes

that become trapped in low field region behind the zone.

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At A, charge carriers due to thermal generation results in charging of diode.

When sufficient number of carrier is generated, the particle current exceeds the external current and the electric field is depressed throughout the depletion region, causing the voltage to decrease. This portion of the cycle is known by “plasma formation”.

During this time interval ,a dense plasma of electrons and holes is created. The voltage decrease to  D.

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A large time is required to remove the plasma because total plasma charge is large ,At point E plasma is removed.

As the residual charge is removed, the voltage increases from point E to point F . At point F all the charge generated internally has been removed.

At point G the diode current goes 0 for half period and the voltage remains constant.

PERFORMANCE CHARACTERISTICS:- Efficiency- 15 to 40% Frequency- 3 to 50 GHz CW power -1-3 W Pulse powers = 1.2KW Noise figure - >30 dB Operating voltage- 60-150V

 

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ADVANTAGES:- High power capability High levels of performance Highly stable frequency 

DISADVANTAGES:- Very noisy (avalanche is noisy process) Poor tuning range than Gunn diodesAPPLICATIONS:- In low power doppler radar Local oscillator for radar Radio altimeter Phased array radar

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BARITT It is the latest addition to the family of active

microwave diodes. It has long drift region similar to IMPATT. BARITT devices are improved version of

IMPATT devices. BARITT diode uses thermionic emission

rather than avalanche multiplication. The BARITT diode uses Injection and Transit-

time properties of minority carriers to produce a negative resistance at microwave frequencies.

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BARITT consists of an emitter, base intermediate drift or depletion region and collector.

An essential requirement of BARITT is the intermediate drift region by entirely depleted to cause punch through to emitter base junction.

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A rapid increase in current with applied voltage is due to the thermionic hole injection into semiconductor.

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PERFORMANCE CHARACTERISTICS:- Efficiency- 1.8% Frequency- 4 to 10 GHz Power – 50mW Noise figure - 9dB

ADVANTAGES:- Less noisy than impatt diodes NF of 15dB at C band using Baritt amplifierDISADVANTAGES:- Narrow bandwidth Limited few mW of power output

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APPLICATIONS Mixer Oscillator Small signal amplifier***********************************************