IMPATT DIODE

Created by:Doshi Jay

Chaudhary Dhaval

Introduction• When the pn junction diode is reverse-biased, then current does

not flow.• However when the reverse voltage exceeds a certain value,

the junction breaks down and current flows with only slight increase of voltage. This breakdown is caused by avalanche multiplication of electrons and holes in the space charge region of the junction.

• The pn junction in the avalanche breakdown condition exhibits negative resistance characteristics in the microwave frequency range.

• Since the negative resistance is based upon avalanche multiplication and transit-time effect of carriers, the device has been called the IMPATT (Impact Avalanche Transit-Time) Diode.

Device Structure

• The original suggestion for a microwave device employing transit-time effect was made by W. T. Read and involved an n+-p-i-p+ structure such as that shown in figure. This device operates by injecting carriers into the drift region and is called an IMPATT diode.

The device consists essentially of two regions: 1)The n+p region, at which avalanche multiplication occurs, and 2) The I (essentially intrinsic) region, through which

generated holes must drift in moving to p+ contact.

Principle of operation

• In Impatt diode extremely high voltage gradient is applied(400kv/cm)which a normal which a normal pn junction can't withstand.

• Such a high potential graidient,back-biasing the diode cause a flow of minority carrier across the junction.

• The ac current is approxi 180 degree out of phase with the applied voltage this gives rise to negative conduction and oscillation is resonant circuit.

V-I characterstics

Construction

• IMPATT diode are made of silicon as it is cheaper and easier to fabricate using epitaxial groth.

• below figure is showing a typical Impatt diode.The gold alloy

contact is used as it has low ohmic and thermal resistance.

Classification• Device structure is based on the doping profile. The three

basic types of Impatt diodes are:-1)Single drift region (SDR) - The SDR diode consists of a single avalanche zone and a single drift zone with p+nn+ structure.2)Double drift region (DDR) – A DDR diode has a p+pnn+structure that consist of two drift layers, one for electrons and other for holes on either side of the central avalanche zone.3) Double avalanche region (DAR) – The DAR diode has a p+nipn+ structure that consist of one drift zone sandwiched between two avalanche zones. The electrons and holes from the two junctions travel across the central i-region in opposite directions and deliver power.

Disadvantages

• The disadv. Of IMPATT diode is that it is very noisy because avalanche is a noisy process.

• Noise fig for the IMPATT diode is 30db arer not as good as klystron/gunn diode/twt amp.

• Tuning range is not as good as gunn diodes.

Application These diodes make excellent microwavegenerators for many

applications like:-1. Parametric amplifier,2. Parametric up converter,3. Parametric down converter,4. Negative resistance parametricamplifier.

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