Impatt-Diode.ppt (Size: 1,009.5 KB / Downloads: 268)
Presented By:Name:- SABYASACHI SABAT
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.
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
IMPATT I-V Characteristics
Small Signal model
Some IMPATT Circuits
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.
These diodes make excellent microwave generators for many applications like:-
1. Parametric amplifier,
2. Parametric up converter,
3. Parametric down converter,
4. Negative resistance parametric amplifier.
IMPATT stands for Impact Avalanche And Transit Time
Operates in reverse-breakdown (avalanche) region
Applied voltage causes momentary breakdown once per cycle
This starts a pulse of current moving through the device
Frequency depends on device thickness
IMPact Ionization Transit Time
IMPATT devices can be used for oscillator and amplifier applications
They can be fabricated with Si, GaAs, and InP
Can be used up 400 GHz.
In general, IMPATTs have 10 dB higher AM noise than that of Gunn diodes
IMPATT diode is not suitable for use as a local oscillator in a receiver.