Avalanche Diode
Understanding Avalanche Diodes
IMPATT diodes were historically the highest-power solid-state sources at millimeter-wave frequencies before GaN. They exploit the negative resistance created by the combination of the avalanche multiplication delay (180 degrees) and the carrier transit delay through the depletion region.
IMPATT Properties
- Frequency range: 3-300+ GHz. Most common 30-100 GHz.
- Output power: 100 mW-10W at microwave. Decreases at higher frequencies.
- Noise: High. The avalanche process is inherently noisy. Not suitable for LO applications requiring low phase noise.
- Efficiency: 10-30%.
IMPATT Types
- Single-drift: N+PNP+ structure. Simpler, lower power.
- Double-drift: P+NN+P+. Both electrons and holes contribute. Higher power and efficiency.
- Materials: Si (lower cost, lower frequency), GaAs (higher frequency, higher power).
Frequently Asked Questions
What is an IMPATT diode?
An IMPATT diode generates microwave oscillations through avalanche breakdown and transit-time delay. It produces higher power than Gunn diodes at mmWave frequencies but with higher noise. Used as a power source from 3 to 300+ GHz.
Is IMPATT still used?
IMPATT has been largely displaced by GaN amplifiers for applications below 100 GHz. It remains relevant above 100 GHz where GaN performance is limited, and for some specific applications requiring simple, compact mmWave sources.
Why are IMPATT diodes noisy?
The avalanche multiplication process is inherently random, with statistical fluctuations in the number of impact-ionization events. This produces shot noise amplified by the avalanche gain, resulting in high AM and PM noise in the oscillator output.