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Why and how do breakdowns happen in reverse bias diodes ?

Breakdown in reverse-biased diodes occurs when the voltage applied across the diode in the reverse direction exceeds a critical value. This phenomenon is commonly associated with two types of breakdown mechanisms: Zener breakdown and avalanche breakdown.

1. Zener Breakdown:

  • Mechanism:
    • Zener Effect: Zener breakdown occurs in heavily doped diodes, where the electric field across the depletion region becomes strong enough to cause the generation of electron-hole pairs by the Zener effect.
    • Quantum Tunneling: Electrons gain enough energy to tunnel through the potential barrier of the depletion region, creating an avalanche of charge carriers.
  • Voltage Level:
    • Constant Voltage: Zener breakdown typically occurs at a relatively constant voltage known as the Zener voltage (��VZ​).
    • Controlled Breakdown: In Zener diodes, which are designed for controlled breakdown, the doping levels are precisely controlled to achieve the desired Zener voltage.
  • Applications:
    • Voltage Regulation: Zener breakdown is utilized in Zener diodes for applications such as voltage regulation, where a stable voltage reference is required.

2. Avalanche Breakdown:

  • Mechanism:
    • Impact Ionization: Avalanche breakdown occurs in lightly doped diodes. When a reverse bias voltage is applied, charge carriers gain energy from the electric field, colliding with atoms and generating additional charge carriers through impact ionization.
    • Avalanche Effect: The process creates an avalanche effect, leading to a rapid increase in the number of charge carriers.
  • Voltage Level:
    • Variable Voltage: Unlike Zener breakdown, the voltage at which avalanche breakdown occurs is not fixed. It depends on the doping concentration, temperature, and other factors.
    • Uncontrolled Breakdown: Avalanche breakdown is generally less controlled than Zener breakdown and can occur at higher voltages.
  • Applications:
    • Photodiodes and Avalanche Photodiodes: Avalanche breakdown is intentionally utilized in avalanche photodiodes to amplify the photocurrent in photodetectors.

3. Breakdown Voltage Factors:

  • Doping Level: Doping concentration significantly affects the breakdown voltage. Higher doping levels result in lower breakdown voltages.
  • Temperature: Breakdown voltage is temperature-dependent. An increase in temperature can lead to an increase in breakdown voltage.
  • Reverse Bias Voltage: As the reverse bias voltage increases, the electric field across the depletion region intensifies, making breakdown more likely.

4. Consequences of Breakdown:

  • Current Flow: When breakdown occurs, a sudden increase in current flows through the diode.
  • Potential Damage: If not controlled, breakdown can lead to excessive current and potential damage to the diode or the entire circuit.

5. Conclusion:

Breakdown in reverse-biased diodes, whether Zener or avalanche, is a result of the electric field across the depletion region reaching a critical level. Understanding the breakdown mechanisms is crucial for designing circuits that utilize breakdown for specific applications, such as voltage regulation or photodetection. However, care must be taken to prevent uncontrolled breakdown, which can lead to undesirable consequences, including potential damage to the diode and the surrounding circuitry.

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