Avalanche Breakdown and Zener Breakdown Effect Explained

Summary

This video on the YouTube channel 'ALL ABOUT ELECTRONICS' explains the breakdown mechanisms of PN Junction diodes, focusing on Avalanche and Zener effects. It begins with a recap of diode behavior under forward and reverse bias, then explores how high reverse voltages cause a sudden increase in current. The Avalanche effect occurs due to impact ionization at high voltages, while the Zener effect results from tunneling in heavily doped diodes at lower voltages. The video also covers differences in temperature dependence, breakdown voltage ranges, and practical applications, highlighting why Zener diodes are used for voltage regulation. Clear explanations make complex concepts accessible.

Takeaways

• 😀 PN junction diodes can operate in forward and reverse bias conditions, with current flowing primarily in forward bias.
• 😀 In reverse bias, a small reverse saturation current flows due to minority charge carriers, largely unaffected by increased voltage until breakdown occurs.
• 😀 Avalanche breakdown occurs when reverse voltage is high enough that accelerated carriers knock out bound electrons, leading to a sudden surge in current.
• 😀 Impact ionization in Avalanche breakdown creates more free charge carriers through successive collisions, drastically increasing reverse current.
• 😀 Zener breakdown occurs in heavily doped diodes where a strong electric field enables electrons to tunnel across the narrow depletion region.
• 😀 Zener diodes are designed to operate in breakdown region, maintaining nearly constant voltage while the current increases, making them ideal for voltage regulation.
• 😀 Avalanche effect is predominant in diodes with breakdown voltage above 6 V, while Zener effect dominates below 4 V; both may occur between 4-6 V.
• 😀 Temperature affects breakdown differently: in Avalanche breakdown, higher temperature increases breakdown voltage; in Zener breakdown, higher temperature decreases it.
• 😀 Heavily doped P⁺ and N⁺ regions in Zener diodes result in narrow depletion regions and a stronger built-in electric field, facilitating tunneling.
• 😀 Normal diodes should avoid operating in the breakdown region unless designed as Zener diodes, as excessive current can damage them.
• 😀 The reverse current in Zener diodes increases with applied voltage beyond Zener voltage, but the voltage across the diode remains nearly constant.

Q & A

What happens when a PN Junction diode is operated in forward bias?

In forward bias, when the applied voltage exceeds the diode's threshold or barrier potential, current flows from the P-side to the N-side.

Why is there hardly any current in a reverse-biased diode?

In reverse bias, only a small reverse saturation current flows due to minority charge carriers, as the depletion region prevents majority carriers from moving.

What is the Avalanche breakdown effect?

Avalanche breakdown occurs when high reverse voltage accelerates minority carriers, causing collisions with silicon atoms that generate more charge carriers. This results in a sudden surge of reverse current.

How does impact ionization cause Avalanche breakdown?

Impact ionization occurs when accelerated electrons collide with bound electrons in silicon atoms, knocking them free and creating more electron-hole pairs, leading to an exponential increase in current.

What distinguishes Zener breakdown from Avalanche breakdown?

Zener breakdown occurs in heavily doped diodes with a narrow depletion region, where a strong electric field causes tunneling of electrons across the junction, whereas Avalanche breakdown occurs due to impact ionization in normally doped diodes.

At what breakdown voltages are Zener and Avalanche effects predominant?

Zener effect predominates at breakdown voltages below 4V, Avalanche effect dominates above 6V, and both effects can occur between 4V and 6V.

Why is the Zener diode used as a voltage regulator?

Because beyond the Zener voltage, the diode maintains almost constant voltage while allowing current to increase, stabilizing voltage in circuits.

How does temperature affect the breakdown voltage of Zener and Avalanche diodes?

For Zener breakdown, the temperature coefficient is negative, so breakdown voltage decreases with temperature. For Avalanche breakdown, the coefficient is positive, so breakdown voltage increases with temperature.

What is the role of heavy doping in Zener diodes?

Heavy doping reduces the width of the depletion region, increases the number of immobile ions, and strengthens the electric field, enabling the tunneling effect responsible for Zener breakdown.

Why should normal diodes avoid operating in the breakdown region?

Exceeding the breakdown voltage in normal diodes can cause excessive current and power dissipation, potentially damaging the diode.

What is reverse saturation current and why does it hardly change with voltage?

Reverse saturation current is the small current due to minority carriers in a reverse-biased diode. It hardly changes with applied voltage because it depends on the minority carrier concentration, which is very low.

What happens to the kinetic energy of minority carriers as reverse voltage increases?

As reverse voltage increases, minority carriers accelerate due to a stronger electric field, and if the breakdown voltage is reached, they can generate additional charge carriers through collisions.