Understanding Avalanche Breakdown in Semiconductors

Avalanche breakdown is a critical phenomenon observed in p-n junction diodes when they are reverse biased at a sufficiently high voltage. It leads to a rapid increase in reverse current.

What Causes Avalanche Breakdown?

The primary cause of avalanche breakdown is a process called impact ionization. Let's break down how this happens:

• When a p-n junction is reverse biased, a depletion region is formed.
• As the reverse voltage increases, the electric field across the depletion region becomes very strong.
• Any free charge carriers (like electrons or holes) minority carriers that enter this region are accelerated by this strong electric field.
• If the electric field is strong enough, these accelerated carriers gain significant kinetic energy.
• These high-energy carriers then collide with the atoms in the crystal lattice.
• A sufficiently energetic collision can knock loose a valence electron from the atom, creating a new electron-hole pair. This is impact ionization.
• The newly created electron and hole are also accelerated by the electric field and can cause further ionization by colliding with other atoms.
• This process cascades, with each ionization event leading to more carriers and subsequent ionizations. This multiplication effect is similar to an avalanche in snow, hence the name 'avalanche breakdown'.
• The result is a sudden, large increase in the reverse current through the diode at the breakdown voltage.

Analyzing the Options

Let's look at why 'Impact of ionization' is the correct cause and why the other options are not the primary cause of avalanche breakdown:

• Emission of electrons: While electrons are involved as charge carriers, simple emission (like thermionic emission) is not the mechanism that triggers the sudden, regenerative current increase characteristic of avalanche breakdown. The breakdown is due to *multiplication* of carriers via ionization.
• Impact of ionization: As explained above, this is the core physical process where high-energy carriers collide with lattice atoms, creating new electron-hole pairs, leading to carrier multiplication and the avalanche effect. This is the direct cause.
• Strong electric field: A strong electric field is necessary to provide the energy for the carriers to cause impact ionization. However, the strong electric field itself is not the breakdown mechanism; it's the *condition* that enables impact ionization to occur. Impact ionization is the mechanism triggered by the strong field.
• High temperature: Temperature has an effect on breakdown voltage, but it's not the cause of the avalanche mechanism itself. In fact, avalanche breakdown voltage *increases* with temperature because collisions with phonons (lattice vibrations) become more frequent, reducing the mean free path of carriers and requiring a higher field for them to gain enough energy for ionization. High temperature is more directly linked to phenomena like increased leakage current or, in specific diodes, Zener breakdown voltage variation, but not the cause of avalanche breakdown.

Therefore, the rapid multiplication of carriers through impact ionization is the fundamental reason for avalanche breakdown.

Revision Table: Breakdown Mechanisms

Additional Information: Zener vs. Avalanche Breakdown

It's important to distinguish avalanche breakdown from Zener breakdown, another type of breakdown in reverse-biased diodes. Both lead to a sudden increase in reverse current, but the underlying physics is different.

• Zener Breakdown: Occurs in heavily doped p-n junctions. The depletion region is very narrow. A strong electric field exists even at relatively low reverse voltages (typically below 5V). This strong field directly causes electrons to tunnel from the valence band on the p-side to the conduction band on the n-side. This is a field emission process, not caused by collisions.
• Avalanche Breakdown: Occurs in more lightly doped junctions (compared to Zener diodes). The depletion region is wider. A higher reverse voltage (typically above 5V) is needed to create a strong enough field to accelerate carriers to cause impact ionization.
• In practice, breakdown in diodes around 5-8V can be a combination of both Zener and avalanche effects. However, at higher breakdown voltages, avalanche breakdown dominates, and at lower voltages (<5V), Zener breakdown dominates.