Understanding Avalanche Breakdown in PN Junction Diodes

🧱 The Widening of the Depletion Region

As the depletion layer widens:

• More fixed positive ions appear on the N-side (because electrons leave that region).
• More fixed negative ions appear on the P-side (because holes leave that region).

This builds a stronger electric field across the junction that opposes any current flow.

Because of this strong electric field, it becomes very difficult for majority carriers to cross the junction.

Hence, almost no current flows — this is why a diode “blocks” current in reverse bias.

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What Are “Thermally Generated Free Electrons”?

Even when no external voltage is applied to a semiconductor, atoms inside the crystal lattice are constantly vibrating due to temperature (heat energy).

This is because the atoms in a solid are not completely stationary — they always have some thermal energy that increases with temperature.

At any non-zero temperature (even at room temperature, ~300 K):

• Some of this thermal energy is large enough to break the covalent bonds between silicon atoms.
• When a covalent bond breaks, one electron becomes free to move in the lattice.
• The atom that lost the electron becomes a hole (a vacant bond that can accept another electron).

So, this process creates electron-hole pairs (EHPs) even without any external energy source like light or voltage.

These are called thermally generated charge carriers:

• The free electron → goes to the conduction band (can move freely).
• The hole → remains in the valence band (acts as a positive charge).

However, this happens only occasionally — because most electrons still stay bonded — so the number of thermally generated carriers is small.

Still, they exist everywhere in the diode.

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⚡ Now, What Happens During Reverse Bias?

When a reverse voltage is applied:

• The electric field across the depletion region becomes very strong.
• This field pulls the thermally generated electrons (minority carriers) that appear near the junction.

Now, these few free electrons are accelerated by the electric field — meaning they gain kinetic energy as they move through the depletion region.

The stronger the electric field, the faster these electrons move.

At low reverse voltages, the field is not strong enough to cause anything major — these electrons just move across, creating a small, constant reverse saturation current (I₀).

But as the reverse voltage increases, the electric field strength becomes enormous — in the order of 10⁵ to 10⁶ V/cm.

Now the electrons gain so much energy that something dramatic happens.

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💥 Impact Ionization — The Start of Avalanche Breakdown

A moving electron in a crystal normally bounces off atoms, but if it’s moving very fast (has high kinetic energy), each collision becomes violent enough to:

• Knock out an electron from the valence bond of a silicon atom.
• This atom loses one of its bonded electrons → creating a new hole.
• The knocked-out electron becomes a new free electron.

So from one energetic electron, now we have two free electrons and one hole.

This process is called impact ionization.

You can think of it like a cue ball in billiards — when a fast-moving ball hits others, it scatters them and creates more motion.

Similarly, one high-speed electron “hits” the atomic lattice and liberates more electrons.

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⚙️ The Chain Reaction (Avalanche Effect)

Now, the newly freed electrons also experience the same strong electric field.

They too get accelerated to high speeds and collide with more atoms, knocking out more electrons.

This becomes a chain reaction:

1 electron → 2 → 4 → 8 → 16 → 32 …

Each step doubles the number of charge carriers.

Within a very short time, millions of new electrons and holes are created in the depletion region.

This rapid multiplication of carriers is called avalanche multiplication — because it grows like a snow avalanche, starting from a few carriers and exploding into many.

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⚠️ What This Means Inside the Diode

Inside the diode during avalanche breakdown:

• The depletion region is flooded with newly created free electrons and holes.
• These carriers now contribute to a large current, even though the applied voltage hasn’t increased much.
• The crystal lattice experiences frequent collisions → which generate heat.
• If not controlled by a resistor or external circuit, this can overheat and permanently damage the junction.

That’s why normal diodes are not meant to operate in breakdown — except in special cases (like Zener diodes) where it’s designed to handle it.

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Hence, in reverse bias, the diode shows almost no current because the widened depletion region blocks majority carriers. Only a very small reverse saturation current (I₀) flows due to thermally generated minority carriers, which appears as a nearly flat line on the V–I graph.

As the reverse voltage increases, this current remains almost constant until the breakdown voltage (V_BR) is reached. At this point, the electric field becomes strong enough to cause impact ionization, leading to avalanche breakdown. The current then rises sharply with only a small increase in voltage, producing the steep bend on the reverse side of the V–I characteristic.

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What’s Next?

In this blog, we studied how a very strong reverse electric field can accelerate charge carriers and cause avalanche multiplication, leading to avalanche breakdown in a diode.

But avalanche breakdown is not the only type of reverse breakdown mechanism. In heavily doped PN junctions, another important phenomenon called Zener breakdown occurs due to quantum mechanical tunneling.

In the next blog, we will study:

• What is a Zener diode
• Zener breakdown mechanism
• Tunneling effect
• V–I characteristics of a Zener diode
• Voltage regulation using Zener diodes

👉 Click below to continue to the next part:

What is a Zener Diode? Working and Breakdown Explained

Forward Bias vs Reverse Bias in a Diode Explained

Let’s now see what happens when we apply an external voltage to the diode. This process is called biasing.

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1️⃣ Forward Bias

In forward bias, the positive terminal of the battery is connected to the P-side, and the negative terminal to the N-side.

Now, think of what’s happening internally:

• The positive terminal repels holes in the P-side towards the junction.
• The negative terminal pushes electrons in the N-side towards the junction.
• As a result, the depletion region becomes thinner, and the barrier potential decreases.

When the applied voltage becomes equal to or greater than the barrier potential (0.7 V for silicon), the junction becomes conductive.

Electrons now have enough energy to cross the barrier, recombine with holes, and current begins to flow through the diode.

This current is called Forward Current, and the diode is said to be in its conducting state.

So, in simple words — in forward bias, the diode allows current to flow.

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2️⃣ Reverse Bias

Now let’s reverse the connections — connect the positive terminal to the N-side, and the negative terminal to the P-side.

What happens?

• The positive terminal pulls electrons away from the junction.
• The negative terminal pulls holes away from the junction.
• The depletion region widens, and the barrier potential increases.

As a result, no significant current flows. Only a tiny current (called leakage current) flows due to the minority charge carriers.

This current is extremely small and often negligible.

So, in reverse bias, the diode acts like an open switch — it blocks current.

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What’s Next?

Now that we understand how a diode behaves under forward and reverse bias conditions, the next step is to study how the current through the diode changes with applied voltage.

In the next blog, we will study:

• V–I characteristics of a diode
• Cut-in (threshold) voltage
• Forward and reverse current behavior
• Reverse saturation current
• Breakdown region

👉 Click below to continue to the next part:

V–I Characteristics of a Diode Explained

What is a Diode? PN Junction, Depletion Region and Barrier Potential Explained

What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow in only one direction — from the Anode (P-side) to the Cathode (N-side) — and blocks it in the opposite direction. Diodes are one of the most fundamental semiconductor devices used in electronics. They are widely used in rectifiers, voltage regulators, signal clipping circuits, and digital logic.

Construction: How a Diode is Made

A diode is created by joining a P-type and an N-type semiconductor crystal together — forming a PN junction.

Let’s quickly recall what these materials are:

• P-type semiconductor: This is a material doped with trivalent atoms (like Boron). It has an abundance of holes (missing electrons) — which act as positive charge carriers.
• N-type semiconductor: This is doped with pentavalent atoms (like Phosphorus). It has extra free electrons, which are negative charge carriers.

When P type and N type are placed together:

1. Before contact
   • N-type region: has many free electrons (majority carriers).
   • P-type region: has many holes (majority carriers).
   • Both sides are neutral overall.
2. After contact (joining P and N)
   • There is a concentration gradient:
   • Electrons are high in N-side, low in P-side.
   • Holes are high in P-side, low in N-side.
3. Electron diffusion
   • Electrons from the N-side diffuse (move) across the junction into the P-side (because they move from high → low concentration).
   • When they cross and recombine with holes in the P-side, they leave behind positive donor ions (fixed) on the N-side.
4. Hole diffusion
   • Holes from the P-side also move slightly toward the junction and some cross into the N-side (though fewer, because electron mobility is higher).
   • Those holes that cross recombine with electrons near the junction, leaving negative acceptor ions (fixed) behind on the P-side.

As a result, a narrow region near the junction is formed that contains immobile positive ions on the N-side and immobile negative ions on the P-side.

This region is called the Depletion Region, as it is depleted of free charge carriers (electrons and holes).

Formation of the Electric Field

The presence of fixed positive and negative ions on either side of the junction produces an electric field (E) directed from the N-region (positive ions) toward the P-region (negative ions).

This electric field opposes further diffusion of electrons and holes across the junction.

As a result:

• Electrons experience a force opposite to this field (P → N).
• Holes experience a force along the field (P → N).
• The diffusion of majority carriers (electrons and holes) decreases.
• Diffusion current arises due to concentration gradients of charge carriers — basically, charges move from high concentration → low concentration.

Electrons naturally move from N → P (high electron concentration in N, low in P).

Holes naturally move from P → N (high hole concentration in P, low in N).

This movement of carriers creates a diffusion current.

• Drift current arises when charge carriers move due to an electric field.

In a PN junction, the depletion region has an electric field (from N → P). Drift current occurs because the electric field in the depletion region forces charge carriers to move.

Electrons move opposite to the electric field.

Holes move in the direction of the electric field.

This movement of carriers due to the electric field forms the drift current. Holes move along the field (N → P side). This movement due to the electric field is called drift current.

• An equilibrium condition is eventually reached where the diffusion current is exactly balanced by the drift current caused by this electric field.
• The potential difference across the junction is called the barrier potential (V₀).

Electrons that have crossed recombine immediately near the junction; they do not travel deep into the opposite side. The immobile ions and electric field maintain the depletion region.

Barrier Potential (V₀)

The barrier potential is the built-in potential difference across the depletion region created by the electric field of the immobile ions.

An external voltage must reduce or overcome this barrier sufficiently to significantly reduce the barrier and allow conduction.

Typical values are:

• For Silicon (Si) → around 0.7 volts
• For Germanium (Ge) → around 0.3 volts

This is why a silicon diode does not conduct until the voltage across it reaches roughly 0.7 V — that’s the point where the external voltage breaks down the built-in electric field and allows current to start flowing.

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What’s Next?

Now that we understand how a PN junction diode is formed, how the depletion region develops, and how the barrier potential controls current flow, the next step is to understand what happens when an external voltage is applied to the diode.

In the next blog, we will study:

• Forward Bias
• Reverse Bias
• Carrier movement inside the diode
• How the depletion region changes during biasing

👉 Click below to continue to the next part:

Understanding Forward Bias and Reverse Bias in a Diode