🧱 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.
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.
⚡ 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.
💥 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.
⚙️ 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.
⚠️ 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.
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 (VBR) 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.
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:
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