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:
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Before contact
- N-type region: has many free electrons (majority carriers).
- P-type region: has many holes (majority carriers).
- Both sides are neutral overall.
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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.
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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.
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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.
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:
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