Diodes

Objectives

  • Define the basic function of a rectifier diode
  • Discuss Light-Emitting Diodes (LED)
  • Explain a flyback diode application

What is a Diode?

In very simple terms, a diode is a device that allows current to flow in one direction only.

Current flows in the direction of the arrow.

Electrical symbol for a diode

A similar device in the hydraulic world is the check valve, which allows fluid to flow through it in one direction only.

How Does It Work?

A diode is created combining two slightly different layers of semiconducting material.

A typical diode

A semiconducting material is usually created using silicon because each silicon atom has four covalent electrons.

These are electrons that can bond with a neighboring atoms covalent electron to create a covalent bond.

This allows silicon atoms to combine to form stable crystal-like structures where positive and negative charges of each atom is balanced.

Highlighted: a covalent bond

This stability means that the electrons want to stay where they are, rather then move around, which is needed for electric current to flow.

Thus, silicon is an insulator rather than a conductor.

In order to give silicon the ability to conduct electricity impurities must be introduced to its structure.

This process of introducing impurities is known as doping.

Even with doping, silicon never becomes a great conductor of electricity.

This is why it's call a semiconductor!

Holes created by doping with boron

Doping is used to create two general types of semiconductor.

The first type is known as a p-type semiconductor.

A p-type semiconductor is created by doping the silicon with an impurity that only has three covalent electrons.

The effect is the creation of holes in the crystal structure where there are not enough electrons to go around.

This gives the semiconductor a slight positive charge, hence the name, p-type.

Doping with phosphorus adds extra, unbonded electrons.

The second type is known as an n-type semiconductor.

An n-type semiconductor is created by doping the silicon with an impurity that has five covalent electrons.

The extra electron has nothing to bond to so it just floats around, looking for a reason to zip away.

These extra electrons give the semiconductor a slight negative charge.

Back to the diode!

A diode is made by combining a p-type semiconductor and an n-type semiconductor into a single device.

P-Type
Semiconductor
N-Type
Semiconductor

At the point where the p-type and n-type semiconductors meet an electron exchange occurs.

Free electrons on the n-type side begin to fill in the holes on the p-type side.

This is called the p-n junction.

As free electrons fill holes a buffer zone is created where all the holes have been filled and there are no more free electrons.

This zone is known as the depletion region.

Depletion Region

Once all the immediately accessible holes have been filled, electrons will need to cross the depletion region to reach holes on the far side.

An equilibrium is reached when the depletion region becomes too thick for electrons to cross in order to reach the holes on the far side.

To overcome the depletion region, electrons will need to be given a boost by an external voltage source.


Different doping materials will create depletion regions of different thicknesses.

The thicker the depletion region the higher the voltage required to cross it.

Review

A diode only allows current to flow in one direction?

The process of introducing impurities to silicon is known as…

A P-type semiconductor has an excess of electrons.

N-type semiconductors have a ______ charge.

Forward Bias

As we stated at the beginning, diodes allow current to flow in only one direction.

But how is this done?

The secret lies in the depletion region and the polarity of the diode!

For current to flow through the diode, the anode needs to be more positive than the the cathode.

Anode
Cathode

The positive polarity of the supply repels the holes of the p-type semiconductor.

This forces the holes toward the depletion region.

At the same time, the negative polarity of the supply is pushing the free electrons of the p-type semiconductor into the deletion region.

The end effect of this is the narrowing of the depletion region.


New Depletion Region

Original Depletion Region

Because of this narrowing it now takes far less voltage to get current to flow across the depletion region. This is known as Forward Bias.

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