#CG

What is a Diode.

A diode is a pn junction device which allow current to pass through it in only one direction that is forward direction.

A good example is a blocking diode found in charge controllers of solar panels which only allow current to flow from solar panel to the battery and prevent movement of current from battery back to solar panel when there is no sunlight.

Diode symbol

Diode construction

A diode in construction consist of two doped semiconductor materials joined together, that are n-type material and p-type material. The boundary seperating the two materials is called pn juction.

The p-type material region of a diode is also called Anode and the n-type material is also called the Anode.

Semiconductors are poor conductors of electricity at room temperature, however they are doped (adding of controled amount of impurities to semiconductor material) to improve their conductivity to become important in electronic engineering.

NB - It is important to know that the conduction of electricity in doped semiconductors is slightly different from that in conductors that is why semiconductors are important in electronic engineering.

Silicon and germanium are the semiconductor materials most used in diodes.

P-type material is a semiconductor material doped with atoms of group three such as boron(B) and n-type material material is a semiconductor doped with atoms of group 5 such as phosphorus(P).

How doping improve conductivity in semiconductors materials.

When a semiconductor material is doped with group five atoms they is creation of free electrons or negative charges since negative charges are found on elections.

These free electrons will initiate conduction in the n-type material when connected to a source of voltage.

When semiconductor material is doped with group three atoms they is creation of holes or positive charges since positive charges are found on holes.

The term holes is same as protons. These holes will initiate conduction in the p-type material when connected to a source of voltage.

We have covered this topic in detail on how pn junction, depletion region and barrier potential are formed, you can check out.

When p-type and n-type materials are joined together, the region seperating them is called pn junction.

As soon as the pn junction is formed, there are some activities which take place near the junction which causes creation of layers of opposite charges on the sides of a pn junction.

The two layers of opposite charges forms a depletion region. The two layers of opposite charges create an electric field which prevents movement of electrons across the junction.

The voltage of the electric field which prevents electrons to cross the junction is called barrier potential.

In diodes made of silicon the barrier potential is approximately 0.7V and in germanium diodes the barrier potential is approximately 0.3V.

The barrier potential depends on the semiconductor used, level of doping and temperature of the material.

How does a diode works

As previously mentioned, a diode is a device which allow current to pass through it in only one direction.

To understand how a diode works and other electronic devices we need first to look on electric current, keep in mind that electrons flow from negative to positive although current is said to flow from positive to negative.

The flow of current from positive to negative was before the discovery of electrons, this flow was said it is due to electric fluid which flow from positive to negative.

After the discovery of electrons it was then found that it is electrons which cause current not electric fluid but the direction of current was not changed due to some reasons.

Let us look on what happen when a diode is connected in forward direction and in reverse direction. The process of applying a dc source of voltage to a device(diode) to study how it works is called biasing.

Forward Biasing.

It is a condition which allow current through a diode. To connect a diode in forward direction, the p-type side of a diode is connected to the positive side of a battery and the n-type to the negative side of a battery.

The resistor in the circuit limits current to a value that does not cause damage of a diode.

What anable current to pass through a diode in forward direction.

As the negative side of a battery is connected to the n-type side of a diode it repels free electrons in the n-type side pushing them towards the p-n junction and also provide a continuous supply of electrons to the n-type side.

If these free electrons moving towards the p-n junction are given a force of voltage equal to barrier potential, they will use that energy to pass through depletion region.

If the free elections use all energy available to cross the junction they will combine with holes in p-type region. In the p-type region the elections are attracted by the positive side of a battery and start to move from hole to hole towards the positive side of a battery. This is how a diode allows current in forward direction.

Reverse biasing.

It is a condition which does not allow current through a diode. To connect a diode in reverse direction, the p-type side of a diode is connected to the negative side of a battery and the n-type to the positive of a battery.

What prevent current through a diode in reverse direction.

As the positive side of a battery is connected to n-type region, it attract free electrons from the n-type side away from the junction creating more holes near the pn junction in the n-type material

And also at the same time the negative side of a battery supply free elections to the p-type material adding more electrons near the junction.

The addition of holes in the n-type and electrons in the p-type material near the junction cause widening of depletion region and creation of electric field with voltage equal to the supply which result in no flow of current.

In reverse biasing if the supply voltage is increased to diode break down voltage, current will start to flow in reverse and the diode will be dead.

Break down voltage (VBR) is the minimum voltage at which the pn junction breaks down resulting in sudden rise in reverse current.

V-l characteristic curve..

The results of forward and reverse connecting a diode to a variable voltage supply can be plotted on a graph called V-I characteristic curve. The vertical axis shows current values and the horizontal axis shows voltage values.

NB - A variable voltage supply is a supply which we can tune the voltage to see what happen to a diode at some certain values of voltage.

The left side of vertical axis of a graph shows a diode connected in reverse direction and the right side shows a diode connected in forward.

V-I characteristic curve explanation in context of a complete silicon diode model.

NB - When explaining how a diode works, doing calculations, trouble shooting a diode - maybe you are finding out whether a diode is live or dead etc, they are diode models which are used.

For example our explanations in this article are based on the Complete diode model. They are three diode models which are the ideal model, practical diode model and the complete diode model.

In complete diode model all the characteristics of a diode are taken into account while on other models some characteristics are omitted.

The graph shows that:
In forward direction when the supply voltage is 0V, there is no forward current. When the supply voltage is gradually increased, the current through the diode and voltage across a diode(voltage used to pass current through a diode) gradually increases.

This very small current flowing below the barrier potential of a diode is leakage current which is common in diodes since everything is not perfect.

When the supply voltage increased such that the voltage across a diode equals to barrier potential(0.7V) the current through a diode increases rapidly.

When supply voltage continued to be increased, current continues to increase rapidly but voltage across a diode increases very slightly.

This slight increase in voltage above 0.7V is due to the resistance of the doped semiconductor materials often called Dynamic resistance.

In reverse direction when supply voltage is 0V, there is no reverse current. When reverse voltage is gradually increased they is only a small reverse current and voltage across a diode gradually increases.

When the supply voltage is increased such that the voltage across a diode equals to breakdown voltage the current through a diode increases rapidly.

When supply voltage is continued to increased above breakdown voltage, current continues to increase rapidly but voltage increases very slightly.

Diode other than zener diodes are not designed to operate at breakdown voltage or above and they usually have a large value of of breakdown voltage for example about 50V.