Diode (p-n junction)

This tutorial discusses how diodes, or p-n junctions, work. Semiconductors conduct due to the presence of free electrons and "holes". Electron and hole pairs are called charge carriers. Electrical current is the flow of free electrons from hole to hole. Impurity atoms are often added to semiconductors, called doping, which can greatly increase the number of charge carriers. A semiconductor with more holes than free electrons is called a p-type semiconductor while semiconductors with more free electrons is called an n-type semiconductor. This is illustrated in Figure 1.


Figure 1: Diagram of a n-type and p-type semiconductors showing negative ion (free electrons) and positive ion (holes) distributions.

Diodes are made from two pieces of semiconductors; one is a p-type semiconductor while the other is an n-type semiconductor. When you join a p-type semiconductor to an n-type semi-conductor, you create what is called a p-n junction. At the junction, electrons from the n-type are attracted to the positively charged holes of the p-type semiconductor. The free electrons from the n-type semiconductor diffuse through the junction and fill up the available holes in the p-type semiconductor and leaving holes behind in the n-type semiconductor. This creates what is called the depleted zone where electrons have taken up all of the available holes. Figure 2 shows the free electrons accumulated near the p-type semi-conductor near the junction. This depleted zone acts like a barrier, as there are no holes for free electrons to pass through, i.e., current cannot flow.


Figure 2: A p-n junction showing the depleted zone.

When a voltage differential is applied to the p-n junction, with the positive of the voltage difference on the n-type as shown in Figure 3, it attracts the electrons collected at the junction of the p-type semi-conductor's side of the depleted zone. This is called forward bias. This acts to reduce the size of the depleted zone as the electrons are also attracted to the positive voltage (opposites attract). When the voltage differential is large enough to reduce the depleted zone to nil, current becomes free to flow across the two semiconductors. The voltage required to reduce the depleted zone to zero is called the diode forward voltage drop.


Figure 3: The reduction of the size of the depleted zone from forward biasing a p-n junction diode.

If a voltage differential is applied the opposite way, with the positive voltage difference on the p-type semi-conductor, it attracts the electrons diffused in the p-type semi-conductor side of the depleted zone. This acts to increase the size of the depleted zone. Thus current is unable to flow across the junction. This is called reverse bias.


Figure 4: The increase in the size of the depleted zone from reverse biasing a p-n junction diode.

References

http://en.wikipedia.org/wiki/Diode
http://en.wikipedia.org/wiki/P–n_junction
http://en.wikipedia.org/wiki/Semi-conductor
http://en.wikipedia.org/wiki/Charge_carrier

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