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The P-N junction is the basic building block of almost all integrated circuits and other types of semiconductor devices that are manufactured nowadays, such as transistors, diodes, LEDs, etc.

The PN junction forms the basis of much of today's semiconductor technology.

PN Junction

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The unique properties of the junction enable it to perform a wide variety of functions in many different configurations. The P-N junction has the very useful property that current is allowed to flow only in one direction across the junction, but it is blocked from flowing in the other direction.

Since current consists of a flow of electrons, this means that electrons can flow in only one direction.

If a block of a P-type semiconductor is connected in series with a separate block of an N-type semiconductor through wiring, the resulting circuit would have no unique properties. They would both conduct normally in both directions, as the number of electrons is balanced by the number of protons in both blocks, and neither block has any net charge.

However, when a single semiconductor crystal is manufactured such that it is doped with P-type material on one end, and with N-type material on the other end with a junction in between, there is a drastic change in the semiconductor’s properties.

The N-type material contains an excess number of free electrons, which are negative majority charge carriers, and these are free to move around in the crystal - a few of them can even wander across the junction into the P-type material, and combine with some of the holes in the region.

The free holes in the P-type material are positive majority charge carriers. Because the electrons have moved across the junction from the N-type material to the P-type material, they leave behind positively charged donor ions on the negative side and the holes from the P-type material have essentially migrated across the junction in the opposite direction into the N-region where there are large numbers of free electrons. This charge transfer of electrons and holes across the junction is known as diffusion.

Eventually, an equilibrium state is reached, as atoms in the P-type material near the junction are able to receive an extra electron, and atoms in the N-type material are able to rid themselves of an extra electron.  Because of this, the thin region of the P-type material near the junction has a net negative charge because of the electrons attracted. Since electrons have been removed from the N-type region near the junction, it takes on a localized positive charge. The N-side has a positive voltage relative to the P-side. The total charge on each side of the junction must be equal and opposite to maintain a neutral charge condition around the junction.

After the charge on both sides reaches a certain level, it will prevent any additional electrons and holes from crossing the junction, because of the repelling nature in electric fields of the same polarity. This thin layer on both sides of the junction where charge has been built up is known as the depletion region, and has been depleted of positive and negative majority charge carriers. The small voltage potential across this junction creates a kind of insulator which separates the conductive P and N doped regions.

Any free charge that happens to wander into the depletion area will only find positive charges (the donor atoms) on the N-type side and a negative charges (the acceptor atoms) on the P-type side.

These exert a force on the free charges, driving them back to their 'own side' of the junction away from the depletion region. The charges built up on both sides of the junction tend to clear the depletion region of any free charges. A free charge now requires some extra energy to cross the junction and overcome the forces exerted by the donor/acceptor atoms.

If a positive voltage (forward bias) is applied between the two ends of the PN junction such that the P side is positive and the N side is negative, it would be able to supply the free electrons and holes with the extra energy needed to cross the junction. Electrons in the N side are attracted towards the positive voltage and are assisted to jump across the depletion layer. Likewise, holes in the P side move towards the negative voltage and jump the depletion layer.  Once the voltage reaches this barrier potential, the P-N junction will freely conduct current, limited only by the external source.

The external voltage required to overcome this potential barrier depends on the type of semiconductor material, the amount of doping, and temperature.

Typically at room temperature the voltage across the depletion layer for silicon is about 0.6 - 0.7 volts and for germanium is about 0.3 - 0.35 volts. Silicon is usually the better choice, because it has lower leakage currents when reverse biased. PN junction diodes can also be manufactured from other semiconductor materials, but these are usually for specialized applications.

If a voltage is applied to the ends of the PN junction in the opposite direction, such that the N side is positive and the P side is negative, there would be essentially no current through the device. This is because the holes in the P type region are attracted towards the negative potential that is applied to it. Likewise, the electrons are attracted towards the positive potential which is applied to the N type region. This actually pulls the holes and electrons away from the junction itself and the depletion region will increase in width.

In effect, this increases the barrier potential according to the applied (reverse bias) voltage. The width of the depletion region is only able to increase up to a certain point, and if the reverse bias voltage is large enough, it will overcome this depletion layer potential, and freely conduct current in the same way as with forward bias. This upper limit is known as the breakdown voltage, and a large current here could actually damage the device because of the excess power drawn.

In looking at the characteristic voltage-current plot of the PN junction, it can be seen that in the forward direction (forward biased), current is blocked until the forward barrier potential is reached, and in the negative direction (reverse biased), current is blocked until the breakdown potential is reached.

While the PN junction provides excellent rectifying characteristics, it is not a perfect diode having infinite resistance in the reverse direction and zero resistance in the forward direction. In reality a small amount of reverse current does flow, although it is usually very small, in the region of a few pico amps or microamps.

The reverse leakage current results from what are called minority carriers, which are the small number of electrons found in a P type region and holes in an N type region. This is caused by impurities in the manufacturing process.  The new methods of manufacturing nowadays have reduced the number of minority carriers, as well as the levels of reverse currents.

- James Reinholm