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What is a semiconductor

PN Junction

PN Junction

- the basis of the semiconductor PN junction used in diodes and transistors

Before reading this page, it is worth reading the page entitled "What is a semiconductor" - see the related articles list below the left menu. This will explain some of the terms used on this page.

The PN junction is one of the most important structures in today's electronics scene. It forms the basis of many components including:

• Diode


• Bipolar transistor


• Junction FET


• Diac


• Triac

The PN junction has the very useful property that electrons are only able to flow in one direction. As current consists of a flow of electrons, this means that current is allowed to flow only in one direction across the structure, but it is stopped from flowing in the other direction across the junction.

PN Junction

A PN junction is made from a single piece of semiconductor that is made to have two differing areas. One end is made to be P-type and the other N-type. This means that both ends of the PN-junction have different properties. One end has an excess of electrons whilst the other has an excess of holes. Where the two areas meet the electrons fill the holes and there are no free holes or electrons. This means that there are no available charge carries in this region. In view of the fact that this area is depleted of charge carriers it is known as the depletion region.

The semiconductor diode PN junction with no bias applied

The depletion region is very thin - often only few thousandths of a millimetre - but this is enough to prevent current flowing in the normal way. However it is found that different effects are noticed dependent upon the way in which the voltage is applied to the junction.

The semiconductor diode PN junction with forward bias

• Current Flow - If the voltage is applied such that the P type area becomes positive and the N type becomes negative, holes are attracted towards the negative voltage and are assisted to jump across the depletion layer. Similarly electrons move towards the positive voltage and jump the depletion layer. Even though the holes and electrons are moving in opposite directions, they carry opposite charges and as a result they represent a current flow in the same direction.



• No current flow - If the voltage is applied to the PN junction in the opposite sense no current flows. The reason for this is that the holes are attracted towards the negative potential that is applied to the P type region. Similarly the electrons are attracted towards the positive potential which is applied to the N type region. In other words the holes and electrons are attracted away from the junction itself and the depletion region increases in width. Accordingly no current flows across the PN junction.

The semiconductor diode PN junction with reverse bias

PN junction characteristics

While the PN junction provides an excellent rectifying action, it is not a perfect diode having infinite resistance in the reverse direction and zero resistance in the forward direction. In order that the PN junction can be used, it is necessary to know a little about its properties and characteristics with forward and reverse bias.

Looking at the characteristic plot of the PN junction, it can be seen that in the forward direction (forward biased) it can be seen that very little current flows until a certain voltage has been reached. This represents the work that is required to enable the charge carriers to cross the depletion layer. This voltage varies from one type of semiconductor to another. For germanium it is around 0.2 or 0.3 volts and for silicon it is about 0.6 volts. It is possible to measure a voltage of about 0.6 volts across most small current diodes when they are forward biased as most diodes are silicon. A small number will show a lower voltage and are likely to be germanium. Power rectifier diodes normally have a larger voltage across them but this is partly due to the fact that there is some resistance in the silicon, and partly due to the fact that higher currents are flowing and they are operating further up the curve.

The characteristic of a diode PN junction

In the reverse direction, a perfect diode would not allow any current to flow. In reality a small amount of current does flow, although this is likely to be very small and in the region of pico amps or microamps. It has been exaggerated on the diagram so that it can be seen. Although it is normally very low, the performance of any diode will degrade at higher temperatures and it is also found that germanium is not as good as silicon.

This reverse current results from what are called minority carriers. These are a very small number of electrons found in a P type region or holes in an N type region. Early semiconductors has relatively high levels of minority carriers, but now that the manufacture of semiconductor materials is very much better the number of minority carriers is much reduced as are the levels of reverse currents.

Summary

The basic diode PN junction is used throughout the whole of the electronics industry today. Even it its basic form as a diode, it is used in enormous quantities, but beyond that the PN junction forms the bedrock of much of today's high-tech transistors and integrated circuits. Without the PN junction, life today would be very different, and electronics would be a very different scene.