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What is a semiconductor

PN Junction






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How a PN Junction Diode Works

- summary of the basics or a tutorial of how a PN junction or diode works showing how does the current flow in only one direction, and how diodes can be used on their own and in 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 basics of semiconductors and 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 today's semiconductor technology, and was the first semiconductor device to be used. The first semiconductor diode to be used was the Cat's Whisker wireless detector used in early wireless sets. It consisted of a wire placed onto a material that was effectively a semiconductor. The point where the wire met the semiconductor then formed a small PN junction and this detected the radio signals.

The diode or PN junction was the first form of semiconductor device to be investigated in the early 1940s when the first real research was undertaken into semiconductor technology. It was found that small point contact diodes were able to rectify some of the microwave frequencies used in early radar systems and as a result they soon found many uses.

Today, the PN junction has undergone a significant amount of development. Many varieties of diode are in use in a variety of applications. In addition to this, the PN junction forms the basis of much of today's semiconductor technology where it is used in transistors, FETs, and many types of integrated circuit.

The PN junction is found in many semiconductor devices today. These include:

  • 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.

PN junction with no bias applied

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.

PN junction with forward bias applied

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.

PN junction with reverse bias

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.

PN junction diode characteristic

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.


Varieties of PN junction diode

basic PN junction diodes are used in many applications. These diodes are optimised for particular applications and as a result, diodes may be obtained for specific purposes.

Specific types of diode that are in widespread use include the following:

  • Small signal diode:   This type of diode is a standard PN junction and is designed for carrying low levels of current. They are often used as signal detectors, or as general diodes in low power circuitry.


  • Rectifier diode:   This type of diode is used in power supply applications for rectifying the current. These diodes are larger than signal diodes and are able to dissipate more heat.


  • Varactor or varicap diode:   This type of diode is used as a variable capacitor. It is reverse biased and uses the capacitance across the depletion layer. By changing the reverse voltage the thickness of the depletion layer changes and along with it so does the capacitance.


  • Light emitting diode (LED):   It is found that some diodes that use more complicated compounds will actually emit light when current is passed through them. Although RED LEDs were the first to be sold, it is now possible to obtain a variety of different coloured LEDs


  • Laser diode :   This form of diode is able to generate laser light, i.e. a single wavelength or frequency.


  • Photodiode:   This form of diode is used to detect incoming light.


  • Zener diode:   This type of diode uses a reverse breakdown voltage, and it provides a stable reference used in regulated power supplies.


It can be seen that there are many different types of diode that are available for a variety of different tasks. Further details of these and other semiconductor devices can be found in this section (Electronics components) of the website.


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.

 


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