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.
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.
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.
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