PN Junction Diode

- a summary or tutorial giving the essentials or basics of the PN junction diode, including its applications, specifications and parameters and its operation.

The semiconductor diode is widely used within the electronics and semiconductor industry.

It is used in its own right, and as a PN junction it is a critical element in transistors and many other semiconductor devices.

However as a discrete component it is also a key part of many electronic circuits, being used in its own right.

Diodes can be manufactured for a whole variety of applications from very low power signal applications right up to power rectification and the like. The technologies use may also differ and as a result there are many different types of diodes, both in ratings and the functions for which they are intended.

In most cases the basic format for the diode is much the same. The diode contains a PN junction which provides the basic functionality for the device.


What is a diode: the basics

The term PN junction diode is normally reserved for what may be called the basic form of diode, although in reality the term applies to virtually any form of semiconductor diode.

These diodes rely on the properties of semiconductors for their operation.


Note on Semiconductor Theory:

Semiconductors form the basis of today's electronics industry. Everything from simple discrete diodes and transistors through to state of the art microprocessors all relay on semiconductors. The technology relies upon a group of materials called semiconductors which can be doped to give p-type variants where charge carriers are holes and n-type semiconductors where the charge carriers are electrons.

Click on the link for a Semiconductor theory tutorial


Using semiconductor technology, the PN junction diode gains its name from the fact that it is formed from a semiconductor PN junction and by its nature it only allows current to flow in one direction. However the PN junction diode also has other properties that can be used in many other applications. These range from light emission to light detection and variable capacitance to voltage regulation.

The theory behind semiconductor diodes uses the basic semiconductor ideas and applies them to a junction between the two types of semiconductor, p-type where the charge carriers are formed by holes and n-type where the charge carriers are electrons.


Note on PN Junctions:

The PN junction is a basic structure within semiconductors. As the name implies it is a junction between P-Type and N-Type semiconductor areas. This structure allows current to follow in one direction thereby providing a rectification function.

Click on the link for a PN Junction


The basic form of PN junction finds many uses in electronics circuits. The standard PN junction diodes are available in a variety of forms. They are mainly manufactured from silicon, although germanium diodes are also available. PN junction diodes can also be manufactured from other semiconductor materials, but these are generally specialised diodes used for particular applications.

Diagram of some typical diode outlines against the circuit symbol showing how they cross refer in terms of connections
Diode circuit symbol and common package outlines

As the name indicates a diode has two terminals. These are referred to as the anode and cathode. When in circuit, the current flow (conventional current flow) is across the PN junction diode from the anode to the cathode. As the diode is a one way device, current is inhibited from flowing in the other direction.

The PN junction has the characteristic curve shown below. It can be seen that current is blocked in the reverse direction, although at some stage it will break down. In the forward direction current flows once the 'turn-on' voltage has been exceeded. Typically this is 0.6 to 0.7 volts for silicon diodes and 0.3 to 0.4 volts for germanium. Some diodes such as Schottky diodes or those using different materials, etc will have different characteristics and turn-on voltages.

The diode IV characteristic showing the forward turn on voltage and reverse current blocking until reverse breakdown occurs.
Diode IV characteristic


Summary

The PN junction diode may be looked at as the basic or entry level diode. Nevertheless the basic PN junction diode is widely used in many applications from being a signal diode and detector, through being a clamp diode or transient suppressor across inductors or relay coils, through to high power rectification. In all these applications the basic PN junction diode is able to provide a very useful service. As a result, these diodes are use dry the million each day.

By Ian Poole


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