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.

This overview of the different types of diode is split into several pages, each addressing a different type of diode or diode technology:

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. 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. Many of these types of diode are described in other pages on this section of the Radio-Electronics.Com website.

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.

The basic PN junction diodes are able to perform a variety of roles in electronics circuits. These range from applications as small signal diodes, to switching, to those required in applications such as power supplies as high current or high voltage rectifiers.

PN junction diode circuit basics

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.

Key PN junction diode specifications

There are many parameters that can be specified for any form of diode. Some of the key PN junction diode specifications or parameters are outlined below:

• Semiconductor material:   The semiconductor material used in the PN junction diode is of paramount importance because the material used affects many of the major properties of the diode. Silicon is the most widely used material as if offers high levels of performance for most applications and it offers low manufacturing costs. The other material that is used is germanium. Other materials are generally reserved for more specialist diodes.
• Forward voltage drop:   Any electronics device passing current will develop a resulting voltage across it. The voltage across a PN junction diode arise for two reasons. The first of the nature of the semiconductor PN junction. Broadly speaking, 0.6 volts is required for silicon and about 0.2 to 0.3 volts across a germanium diode to enable the depletion layer to be overcome and for current to flow. The second arises from the normal resistive losses in the device. As a result a figure for the forward voltage drop are a specified current level will be given. This figure is particularly important for rectifier diodes where significant levels of current may be passed.
• Peak Inverse Voltage (PIV):   This is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the deice may fail. This voltage is not simply the RMS voltage of the incoming waveform. Each circuit needs to be considered on its own merits, but for a simple single diode half wave rectifier with some form of smoothing capacitor afterwards, it should be remembered that the capacitor will hold a voltage equal to the peak of the incoming voltage waveform. The diode will then also see the peak of te incoming waveform in the reverse direction and therefore under these circumstances it will see a peak inverse voltage equal to the peak to peak value of the waveform.
• Maximum forward current:   When designing a circuit that passes any levels of current it is necessary to ensure that the maximum current levels for the diode are not exceeded. As the current levels rise, so additional heat is dissipated and this needs to be removed.
• Leakage current:   If a perfect diode were available, then no current would flow when it was reverse biased. It is found that for a real PN junction diode, a very small amount of current flow in the reverse direction as a result of the minority carriers in the semiconductor. The level of leakage current is dependent upon three main factors. The reverse voltage is obviously significant. It is also temperature dependent, rising appreciably with temperature. It is also found that it is very dependent upon the type of semiconductor material used - silicon is very much better than germanium.
  
  The leakage current for a PN junction diode is specified at a certain reverse voltage and particular temperature. It is normally measured in microamps or picoamps.
• Junction capacitance:   All PN junction diodes exhibit a junction capacitance. The depletion region is the dielectric spacing between the two plates which are effectively formed at the edge of the depletion region and the area with majority carriers. The actual value of capacitance being dependent upon the reverse voltage which causes the depletion region to change (increasing reverse voltage increases the size of the depletion region and hence decreases the capacitance). This fact is used in varactor or varicap diodes to good effect, but for many other applications, especially RF applications this needs to be minimised. As the capacitance is of importance it is specified. The parameter is normally detailed as a given capacitance (in pF) at a given voltage or voltages. Also special low capacitance diodes are available for many RF applications.
• Package type:   Diodes can be mounted in a variety of packages according to their applications. High power diodes may require packages that can be bolted to heatsinks, whereas small signal diodes may be available in leaded formats or as surface mount devices.

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.

Further pages from this tutorial
Page [1] > [2] > [3] > [4] > [5] > [6] > [7] > [8] > [9] > [10] > [11] > [12] > [13] > [14] >