Understanding Diode Specifications, Ratings & Parameters

Diodes may appear straightforward but they have many datasheet specifications, parameters and ratings which need to be understood when selecting one.

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Understanding diode specifications, parameters and ratings can be key to selecting the right component. With a huge variety of diodes available on the market, selecting the required one may not always be easy.

Most of the specifications, ratings and parameters are relatively easy to understand, but a few may require a little more explanation, or they may be applicable to a limited number of diodes.

Diode specifications ratings and parameters

The list below provides details of the various diode characteristics, and diode parameters found in the datasheets and specifications for diodes.

  • Semiconductor material:   The semiconductor material used in the PN junction diode is of paramount importance because the material used affects many of the major diode characteristics and properties. Silicon and germanium are two widely used materials:
    • Silicon:   Silicon is the most widely used material as if offers high levels of performance for most applications and it offers low manufacturing costs.
    • Germanium:   Germanium is less widely used and but offers a low turn on voltage of around 0.2 to 0.3 V.
    Other materials are generally reserved for more specialist diodes. For example LEDs use compound materials to provide the different colours.
  • Forward voltage drop, Vf:   Any electronics device passing current will develop a resulting voltage across it and this diode characteristic is of great importance, especially for power rectification where power losses will be higher for a high forward voltage drop. Also RF diodes often need a small forward voltage drop as signals may be small but still need to overcome it.

    The voltage across a PN junction diode arise for two reasons. The first of the nature of the semiconductor PN junction and results from the turn-on voltage mentioned above. This voltage enables 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 diode characteristics is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the device 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 the 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 characteristic or specification for a PN junction diode is specified at a certain reverse voltage and particular temperature. The specification is normally defined in terms of in microamps, µA or picoamps, pA.
  • 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, and in some circumstances, especially RF applications, the package is a key element in defining the overall RF diode characteristics. Also for power applications where heat dissipation is important, the package can define many of the overall diode parameters because 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.

The vast number of diodes have a vast number of different characteristics. Some diodes may be designed purely for rectification, whereas others may be designed to emit light, detect light, act as a voltage reference, provide variable capacitance and the like. Whatever the type of diode, many of the basic specifications, parameters and ratings mentioned above will be important. Understanding the key parameters and ratings when looking at the specifications in the datasheets is key to selecting the right diode.

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