PIN diode

- an overview or tutorial of the basics of the PIN diode, PIN diode technology, their structure, characteristics and applications.

The PIN diode, p-i-n diode is essentially a refinement of the ordinary PN junction diode. Its development arose from the original PN diode development activities and applications for the new diode were soon found.

The PIN diode differs from the basic PN junction diode in that the PIN diode includes a layer of intrinsic material between the P and N layers. As a result of the intrinsic layer, PIN diodes have a high breakdown voltage and they also exhibit a low level of junction capacitance. In addition to this the larger deletion region of the PIN diode is ideal for applications as a photodiode.

PIN diode development

After the PN junction was understood and further developed in the 1940s, other research into variants of the basic PN junction was undertaken. The first references to this was a low frequency high power rectifier that was developed in 1952 by Hall , and some later developments undertaken by Prince in 1956.

Although the PIN diode saw some initial applications as power rectifiers it was later realised that the lower junction capacitance could be utilised in microwave applications. In 1958 some of the first microwave devices were developed, and later during the 1960s they gained more widespread acceptance in this role.

With the introduction of semiconductors as photo devices the PIN diode saw its use increase as a photodetector. Its large depletion area was ideal for its use in this role.

PIN diode basics and operation

The PIN diode can be shown diagrammatically as being a PN junction, but with an intrinsic layer between the PN and layers. The intrinsic layer of the PIN diode is a layer without doping, and as a result this increases the size of the depletion region - the region between the P and N layers where there are no majority carriers. This change in the structure gives the PIN diode its unique properties.

Basic PIN diode structure
Basic PIN diode structure

The PIN diode operates in exactly the same way as a normal diode. The only real difference is that the depletion region, that normally exists between the P and N regions in an unbiased or reverse biased diode is larger.

In any PN junction, the P region contains holes as it has been doped to ensure that it has a predominance of holes. Similarly the N region has been doped to contain excess electrons. The region between the P and N regions contains no charge carriers as any holes or electrons combine As the depletion region has no charge carriers it acts as an insulator.

Within a PIN diode the depletion region exists, but if the diode is forward biased, the carriers enter the depletion region (including the intrinsic region) and as the two carrier types meet, current starts to flow.

When the diode is forward biased, the carrier concentration, i.e. holes and electrons is very much higher than the intrinsic level carrier concentration. Due to this high level injection level, the electric field extends deeply (almost the entire length) into the region. This electric field helps in speeding up of the transport of charge carriers from p to n region, which results in faster operation of the diode, making it a suitable device for high frequency operations.

PIN diode uses and advantages

The PIN diode is used in a number of areas as a result of its structure proving some properties which are of particular use.

  • High voltage rectifier:   The PIN diode can be used as a high voltage rectifier. The intrinsic region provides a greater separation between the PN and N regions, allowing higher reverse voltages to be tolerated.
  • RF switch:   The PIN diode makes an ideal RF switch. The intrinsic layer between the P and N regions increases the distance between them. This also decreases the capacitance between them, thereby increasing he level of isolation when the diode is reverse biased.
  • Photodetector:   As the conversion of light into current takes place within the depletion region of a photdiode, increasing the depletion region by adding the intrinsic layer improves the performance by increasing he volume in which light conversion occurs.

These are three of the main applications for PIN diodes, although they can also be used in some other areas as well.

The PIN diode is an ideal component to provide electronics switching in many areas of electronics. It is particularly useful for RF design applications and for providing the switching, or attenuating element in RF switches and RF attenuators. The PIN diode is able to provide much higher levels of reliability than RF relays that are often the only other alternative.

By Ian Poole

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