Editing PIN diode (section)

== Applications ==
PIN diodes are useful as [[RF switch]]es, [[attenuator (electronics)|attenuators]], [[photodetector]]s, and phase shifters.<ref>https://srmsc.org/pdf/004430p0.pdf (transcript version: http://www.alternatewars.com/WW3/WW3_Documents/ABM_Bell/ABM_Ch8.htm)</ref>

=== RF and microwave switches ===
[[File:Microwave Switch.png|thumb|right|A PIN diode RF microwave switch]]

Under zero- or reverse-bias (the "off" state), a PIN diode has a low [[capacitance]]. The low capacitance will not pass much of an [[RF signal]]. Under a forward bias of 1 mA (the "on" state), a typical PIN diode will have an RF resistance of about {{nowrap|1 [[ohm]]}}, making it a good conductor of RF. Consequently, the PIN diode makes a good RF switch.

Although RF relays can be used as switches, they switch relatively slowly (on the order of {{nowrap|tens of milliseconds}}). A PIN diode switch can switch much more quickly (e.g., {{nowrap|1 microsecond}}), although at lower RF frequencies it isn't reasonable to expect switching times in the same order of magnitude as the RF period.

For example, the capacitance of an "off"-state discrete PIN diode might be {{nowrap|1 pF}}. At {{nowrap|320 MHz}}, the capacitive reactance of {{nowrap|1 pF}} is {{nowrap|497 ohms}}:

:<math>\begin{align}
Z_\mathrm{diode} &= \frac{1}{2\pi fC}\\
&= \frac{1}{2\pi(320\times10^6\,\mathrm{Hz})(1\times10^{-12}\,\mathrm{F})}\\
&= 497\,\Omega
\end{align}
</math>

As a series element in a {{nowrap|50 ohm}} system, the off-state attenuation is:
:<math>\begin{align}
A &= 20\log_{10}\left(\frac{Z_\mathrm{load} + Z_\mathrm{source}} {Z_\mathrm{source} + Z_\mathrm{diode} + Z_\mathrm{load}}\right)\\
&= 20\log_{10}\left(\frac{50\,\Omega + 50\,\Omega} {50\,\Omega + 497\,\Omega + 50\,\Omega}\right)\\
&= {15.52}\,\mathrm{dB}
\end{align}</math>

This attenuation may not be adequate. In applications where higher isolation is needed, both shunt and series elements may be used, with the shunt diodes biased in complementary fashion to the series elements. Adding shunt elements effectively reduces the source and load impedances, reducing the impedance ratio and increasing the off-state attenuation. However, in addition to the added complexity, the on-state attenuation is increased due to the series resistance of the on-state blocking element and the capacitance of the off-state shunt elements.

PIN diode switches are used not only for signal selection, but also component selection. For example, some low-[[phase noise|phase-noise]] oscillators use them to range-switch inductors.<ref>{{cite web |url=http://www.herley.com/index.cfm?act=app_notes&notes=switches |title=Microwave Switches: Application Notes |website=Herley General Microwave |url-status=unfit |archive-url=https://web.archive.org/web/20131030102546/http://www.herley.com/index.cfm?act=app_notes&notes=switches |archive-date=2013-10-30}}</ref>

=== RF and microwave variable attenuators ===
[[File:General Microwave Modulator.png|thumb|right|An RF microwave PIN diode attenuator]]

By changing the bias current through a PIN diode, it is possible to quickly change its RF resistance.

At high frequencies, the PIN diode appears as a resistor whose resistance is an inverse function of its forward current. Consequently, PIN diode can be used in some variable attenuator designs as amplitude modulators or output leveling circuits.

PIN diodes might be used, for example, as the bridge and shunt resistors in a bridged-T attenuator. Another common approach is to use PIN diodes as terminations connected to the 0 degree and -90 degree ports of a quadrature hybrid. The signal to be attenuated is applied to the input port, and the attenuated result is taken from the isolation port. The advantages of this approach over the bridged-T and pi approaches are (1) complementary PIN diode bias drives are not needed—the same bias is applied to both diodes—and (2) the loss in the attenuator equals the return loss of the terminations, which can be varied over a very wide range.

=== Limiters ===
PIN diodes are sometimes designed for use as input protection devices for high-frequency test probes and other circuits. If the input signal is small, the PIN diode has negligible impact, presenting only a small parasitic capacitance. Unlike a rectifier diode, it does not present a nonlinear resistance at RF frequencies, which would give rise to harmonics and intermodulation products. If the signal is large, then when the PIN diode starts to rectify the signal, the forward current charges the drift region and the device RF impedance is inversely proportional to the signal amplitude. That signal amplitude varying resistance can be used to terminate some predetermined portion of the signal in a resistive network dissipating the energy or to create an impedance mismatch that reflects the incident signal back toward the source. The latter may be combined with an isolator, a device containing a circulator which uses a permanent magnetic field to break reciprocity and a resistive load to separate and terminate the backward traveling wave. When used as a shunt limiter the PIN diode impedance is low over the entire RF cycle, unlike paired rectifier diodes that would swing from a high resistance to a low resistance during each RF cycle clamping the waveform and not reflecting it as completely. The ionization recovery time of gas molecules that permits the creation of the higher power spark gap input protection device ultimately relies on similar physics in a gas.

=== Photodetector and photovoltaic cell ===
The PIN photodiode was invented by [[Jun-ichi Nishizawa]] and his colleagues in 1950.<ref>{{cite book|url=https://books.google.com/books?id=PbYgBQAAQBAJ&pg=PA137|title=Electronic Inventions and Discoveries: Electronics from Its Earliest Beginnings to the Present Day|first=G. W. A.|last=Dummer|date=22 October 2013|publisher=Elsevier|isbn=9781483145211|access-date=14 April 2018|via=Google Books}}</ref>

PIN photodiodes are used in fibre optic network cards and switches. As a photodetector, the PIN diode is reverse-biased. Under reverse bias, the diode ordinarily does not conduct (save a small dark current or I<sub>s</sub> leakage). When a [[photon]] of sufficient energy enters the [[depletion region]] of the diode, it creates an [[electron-hole pair]]. The reverse-bias field sweeps the carriers out of the region, creating current. Some detectors can use [[avalanche photodiode|avalanche multiplication]].

The same mechanism applies to the PIN structure, or '''p-i-n junction''', of a [[solar cell]]. In this case, the advantage of using a PIN structure over conventional semiconductor [[p–n junction]] is better long-wavelength response of the former. In case of long wavelength irradiation, photons penetrate deep into the cell. But only those electron-hole pairs generated in and near the depletion region contribute to current generation. The depletion region of a PIN structure extends across the intrinsic region, deep into the device. This wider depletion width enables electron-hole pair generation deep within the device, which increases the [[quantum efficiency]] of the cell.

Commercially available PIN photodiodes have quantum efficiencies above 80-90% in the telecom wavelength range (~1500&nbsp;nm), and are typically made of [[germanium]] or [[indium gallium arsenide|InGaAs]]. They feature fast response times (higher than their p-n counterparts), running into several tens of gigahertz,<ref>{{Cite web |url= https://www.discoverysemi.com/Product%20Pages/40G_Products/40G_Modules.php |title= Discovery semiconductor 40G InGaAs photodetector modules}}</ref> making them ideal for high speed optical telecommunication applications. Similarly, [[silicon]] p-i-n photodiodes<ref>{{cite web |title = Si photodiodes {{!}} Hamamatsu Photonics |url= https://www.hamamatsu.com/eu/en/product/optical-sensors/photodiodes/si-photodiodes/index.html |website= hamamatsu.com |access-date= 2021-03-26}}</ref> have even higher quantum efficiencies, but can only detect wavelengths below the bandgap of silicon, i.e. ~1100&nbsp;nm.

Typically, [[amorphous silicon]] [[thin-film solar cell|thin-film cells]] use PIN structures. On the other hand, [[CdTe]] cells use NIP structure, a variation of the PIN structure. In a NIP structure, an intrinsic CdTe layer is sandwiched by n-doped CdS and p-doped ZnTe; the photons are incident on the n-doped layer, unlike in a PIN diode.

A PIN photodiode can also detect [[ionizing radiation]] in case it is used as a [[semiconductor detector]].

In modern fiber-optical communications, the speed of optical transmitters and receivers is one of the most important parameters. Due to the small surface of the photodiode, its parasitic (unwanted) capacitance is reduced. The bandwidth of modern pin photodiodes is reaching the microwave and millimeter waves range.<ref>Attila Hilt, Gábor Járó, Attila Zólomy, Béatrice Cabon, Tibor Berceli, Tamás Marozsák: "Microwave Characterization of High-Speed pin Photodiodes", Proc. of the 9th Conference on Microwave Techniques COMITE’97, pp.21-24, Pardubice, Czech Republic, 16-17 Oct. 1997.</ref>