Editing PIN diode (section)

== Characteristics ==
The PIN diode obeys the standard diode equation for low-frequency signals. At higher frequencies, the diode looks like an almost perfect (very linear, even for large signals) resistor. The P-I-N diode has a relatively large stored charge adrift in a thick [[intrinsic semiconductor|intrinsic region]]. At a low-enough frequency, the stored charge can be fully swept and the diode turns off. At higher frequencies, there is not enough time to sweep the charge from the drift region, so the diode never turns off. The time required to sweep the stored charge from a diode junction is its [[diode#Reverse-recovery effect|reverse recovery time]], and it is relatively long in a PIN diode. For a given semiconductor material, on-state impedance, and minimum usable RF frequency, the reverse recovery time is fixed. This property can be exploited; one variety of P-I-N diode, the [[step recovery diode]], exploits the abrupt impedance change at the end of the reverse recovery to create a narrow impulse waveform useful for [[frequency multiplier|frequency multiplication]] with high multiples.{{Cn|date=July 2022}}

The high-frequency resistance is inversely proportional to the DC bias current through the diode. A PIN diode, suitably biased, therefore acts as a variable resistor. This high-frequency resistance may vary over a wide range (from {{nowrap|0.1 Ω}} to {{nowrap|10 kΩ}} in some cases;<ref name="PDFun">{{Citation |first=Bill |last=Doherty |url=https://www.microsemi.com/sites/default/files/micnotes/701.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.microsemi.com/sites/default/files/micnotes/701.pdf |archive-date=2022-10-09 |url-status=live |title=MicroNotes: PIN Diode Fundamentals |location=Watertown, MA |publisher=Microsemi Corp. |id=MicroNote Series 701}}</ref> the useful range is smaller, though).

The wide intrinsic region also means the diode will have a low capacitance when [[p–n diode#Reverse bias|reverse-biased]].

In a PIN diode the depletion region exists almost completely within the intrinsic region. This depletion region is much larger than in a PN diode and almost constant-size, independent of the reverse bias applied to the diode. This increases the volume where electron-hole pairs can be generated by an incident photon. Some [[photodetector]] devices, such as PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode), use a PIN junction in their construction.

The diode design has some design trade-offs. Increasing the cross-section area of the intrinsic region increases its stored charge reducing its RF on-state resistance while also increasing reverse bias capacitance and increasing the drive current required to remove the charge during a fixed switching time, with no effect on the minimum time required to sweep the charge from the I region. Increasing the thickness of the intrinsic region increases the total stored charge, decreases the minimum RF frequency, and decreases the reverse-bias capacitance, but doesn't decrease the forward-bias RF resistance and increases the minimum time required to sweep the drift charge and transition from low to high RF resistance. Diodes are sold commercially in a variety of geometries for specific RF bands and uses.