The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the normal function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications.
A PiN diode operates under what is known as high-level injection. In others words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. Its function can be likened to filling up a water bucket with a hole on the side. Once the water reaches the hole's level it will begin to pour out. Similarly, the diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the amount of electrons are equal to the amount of holes in the intrinsic region. When the diode is forward biased, the injected carrier concentration is typically several orders of magnitudes higher than the intrinsic level carrier concentration.
A 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. There is a lot of stored charge in the intrinsic region. At low frequencies, the charge can be removed and the diode turns off. At higher frequencies, there is not enough time to remove the charge, so the diode never turns off. The PIN diode has a poor reverse recovery time.
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 0.1 ohm to 10 kΩ in some cases; the useful range is smaller, though).
The wide intrinsic region also means the diode will have a low capacitance when 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 area where electron-hole pairs can be generated. For these reasons many photodetector devices include at least one PIN diode in their construction, for example PIN photodiodes and phototransistors (in which the base-collector junction is a PIN diode).
The diode design has some design tradeoffs. Increasing the dimensions of the intrinsic region (and its stored charge) allows the diode to look like a resistor at a lower frequencies. It adversely affects the time needed to turn off the diode and its shunt capacitance. PIN diodes will be tailored for a particular use.
Under zero or reverse bias, 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, a typical PIN diode will have an RF resistance of about 1 ohm, making it a good RF conductor. The PIN diode makes a good RF switch.
Although RF relays can be used as switches, they switch very slowly (on the order of 10 milliseconds). A PIN diode switch can switch much more quickly (e.g., 1 microsecond).
The capacitance of an off discrete PIN diode might be 1pF. At 160MHz, the reactance of 1pF is 1000 ohms. In a 50 ohm system, this can allow excessive RF through. In applications that need high isolation, switches are cascaded to improve the isolation.
PIN diode switches might be used for signal selection, but they are also used for component selection. Some low phase noise oscillators use PIN diodes to range switch inductors.
By changing the bias current through a PIN diode, it's possible to quickly change the 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. Consequently, PIN diodes are used for amplitude modulators or output leveling circuits.
PIN diodes might be used, for example, as the bridge and shunt resistors in a bridged-T attenuator.
PIN diodes are sometimes used as input protection devices for high frequency test probes. If the input signal is within range, the PIN diode has little impact as a small capacitance. If the signal is large, then the PIN diode starts to conduct and becomes a resistor that shunts most of the signal to ground.
Other more specialized PIN diodes are used in fibre optic network cards and switches. In this case, the PIN diodes are used as photodiodes.
As a photodetector, the PIN diode is reverse biased. Under reverse bias, the diode ordinarily does not conduct (save a small dark current or Is leakage). A photon entering the intrinsic region frees a carrier. The reverse bias field sweeps the carrier out of the region and creates a current.
The reverse bias can be quite large. Higher voltage create higher fields and sweep the carriers out more quickly.
Some detectors can use avalanche multiplication.
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