Layers of a PIN diode
A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.
The wide intrinsic region is in contrast to an ordinary p–n diode. The wide intrinsic region makes the PIN diode an inferior rectifier
(one typical function of a diode), but it makes it suitable for
attenuators, fast switches, photodetectors, and high voltage power
electronics applications.
Operation
A PIN diode operates under what is known as high-level injection.
In other 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 number of electrons is equal to the number
of holes in the intrinsic region. When the diode is forward biased,
the injected carrier concentration is typically several orders of
magnitude higher than the intrinsic carrier concentration. Due to this
high level injection, which in turn is due to the depletion process,
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 the P to the N region, which results in faster
operation of the diode, making it a suitable device for high frequency
operations.
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 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 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 multiplication with high multiples.
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 Ω 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 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 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, 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.
Applications
RF and microwave switches
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 1 ohm, making it a good RF conductor. 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 10s of milliseconds). A PIN diode switch can switch much more quickly (e.g., 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 1 pF. At 320 MHz, the capacitive reactance of 1 pF is 497 ohms:
As a series element in a 50 ohm system, the off-state attenuation in dB is:
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 oscillators
use them to range-switch inductors.
RF and microwave variable attenuators
An RF Microwave PIN diode Attenuator.
By changing the bias current through a PIN diode, it is 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 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 a resistance
inversely proportional to the signal amplitude. That signal amplitude
varying resistance can be used to terminate some predetermined portion
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. It should be emphasized that when used as a shunt limiter the PIN
diode is a low impedance 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.
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 Is 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 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 nm), and are
typically made of germanium or InGaAs. They feature fast response times (higher than their p-n counterparts), running into several tens of gigahertz, making them ideal for high speed optical telecommunication applications. Similarly, silicon p-i-n photodiodes have even higher quantum efficiencies, but can only detect wavelengths below the bandgap of silicon, i.e. ~1100 nm.
Typically, amorphous silicon 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 X-ray and gamma ray photons.
Example PIN photodiodes
SFH203 and BPW43 are cheap general purpose PIN diodes in 5 mm clear plastic cases with bandwidths over 100 MHz. RONJA telecommunication systems are an example application.
