of PIN diodes
The figure below
shows a horizontal PIN diode, sometimes called H-PIN. Here the
P and N layers are formed on top of the I layer.
Here's what is
known as a vertical PIN diode or V-PIN. Here the diode is formed
of a stack of the three materials, from top to bottom, P, I, N.
Last, here's a
structure known as a NIP diode. It's just a PIN diode, upside
PIN diode response
A PIN diode acts
like a current controlled resistor. The more current that you
inject through the I region, the lower the RF resistance. The
current/resistance characteristic is ideally R=K/I (where K is
a constant), which looks like a straight line when graphed on
log-log scales. Thanks for the correction, Morten!
Look at the range
of resistance that you can obtain, from 0.1 ohm to 10,000 ohms...
yes, the PIN diode pretty much covers the entire horizontal axis
of the Smith chart! (Look
at the chart above, and notice that 50 ohms occurs very close
to the center of the response). This is what makes it such a versatile
device, it can behave like an open circuit, a short circuit, or
any reflection coefficient in between. You can use it to create
switches, phase shifters and variable attenuators, and we'll show
you how if you follow the links at the top of the page.
limitation of PIN diodes
PIN diodes have
a low frequency limitation due to carrier lifetime. What's a carrier?
Who cares? The problem can be explained without worrying about
diode acts like a rectifier, no matter what the frequency. A rectifier
has that familiar nonlinear I-V curve. While under forward bias,
the current takes off for the moon after a half-volt or so. Under
reverse bias, there is essentially zero current for many volts,
until breakdown occurs. A Schottky diode is an excellent example
of a rectifier diode, that's why it is used as a detector. RF
enters, DC leaves.
A PIN diode only
acts like a rectifier at low frequencies. At microwave frequencies,
the IV curve undergoes a change, so that it behaves like a resistor,
whose resistance value is determined by the level of DC current
that is present in the I-region. Thus a PIN diode is essentially
a DC-controlled high-frequency resistor. Just as important, if
no DC current is present, the diode behaves like an open circuit.
at which the PIN diode transitions from acting like a diode to
acting like a resistor is a function of the thickness of the I-region.
Thicker diodes can be used as switches to lower frequencies. By
carefully selecting diodes, you can make a PIN diode switches
operate down to 1 MHz.
As if this behavior
wasn't outstanding enough, wait, there's more! The DC control
current can be small, while the microwave current can be huge...
a few milliamps of current at DC can cause the PIN diode to short
out an amp or more of RF current. This is a huge advantage to
RF switch designers that need to consider power handling.
PIN diode limiters
PIN diodes can
also be used to create limiters
(a type of nonlinear device ),
usually as one or more shunt elements across a transmission line.
Often the diodes are spaced apart by the magic
quarterwave to improve small signal response. A limiter is
(usually) a passive device that has low loss for small signals,
then increases its attenuation as power levels increase.
Some PIN diode
limiters are passive, meaning that the PIN diode creates the nonlinear
response by itself. An "active limiter" adds a detector
circuit that applies DC current to the PIN diode to turn it
on harder, at lower power. The detector uses a Schottky diode.
A switched limiter uses a DC control signal to turn on the PIN