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PIN diode (switch)
PIN stands for P-type,
intrinsic, and N-type semiconductor material layers and it can be made using
either GaAs or Silicon as
the semiconductor substrate. Microwave switches are
often made using PIN diode, where P is the anode, and N is the cathode. Conventionally
the current injection side (under forward bias) is called the anode. In the
schematic symbol the anode is the side with the arrow, the cathode is the side with
the plate.
Classification of PIN diodes:
Following figure shows a horizontal PIN diode also called as H-PIN. Here the
P and N layers are formed on top of the I layer.
However in the vertical
PIN diode or V-PIN, the diode has a stack of the three materials, P layer at
the top, N-layer at the bottom (on substrate) and I layer sandwiched between
them.
If we reverse the order
of stacking the above semiconducting layers, we make a NIP (a PIN reversed
upside down) that looks as:
Characteristics of PIN diode: Following diagram shows the variation in the resistance
of PIN diode as a function of dc current flowing
through it. The higher the current injected through its middle layer (I region),
the lower is its RF resistance. Thus it acts like a current controlled
resistor. Its ideal resistance (R) – current (I) relationship is R = K / I, where
K is a constant. If plotted on a log-log scale it looks like a straight line.
One can obtain a
resistance ranging from 0.1 Ω to 10 KΩ for the PIN diode, which covers the entire
horizontal axis of the Smith chart. And the fact that the resistance value of 50 Ω appears almost in the middle of the response, makes it such a
versatile device. It offers itself as an open circuit, a short circuit, or provides
any reflection coefficient between these extremes. Thus it can be used to make switch,
phase shifter and variable attenuator.
Limitations:
Conventional diode has non-linear I-V characteristics and it rectifies
a signal, irrespective of the frequency of the input signal. A forward biased
diode allows very-very high current to flow through it for a signal voltage of
~1 volt or more. And a reverse biased diode allows no current for finite values
of signal voltage until the breakdown occurs. An example is Schottky diode,
which is used as a detector taking RF as input and giving dc as output.
A PIN diode also behaves like a rectifier at low frequencies. At µwave
frequencies, its I–V curve undergoes a change, and it starts behaving like a resistor,
whose resistance is determined by the amount of DC current flowing through the
I-region. Thus a PIN diode is essentially a DC-controlled high-frequency
resistor. If no DC current is flowing through the I-region, PIN diode behaves
like an open circuit.
PIN
diode has a low frequency limitation due to carrier lifetime. The frequency at which the PIN diode makes a transition
from behaving like a diode to a current controlled resistor is a function of
the thickness of the I-region. Thicker diode can be used as switch at lower
frequencies. By choosing its fabrication parameters, a PIN diode can be made to
work as switch operating at a frequency as low as 1 MHz. For a DC control
current as small as a few mA, a PIN diode can work as switch to carry µwave signals
of very large amplitude, for instance like 1000 mA. Thus PIN diode is a
wonderfully efficient device for designing RF switch handling substantial power.
Limiters:
A
limiter is a device which offers low attenuation to small signals and its
attenuation increases as the power level rises. PIN diodes can also be used to
create such a nonlinear device viz. limiter. For this purpose, PIN diode is
put as shunt element across a transmission line and put at a gap of quarter
wave to improve small signal response. Some PIN diode limiters are passive and the
PIN diode creates the nonlinear response by itself. On the other hand, an
active limiter includes a Schottky diode detector that
applies DC current to the PIN diode to turn it on at lower power.
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