PIN Diode
Definition: The diode in which the intrinsic layer of high resistivity is sandwiched
between the P and N-region of semiconductor material such type of diode is known
as the PIN diode. The high resistive layer of the intrinsic region provides the large
electric field between the P and N-region. The electric field induces because of the
movement of the holes and the electrons. The direction of the electric field is from
n-region to p-region.
The high electric field generates the large electron holes pairs due to which the
diode process even for the small signals. The PIN diode is a type of photodetector
used for converting the light energy into the electrical energy.
The intrinsic layer between the P and N-type regions increases the distance
between them. The width of the region is inversely proportional to their
capacitance. If the separation between the P and N region increases their
capacitance decreases. This characteristic of diode increases their response time
and makes the diode suitable for works like a microwaves applications.
Symbol of PIN Diode
The symbolic representation of the PIN diode is shown in the figure below. The
anode and cathode are the two terminal of the PIN diode. The anode is the positive
terminal and cathode represent their negative terminals.
PIN Diode Structure
The diode consists the P-region and N-region which is separated by the intrinsic
semiconductor material. In P-region the hole is the majority charge carrier while in
n-region the electron is the majority charge carrier. The intrinsic region has no free
charge carrier. It acts as an insulator between n and the p-type region. The i-region
has the high resistance which obstructs the flow of electrons to pass through it.
Working of PIN Diode
The working of the PIN diode is similar to the ordinary diode. When the diode is
unbiased, their charge carrier will diffuse. The word diffusion means the charge
carriers of the depletion region try to move to their region. The process of diffusion
occurs continue until the charges become equilibrium in the depletion region.
Let the N and I-layer make the depletion region. The diffusion of the hole and
electron across the region generates the depletion layer across the NI-region. The
thin depletion layer induces across n-region, and thick depletion region of opposite
polarity induces across the I-region.
Forward Biased PIN Diode
When the diode is kept forward biased, the charges are continuously injected into
the I-region from the P and N-region. This reduces the forward resistance of the
diode, and it behaves like a variable resistance.
The charge carrier which enters from P and N-region into the i-region are not
immediately combined into the intrinsic region. The finite quantity of charge
stored in the intrinsic region decreases their resistivity.
Consider the Q be the quantity of charge stored in the depletion region. The τ be
the time used for the recombination of the charges. The quantity of the charges
stored in the intrinsic region depends on their recombination time. The forward
current starts flowing into the I region.
Where, IF – forward current
τ- recombination time
The resistance (Rs) of the current under forwarding biased is inversely proportional
to the charge Q stored in the intrinsic region.
Where, w – width region
μ – electron mobility
μ0 – hole mobility
From equation (1) and (2), we get
The above equation shows that the resistance of the intrinsic region depends on the
width of the region.
Reversed Biased PIN Diode
When the reverse voltage is applied across the diode, the width of the depletion
region increases. The thickness of the region increases until the entire mobile
charge carrier of the I-region swept away from it. The reverse voltage requires
for removing the complete charge carrier from the I-region is known as the
swept voltage.
In reverse bias, the diode behaves like a capacitor. The P and N region acts as the
positive and negative plates of the capacitor, and the intrinsic region is the
insulator between the plates.
Where, A – junction diode
w – intrinsic region thickness
The lowest frequency at which the effect starts to begins is expressed as
Where, ε – silicon dielectric constant
Applications of PIN Diode
• High Voltage Rectifier – It is used as a high voltage rectifier. The diode has a
large intrinsic region between the N and P-region which can tolerate the high
reverse voltage.
• Photo-detector – The PIN diode is used for converting the light energy into
the electrical energy. The diode has large depletion region which improves
their performance by increasing the volume of light conversion.
The PIN diode is most suitable for low voltage applications.
