PN Junction Diode
A two-terminal or two-electrode semiconductor device which allows the electric current to flow only in
one direction while blocking the electric current in the opposite or reverse direction, is known as PN
junction diode. This device is forward biased, allowing the electric current to flow. On the other hand, in
reverse bias conditions, it blocks the electric current flow.
PN Junction Diode Symbol
Formation of PN Junction Diode
In a PN junction diode, an ionized donor is left behind on the N-side when an electron diffuses from the
N-side to the P-side and a layer of positive charge develops on the N-side of the junction. When a hole
moves from the P-side to the N-side, an ionized acceptor is left behind on the P-side, causing a layer of
negative charges to accumulate on the P-side of the junction. The depletion area is defined as a region of
positive and negative charge on each side of the junction. An electric field with a direction from a positive
charge to a negative charge develops on either side of the junction.
The electric potential between P and N-regions changes when an external potential is supplied to the PN
junction terminals. As a result, the flow of the majority of carriers is altered, allowing electrons and holes
to diffuse through the PN junction.
The diode is thought to be in the forward bias state if the applied voltage reduces the width of the
depletion layer, and reverse bias if the applied voltage increases the width of the depletion layer. The
diode is said to be in the zero bias or unbias state if the breadth of the depletion layer remains
unchanged.
Biasing Conditions of PN Junction Diode
To connect external battery across pn junction known as biasing.
1-Forward Bias
The PN junction is forward-biased when the P-type is connected to the positive terminal of the battery
and the N-type is connected to the negative terminal. In this condition, the applied electric field and the
built-in electric field at the PN junction are in opposing directions.
2-Reverse Bias
The PN junction is reverse biased when the P-type is connected to the negative terminal of the battery
and the N-type is connected to the positive side. In this condition, the applied electric field and the built-
in electric field are both in the same direction. The resultant electric field and the built-in electric field are
also in the same direction, resulting in a more resistive, thicker depletion area. Increasing the applied
voltage results in a thicker and more resistant depletion area.
PN Junction Formula
The PN junction formula, which is derived from the built-in potential difference created by the electric
field, is expressed as follows:
where,
The zero-bias junction voltage is E0.
At room temperature, VTVT is the thermal voltage of 26 mV.
The impurity concentrations are denoted by the letters NDND and NANA.
The intrinsic concentration is denoted by ni
Current Flow in PN Junction Diode
As the voltage applied to the PN junction increases, electrons from the n-side are driven towards the p-
side. Simultaneously, holes from the p-side migrate towards the n-side. This movement of charge carriers
creates a concentration gradient across the junction. Due to this gradient, charge carriers diffuse from
regions of higher concentration to regions of lower concentration, resulting in a current flow within the
PN junction.
V-I Characteristics of PN Junction
The relationship between the voltage across the junction and current through the circuit is known as the
volt-ampere (V-I) characteristics of a PN junction or semiconductor diode. Normally, voltage is measured
along the x-axis, whereas the current is measured along the y-axis.
The V-I characteristics of the PN junction can be explained in three cases:
Zero bias or unbias
Forward bias
Reverse bias
No movement of holes or electrons occurs at zero bias state as no potential is applied externally which
prevents the passage of electric current to flow in the diode.
When the PN junction diode is in the forward bias, the P-type is linked to the positive terminal of the
external voltage, while the N-type is connected to the negative terminal. This arrangement of diodes
reduces the potential barrier. When the voltage is 0.7 V for silicon diodes and 0.3 V for germanium diodes,
the potential barriers diminish, and current flows.
The current grows slowly while the diode is in the forward bias, and the curve formed is non-linear
because the voltage supplied to the diode surpasses the potential barrier. Once the diode has broken
over the potential barrier, it operates normally, and the curve climbs steeply as the external voltage rises,
yielding a linear curve.
When the PN junction diode is in negative bias, the P-type is linked to the negative terminal of the
external voltage, while the N-type is connected to the positive terminal which leads to the higher
potential barrier. Because minority carriers are present at the junction, a reverse saturation current occurs
at first.
Applications of PN Junction Diode
Some of the most exciting applications of PN diodes are as follows
PN junction diode is utilized as a more triple, voltage doubler, and quadruple in voltage multiplier circuits
as well as a switch in various electrical circuits.
These are used in numerous circuit rectifiers, and varactors for voltage-controlled oscillators.
While the PN junction diode produces light when biased with a current, hence it is employed (LED) and
photodiode applications.
PN junction diodes can also be used for another diode termed a light amplification stimulated emission of
radiation.
In power electronics engineering, it can be employed in solar cells.
It is employed in the detector as well as the demodulator circuit thus it can be used as a detector for the
demodulation circuit.
They are used as clamps to adjust the reference voltage.
The voltage across the PN junction diode is used to produce temperature sensors and reference voltages.
