P-N Junction

Semiconductors are materials whose conductivity lies between conductors

and insulators. Semiconductors are classified as intrinsic semiconductors
and extrinsic semiconductors. Extrinsic semiconductors are further
classified as N-type and P-type semiconductors.

The P-N junction is formed between the p-type and the n-type
semiconductors. In this session, let us know more about the P-N Junction.

Table of Contents:

 What is P-N Junction?

 Formation of P-N Junction

 Biasing Conditions for the P-N Junction Diode

 Forward Bias

 Reverse Bias

 P-N Junction Formula

 How does current flow in P-N junction diode?

 V-I Characteristics of P-N Junction Diode

 Applications of P-N Junction Diode

 Frequently Asked Questions – FAQs

What is P-N Junction?

Definition: A P-N junction is an interface or a boundary between two

semiconductor material types, namely the p-type and the n-type, inside a
semiconductor.

In a semiconductor, the P-N junction is created by the method of doping.

The p-side or the positive side of the semiconductor has an excess of
holes, and the n-side or the negative side has an excess of electrons. The
process of doping is explained in further detail in the next section.

Formation of P-N Junction

As we know, if we use different semiconductor materials to make a P-N

junction, there will be a grain boundary that would inhibit the movement
of electrons from one side to the other by scattering the electrons and
holes and thus, we use the process of doping. We will understand the
process of doping with the help of this example. Let us consider a thin p-
type silicon semiconductor sheet. If we add a small amount of pentavalent
impurity to this, a part of the p-type Si will get converted to n-type silicon.
This sheet will now contain both the p-type region and the n-type region
and a junction between these two regions. The processes that follow after
forming a P-N junction are of two types – diffusion and drift. There is a
difference in the concentration of holes and electrons at the two sides of a
junction. The holes from the p-side diffuse to the n-side, and the electrons
from the n-side diffuse to the p-side. These give rise to a diffusion current
across the junction.

Read More: Semiconductor Materials

Also, when an electron diffuses from the n-side to the p-side, an ionised
donor is left behind on the n-side, which is immobile. As the process goes
on, a layer of positive charge is developed on the n-side of the junction.
Similarly, when a hole goes from the p-side to the n-side, an ionized
acceptor is left behind on the p-side, resulting in the formation of a layer
of negative charges in the p-side of the junction. This region of positive
charge and negative charge on either side of the junction is termed as the
depletion region. Due to this positive space charge region on either side of
the junction, an electric field with the direction from a positive charge
towards the negative charge is developed. Due to this electric field, an
electron on the p-side of the junction moves to the n-side of the junction.
This motion is termed the drift. Here, we see that the direction of the drift
current is opposite to that of the diffusion current.

Biasing Conditions for the P-N Junction Diode

There are two operating regions in the P-N junction diode:

  P-type

 N-type

There are three biasing conditions for the P-N junction diode, and this is
based on the voltage applied:

 Zero bias: No external voltage is applied to the P-N junction diode.

 Forward bias: The positive terminal of the voltage potential is

connected to the p-type while the negative terminal is connected to
the n-type.

 Reverse bias: The negative terminal of the voltage potential is

connected to the p-type and the positive is connected to the n-type.

Forward Bias

When the p-type is connected to the battery’s positive terminal and the n-
type to the negative terminal, then the P-N junction is said to be forward-
biased. When the P-N junction is forward biased, the built-in electric field
at the P-N junction and the applied electric field are in opposite directions.
When both the electric fields add up, the resultant electric field has a
magnitude lesser than the built-in electric field. This results in a less
resistive and thinner depletion region. The depletion region’s resistance
becomes negligible when the applied voltage is large. In silicon, at the
voltage of 0.6 V, the resistance of the depletion region becomes
completely negligible, and the current flows across it unimpeded.

Reverse Bias

When the p-type is connected to the battery’s negative terminal and the
n-type is connected to the positive side, the P-N junction is reverse biased.
In this case, the built-in electric field and the applied electric field are in
the same direction. When the two fields are added, the resultant electric
field is in the same direction as the built-in electric field, creating a more
resistive, thicker depletion region. The depletion region becomes more
resistive and thicker if the applied voltage becomes larger.

P-N Junction Formula

The formula used in the P-N junction depends upon the built-in potential
difference created by the electric field is given as:

E0=VTln[ND.NAni2]

Where,

 E0 is the zero bias junction voltage

 VT is the thermal voltage of 26mV at room temperature

 ND and NA are the impurity concentrations

 ni is the intrinsic concentration.

Read More: Electrostatic Potential

Watch the video and learn more about P-N Junction Diode and
Rectifier

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How does current flow in the PN junction diode?

The flow of electrons from the n-side towards the p-side of the junction
takes place when there is an increase in the voltage. Similarly, the flow of
holes from the p-side towards the n-side of the junction takes place along
with the increase in the voltage. This results in the concentration gradient
between both sides of the terminals. Due to the concentration gradient
formation, charge carriers will flow from higher-concentration regions to
lower-concentration regions. The movement of charge carriers inside the
P-N junction is the reason behind the current flow in the circuit.

V-I Characteristics of P-N Junction Diode

VI characteristics of P-N junction diodes is a curve between the voltage
and current through the circuit. Voltage is taken along the x-axis while the
current is taken along the y-axis. The above graph is the V-I characteristics
curve of the P-N junction diode. With the help of the curve, we can
understand that there are three regions in which the diode works, and
they are:

 Zero bias

 Forward bias

 Reverse bias

When the P-N junction diode is in zero bias condition, there is no external
voltage applied and this means that the potential barrier at the junction
does not allow the flow of current.

When the P-N junction diode is in forward bias condition, the p-type is
connected to the positive terminal while the n-type is connected to the
negative terminal of the external voltage. When the diode is arranged in
this manner, there is a reduction in the potential barrier. For silicone
diodes, when the voltage is 0.7 V and for germanium diodes, when the
voltage is 0.3 V, the potential barriers decrease, and there is a flow of
current.

When the diode is in forward bias, the current increases slowly, and the
curve obtained is non-linear as the voltage applied to the diode
overcomes the potential barrier. Once the diode overcomes the potential
barrier, the diode behaves normally, and the curve rises sharply as the
external voltage increases, and the curve obtained is linear.
When the P-N junction diode is in negative bias condition, the p-type is
connected to the negative terminal while the n-type is connected to the
positive terminal of the external voltage. This results in an increase in the
potential barrier. Reverse saturation current flows in the beginning as
minority carriers are present in the junction.

When the applied voltage is increased, the minority charges will have
increased kinetic energy which affects the majority charges. This is the
stage when the diode breaks down. This may also destroy the diode.

Applications of P-N Junction Diode

 P-N junction diode can be used as a photodiode as the diode is

sensitive to the light when the configuration of the diode is reverse-
biased.

 It can be used as a solar cell.

 When the diode is forward-biased, it can be used in LED lighting

applications.

 It is used as rectifier in many electric circuits and as a voltage-

controlled oscillator in varactors.