PN Junction Diodes
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.
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.
V-I Characteristics of P-N Junction 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.
