A Pn-junction obtained from a pure semiconductor looks
like as shown.
There exists a potential difference Vo
, called barrier potential, across the junction. Its value for silicon based
junction is 0.7 V and 0.3 V for germanium based junction.
Let us now analyse what happens if a battery is
connected to the ends of this Pn-junction.
Forward Biasing of Pn-junction
If a battery is connected to the junction with its
positive terminal connected to the P-side and the negative terminal to the
n-side, the Pn-junction is said to be forward biased.
In other words, the voltage on p-side must be
relatively higher than whatever be the voltage on n-side.
The applied voltage V opposes barrier voltage Vo
. As a result, effective barrier potential decreases to ( V-Vo ) . Majority charge
carriers i.e holes from p-side and electrons from n-side begin to flow towards
the junction.
The diffusion of electrons and holes into the
depletion layer decreases its width.
The effective resistance across the pn-junction
becomes almost zero and it behaves like a closed switch i.e free current flow.
Reverse Biasing of Pn-junction
If the p-side of the junction is connected to the
negative terminal and n-side to the positive terminal of the battery, the
junction is said to be reverse biased.
In other words, the voltage on n-side must be
relatively higher than whatever be the voltage on p-side.
The applied voltage V aids barrier voltage Vo
. As a result, effective barrier potential increases to ( V+Vo ) and hence energy barrier
across the junction increases. Majority charge carriers i.e holes on p-side and electrons on n-side move away from
the junction, increasing the width of the depletion layer.
The resistance of the pn-junction becomes very large
and it behaves like an open switch i.e no current flow.
In fact, a negligibly small amount of current of the
order of micro-amperes flows from n-side to p-side.
What is a Pn junction diode ?
From the above 2 cases of forward biased pn junction diode and reverse biased pn junction diode, we found that a pn junction allows
current to flow from p-side to n-side only and blocks it in opposite direction.
Thus behaving like a diode (any device that conducts only in one direction) and
therefore is commonly known as Pn
junction diode.
Pn junction diode Symbol
Pn junction diode is symbolised as shown.
The arrow pointing from p-side to n-side indicates the
direction in which convetional current can flow easily without much resistance.
It is a trick to remember permissible current flow direction.
