Complete Note: Semiconductor Basics, P-N Junction
Formation, and Biasing of Diodes
Semiconductor Basics
1. Intrinsic and Extrinsic Semiconductors:
Intrinsic Semiconductors: These are pure semiconductors without any impurity
doping. They have a balanced number of electrons and holes (the absence of an
electron). At absolute zero temperature, all atoms in an intrinsic semiconductor are in
the valence band, and there are no free charge carriers. As temperature increases,
electrons gain enough energy to jump into the conduction band, creating holes in the
valence band.
Extrinsic Semiconductors: To increase conductivity, intrinsic semiconductors are
doped with impurities. The process of adding impurities is known as doping:
N-Type Semiconductors: Doping an intrinsic semiconductor with elements
that have more valence electrons than the semiconductor material (like
phosphorus in silicon) creates extra electrons in the conduction band. This
material is known as an N-type semiconductor.
P-Type Semiconductors: Doping with elements that have fewer valence
electrons (like boron in silicon) creates holes (missing electrons) in the
valence band, making it a P-type semiconductor.
2. Doping and its Effect on Conductivity:
Doping significantly alters the electrical properties of a semiconductor. In N-type
material, the excess electrons in the conduction band increase the conductivity. In P-
type material, the presence of holes in the valence band allows easy movement of
electrons, thus enhancing conductivity.
P-N Junction Formation
The Creation of the Depletion Region:
A P-N junction is formed by joining P-type and N-type semiconductors. The junction
results in the diffusion of electrons from the N-region to the P-region and holes from
the P-region to the N-region, leading to recombination and the creation of a depletion
region. This region is devoid of charge carriers and acts as an insulator.
Biasing of Diodes
Forward Bias and its Effect on the Depletion Region:
In forward bias, a positive voltage is applied to the P-type material and a negative
voltage to the N-type material of a diode. This reduces the barrier potential by pulling
the positive charges of the P-side and the negative charges of the N-side away from
the junction. As a result, the depletion region width decreases, and the current can
flow easily through the diode.
Mechanism of Current Flow under Forward Bias (Reduction of Barrier Potential):
When a diode is forward-biased, electrons in the N-type material have enough energy
to cross the junction into the P-type material. The holes in the P-type material also
move towards the N-side. This movement of electrons and holes across the junction
constitutes the forward current. The reduction in the barrier potential (potential
difference) due to the applied voltage enables the movement of charge carriers,
allowing current flow through the diode.
