Today, we are going to discuss the basics of semiconductors
and their role in the field of electronics.
Before semiconductor devices even existed, vacuum tubes were the only devices
available for signal amplification, switching and other applications.
Though vacuum tubes are functional, they're bulky,
require a high operating voltage and are completely inefficient.
When semiconductor devices like transistors were invented, semiconductors
started to acquire a dominating role in electronics.
Semiconductors are materials that are in between conductors and insulators when it
comes to the ability to conduct electrical current, which explains the name, semiconductors.
The most commonly used semiconductor material in the electronics industry is silicon.
After that, it’s a compound known asgallium arsenide.
Semiconductor materials have two current carriers, free electrons and holes.
In an intrinsic semiconductor material, free electrons are produced
when the material receives sufficient thermal energy
that provides valence electrons from the valence band
enough energy to jump to the conduction band and turn it into free electrons.
When valence electrons jump to the conduction band,
they leave vacancies in the valence band.
And these vacancies are called holes.
The number of holes in the valence band is just equal to the number of
free electrons in the conduction band in this undoped, intrinsic material.
A semiconductor material becomes a useful electronic component
by controlling its conductivity.
However, semiconductor materials, in their intrinsic state, do not conduct current well.
This is because of the limited number of free electrons and holes in it.
But through a process known as doping, the conductivity of a semiconductor can be increased.
Doping increases the number of current carriers by adding impurities with either
more free electrons or free holes to the intrinsic semiconductor material.
The number of free electrons in an intrinsic semiconductor material
is increased in the doping process by adding pentavalent impurity atoms
or atoms with five valence electrons such as arsenic, phosphorus, bismuth, or antimony.
To visualize this, let’s examine an antimony atom covalently bonded
with four adjacent silicon atoms.
As we can see, only four valence electrons of the antimony atom are used to form
covalent bonds with the silicon atoms, leaving an extra electron that becomes a free electron.
So by adding pentavalent impurity atoms to an intrinsic semiconductor material,
the number of free electrons can be increased
as well as the conductivity of the semiconductor material.
Semiconductors doped with pentavalent atoms are N-type semiconductors
since the majority of its current carriers are electrons.
In order for an intrinsic semiconductor material to have more holes,
they are doped with trivalent impurity atoms.
These are atoms with three valence electrons in their valence shell
like boron, indium, and gallium.
To understand how trivalent impurity atoms increase the number of holes
in an intrinsic semiconductor material, let’s see a boron atom
form covalent bonds with four adjacent silicon atoms.
In this case, when the boron covalently bonds
with the four silicon atoms, a hole is produced.
This is because, each of the four silicon atoms requires one electron
from the boron atom, but it only has three valence electrons.
In this case, we can say that by adding more trivalent impurity atoms
to an intrinsic semiconductor material, it increases the number of holes
and improves the conductivity of the semiconductor material.
Semiconductors doped with trivalent atoms are P-type semiconductors
since the majority of its current carriers are holes.
The doping process converts an intrinsic semiconductor material into extrinsic
and produces either an N-type or a P-type semiconductor material.
Combining the N-type and P-type semiconductor materials,
creates a boundary known as P-N junction.
This P-N junction is the basis for different semiconductor devices widely used today
like diodes, transistors, and thyristors.
In this video, we talked about the basics of semiconductors,
the intrinsic semiconductor and its poor conductivity,
how doping increases the number of current carriers
in a semiconductor material and improves its conductivity.
We also briefly mentioned how different semiconductor devices
were created based on the P-N junction.
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