Today, we are going to discuss the basics of semiconductors and their role in the field
of electronics.
Before semiconductor devices existed, vacuum tubes were the only devices available for
signal amplification, switching, and other applications.
Though vacuum tubes are functional, they are bulky, require a high operating voltage, and
are 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.
The most commonly used semiconductor material in the electronics industry is silicon.
After that, it’s a compound known as gallium arsenide.
Though germanium was used extensively in the early years of semiconductor technology, it
is unstable at high temperatures, so silicon became more widely used.
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 into free electrons.
When valence electrons jump to the conduction band, they leave vacancies in the valence
band.
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 holes to the intrinsic semiconductor material.
The number of free electrons in an intrinsic semiconductor material with four valence electrons,
such as silicon, is increased in the doping process by adding pentavalent impurity atoms,
or atoms with five valence electrons such as arsenic, phosphorus, bismuth, or antimony.[a]
To visualize this, let’s examine a phosphorus atom covalently bonded with four adjacent
silicon atoms.
As we can see, only four valence electrons of the phosphorus atom are used to form covalent
bonds with the silicon atoms, leaving an extra electron.
At lower temperatures, that extra electron stays with the impurity or donor atom but
when the temperature increases, it becomes a free electron.
Once the electron has moved into the conduction band, the dopant atom is now positively charged
as there are more protons in the nucleus than electrons in orbit.
Finally, while the electron is free to move about, the atom itself is fixed in place and
will not move, even with an applied voltage across the material.
[b]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[c] with four valence electrons, such as silicon,
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 attempt to form covalent bonds with the
four adjacent silicon atoms.[d] In this case, at lower temperatures, when the boron covalently
bonds with its neighbors, the fourth silicon atom doesn’t make a bond.
As the temperature increases, an adjacent silicon atom donates an electron to complete
the fourth bond between the boron and the silicon atoms.
There is now a hole where the adjacent silicon atom gave up its electron and the boron atom
is negatively charged as it has an additional electron.
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.
When you dope an intrinsic semiconductor p-type and then, directly adjacent to that, n-type,
the boundary where the p-type and n-type doped material touches is known as a p-n junction.
[e]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.[f] We also briefly mentioned how different semiconductor
devices were created based on the p-n junction.
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