In this video, we are going to discuss about band gap and semiconductor current carriers,
which will help us understand how current is produced in a semiconductor.
If we recall some basic topics in chemistry, we’ll remember that all atoms consist of
neutrons, protons, and electrons, except for a normal hydrogen atom which doesn’t have
a neutron.
Using the Bohr model, we can visualize that an atom has a central nucleus consisting of
protons and neutrons that is surrounded by orbiting electrons.
The orbits surrounding the nucleus are grouped into energy levels known as shells and the
outermost shell is called the valence shell.
The valence shell of an atom represents a band of energy levels, which is why it’s
also called a “valence band” and valence electrons are confined to that band.
When a valence electron gains enough energy from an external source, it can escape from
the valence band and goes to the conduction band.
Band Gap The difference in energy between the valence
band and the conduction band is called “band gap”.
It is the amount of energy a valence electron must possess so that it can jump from the
valence band to the conduction band, wherein the electron is free to move throughout the
material.
If the band gap is really big, electrons will have a hard time jumping to the conduction
band, which is the reason of material’s poor conductivity.
Energy Diagrams Let’s try to examine the energy diagram
of the three types of materials used in electronics and discuss the conductivity of each material
based on their band gap.
As we can see, the band gap between the valence band and conduction band in an insulator is
really big.
That is why it doesn’t conduct current.
The band gap in a semiconductor is smaller compared to an insulator and allows valence
electrons in the valence band to jump into the conduction band if it receives external
energy.
In a conductor, like copper, there’s no band gap.
Actually, the conduction band and valence band overlaps, which means that electrons
can freely move into the conduction band.
This is why you may hear the electrons in metal referred to as a “sea of electrons”
- they’re just floating around.
Current Carriers Now that we know more about band gap, let’s
discuss the two types of current carriers in a semiconductor, free electrons and holes,
and see how they produce current in a semiconductor.
Atoms may combine to form a solid crystalline material through covalent bonding.
For example, a silicon atom covalently bonds with four adjacent silicon atoms to form an
intrinsic silicon crystal.
Intrinsic because it doesn’t contain impurities and a crystal because there is a pattern in
how the atoms are connected.
Electron Current At room temperature, intrinsic silicon crystal
gains enough heat energy that enables some of the valence electrons to jump into the
conduction band, becoming free electrons.
When this happens, vacancies are left in the valence band within the crystal.
These vacancies are known as holes.
Now, if we put a voltage source across an intrinsic silicon material, the thermally-generated
free electrons in the conduction band will be attracted to the positive end of the voltage
source.
They will move toward the positive end and this movement produces current in the material.
This type of current is called electron current.
Hole Current While electron current happens in the conduction
band, the other type of current, hole current, happens in the valence band.
Remember that as valence electrons jumped into the conduction band, vacancies or holes
are left in the valence band.
Electrons that remain in the valence band can move into a nearby hole when it receives
a small amount of energy.
This movement produces a current in the valence band called hole current.
Though the current is produced by valence electrons moving into a nearby hole, it is
called hole current so that it won’t be confused with the electron current produced
in the conduction band.
In this video, we learned about band gap and the current carriers in a semiconductor.
We examined the energy diagram of the three types of materials used in electronics, insulators,
semiconductors, and conductors and briefly discussed their conductivities.
We also talked about the two types of semiconductor current carriers, free electrons and holes,
and mentioned how they produce current in a semiconductor material.
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