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Understanding Band Gap and Semiconductor Current Carriers in Electronics
Learn about band gap and semiconductor current carriers in this intermediate electronics discussion. Explore how current is produced in semiconductors through the concept of band gaps and the movement of valence electrons to the conduction band.
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Video Transcript
We are going to talk about bandgap 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 will 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.
The difference in energy between the valence band and the conduction band is called band
gap.
It is the amount of energy a valence electrode can produce.
must possess so that it can jump from a 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 why a material may have poor conductivity. Let's try to examine the
energy diagram of 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. This is why it doesn't conduct current. The band gap in a semiconductor is
smaller compared to an insulator and allows a valence electron in the valence band to
jump into the conduction band if it receives external energy. In a conductor, like copper,
there is no band gap. Actually, the conduction band and valence band overlap, which means
the electrons can freely move into the conduction band. This is why you may hear the electrons
and metal referred to as a sea of electrons. They're just floating around.
Now that we know more about bandgap let's discuss the two types of
current carriers in a semiconductor, free electrons and holes, and see how they produce
currents 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.
At room temperature, intrinsic silicon crystal gains enough heat energy that enables some
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.
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 towards the positive end and this movement produces current in the material.
This type of current is called electron 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.
