Grade 11 → Electronics and Communication ↓
Semiconductors
In the world of physics and electronics, semiconductors play a vital role in the functioning of myriad devices, from simple diodes to complex computer chips. Understanding semiconductors is important for anyone interested in electronics, as they are the backbone of modern technology.
What are semiconductors?
Semiconductors are materials whose electrical conductivity lies between conductors and insulators. This means they can conduct electricity under some conditions but not under others. This property makes them invaluable for controlling electrical currents in electronic devices.
Intrinsic semiconductors
Intrinsic semiconductors are pure forms of semiconductor materials. The most common examples include silicon (Si
) and germanium (Ge
). In these materials, the number of charges is typically low and there are no significant impurities.
An intrinsic semiconductor has an equal number of electrons and holes. A "hole" is simply the absence of an electron and acts as a positive charge carrier. When energy is applied, it can excite an electron to jump from the valence band to the conduction band, leaving a hole behind:
This transition contributes to electrical conductivity.
Extrinsic semiconductors
When impurities are added to intrinsic semiconductors, they become extrinsic semiconductors. This process is called doping and it significantly increases the number of charge carriers in the material, increasing its conductivity.
n-type semiconductor
N-type semiconductors are created by adding an element with extra electrons to the semiconductor material. For example, combining silicon with phosphorus (which has five valence electrons) creates extra free electrons:
The extra electrons act as negative charge carriers, hence the name "n-type".
p-type semiconductor
P-type semiconductors are formed when an element with fewer electrons is added. For example, when silicon is combined with boron, which has three valence electrons, "holes" or positive charge carriers are created:
The lack of electrons creates "holes" that can move around, making the material a P-type semiconductor.
PN junction
A PN junction is formed by placing n-type and p-type semiconductors together. This junction is important for diode and transistor operation. Upon joining, electrons from the n-type region move to fill the holes in the p-type region, and a depletion region with a built-in electric field is formed:
This arrangement allows current to flow easily in one direction, but not in the other, thus acting like a diode.
Applications of semiconductors
Semiconductors are essential components in a variety of electronic devices due to their unique properties. Here are some examples:
Diode
A diode is perhaps the simplest semiconductor device. Its main purpose is to allow current to flow in one direction and block it in the opposite direction. This is due to the PN junction which allows current only when the P-side is at a higher voltage than the N-side:
Transistor
Transistors are more complex and are used to amplify or switch signals. They can control a large current with a small current input and are the building blocks for all modern electronic circuits. There are different types of transistors, including bipolar junction transistors (BJTs) and field effect transistors (FETs).
Bipolar junction transistors (BJTs)
A BJT consists of three layers of semiconductor material that form two PN junctions. The three parts are called the emitter, base, and collector:
There are two types of BJTs: NPN and PNP transistors. The NPN type uses electrons as charge carriers while the PNP type uses holes.
Field effect transistors (FETs)
FETs use an electric field to control the size of the channel, and thus the conductivity, of one type of charge carrier in a semiconductor material. Different types include:
- JFET (Junction FET)
- MOSFET (metal–oxide–semiconductor FET)
Conductivity in semiconductors
The conductivity of a semiconductor can be controlled by several factors:
- Doping: Adding impurities to increase charge carriers.
- Temperature: Higher temperatures can increase the energy in the electrons, which increases conductivity.
For example, the electrical conductivity σ
can be expressed as:
σ = q * n * μ
Where:
q
= charge of the electronn
= number of charge carriersμ
= mobility of charge carriers
Conclusion
Semiconductors are the cornerstone of modern electronics, from the simplest circuits to the most complex computing systems. Their ability to conduct electricity under certain conditions makes them versatile and indispensable in our technological society. Understanding their properties and how they can be manipulated opens up the world of electronics and communications to many applications and innovations.