Semiconductors are one of revolutionizing invention in human history that opened doors for a whole new developed generation. A semiconductor is a material that has a conductivity in between conductors and insulators. It is playing a major role in electronic world.
Written by: R.D. Jayathri Madhushika Ranasinghe, (undergraduate), Department of Computer Engineering, University of Peradeniya,
Alessandro Volta is the first person who came up with the word, "semiconducting" in 1782. Then in 1833 Michael Faraday observed this effect of semi conductance.
In 1874 Karl Braun discovered the first semiconductor diode effect. He is the one who observed that current flows freely only in a one direction when there is a contact between two metals.
Next main discovery in semiconductor world was the first semiconductor device, called "cat whiskers" which was invented by Jagadis Chandra Bose. This cat whiskers were used for detecting radio waves and it was a point-contact semiconductor rectifier.
Then in 1947 at the Bell Labs of US, an amazing device was invented. The transistor! It was composed with semiconductor material and John Bardeen, Walter Brattain and William Shockley co-invented this transistor back then.
When we talk about semiconductors we mainly focus about silicon and germanium. These materials have a crystalline structure which has arranged orderly manner. In both silicon and germanium, they have four electrons orbiting in the outer shell. These electrons are called valency electrons.
When the atoms are packed, four valence electrons of each atom are shared with adjacent four atoms. So, these electron pairs are referred as covalent bonds. It is understandable that there are no free electrons left then pure silicon and germanium behave as perfect insulators. But here we cannot say that those are exactly insulators since at normal atmospheric temperature some of covalent bonds are broken, but as an approximation, it can be assumed that pure silicon and germanium are insulators.
Then, with the introduction of impurities to th pure silicon and germanium, we can produce semiconductor rectifiers.
The process of adding impurity atoms to semiconductor atoms is called doping.
Elements such as phosphorus, arsenic and antimony are called pentavalent since their each atom has five valence electrons. So, pentavalent atom can be represented as an ion that has a net positive charge of 5e.
When this type of atom is introduced to pure silicon crystal, it replaces a silicon atom. When it takes silicon's place it only needs 4 electrons to develop the covalent bonds and this provides a free electron. Also, these free electrons can be called as mobile electrons as well.
These pentavalent impurity atoms are called donors since they are responsible for donating free electrons for the crystal. So, a crystal which is doped with these types of impurities are called n-type semiconductors.
Another fact about doped crystal is that it is neutral because the donors provide fixed positively charge ions with the equal number of free electrons.
When the amount of impurity in the pure crystal is high, then the number of free electrons is also high and then conductivity of the semiconductor also becomes greater.
Elements such as indium, gallium, boron and aluminum are called trivalent since each atom of these elements has only three valence electrons. Also, these atoms are called acceptors. So pentavalent atom can be represented as an ion that has a net positive charge of 3e.
When boron is added to pure silicon, the boron is replacing the silicon atoms and it gives an incomplete valence bond because of the 3 valence electrons. Here this gives a vacancy and it is referred to a hole. This hole has the capability to attract a covalent electron from a nearby silicon atom. Once it attracts an electron it creates another hole. This movement of the hole happens randomly when there is no any external electric field.
Once the silicon or germanium crystals are doped with an impurity like above which are responsible for the formation of the holes, it is referred as p-type semiconductors.
The polarity is occurred as A is positive relative to B. Due to the electrical field across the semiconductor it modifies the earlier random movement of the electrons. Then electrons drift towards the positive electrode A.
With the p-type semiconductor, the negative ions are fixed in the crystal structure but the holes have the ability to move in the direction of the electrical field towards B. So, current can pass through p-type semiconductor material due to the drift of holes.
According to this above image of periodic table, semiconductor materials are in the element group 14. Also, the doping agent elements used for n-type semiconductors are in the group 15 while the doping agent elements of p-type semiconductors are in group 13.
The most commonly used semiconductor materials are silicon and germanium. But from these two, silicon is the mostly preferred material due to some reasons.
Those reasons can be mentioned as follows,
Even though usage of silicon is mostly preferred, germanium also have some specific great qualities. Germanium has the ability to offer a much better charge carrier mobility compared to the silicon.
There are many other materials like Gallium arsenide, Silicon carbide, Gallium nitride and Gallium phosphide as well.
Semiconductors play a major role in manufacturing various type of electronic devices like diodes, transistors and IC (integrated circuits).