Technological revolution of the last few decades would have been a daydream if not for the discovery of semiconductors. Semiconductors form a crucial component of almost everything from light bulbs to smartphones and satellites. The building blocks of LED TVs, LED screens in laptops and smartphones are Light-emitting Diodes (LEDs) that are made of doped semiconducting materials. Given the way digital revolution embraced these materials, the underlying science is worth discussing.
Band theory says, just like there are different energy levels for electrons in atoms, there are different energy bands in solids. An electron can have a range of energy values and hence the term band. There are two such bands valence band and conduction band; depending on the energy an electron possess, it can exist in either of the bands. The energy ranges in the conduction band are higher than those in the valence band. Based on the population of electrons in each of these bands the current conducting property of a solid varies. These populations are in turn regulated by energy difference, also called as energy gap, between the two bands. In a conductor like copper, the energy gap between the two bands is very low and hence there are lot of electrons in the conducting band, consequently the material readily conducts electricity. It is opposite in insulators hence they lack electrons in the conducting band and are devoid of conducting ability. Semiconductors fall in between the conductors and insulators with respect to the energy gap. Hence there is a definite number, though very less in magnitude, of electrons available in the conduction band. Triggers like temperature, doping, light etc. can increase the population of electrons in the conduction band of semi conductors.
Silicon (semiconductor) wafers can be doped with materials like Boron, Aluminium, Phosphorous, Arsenic etc. Materials like Boron, Aluminium lack electrons in the outer shell of their atoms and hence they consume electrons already present in the semiconductors and this results in the formation of positive charge (holes) in the system resulting in p-type semiconductors. Phosphorous and Arsenic donate electrons and increase the negative charge in the system resulting in n-type semiconductors. When a p-type semiconductor is placed on n-type semiconductor, due to the presence of excess electrons in n-type semiconductor a movement of charge is observed. This movement eventually stalls and results in the formation of a layer, at the junction of two materials, called as depletion region. Because there are already electrons moved to the p-type semiconductor’s depletion region, for a new electron to move from n-type semiconductor to p-type semiconductor requires energy. The energy required for one unit charge in n-type semiconductor to move across the depletion region is called as junction potential and junction formed is called as p-n junction.
LED is nothing but p-n junction which is taught rigorously during our A Level Physics tuition classes for JC syllabus. When the p-end of junction is connected to positive terminal and n-end to negative terminal of a battery; the electrons get enough energy to jump across the junction. When these electrons bind to the holes in p-type semiconductor, energy is released in the form of light and this phenomenon is called as electro-luminescence.
