In this lab you will be using Light-Emitting Diodes (LEDs) as a source of photon

Question

In this lab you will be using Light-Emitting Diodes (LEDs) as a source of photons^A diode is a semi-conductor device that usually only passes current in one direction, a very useful property in circuits. Semiconductors come in several varieties In a diode there are two adjacent materials, one that has a slight excess of free electrons (known as n-type, because it has more negative carriers), and one that has extra "holes tor electrons to go (known as "p-type", because it has a deficit of free electrons, and therefore has net positive carriers). (You can think of a "hole" like an empty spot on a crowded freeway - one car can move into it, but that leaves the gap somewhere else. A "hole" has a positive charge of 1.6 times 10^-19 Coulombs, just the opposite of an electron. Both electrons and holes can carry current.] In a diode with no voltage across it (as shown at right), there is a small region, known as the depletion layer, where the extra electrons of the n-type material leak over to "fill in" the holes of the p-type material. In this region there are essentially no free charge carriers, and therefore, no possibility to conduct a current through the diode. What happens when we put a voltage across the diode? Considering the diagrams below, circle your answer to what will happen to the thickness of the depletion layer for each orientation of the battery.

Explanation / Answer

Depletion Region:

The thin layer region on both sides of a p-n junction which has immobile ions and is devoid of any charge carrier is called depletion region or depletion layer. In this layer some holes have crossed over to n-type semiconductor and some electrons into to the p-type semiconductor. Thus a voltage barrier is built up for further hole movement from p-type to n-type and electrons from n-type to p-type.

Forward Bias:

A p-n junction is said to be forward biased if the positive terminal of the external battery is connected to p-side and the negative terminal to the n-side of p-n junction.In forward biasing the potential barrier for hole movement(from p-type to n-type)and electron movement from (from n-type to p-type) reduces and as a result the diffusion of holes and electrons across the junction increases.It makes the depletion layer thin and as such the junction diode offers low resistance during forward bias.

Reverse Bias:

A p-n juction is said to be reverse biased if the positive terminal of the battery is connected to n-side and the negative terminal to the p-side of the p-n junction.

In reverse biasing the applied voltage of battery mostly drops across the depletion region of the p-n junction and its direction of voltage is same as that of the potential barrier.Due to it, the reverse bias voltage supports the potential barrier.

i..e during reverse biasing the applied dc voltage aids barrier enhancement across the junction. Due to this the potential drop across the junction increases and as a result the diffusion of holes and electrons across the junction decreases. It makes the depletion layer thick and the juction diode offers high resistance during reverse bias.

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