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" for 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

In a pn junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction.After joining p-type and n-type semiconductors, electrons from the n region near the pn interface tend to diffuse into the p region leaving behind positively charged ions in n region and being recombined with holes so leaving negative charged ions in p region. Likewise, holes from the p-type region near the p–n interface begin to diffuse into the n-type region, leaving behind negative charged ion in p region and being recombined with electrons leaving positive ions in n region. The regions nearby the p–n interfaces lose their neutrality and mobile carriers, forming the depletion layer.

In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and neutralize the depletion zone, reduces its width. The positive potential applied to the p-type material repels the holes, while the negative potential applied to the n-type material repels the electrons. This lowers the barrier in potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin

In reverse bias when the p-type material is connected to the negative terminal of the power supply, the 'holes' in the p-type material are pulled away from the junction, leaving behind charged ions causing the width of the depletion region to increase. Likewise, because the n-type region is connected to the positive terminal, the electrons will also be pulled away from the junction, leaving behind charged ions causing the width of the depletion region to increase.

Left figure is in reverse bias so depletion layer thickness will increase

Right figure is in forward bias so depletion layer thickness will decrease

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