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Question
The diffusion current in a p-n junction is
Options
from the n-side ot the p-side
from the p-side to the n-side
from the n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse-biased
from the p-side to the n-side if the junction is forward and in the opposite direction if it is reverse-biased.
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Solution
from the p-side to the n-side
When a p‒n junction is formed then because of the difference in the concentration of charge carriers in the two regions, electrons from the n region move to the p region and holes from the p region move to the n region. Since the direction of the current is always opposite to the motion of electron, the direction of the current is from the p side to the n side.
Similarly, when the junction is forward biassed, the positive terminal of the battery is connected to the p side of the p‒n junction and the negative terminal of the battery is connected to the n side of the p‒n junction. As a result, electrons in the n side of the p‒njunction are repelled by the negative terminal of the battery and they move to the p side, where the positive terminal of the battery attracts them. Similarly, holes from the p side of the p‒n junction are repelled by the positive terminal of the battery and they move to the n side, where the negative terminal of the battery attracts them. Thus, they give diffusion current from the p side to the n side across the p‒n junction.
In reverse biassing, there is no flow of majority carriers across the junction; hence, there is not diffusion current. Here, the flow of majority carriers is opposed by the applied voltage.
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