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How does a diode allow current to flow in one direction only?

1 Answer

A diode is a semiconductor device that allows current to flow in one direction only, from its anode to its cathode. It acts as a one-way valve for electric current. This behavior is due to the way it is constructed and the unique properties of the semiconductor material it's made of.

The most common type of diode is the p-n junction diode, which consists of two regions of semiconductor material: the p-type and the n-type. The p-type semiconductor is doped (contaminated) with impurities that introduce positively charged "holes" as majority carriers, while the n-type semiconductor is doped with impurities that introduce negatively charged electrons as majority carriers.

When a p-n junction is formed, the free electrons in the n-region move towards the p-region, attracted by the positively charged holes. At the junction, some of these electrons combine with the holes, leaving behind positively charged ions in the n-region and negatively charged ions in the p-region. This creates a region near the junction called the depletion region, which lacks free charge carriers.

Now, here's how the diode operates in each direction:

Forward Bias:
When a positive voltage is applied to the anode (the p-region) and a negative voltage is applied to the cathode (the n-region), the potential difference across the diode reduces the width of the depletion region. This reduction in the depletion region allows current to flow easily through the diode. The majority carriers (electrons in the n-region and holes in the p-region) can now move across the junction and conduct electricity.

Reverse Bias:
Conversely, if a negative voltage is applied to the anode and a positive voltage is applied to the cathode, the potential difference increases the width of the depletion region. This widening of the depletion region prevents the majority carriers from crossing the junction, effectively blocking the flow of current. Only a very small reverse current, known as the leakage current, can flow under reverse bias, but it is typically negligible in well-designed diodes.

In summary, the diode's intrinsic property of creating a depletion region at the p-n junction is what allows it to function as a one-way valve for current flow, permitting current to pass easily in the forward direction (forward bias) but largely blocking it in the reverse direction (reverse bias). This characteristic is crucial for many electronic circuits and devices, such as rectifiers that convert alternating current (AC) to direct current (DC) and protect circuits from reverse current damage.
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