Define potential barriers and potential depletion region for a pn junction diode. Explain how thickness of a depletion region change when pn junction is forward bias.
A potential barrier is simply a resistant force to something. For example if you wanted to throw a ball in to space, you would need to overcome the Gravitational potential of the earth- the gravitational field can be thought of as a potential barrier of the ball. Another example is driving a car through a brick wall, the wall is acting as a potential barrier meaning that it will resist the motion of the car when it the car collides with it.
When a P-N junction is formed, the majority carriers at both sides near the junction diffuse to the opposite side. That is, electrons from the N-side diffuse towards the P-side and holes from the P-side diffuse towards the N-side. This diffusion leaves behind ionized donors on the N-side and ionized acceptors on the P-side. The ionized donors and acceptors are immobile, as they are bonded to the surrounding atoms of the crystal lattice. This layer of immobile ions is known as depletion region or space charge region. That is, there are no free electrons or holes in this region. The thickness of the depletion region is of the order of one-tenth of a micrometer.
Due to this space charge region, an electric field directed from positive charge towards negative charge develops. This field gives rise to the flow of minority carriers. That is, holes in the N-side are attracted towards the P-side and electrons in the P-side are attracted towards the N-side. The motion of charge carriers due to the electric field is known as drift.
A pn junction is said to be forward biased if the positive terminal of the external battery is connected to p side and the negative terminal is the n side.
In forward biasing, the applied voltage mostly drops across the depletion region and the voltage drop across the p side and n side of the pn junction is negligibly small. This is due to the fact that resistance of depletion region is very high as it has no free charge carriers.
The majority carriers , electrons in the n region are repelled by negative terminal of the battery and move towards the junction. Similarly holes from p region move towards junction. The positive terminal of p region attracts electrons from n region and vice versa. As a result diffusion of majority carriers takes place across the junction which makes the junction thinner.
Thus the electric current is called forward current and it is due to majority carriers.
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