Forward Bias: Why does width of depletion layer decrease?

In summary, the depletion layer in a diode is formed by the movement of electrons and holes, creating a barrier potential that prevents current flow. When the diode is forward biased, the barrier potential decreases and allows for the movement of electrons again. However, this raises the question of why the width of the depletion layer decreases when more electrons are moving from the n side to the p side. The flaw in this understanding lies in the fact that to maintain a forward bias, a current must be supplied to the diode, which replaces the electrons that are moving from the n side to the p side. This means that the depletion width must be smaller in the forward bias case in order to maintain equilibrium.
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takando12
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The depletion layer is formed by the movement of electrons from the n to p side and holes from p to n side of the diode. The layer consists of positively charged donor ions close to the n side and negatively charged acceptor ions close to the p side. When in equilibrium,the barrier potential prevents this flow of carriers and the current is 0. Now when we forward bias a diode, the external voltage is in the opposite direction of the barrier potential and hence decreases the barrier potential. As a result the movement of electrons from n to p dominates once again. But keeping in mind that this flow was responsible for the creation of the depletion layer in the first place, why is the width of the same decreasing? If electrons are going to move from n to p because of reduction of the barrier potential , shouldn't it just create more donor ions and acceptor ions on either side and hence increase the width?
Please help me find the flaw in my understanding.
 
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In order to maintain a forward bias, you need to supply a current to the diode. So the electrons that move from the n side to the p side are replaced by new electrons flowing in from the terminal connected to the n-side. If no current is supplied, the equilibrium condition is quickly re-established. In order to establish the forward bias in the first place, you need to flow extra electrons into the n side to "cover" the fixed donor ions. It is easy to see that the depletion width must be smaller in the forward bias case by realizing that the integral of the electric field across the depletion region must equal the barrier potential difference.
 

FAQ: Forward Bias: Why does width of depletion layer decrease?

1. What is forward bias and how does it affect the depletion layer width?

Forward bias is a condition in a p-n junction where the p-side is connected to the positive terminal of a battery and the n-side is connected to the negative terminal. This results in a decrease in the width of the depletion layer, as the positive voltage on the p-side repels holes and attracts electrons, while the negative voltage on the n-side repels electrons and attracts holes, reducing the size of the depletion layer.

2. What is the depletion layer and why is it important in a p-n junction?

The depletion layer is a region of the p-n junction where there are no free charge carriers due to the diffusion of majority carriers from one side to the other. It is important because it acts as a barrier to the flow of current in the reverse bias direction, preventing the flow of majority carriers.

3. How does the depletion layer width affect the current flow in a p-n junction?

The depletion layer width affects the current flow by acting as a barrier to the flow of majority carriers. A narrower depletion layer means a smaller barrier and therefore, a higher current flow in the forward bias direction. In the reverse bias direction, a wider depletion layer means a larger barrier and therefore, a lower current flow.

4. What factors affect the width of the depletion layer in a p-n junction?

The width of the depletion layer is affected by the doping levels of the p and n regions, the built-in potential of the p-n junction, and the applied voltage in the forward bias direction. Higher doping levels result in a narrower depletion layer, while a higher built-in potential or applied voltage results in a wider depletion layer.

5. How does the decrease in depletion layer width affect the diode's characteristics?

The decrease in depletion layer width in a forward biased p-n junction results in a lower barrier for majority carriers, allowing them to flow more easily. This leads to an increase in the forward current of the diode, as well as a decrease in the forward voltage, making the diode more conductive. It also decreases the capacitance of the p-n junction, resulting in faster switching speeds.

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