Question

What is the effect of forward and reverse biasing on the depletion layer?

Answer 1

When we keep materials of p-type and n-type very close with each other there will be a junction between them that behaves differently from either side of the material alone. The electrons and holes from p-type and n-type of material are close to each other. There is a force that exists between negative electrons and positive holes according to coulomb’s law, when the junction is formed between these two a few electrons from the n-type will try and diffuse through the junction and then combines with the holes that are present in the p-side to form negative ions and leaves behind the positive ions in the n-side. This will result in the formation of the depletion layer.

Complete answer:
We can say a p-n junction is forward biased when we connect the positive terminal of the battery or voltage source to the p-side of the junction and the negative terminal to the n-side of the junction. In the p-n junction diode when it is forward biased we can see that the built-in electric field and the battery-induced electric field are in the opposite direction. Since the resultant field is lesser than the built-in voltage there will be depletion formed and will be thinner and less resistive.

Now for the reverse-biased p-n junction, the positive terminal is connected to the n-side of the diode and the negative terminal is connected to the p-side of the diode. In this case, the applied electric field and the built-in electric field are on the same side. Therefore we can see that the resultant field will be in the same direction as the applied electric field. This creates the depletion layer to be thicker and more resistive.

Note: Therefore we can say that the depletion layer is thicker in the reverse biased mode and thinner in the forward-biased mode. Forward bias will make the diode’s resistance decrease and the reverse bias will make the diode’s resistance increase. However, it is not possible to eradicate this depletion layer completely given how high the diode is forward biased. There will always be a very low resistance in the diode.