Question

How a p-n junction is formed? Explain in full detail indicating reverse bias and forward bias conditions?  

Can you explain ?

Answer 1

Formation of the P-N junction:-

Imagine a thin p-type silicon (p-Si) semiconductor wafer. Now, we add a pentavalent impurity to it. Due to this, a small part of the p-Si wafer is converted into an n-Si. Now, the wafer contains a p-region and an n-region with a metallurgical junction between the two.

(This is one of the simplest way to form p-n junction. Although there re other methods also.)

Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side.In a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n-type side, positive ions are exposed. On the p-type side, negative ions are exposed. An electric field forms between the positive ions in the n-type material and negative ions in the p-type material. This region is called the depletion region since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers.

The current caused by the diffusion of carriers across the junction is called a diffusion current. Minority carriers which reach the edge of the diffusion region are swept across it by the electric field in the depletion region. This current is called the drift current. In equilibrium, the net current from the device is zero. The electron drift current and the electron diffusion current exactly balance out .Similarly, the hole drift current and the hole diffusion current also balance each other out.

( I will include an hand-made diagram at the end which will give you an better idea of diffusion and drift current directions.)

Blockrans drift Electrons Diffusion 00 ODO TODO AC bepletion region diffusion Hole dult

Forward Bias:-

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, the potential barriers opposition will be overcome and current will start to flow. This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage.

It increases the diffusion current.While the diffusion current increases, the drift current remains essentially unchanged since it depends on the number of carriers generated within a diffusion length of the depletion region or in the depletion region itself. Since the depletion region is only reduced in width by a minor amount, the number of minority carriers swept across the junction is essentially unchanged.

Reverse Bias:-

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.

Hence the diffusion current decreases. As in forward bias, the drift current is limited by the number of minority carriers on either side of the p-n junction and is relatively unchanged by the increased electric field. A small increase in the drift current is experienced due to the small increase in the width of the depletion region.