SE1x_2022_Week_2_6_Semiconductor_Junction-video

Summary

This video lecture introduces the concept of a p-n junction, a fundamental component in solar cell technology. The lecture explains the behavior of charge carriers in p-type and n-type semiconductors, highlighting the effects of diffusion and drift in thermal equilibrium. It covers the formation of the space charge region and the electric field that governs carrier movement. The discussion also touches on the role of the Fermi level and band diagram, and introduces how applying voltage or illumination can alter the junction's equilibrium, leading to the creation of a solar cell. The working principle of solar cells is promised for further exploration in the next video.

Takeaways

• 😀 The p-n junction is the fundamental structure in most solar cells, formed by joining n-type and p-type semiconductor materials.
• 😀 In n-type material, the majority charge carriers are electrons, while in p-type material, the majority charge carriers are holes.
• 😀 At thermal equilibrium, the Fermi level in p-type is closer to the valence band, and in n-type, it is closer to the conduction band.
• 😀 The p-n junction creates a density gradient at the interface, with holes predominantly in the p-region and electrons in the n-region.
• 😀 Diffusion of charge carriers occurs due to these density gradients, but it is balanced by drift caused by an internal electric field.
• 😀 The space charge region (also called the depletion zone) forms when charge carriers recombine, leaving only fixed charges behind.
• 😀 In the space charge region, the p-side becomes negatively charged, and the n-side becomes positively charged, creating an electric field.
• 😀 This internal electric field drives the minority charge carriers in each region to move against the direction of diffusion.
• 😀 Diffusion current density is controlled by the majority charge carrier density, while drift current density is controlled by the minority charge carrier density.
• 😀 Under thermal equilibrium, no current flows through the p-n junction unless an external voltage or light (solar radiation) is applied, which disturbs the equilibrium and leads to current generation in solar cells.

Q & A

What is a p-n junction in a solar cell?

A p-n junction is formed by joining p-type and n-type semiconductor materials. It is the core mechanism in most solar cells, where the p-type material has an abundance of holes (positive charge carriers) and the n-type material has an abundance of electrons (negative charge carriers).

What happens when a p-type and an n-type material are combined to form a p-n junction?

When p-type and n-type materials are joined, the charge carriers (electrons and holes) begin to diffuse across the junction. This results in the formation of a depletion zone where mobile charge carriers are depleted, creating an electric field that opposes further diffusion.

Why is there a depletion zone at the p-n junction?

The depletion zone forms because the electrons from the n-region and holes from the p-region recombine near the junction. This recombination leaves behind fixed charges from the donor and acceptor atoms, creating an electric field and depleting mobile charge carriers in the region.

What are the two transport mechanisms that occur at a p-n junction?

The two transport mechanisms are diffusion and drift. Diffusion is driven by charge carrier density gradients, while drift is caused by the electric field in the depletion zone, which influences the movement of minority charge carriers.

What role does the electric field play in a p-n junction?

The electric field in the depletion zone forces the minority charge carriers to move in the opposite direction of the diffusion. For example, in the p-region, the electric field pushes minority electrons toward the n-region, and in the n-region, it pushes minority holes toward the p-region.

What is the significance of the Fermi level in a p-n junction?

The Fermi level represents the energy level at which the probability of finding an electron is 50%. In a p-n junction, the Fermi levels of the p-type and n-type materials align when in thermal equilibrium, causing the conduction and valence bands to bend across the junction.

How can the equilibrium in a p-n junction be disturbed?

The equilibrium can be disturbed by applying a bias voltage or by illuminating the junction. Applying a voltage can shift the charge carrier densities, while illumination increases the density of minority charge carriers, which is key to solar cell operation.

What happens when a solar cell is exposed to light?

When a solar cell is exposed to light, it increases the density of minority charge carriers in the p-n junction. This generates a photocurrent, as the increased carrier density causes the movement of charge carriers, which is harnessed as electrical energy.

What is the space charge region in a p-n junction?

The space charge region, also known as the depletion zone, is a region around the p-n interface where mobile charge carriers are absent. This area is charged due to the fixed charges from the donor and acceptor atoms and creates an internal electric field.

How does the concept of diffusion differ from drift in a p-n junction?

Diffusion is the movement of charge carriers due to density gradients, driven by the natural tendency of particles to spread out. Drift, on the other hand, is the movement of charge carriers under the influence of an electric field, which in the case of a p-n junction, results from the built-in field in the depletion zone.