SE1x_2022_Week_2_7_Semiconductor_Junction_The_Solar_Cell-video

Summary

This video explains the operation of a p-n junction, focusing on how it behaves under forward and reverse bias conditions, as well as when exposed to light. It discusses the effects of biasing on the depletion zone, current density, and the dominance of diffusion and drift currents. The lecture also introduces the concept of a solar cell, explaining how light absorption generates electron-hole pairs, leading to current generation under a short-circuit condition and voltage under an open-circuit condition. The video provides a comprehensive overview of the photovoltaic effect, emphasizing the key steps in the process.

Takeaways

• 😀 A pn junction in thermal equilibrium has balanced diffusion and drift over the depletion zone.
• 😀 Applying a forward bias to a pn junction reduces the effective electric field across the depletion zone, making diffusion more dominant than drift.
• 😀 Under forward bias, the width of the depletion zone becomes narrower, increasing the diffusion of charge carriers.
• 😀 In forward bias, the pn junction generates a net current, where electrons and holes move from their respective regions and create current flow.
• 😀 In reverse bias, the electric field across the depletion zone increases, causing the depletion zone to widen, reducing diffusion and increasing drift.
• 😀 Reverse bias results in a very small current due to the dominance of drift current over diffusion current, which is characteristic of diodes.
• 😀 Shining light on a pn junction generates electron-hole pairs, which increases the density of minority charge carriers, enhancing the drift current.
• 😀 The current generated in an illuminated pn junction can be seen in the form of short-circuit current, which occurs when the junction is part of a closed circuit.
• 😀 Under open circuit conditions, the pn junction builds up an electric field, creating a voltage known as the open-circuit voltage, instead of generating current.
• 😀 The photovoltaic effect involves three essential steps: 1) generation of electron-hole pairs due to light, 2) separation of electrons and holes at the depletion zone, and 3) collection of charge carriers at the terminals.
• 😀 In the following weeks, students will explore how to determine solar cell performance using external parameters like short-circuit current density and open-circuit voltage.

Q & A

What happens when a pn junction is in thermal equilibrium in the dark?

In thermal equilibrium, the diffusion and drift currents across the depletion zone are balanced. This results in no net current flow through the pn junction.

How does applying forward bias affect the pn junction?

Applying forward bias causes the p-region to become more positive and the n-region more negative. This reduces the electric field across the depletion zone, narrowing it, which in turn increases the diffusion of charge carriers over drift, leading to a net current.

What is the role of the depletion zone in a pn junction?

The depletion zone is where the built-in electric field is located, and it is the region with the highest resistivity. The electric field in this zone controls the movement of charge carriers and is influenced by external bias.

How do forward bias and reverse bias influence the width of the depletion zone?

Under forward bias, the depletion zone narrows due to the opposing electric field from the external bias, which reduces the electric field. Under reverse bias, the depletion zone widens as the electric field from the external bias adds to the built-in field.

What is the effect of illumination on the pn junction?

Illumination of the pn junction generates electron-hole pairs by photon absorption. This increases the density of minority charge carriers, which enhances drift current, and leads to the generation of a net current.

How does a solar cell work under illumination?

A solar cell operates by generating electron-hole pairs when illuminated. The movement of these charge carriers, influenced by the electric field in the depletion zone, leads to current generation in an external circuit, particularly under short-circuit conditions.

What is the difference between forward bias and reverse bias in terms of current flow?

Under forward bias, diffusion current is dominant, allowing significant current to flow through the pn junction. Under reverse bias, drift current is dominant but much smaller, resulting in a very low current, essentially blocking the flow of current.

What is meant by the quasi Fermi levels in a pn junction under forward bias?

Quasi Fermi levels refer to the separate energy levels for electrons and holes in the depletion zone when the system is not in equilibrium due to forward bias. These levels represent the populations of electrons and holes and are split by an amount proportional to the applied bias voltage.

What happens to the electric field and current density under reverse bias?

Under reverse bias, the electric field across the depletion zone increases, which widens the depletion zone. This reduces the diffusion current density and increases the drift current, but the total current is extremely small, effectively blocking the flow of current.

What is the open circuit voltage in a solar cell?

The open circuit voltage is the voltage that builds up across the solar cell when it is illuminated but not connected to an external circuit. This voltage is created due to the accumulation of charge carriers in the p- and n-regions, creating an electric field that opposes further drift current.