Lab 4 and 5: PN junctions and Solar Cells

Summary

This video lecture provides a concise overview of PN junction diodes and their operation. It explains the basic concepts of diodes as electronic components that allow current to flow in one direction, akin to a plumbing valve. The lecture revisits the principles of p-type and n-type semiconductors, the formation of depletion regions, and the role of built-in electric fields. It also touches on forward and reverse biasing of diodes, leading to their rectification properties, which are crucial for various semiconductor devices. The lecture concludes with a brief mention of upcoming topics on solar cells.

Takeaways

• 📘 The lab focuses on experiments with PN junction diodes and solar cells, with a brief theory recap.
• 🧠 A diode is compared to a one-way valve in plumbing, allowing current to flow in one direction while blocking it in the opposite direction.
• ⚙️ A diode is made from p-type and n-type semiconductor materials, typically silicon, which are joined together to form a junction.
• 🔋 In intrinsic silicon, there are no free electrons, making it an insulator; doping silicon with trivalent or pentavalent atoms turns it into p-type or n-type silicon, respectively.
• ⚡ P-type silicon has an excess of holes (positive charge carriers), while n-type silicon has an excess of electrons (negative charge carriers).
• 🔄 When p-type and n-type silicon are joined, electrons and holes diffuse across the junction, creating a space charge region or depletion region where no free carriers exist.
• ⚖️ The built-in electric field created by the separation of charges in the depletion region prevents further diffusion, maintaining equilibrium.
• ➡️ Applying a forward bias (positive on p-side, negative on n-side) reduces the built-in electric field, allowing current to flow.
• ⬅️ Applying a reverse bias (negative on p-side, positive on n-side) strengthens the built-in electric field, blocking current flow.
• 📈 In the forward bias, the current increases exponentially with voltage, while in reverse bias, the current remains negligible and constant.

Q & A

What is the primary function of a diode?

A diode acts as an electronic switch, allowing current to flow in one direction (forward bias) and blocking current flow in the opposite direction (reverse bias).

What happens when a P-type and an N-type semiconductor material are brought into close contact?

When a P-type and an N-type semiconductor material are brought into close contact, a PN junction is formed. This junction creates a depletion region where the majority of free charge carriers (electrons and holes) are depleted due to diffusion of electrons from the N-side to the P-side and holes from the P-side to the N-side.

Why is silicon commonly used as a semiconductor material?

Silicon is commonly used as a semiconductor material because it is a group 4 element with four valence electrons, which allows for the formation of a perfect lattice structure. It can be easily doped to create P-type or N-type semiconductors by adding impurities like boron (trivalent) or phosphorus (pentavalent).

How does doping with a trivalent atom like boron create a P-type semiconductor?

Doping with a trivalent atom like boron creates a P-type semiconductor because boron has three valence electrons, which leaves an unpaired 'hole' when it bonds with neighboring silicon atoms. This hole can move through the lattice and act like a positively charged entity, thus creating P-type material.

What is the role of the built-in electric field in a PN junction?

The built-in electric field in a PN junction acts as a barrier to prevent further diffusion of charge carriers (electrons and holes). It is generated due to the separation of charges in the depletion region and helps maintain equilibrium in the system.

What is the effect of applying a forward bias to a PN junction diode?

Applying a forward bias to a PN junction diode reduces the barrier created by the built-in electric field, allowing current to flow more easily from the P-side to the N-side. This results in an exponential increase in current with increasing voltage.

How does reverse bias affect the current flow in a PN junction diode?

Applying a reverse bias to a PN junction diode strengthens the built-in electric field, which further prevents the flow of charge carriers. This results in a very small, almost negligible, and constant current flow that does not significantly change with voltage.

What is the significance of the depletion region in a PN junction?

The depletion region in a PN junction is significant because it is devoid of free charge carriers (electrons and holes). This region is crucial for the diode's rectification property, allowing current to flow in one direction and blocking it in the other.

What will be observed during the lab experiment with PN junction diodes?

During the lab experiment with PN junction diodes, students will observe the IV characteristics of the diode. In forward bias, they will see an exponential increase in current with voltage, while in reverse bias, the current will remain negligible and almost constant.

What is the difference between equilibrium and non-equilibrium conditions in a PN junction?

In equilibrium conditions, no external bias is applied, and the system is in a balanced state with a built-in electric field preventing further diffusion of charge carriers. In non-equilibrium conditions, an external bias is applied, which can either counteract (forward bias) or strengthen (reverse bias) the built-in electric field, affecting the flow of current.