What Are Mobility and Current Density in Semiconductors?

TL;DR

Mobility in semiconductors refers to the drift velocity of electrons per unit electric field and is directly proportional to electric field strength. Electrons typically have greater mobility than holes. Conductivity is the reciprocal of resistivity, with the SI unit for resistivity being ohm-meter, while drift and diffusion currents coexist within semiconductor materials.

Transcript

hello friends in today's class we'll be studying about current density and mobility well current density and mobility forms the fundamental principles of how current transmission in semiconductors takes place let's start with mobility mobility when an electric field is applied across a solid it accelerates the electrons in the direction of the elec... Read More

Key Insights

• 🏑 Mobility represents the drift velocity of electrons per unit electric field in semiconductors.
• 🕳️ The mobility of electrons is greater than the mobility of holes in semiconductors.
• 🤒 Conductivity is the reciprocal of resistivity, and the SI unit for resistivity is ohm-meter.
• 🕳️ Drift current occurs when an external electric field is applied, causing electrons and holes to drift in opposite directions.
• 🕳️ Diffusion current is observed in pn junctions and arises due to the diffusion of electrons and holes through the junction.
• 💐 Drift current and diffusion current coexist in semiconductors and contribute to the total current flowing through the material.
• ❓ Resistivity determines a material's opposition to electrical current, while conductivity measures its ability to conduct current.

Questions & Answers

Q: What is mobility in semiconductors?

Mobility in semiconductors refers to the drift velocity of electrons per unit electric field. It determines how easily electrons can move through the material when an electric field is applied.

Q: How is mobility related to the drift velocity and electric field?

Drift velocity is directly proportional to the electric field strength. The constant of proportionality, known as mobility, represents how fast electrons drift in response to the electric field.

Q: What is the relationship between the mobility of electrons and holes in semiconductors?

In general, the mobility of electrons is almost equal to three times the mobility of holes in semiconductors. This means that electrons can move more easily through the material compared to holes.

Q: What is resistivity and how is it related to conductivity?

Resistivity is a property of a material that determines its opposition to electrical current. It is the reciprocal of conductivity, which is a measure of how well a material can conduct electrical current.

Q: How is conductivity calculated?

Conductivity can be calculated by dividing the product of length (l) and area (a) of a conductor by its resistance (r). It is also given by the formula sigma = 1/rho, where sigma is conductivity and rho is resistivity.

Q: How is drift current different from diffusion current?

Drift current occurs when an external electric field is applied, causing electrons and holes to drift in opposite directions. Diffusion current, on the other hand, is observed in pn junctions and arises due to the diffusion of electrons and holes through the junction.

Q: What causes diffusion current in a pn junction?

When a pn junction is forward biased, the positive terminal attracts holes from the p-side and the negative terminal attracts electrons from the n-side. This diffusion of carriers across the junction constitutes diffusion current.

Q: Do drift current and diffusion current coexist in semiconductors?

Yes, both drift current and diffusion current coexist in semiconductors. They contribute to the total current flowing through the material and need to be considered together in calculations.

Summary & Key Takeaways

• Mobility is the drift velocity of electrons per unit electric field and is proportional to the electric field strength.

• The mobility of electrons is always greater than the mobility of holes in semiconductors.

• Conductivity is the reciprocal of resistivity, and the SI unit for resistivity is ohm-meter.