SILICON

A conductor you can tune

Si · 1.12 eV

Silicon's conductivity sits between a metal's and an insulator's. Its band gap is 1.12 eV, so at room temperature only about 10¹⁰ electrons per cm³ are free, compared with roughly 5×10²² silicon atoms per cm³. Adding impurities changes that by many orders of magnitude.

Pure silicon barely conducts. Doping makes it useful.

DOPING

Choose your carrier

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n-type

Donors such as phosphorus or arsenic add free electrons

➕

p-type

Acceptors such as boron leave mobile holes behind

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Majority and minority

The dominant carrier is the majority; the other is the minority

Typical doping runs from about 10¹⁵ to 10²⁰ atoms per cm³.

MASS ACTION

Fewer holes, more electrons

n · p = nᵢ²

In equilibrium the electron and hole concentrations multiply to a constant, nᵢ². In p-type silicon doped with 10¹⁶ boron atoms per cm³, holes number about 10¹⁶ per cm³, so electrons drop to (10¹⁰)² / 10¹⁶ = 10⁴ per cm³.

Minority carriers in the substrate are nearly absent, until a gate field pulls them in.

THE P–N JUNCTION

Where n meets p

Electrons diffuse from the n side into the p side, and holes go the other way. That leaves behind fixed, charged dopant ions, forming a depletion region with a built-in electric field. Reverse bias widens it; forward bias narrows it.

A reverse-biased junction blocks current, and that's how a MOSFET's source and drain stay isolated from the body.

MOVING CHARGE

Drift, diffusion, and mobility

Drift Charges pushed along by an electric field

Diffusion Charges spreading from high concentration to low

Mobility How easily a carrier moves: about 1400 cm²/V·s for bulk electrons, 450 for holes

Inversion layers Mobility at the surface is lower, often a few hundred

Electrons move faster than holes, which is why NMOS transistors beat PMOS in speed.

UNIT 2 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered electrons, holes, doping, and the p–n junction.