THE OLDEST METHOD

Push dopant in with heat

1000+ degrees C, minutes to hours

Diffusion doping heats the wafer with a dopant source at its surface. Dopant atoms wander into the crystal the way a drop of ink spreads through water, moving from high concentration toward low. It was the standard doping method for decades, and it is still used where fine depth control does not matter.

Given enough heat and time, dopant atoms find their own way into the crystal.

DOPANT SOURCES

Three ways to supply the surface

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Solid source

A disc of dopant-glass sits face to face with the wafer in a furnace

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Liquid source

A carrier gas bubbles through a liquid dopant compound

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Gas source

A gas such as phosphine or diborane flows over the wafer directly

All three do the same job: keep the surface supplied while heat does the work.

TWO STEPS

Predeposition, then drive-in

Predeposition A shorter, lower-temperature step that loads a fixed dose at the surface

Drive-in A longer, hotter step that pushes that dose deeper and shapes its profile

Why split it Predep controls the total dose; drive-in controls the final depth

Two separate steps give independent control of dose and depth.

THE PROFILE

Concentration fades smoothly with depth

Diffusion leaves a graded profile: concentration peaks at the surface and falls off smoothly with depth. The junction depth is where the falling dopant concentration crosses the silicon's original background doping.

A diffused junction has no sharp edge: it's a gradient, not a step.

WHY IT FADED

Heat spreads sideways too

same heat, same spread, every direction

Diffusion pushes dopant sideways under a mask edge by roughly as much as it pushes down, which was fine when features were microns wide. At the width of a modern transistor gate, that sideways spread eats into space the design can't spare, which is why ion implantation replaced diffusion for defining transistors.

Diffusion doesn't know which direction is 'down': it spreads every way at once.

UNIT 20 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the oldest way to move dopant into silicon: predeposition and drive-in.