PART 4 • DOPING AND THE TRANSISTOR

Doping and the Transistor

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Diffusion

The oldest way to move dopant into silicon: heat and time

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Implantation

The modern way: fire dopant atoms in as a beam of ions

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The transistor

Wells, junctions, and a gate stack, put together

By the end of this part, you can explain how a bare silicon wafer becomes a working switch.

A CRYSTAL THAT BARELY CONDUCTS

The whole point of all that purity

9 nines pure, on purpose

Part 1 spent six units getting silicon to nine nines pure, because an impurity anywhere in the crystal is a defect you can't control. Doping flips that: once the crystal is flawless, a fab adds back a tiny, exact, deliberate amount of a different element, in only the spots it wants.

Purity first, so the impurities that follow are the only ones there.

TWO FLAVORS

Pick your carrier

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n-type

Phosphorus or arsenic donates a free electron to the lattice

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p-type

Boron accepts an electron, leaving a mobile hole behind

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Typical dose

As few as one dopant atom per 10,000 to 100,000 silicon atoms

A handful of dopant atoms per hundred thousand is enough to change how silicon conducts.

CARRIERS

Majority and minority

Majority carrier The one doping put there in abundance: electrons in n-type, holes in p-type

Minority carrier The opposite type, present in trace amounts

Mass action n times p is a constant, so more of one type means less of the other

Doping sets which carrier does almost all the conducting.

THE P-N JUNCTION

Where two doped regions meet

Put p-type and n-type silicon side by side and the boundary between them becomes a p-n junction. Mobile carriers near the boundary diffuse across and cancel out, leaving a depletion region with almost no free carriers and a built-in electric field.

A p-n junction blocks current one way and passes it the other: the basis of a diode, and of a transistor's source and drain.

FROM JUNCTION TO TRANSISTOR

A MOSFET is built from junctions

well + source + drain

A transistor's source and drain are doped regions of one type, sitting inside a well of the opposite type. That arrangement is two back-to-back junctions, held off by a gate. The rest of this part covers how a fab actually places that doping: diffusion, implantation, and the finished device.

Every transistor on the chip is, underneath, a careful arrangement of doped regions.

UNIT 19 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered why pure silicon needs controlled impurities, and what a p-n junction does.