BUILDING BOTH FLAVORS

CMOS needs NMOS and PMOS side by side

n-well and p-well, on one substrate

Modern chips use CMOS, meaning both n-channel and p-channel transistors on the same wafer. Since a transistor's well must be the opposite type of its source and drain, a fab implants separate n-wells and p-wells into the substrate before building each transistor.

Wells let one wafer host both flavors of transistor at once.

A COMPLETE CROSS-SECTION

Every doped region in one device

Inside a p-well, two n-plus source and drain regions sit apart, with the gate stack on top defining the channel between them. Thin LDD implants sit just inside the source and drain edges, tucked under the gate.

Every region here is a separate, precisely placed implant.

EXTRA IMPLANTS

Why one source and drain isn't enough

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LDD

A lightly-doped drain implant softens the electric field at the drain edge

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Halo

A tilted implant of the well's own type curls under the edge of the source and drain

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The goal

Both control short-channel effects that get worse as the gate shrinks

A short transistor needs more than a plain source and drain to behave well.

BUILD ORDER

Doping fits a fixed sequence

1. Wells n-well and p-well implants, each masked separately

2. Gate stack Oxide and poly (or metal) gate patterned first, so it can act as its own mask

3. LDD A light implant self-aligned to the gate edge

4. Spacer, then source/drain A sidewall spacer sets back the heavier implant from the gate

The gate goes down before the source and drain, so it masks its own channel.

UNIT 23 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered wells, source and drain, and the extra implants a short transistor needs.