Let's go inside an Intel 4004 microprocessor to see an individual transistor

To make this transistor, first oxide then nitride layers are grown

A shadow mask then exposes a photographic emulsion

Exposed nitride is then etched away

Oxygen reacts with the Si to grow more oxide

By heating in silane, polycrystal Si is then grown

A stripe is exposed in a second photographic emulsion

Exposed polycrystal Si is etched away leaving the transistor gate

Phosphorus ions then penetrate thin oxide to create the source and drain

Heating in oxygen grows an insulating coat on the gate

A third photographic emulsion defines windows above the source and drain