Conductivity in a semiconductor depends on available charge carriers and how easily they can move.
Doping is one way engineers change that behavior.
Charge Carriers
Current in a material is carried by mobile charge.
In semiconductor reasoning, the important carriers are:
- electrons, which carry negative charge
- holes, which behave like mobile positive charge positions
The details come from solid-state physics, but the first useful model is simple:
More available mobile carriers usually means an easier path for current.
Doping
Doping means adding small, controlled amounts of impurity atoms to a semiconductor.
The goal is not contamination by accident. The goal is controlled change in electrical behavior.
Doping can make regions behave as:
- n-type, where electrons are the main mobile carriers
- p-type, where holes are the main mobile carriers
Those regions can then be arranged so device behavior depends on voltage, fields, and junctions.
Why This Matters for Transistors
A transistor is not just a uniform block of material.
It has engineered regions:
- some regions provide carriers
- some regions block or shape movement
- some regions form a controllable path
- some terminals connect the device to the outside circuit
For a MOSFET, source and drain regions are doped so they can provide carriers. The channel region between them can be made conductive or non-conductive depending on the gate condition.
Boundary
Doping does not by itself make a transistor. Structure matters.
The durable model is:
Doping prepares semiconductor regions with different carrier behavior. Transistor action comes from arranging those regions so an electrical condition controls carrier movement.