Semiconductor devices work because material regions are arranged into structures.
The same silicon can behave very differently depending on doping, geometry, insulation, and applied voltage.
A Junction Is a Boundary With Behavior
When differently doped semiconductor regions meet, the boundary is not just a line on a diagram.
Charge distribution near the boundary can create an internal barrier. That barrier can make it harder or easier for carriers to cross depending on the applied voltage.
This is why device diagrams matter. The shape and arrangement of regions are part of the behavior.
Barriers Make Control Possible
A useful transistor must have a way to control carrier movement.
Control can come from:
- changing a barrier
- creating a conductive channel
- removing or reducing carrier movement
- steering current through one path instead of another
The exact mechanism differs between transistor families. A BJT relies heavily on junction behavior and carrier injection. A MOSFET relies on an electric field controlling a channel near an insulated gate.
Why This Series Focuses on MOSFETs
Modern digital integrated circuits are dominated by MOSFET-based CMOS logic.
For a developer trying to understand the physical basis of computation, the most useful path is:
- semiconductor regions can be engineered
- a MOSFET gate can control a channel
- complementary MOSFETs can pull outputs high or low
- those output ranges become digital logic
BJT physics is important historically and practically, but it is not the main path to CMOS logic.
Boundary
Do not treat a transistor as a generic chunk of material.
The durable model is:
Semiconductor structure creates barriers and controllable regions. A transistor works because the structure lets one electrical condition control carrier movement somewhere else.