Field Effect Transistors
Normally, semiconductors don't have many free electrons. Since electric current relies on those free electrons, the amount of current that can travel through an isolated semiconductor is negligible. Put a slab of silicon, for example, in the middle of a circuit and it will stop the current in its tracks.
But things change if you put an electric field near that silicon. Bringing a positively-charged metal plate up close will attract the negatively-charged electrons from inside the body of the semiconductor. These electrons stream to the surfacesuddenly there is an abundance of free electrons creating a pathway for the blocked current.
Where once there was a stopped passage, electricity can now flow. By controlling the voltage on the metal plate, you can easily flip the current through the semiconductor on and off.
On top of that, the current traveling through the semiconductor will be an exact replica of the signal sent to the metal plateonly larger. The transistor has amplified the original signal. Since this transistor depends on an electric field, it's known as a Field Effect transistor.
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