The control terminal. Its voltage decides how much current can flow
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Source
Where the current-carrying charges enter the channel
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Drain
Where those charges leave the channel
A fourth connection, the body, also exists. We'll meet it in lesson 3.
THREE JOBS
What a transistor can be
SwitchGate high means on, gate low means off: the basis of digital logic
AmplifierA small change in gate voltage makes a larger change in current
SensorAnything that changes the gate's effective voltage changes the current
The sensor job is why this course exists.
FIELD EFFECT
Current controlled by a field
field ā current
In a field-effect transistor, the gate's electric field, not a current, controls how many charges sit in the channel. The gate is insulated by an oxide, so almost no DC current flows into it. That makes it cheap to hold in a state and a very sensitive voltage probe.
The insulated gate is what makes a MOSFET both efficient and a good sensor.
THE FET FAMILY
Where the MOSFET fits
MOSFETInsulated gate: an oxide separates the gate from the channel
JFETThe gate is a reverse-biased pān junction instead of an oxide
MESFET, HEMTMetal-semiconductor gates, used mostly in high-frequency circuits
BJTNot a FET at all: a small input current controls a larger one
We focus on the MOSFET, because its oxide surface is what sensing builds on.
UNIT 1 STUDY COMPLETE
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Ready for the Fab Challenge?
You've covered what a transistor does, and why the field-effect version dominates.