At its most basic, a transistor is a semiconductor device that is used to control or amplify an electrical current.
Transistors are divided into groups:
A bipolar junction transistor (BJT) is essentially two diodes combined together.
This is done by sandwiching a P-type semiconductor material between two N-type semiconductor materials to create an NPN transistor.
(The NPN type is by far the most common type of bipolar junction transistor.)
Alternately, sandwiching an N-type semiconductor material between two P-type semiconductor materials creates a PNP transistor.
A bipolar junction transistor has three leads: the collector, the emitter, and the base.
In NPN BJT's the current flows from collector to emitter.
In PNP BJT's, the current flows from emitter to collector.
Both NPN and PNP transistors are controlled by emitter-base current.
The schematic symbol for a NPN transistor.
The schematic symbol for a PNP transistor.
The line with the arrow on it is always the emitter.
The direction of the arrow lets you know whether you are dealing with an NPN transistor or a PNP transistor.
A little trick to help you remember:
NPN means Not Pointing iN.
The transistor functions like a faucet for an electric current.
The base acts as the control valve, limiting how much flow is allowed to pass.
Let's look at how this works in a simple circuit.
When the switch is closed, a path is created for current to flow from the signal source (VBE), across the base-emitter junction, to the ground.
Once a sufficient amount of current begins to flow through a P-N junction, (in this case the base-emitter junction), that junction resistance becomes effectively 0Ω. This is the same way that a diode works.
With the resistance of the base-emitter junction now reduced, current can now flow directly from the collector across to the emitter.
The current across the collector to the emitter is proportionally greater than the current that is flowing across the base to the emitter.
The ratio of the two current flows is dependant on the specific material that the transistor is made from.
When the switch is opened again, the current flow through the base-emitter junction stops and that junction reasserts its resistance. Current flow from the collector to the emitter is no longer possible.
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