collector
The amplified terminal on a bipolar junction transistor.
collector: where current flows out of a transistor
In a bipolar junction transistor (BJT), the collector is the terminal that receives the amplified output current. It sits opposite the emitter in the device structure: current enters through the base (the control terminal), gets amplified inside the transistor, and exits through the collector. The collector is typically doped more lightly than the emitter, and the region between emitter and collector (the base) is extremely thin, often less than 1 micrometer.
The collector needs to be biased at a higher potential than the emitter for an NPN transistor in active mode. This reverse bias on the collector-base junction is essential for the transistor to amplify; it widens the depletion region and allows minority carriers (electrons in an NPN device) to be swept through toward the collector terminal with high velocity. Remove this bias and amplification stops.
Common configurations and pitfalls
The collector current itself is not the controlling signal; it is proportional to the base current, with the ratio called the current gain or beta. In a saturated transistor, both junctions are forward biased and current gain drops sharply, making saturation useful for switching applications where you want the transistor fully on. In the cutoff state, the base current is zero and collector current approaches zero.
Heat dissipation at the collector is often the limiting factor in BJT circuits. High collector current combined with a high collector-emitter voltage produces significant power loss. This is why power transistors have large metal collector tabs and are often mounted to heat sinks. Exceeding the maximum collector current, maximum collector-emitter voltage, or maximum power dissipation causes permanent damage.
The term "collector" reflects the historical function: in early transistor designs, the collector region literally collected charge carriers that had been emitted from the emitter and transported through the base. The name stuck even though modern understanding treats it more precisely as a reverse-biased junction that extracts and amplifies current.