Collector
The collector is the terminal and region of a bipolar junction transistor that collects the carriers sent across the base, delivering the large output current of the device. It is the opposite end of the three-layer sandwich from the emitter: where the emitter is built to inject carriers, the collector is built to gather them, withstand a high voltage, and get rid of heat. Whatever the control signal does at the base, the amplified result appears as current flowing through the collector.
The collector is physically the largest of the three regions and only moderately doped, and both facts follow from its job. Carriers that arrive from the base are swept in by an electric field and then travel on through the collector to its contact, so the region must be big enough to spread the current and dissipate heat. Doping it more lightly than the emitter keeps the depletion region on the collector side wide, which raises the voltage the junction can block before breakdown. Many power and integrated transistors add a heavily doped layer or a buried layer beneath a lighter one to get low series resistance and high voltage rating at once. The emitter is doped heavily, the base lightly, the collector in between; that ordering is what makes the transistor asymmetric, so swapping emitter and collector gives a device that works poorly, with much lower gain.
The base-collector junction is reverse-biased in normal amplifying operation, and that is what makes it a collector. An n-p-n transistor used as an amplifier has its collector held at a more positive voltage than its base, so the junction between them is blocking and would pass almost no current on its own. Carriers arriving from the emitter side are minority carriers in the base, and the reverse field at the junction sweeps them across as though they fell down a slope. Because the field does the collecting, the collector current depends very little on the collector voltage, which is why a transistor behaves like a current source from the collector's point of view, apart from the slight rise linked to the Early effect. The current is roughly the base current times the current gain, and nearly equal to the emitter current, differing from it only by the small base current.
Collector power dissipation, not current alone, usually sets the limit on a transistor. The heat produced is the voltage across the transistor multiplied by the current through it (P = VCE × IC), and nearly all of it is generated at the collector junction. Datasheets therefore list a maximum collector current, a maximum collector-emitter voltage and a maximum power dissipation, and the safe operating area graph shows the combinations that avoid damage, including the second-breakdown regions where current crowds into hot spots. A transistor driven fully on has a small voltage across it and wastes little, and one driven fully off carries no current and wastes none; the dangerous region is in between, which is why linear amplifiers and slow switching edges get hot. The rated power applies only if the case is kept cool, and the allowed dissipation falls as temperature rises.
Power transistors carry the heat out through the collector, and that has practical wiring consequences. In the metal-can TO-3 package, as used by the long-lived 2N3055, the case itself is the collector, and in the TO-220 package the metal tab bolted to a heat sink is also connected to the collector. Bolting that tab directly to a grounded chassis can therefore short the collector to ground, so builders insert an insulating pad and a plastic washer between the tab and the heat sink. Thermal design is a chain of resistances, from the junction to the case, to the heat sink and to the air, and the collector junction temperature has to stay below a rated limit, around 150 degrees Celsius for many silicon devices. Forgetting this chain is a common way to destroy an otherwise correct circuit.
The name collector survives in modern parts, but not always in the same form. Integrated bipolar transistors have their collector contacts on the top surface, with a buried layer underneath to reduce resistance, and in an open-collector output the collector is left unconnected inside the chip so that several outputs can share one line with an external pull-up resistor. The corresponding terminal in a MOS transistor is the drain, which plays an equivalent role but is controlled by a voltage on an insulated gate rather than by base current. Collector current still matters in audio output stages, power supplies, motor drivers and radio-frequency amplifiers, where bipolar and insulated-gate bipolar devices handle large currents.
Read the collector as the transistor's output terminal and heat exit: a moderately doped, reverse-biased region that gathers the carriers the base has allowed through, and that must carry both the amplified current and the waste heat that comes with it.