Emitter
The emitter is the terminal of a bipolar junction transistor that injects charge carriers into the base, and it is the most heavily doped of the transistor's three regions. In an NPN transistor the emitter sends electrons across the forward-biased base-emitter junction, while in a PNP transistor it sends holes, and in both cases it supplies the stream of carriers that the collector will later gather.
The emitter is heavily doped so that almost all the current across its junction is the useful kind. When the base-emitter junction is forward biased, current has two parts: carriers from the emitter into the base, and carriers from the base back into the emitter. Only the first part reaches the collector, so the design goal is to make that part dominate, which is called a high emitter injection efficiency. Doping the emitter far more heavily than the base, commonly by a factor of 100 or more, achieves it. The base is also made thin, so most injected carriers diffuse across before recombining and arrive at the collector. The ratio of collector current to base current, the current gain, is therefore large, often 50 to several hundred in a small-signal silicon transistor. William Shockley worked out this reasoning in 1948 in his theory of the junction transistor, and it explains why the emitter and collector are not interchangeable even though they are both the same doping type.
The arrow in the circuit symbol sits on the emitter and shows the direction of conventional current through it. In an NPN symbol the arrow points away from the base, since conventional current leaves the emitter terminal while electrons actually move into the transistor from it; in a PNP symbol the arrow points toward the base. Because the arrow tracks the polarity of the emitter-base junction, it is also a quick way to read the device type: the arrow points in the direction a diode between base and emitter would conduct. The names go back to the first transistors: the point-contact transistor demonstrated at Bell Labs on 23 December 1947 had an emitter wire and a collector wire pressed onto a germanium slab, and the emitter of the junction transistor inherits that role with a cleaner physical structure.
The emitter follower is the circuit that uses the emitter as the output. In this arrangement, called common-collector, the signal goes into the base and is taken from the emitter, which rides about 0.7 volts below the base voltage and follows it with a voltage gain just under one. What it gains is impedance transformation: it presents a high input impedance and a low output impedance, so it can drive a load that would otherwise sag the signal, and it is a standard buffer stage in analog design. A resistor in the emitter lead also stabilizes the operating point, because as current rises the voltage across it rises and reduces the base-emitter voltage, a feedback that tames the transistor's strong temperature sensitivity. Emitter-coupled pairs, in which two transistors share a common emitter current source, form the differential amplifier and the old emitter-coupled logic family.
Modern bipolar technology still depends on clever emitter engineering. Herbert Kroemer proposed the wide-gap-emitter idea in the 1950s, and heterojunction bipolar transistors, with an emitter of larger band gap than the base, push injection efficiency even further; he shared the 2000 Nobel Prize in Physics in part for heterostructure work. Silicon-germanium heterojunction bipolar transistors, with a graded germanium profile in the base, sit in many radio-frequency chips for cellular phones, radar and fast optical links, usually beside CMOS in BiCMOS processes. Polysilicon emitters and emitter dimensions of tens of nanometers set the speed. Even in digital chips dominated by MOS transistors, parasitic bipolar structures form accidentally in CMOS, where an unwanted emitter-base-collector path can trigger latch-up if care is not taken.
A frequent misconception is that the emitter is the output, or that it always emits electrons. The word describes injection into the base, not the position in the signal chain: the emitter is the output only in a follower, while the collector is the output in the common-emitter amplifier that provides most of the voltage gain. In a PNP transistor, it emits holes. The emitter is also not interchangeable with the collector, since the doping, size and thickness differ, and swapping them leaves a transistor with poor gain. Compared with the base that controls the flow and the collector that gathers it, the emitter is the source of the carriers, and the field-effect transistor's analogue is the source terminal.
Read emitter as the heavily doped injector end of a bipolar transistor, the region whose carrier stream the base steers and the collector collects, marked by the arrow in the symbol.