base

In a bipolar junction transistor (BJT), the base is the thin middle layer of semiconductor material sandwiched between the emitter and collector. It is the controlling terminal: a small current applied to the base modulates a much larger current flowing between emitter and collector, making the BJT useful as an amplifier or switch.

The base is physically a lightly doped region, typically only a few micrometers thick in modern devices. In an NPN transistor, the base is p-type silicon; in a PNP transistor, it is n-type. This doping level is critical: the base must be thin enough for charge carriers to diffuse across it efficiently, but doped lightly enough to maintain useful current gain. The relationship between base current and collector current is expressed as hFE or beta, typically ranging from 50 to 300 in general-purpose transistors.

Base current and amplification

When you apply a voltage to the base with respect to the emitter, you forward-bias the base-emitter junction. This causes emitter current to flow into the base, and most of this current is swept into the collector by the reverse-biased base-collector junction. The base current itself is only a small fraction of the total emitter current because most charge carriers injected from the emitter traverse the base without recombining. This is why the base can control a much larger collector current: a milliamp or two of base current might control hundreds of milliamps at the collector.

The base terminal is brought out to a wire bond or tab on the transistor package. In through-hole devices like the 2N2222 or 2N3904, it is one of three leads. The base is sensitive to leakage and contamination on the die surface, which is why old transistors stored in humid conditions sometimes show degraded gain; modern plastic packages with epoxy sealing have largely resolved this problem.

The term base has no obvious connection to the physics; it is simply historical convention from early semiconductor nomenclature. The emitter "emits" carriers, the collector "collects" them, and the base was named as the intermediate terminal. Confusion with the base of a transistor amplifier circuit (the biasing point around which signals are amplified) sometimes arises in conversation, but they are distinct concepts.