The schematic symbols for pnp- and npn-type BJTs.
A bipolar junction transistor (BJT) is a type of transistor, an amplifying or switching device constructed of doped semiconductor. The BJT is a sandwich of differently doped sections, either NPN or PNP. The center section is called the base of the transistor. By varying the current between the base and one terminal called the emitter, one can vary the current flow between the emitter and a third terminal known as the collector, causing amplification of the signal at that terminal. BJTs can be thought of as current-controlled resistors. BJTs are usually characterized as current amplifiers.
Conceptually, one can understand a bipolar transistor as two diodes placed back to back. In normal operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. In an npn-type transistor for example, electrons from the emitter wander (or "diffuse") into the base. These electrons in the base are in the minority - there are plenty of holes to recombine with. The base is always made very thin such that most of the electrons diffuse over to the collector before they recombine with holes. The collector-base junction is reverse biased to prevent the flow of holes, but electrons meet a more friendly reception - they are swept into the collector by the electric field around the junction. The proportion of electrons able to run the base "gauntlet" and make it to the collector is very sensitive to the current passing through the base. Hence, a small change of the base current can translate to a large change in electron flow between emitter and collector. For example the ratio of these currents (Ic Ib usually called β) in some bipolar transistors is 100 or more.
Structure and use of npn transistor
The diagram opposite is a schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct a current from C to E, a small voltage (around 0.7 volts) is applied across the base-emitter junction. This voltage is called VBE. This causes the upper p-n junction to conduct, allowing a larger current IC to flow into the collector. The total current flowing into the transistor is therefore the base current IB plus the collector current IC. The total current flowing out is simply the emitter current, IE. As with all electrical devices, the total current flowing in must be equal to the total current flowing out, so:
- IE = IB + IC
This behaviour can be used to make a digital switch: if the base voltage is simply switched on and off, it causes the collector current to switch on and off at the same time. If the base voltage is varied slightly around the switch-on voltage, the current flowing from C to E also varies, allowing the transistor to act as an amplifier.
Cross section of an npn-type BJT as found in integrated circuits (simplified). The base region is the pink layer sandwiched between the green emitter region above and the yellow collector volume below.
Transistors have different regions of operation. In the "linear" regime, collector-emitter current is approximately proportional to the base current but many times larger, making this the ideal mode of operation for current amplification. The BJT enters "saturation" when the base current is increased to a point where the external circuitry prevents the collector current from growing any larger. A residual voltage drop of approximately 0.3V between collector and emitter then remains. A transistor is said to operate in the "cut off" regime when the base-emitter voltage is too small for any significant current to flow. In typical BJTs manufactured from silicon, this is the case below 0.7V or so. BJTs that alternate between cut off and saturation can by viewed as electronic switches. The germanium transistor was more common in the 1950s and 1960s, and while it exhibits a lower "cut off" voltage, making it more suitable for some applications, it also has a greater tendency to exhibit thermal runaway.
Because of its temperature sensitivity, the BJT can be used to measure temperature. Its nonlinear characteristics can also be used to compute logarithms.