Bipolar junction transistor

A bipolar junction transistor (BJT) is a semiconductor device that employs both electrons and electron holes as charge carriers. In contrast, unipolar devices such as field-effect transistors (FETs) use only one type of carrier. BJTs are capable of using a small current applied at one terminal to control a much larger current between two other terminals, making them valuable as electronic switches and amplifiers. They are fundamental components in discrete circuitry and integrated circuits, particularly in analogue, mixed-signal, and certain high-performance digital applications.

Structure and Types

BJTs consist of two pn junctions formed within a single crystalline semiconductor. These junctions are created through processes such as controlled doping during crystal growth, alloying, or diffusion of dopants. The two primary BJT types are:

• NPN transistors, which contain a thin p-doped base between two n-doped regions.
• PNP transistors, in which a thin n-doped base sits between two p-doped regions.

The three regions—emitter, base, and collector—are connected to external circuitry via three leads. The emitter is heavily doped to maximise charge-carrier injection, while the base is thin and lightly doped to minimise recombination. The collector is doped more lightly than the emitter, allowing effective collection of minority carriers.

Current Direction and Symbol Conventions

By convention, current flow in circuit diagrams is depicted as the movement of positive charge, termed conventional current. In metal conductors, the physical current consists of electrons moving in the opposite direction, but in semiconductors both electrons (negative carriers) and holes (positive carriers) participate in conduction.
The arrow on a BJT symbol marks the emitter and indicates the direction of conventional current flow through the emitter–base junction: outward for NPN transistors and inward for PNP transistors.

Principles of Operation

BJT operation relies on the diffusion of charge carriers across regions of differing carrier concentration. The base–emitter junction is forward biased in normal operation, allowing carriers to be injected from the emitter into the base. These carriers then diffuse across the thin base region towards the collector.
Because the base is thin and lightly doped, most injected minority carriers reach the collector without recombining. The base–collector junction is reverse biased, and its electric field sweeps carriers into the collector region. This configuration enables a small base current to control a much larger collector current, classifying the BJT as a minority-carrier device.
The efficiency with which carriers cross the base is reflected in transistor parameters α (common-base current gain) and β (common-emitter current gain), the latter often used in analogue design.

Current and Voltage Control Models

The relationship between currents and voltages in BJTs can be interpreted in two complementary ways:

• Current control, where the collector current is modelled as approximately proportional to the base current (β × I_B). This approach offers intuitive linearity for basic circuit design.
• Voltage control, based on the exponential I–V behaviour of the forward-biased base–emitter junction. The collector current increases exponentially with the base–emitter voltage. Models such as the Ebers–Moll formulation describe this relationship and are preferred for precision design, particularly in integrated circuits.

A charge-control perspective also exists, accounting for the distribution and dynamics of stored charge within the base. Although this view is useful for understanding switching behaviour and devices such as phototransistors, it is less commonly used in routine circuit design.

High-Frequency and Dynamic Behaviour

At high frequencies, BJT performance is limited by the time required for minority carriers to traverse the base region. Low-level injection conditions result in ambipolar diffusion, where both carrier types move with the same effective diffusion rate. Advanced models, including the Gummel–Poon model, incorporate these effects for accurate simulation.
During switching, BJTs can exhibit delays due to storage time—the accumulation of excess charge in the base when the device is driven into saturation. Techniques such as the Baker clamp are employed to prevent deep saturation and reduce turn-off delay at the expense of slightly higher on-state voltage.

Applications and Integration

Bipolar transistors were the foundational active devices in early integrated circuits and remained dominant in mainframe and minicomputer systems until MOSFET-based CMOS technology became standard for digital logic due to its lower power consumption. Nevertheless, BJTs retain important roles in:

• Analogue signal amplification
• High-frequency and radio-frequency amplifiers
• High-current and high-voltage switching
• Mixed-signal integrated circuits through BiCMOS technology