Bipolar Junction Transistors

A transistor is a three-terminal active semiconductor device that transforms current flow from a low resistance path to a high resistance path. This transfer is abbreviated as the transistor. The current conduction in a transistor involves two types of charge carriers: electrons and holes. There are two junctions in a transistor: the Emitter-Base Junction and the Collector-Base Junction, resulting in its name, Bipolar Junction Transistor (BJT).

The BJT consists of three regions:

• Emitter: Supplies charge carriers (electrons or holes) to other regions. Heavily doped and larger than the base but smaller than the collector.

• Base: The middle region that forms two PN junctions, thin compared to the emitter and collector, and lightly doped, controlling the flow of charge carriers.

• Collector: Collects charge carriers; larger than both emitter and base, with doping levels intermediate between them.

Types of BJT

• PNP Transistor: Contains 2 P regions supported by an N region (N type semiconductor sandwiched between 2 P semiconductors).

• NPN Transistor: Contains 2 N regions separated by a P region (P type semiconductor sandwiched between 2 N semiconductors).

In circuit symbols, the arrow indicates the direction of conventional current flow from base to emitter. A BJT can be viewed as two PN junction diodes connected back-to-back. For proper operation, the Emitter-Base (EB) junction must be forward biased and the Collector-Base (CB) junction must be reverse biased. This creates low resistance for the forward biased diode and high resistance for the reverse biased one, enabling the transfer of current flow from low to high resistance, hence the name 'Transistor'.

Biasing of Transistors

The application of suitable DC voltages across the transistor terminals is called biasing, which depends on how the two PN junctions get biased.

Working of NPN Transistor

In an NPN transistor, forward bias causes electrons in the N type emitter to flow toward the base, comprising the emitter current (Ie). The base is thin; hence, only a few electrons (5%) recombine with holes, forming the base current (Ib). The remaining 95% enter the collector, resulting in the collector current (IC). The equation is Ie = Ib + Ic. Electrons in the collector are attracted to the positive terminal of the battery (Vcb), while an equal number enter the emitter from the battery's negative side.

Working of PNP Transistor

In a PNP transistor, the positive terminal of the battery connected to the emitter repels holes toward the base, as the emitter-base junction is forward biased. About 5% of holes recombine with electrons at the base, while the remaining 95% flow through to the collector.