Bipolar Junction Transistors (BJTs) are three-layer semiconductor devices that amplify electrical signals. They come in two types: NPN and PNP, with NPN being more common due to better performance. BJTs are crucial for analog and digital circuits.

BJTs operate in three modes: forward-active, saturation, and cutoff. In forward-active mode, they act as current amplifiers. Saturation and cutoff modes are used for switching applications. Key parameters include current gain (β) and emitter injection efficiency.

BJT Structure

Semiconductor layers

• Consists of three semiconductor layers: emitter, base, and collector
• Emitter heavily doped to inject charge carriers into the base region
• Base thin, lightly doped layer that controls the flow of charge carriers from emitter to collector
• Collector moderately doped, collects charge carriers from the base
• Doping levels and thicknesses of each layer crucial for proper transistor operation

Transistor types

• Two main types of BJTs: NPN and PNP transistors
• NPN transistor has a p-type base sandwiched between n-type emitter and collector (emitter and collector are n-type, base is p-type)
• PNP transistor has an n-type base sandwiched between p-type emitter and collector (emitter and collector are p-type, base is n-type)
• NPN transistors more common due to better performance characteristics (higher current gain and switching speeds)

BJT Operation Modes

Forward-active mode

• Base-emitter junction forward-biased, base-collector junction reverse-biased
• Emitter injects charge carriers (electrons for NPN, holes for PNP) into the base region
• Most charge carriers diffuse across the thin base layer and are swept into the collector by the electric field in the base-collector depletion region
• Collector current ($I_C$) controlled by the base current ($I_B$) in this mode
• Transistor acts as a current amplifier, with $I_C = \beta I_B$, where $\beta$ is the current gain

Saturation and cutoff modes

• Saturation mode occurs when both base-emitter and base-collector junctions are forward-biased
• Transistor acts like a closed switch, with low voltage drop between collector and emitter ($V_{CE(sat)}$)
• Cutoff mode occurs when both base-emitter and base-collector junctions are reverse-biased
• Transistor acts like an open switch, with negligible collector current and high collector-emitter voltage
• Saturation and cutoff modes used in digital logic applications (transistor as a switch)

BJT Parameters

Current gain (β)

• Current gain ($\beta$) is the ratio of collector current to base current in forward-active mode
• Defined as $\beta = I_C / I_B$, typically ranges from 50 to 200 for common transistors
• Higher $\beta$ values indicate more efficient transistor operation (small base current can control a large collector current)
• Current gain depends on factors such as temperature, collector current, and transistor geometry

Emitter injection efficiency

• Emitter injection efficiency ($\gamma$) is the ratio of charge carriers injected from the emitter into the base to the total emitter current
• Defined as $\gamma = I_{Cn} / I_E$ for NPN transistors, where $I_{Cn}$ is the electron current injected into the base from the emitter
• High emitter injection efficiency essential for good transistor performance
• Factors affecting emitter injection efficiency include doping levels, emitter-base junction design, and surface recombination effects
• Emitter injection efficiency typically ranges from 0.95 to 0.99 for modern transistors