Bipolar Junction Transistors Overview

A semiconductor device that amplifies or switches electronic signals. It consists of three semiconductor regions: emitter, base, and collector.

A type of transistor with three semiconductor regions: emitter, base, and collector. The base is a thin, lightly doped region.

The heavily doped region in a BJT that emits current carriers, typically electrons in an npn transistor and holes in a pnp transistor.

The lightly doped region in a BJT that collects current carriers. It's typically larger than the emitter.

The thin, lightly doped region in a BJT that controls the flow of current carriers between the emitter and collector.

The ability of a transistor to amplify or increase the strength of a signal. The transistor's output signal is stronger than the input signal.

The ability of a transistor to act like a switch, turning a signal on or off.

The flow of current carriers (electrons or holes) through a transistor. It is controlled by the base region.

The relationship between the input and output signals of a transistor. It describes how the transistor's output current changes in response to changes in its input current.

A pnp transistor with a current flow from the emitter to the collector.

An npn transistor with a current flow from the emitter to the collector.

The central functionality of a Bipolar Junction Transistor (BJT). A small base current controls a significantly larger collector current. This is known as current amplification.

Two tightly connected PN junctions form the structure of a Bipolar Junction Transistor (BJT).

The base-emitter (BE) junction acts like a forward-biased diode, allowing electrons or holes from the emitter to enter the base, forming the emitter current (IE).

The base region in a BJT is intentionally thin and lightly doped. This minimizes the number of charge carriers recombining in the base, resulting in a small base current (IB).

The base-collector (BC) junction is reverse-biased. This characteristic helps most of the charge carriers arriving at the base to not recombine but instead diffuse to the collector, creating the collector current (IC).

The total emitter current (IE) is the sum of the base current (IB) and the collector current (IC), reflecting the conservation of charge within the transistor.

A BJT is often used as a two-port network. Connecting the three transistor terminals (Base, Emitter, Collector) to external circuits, based on which terminal is common, defines the three possible configurations:

A configuration where the emitter is the common terminal. This configuration is useful for high-frequency applications due to its low input impedance.

A configuration where the emitter is the common terminal for both input and output circuits. This configuration provides good current amplification, making it popular in amplifiers.

A configuration where the collector is the common terminal. Useful for high-impedance loads, it works as an emitter follower, providing voltage buffering and a high input impedance.

The total collector current in a transistor is the sum of the current amplified by the emitter and the small reverse saturation current of the collector-base junction. It's practically equal to the amplified emitter current because the reverse saturation current is insignificant.

The current gain of a transistor is the ratio of collector current to emitter current, which is approximately equal to alpha (α). This means a small change in base current causes a much larger change in collector current.

The reverse saturation current (ICBO) is the small current that flows through the collector-base junction when the emitter current is zero. It is caused by minority carriers diffusing across the junction.

In a BJT, the collector current is primarily determined by the emitter current and the current gain (α).

A load line is a graphical representation of the relationship between the collector voltage (VCE) and collector current (IC) for a transistor circuit.

The intersection of the DC load line with the transistor's collector characteristics curves determines the quiescent point (Q-point), also known as the operating point.

The load line equation describes the relationship between VCE and IC based on the collector circuit components, particularly the collector resistor (RC).

The cut-off region of the load line corresponds to zero collector current (IC = 0), where the transistor is essentially turned off.

The saturation region of the load line corresponds to a maximum collector current (IC = Vcc/RC), where the transistor is fully on.

The operating point (Q-point) of a transistor can be shifted by changing the base current (IB), effectively changing the transistor's amplification factor.