Bipolar Junction Transistor

There are two types of BJT, each with its characteristics and uses.

1. NPN Transistor: A thin layer of p-type semiconductor sandwiched between two thicker layers of n-type semiconductor. In this type, electrons serve as the majority charge carriers.

2. PNP Transistor: In contrast, PNP transistors have three layers with an arrangement opposite to NPN transistors: two layers of p-type semiconductors sandwiching an n-type layer. Here, the majority charge carriers are holes.

The two types and their symbols are shown in the image below.

The emitter, the base, and the collector are carefully doped to achieve specific electrical characteristics.

The emitter is heavily doped, either with excess electrons or holes. This heavy doping allows for efficient injection or emission of charge carriers into the base region.

Adjacent to the emitter is the base, which is lightly doped compared to the emitter. The base is usually very narrow to facilitate the diffusion of charge carriers injected from the emitter to the collector.

The collector, located opposite the emitter, is moderately doped. Its primary function is to collect the majority charge carriers emitted by the emitter. During normal operation, the collector-base junction is reverse-biased, allowing it to collect charge carriers that diffuse across the thin base region.

The physical structure of a BJT often involves sandwiching the lightly doped base region between the heavily doped emitter and moderately doped collector. This configuration enables the transistor to amplify and control the current flow between the collector and the emitter based on the small current flowing into the base terminal. The precise doping levels and physical dimensions of these regions determine the transistor’s characteristics and functionality in electronic circuits.

1. Cut-off region: The first region of operation is the cut-off region. In this region, the base-emitter and the base-collector junctions are reverse-biased, resulting in minimal or zero current flow. The transistor is effectively turned off, and no amplification or switching occurs.

2. Active region: Also known as the amplification region, both junctions are forward-biased. It allows a controlled amount of current to flow through the transistor. In this region, small changes in input signals can result in significant output variations, making it ideal for amplification.

3. Saturation region: Both junctions are forward-biased to the extent that the collector current is not dependent on the base current and has reached a maximum. In saturation, the transistor is a near-short circuit between the collector and emitter terminals. The transistor is a closed switch and can be used for switching applications.

BJT operates in three different modes: Common Base (CB), Common Emitter (CE), and Common Collector (CC).

1. Common Base

As the name suggests, the input and output circuits share the base terminal. The input signal is applied between the emitter and the base terminals, while the output is taken across the collector and the base terminals. The base terminal is usually grounded or connected to a reference point.

For the transistor to function in this setup, the current entering the emitter terminal should surpass both the base and collector currents. Consequently, the collector current output is lower than the input emitter, resulting in a current gain of one or less.

2. Common Emitter

In this configuration, the emitter is grounded and common to the base and the collector. The input signal is applied between the base and the emitter, while the output is taken from between the collector and the emitter.

This configuration offers several advantages, including high voltage gain, good input-output phase relationship, and relatively low output impedance, making it the most popular configuration among the three.

3. Common Collector

This configuration is known as a Voltage Follower or Emitter Follower circuit. The collector terminal is common to the input and output circuits. The input signal is applied between the base-collector region, and the output is taken from the emitter-collector region.

This configuration offers a high input impedance and a low output impedance, making it an excellent choice for impedance matching and voltage buffering applications. It provides unit voltage gain but substantial current gain.

The following table illustrates the characteristics of the various BJT configurations:

Characteristics	Common Base	Common Emitter	Common Collector
Power Gain	Low	Very high	Medium
Current Gain	Low	Medium	High
Voltage Gain	High	Medium	Low
Power Gain	Low	Very high	Medium
Input Impedance	Low	Medium	High
Output Impedance	Very high	High	Low
Phase Angle	0°	180°	0°