What is Transistor? – About Transistor Terminals | New Topic [2024]

In this note, we are going to learn a topic called “What is Transistor?“, also Symbol, Transistor Terminals, and Applications. Welcome to Poly Notes Hub, a leading destination for engineering notes for engineering students.

Author Name: Arun Paul.

What is Transistor in Electronics?

A transistor consists of two PN junctions formed by sandwiching either p-type or n-type semiconductors between a pair of opposite types.

It is a semiconductor device that amplifies and switches electronic signals and electrical power. It consists of three layers of semiconductor material, each of which may carry an electric current. These layers are doped to produce either an excess of electrons (n-type) or an electron shortage, sometimes known as “holes” (p-type). The connections between these layers regulate the flow of current.

Accordingly; there are two types of transistor, namely –

1. NPN Transistor
2. PNP Transistor

An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p-type as shown in the figure. However ,a p-n-p transistor is formed by two p-sections separated by a thin section of n-type as shown in the figure below.

Transistors are classified into two types: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). BJTs utilize electron and hole charge carriers, whereas FETs rely primarily on electron flow. Transistors are key building elements of modern electronic devices such as computers, cellphones, and amplifiers, as well as critical components in digital and analog circuits. They transformed electronics by eliminating cumbersome vacuum tubes, resulting in smaller, more efficient, more dependable systems.

In each type of transistor, the following points may be noted:

1. These are two PN Junctions. Therefore, a transistor may be regarded as a combination of two diodes connected back to back.
2. There are three terminals ,one taken from each type of semiconductor.
3. The middle section is a very thin layer. This is most important factor in the function of a transistor.

Symbol of Transistor

📌 In the above picture, we are shown the NPN Transistor Symbol and PNP Transistor Symbol.

What is BJT or Bipolar Junction Transistor ?

There are two basic types of transistor: The Bipolar Junction Transistor (BJT) and The Field Effect Transistor (FET). But note that when we use the term transistor, it means Bipolar Junction Transistor (BJT). The term comes from the fact that in a bipolar transistor, there are two types of charge carrier ( Viz. electrons and holes ) that plays part in conductions. Note that “Bi” means two and polar means polarities. For this reason, it is called Bipolar Junction Transistor or BJT.

Terminals of Transistor

There three types of transistor terminals –

1. Base
2. Collector
3. Emitter

Below, we discuss about these transistor terminals –

1. Base (B)

1. The base terminal is the control terminal of the transistor.
2. It is responsible for regulating the flow of current between the collector and emitter terminals.
3. By applying a small current or voltage to the base, you can control a much larger current flowing between the collector and emitter, allowing the transistor to amplify signals.
4. In an NPN transistor, current flows from the emitter to the collector when the base current is present.

2. Collector(C)

1. The collector terminal is the output terminal of the transistor.
2. It allows the current to flow from the transistor’s collector to its emitter.
3. The collector current is controlled by the base current, making the transistor act as an amplifier or a switch.

3. Emitter (E)

1. The emitter terminal is the input terminal of the transistor.
2. It emits the electrons (in NPN) or holes (in PNP) that combine with the collector current.
3. The emitter terminal provides the pathway for the current to flow out of the transistor.

Some Important Facts about Transistor

Here we listed some important facts about transistor –

1. The transistor has three regions, namely; emitter, base, and collector. The base is much thinner than the emitter while the collector is wider than both.
2. The emitter is heavily doped. The base is lightly doped .The collector is moderately doped.
3. The transistor has two PN Junctions it is like two diodes. The junction between the emitter and base may be called the emitter-base diode or simply the emitter diode. The junction between the base and collector may be called collector-base diode or simply collector diode.
4. The emitter diode is always forward-biased whereas the collector diode is always reverse-biased. As shown in the below image.
5. The resistance of the “emitter diode” is very small compared to the “collector diode“. Therefore forward bias applied to the emitter diode is generally very small whereas reverse bias on the collector diode is much higher.

Applications of Transistor

Here we listed some important transistor applications –

1. Improving electronic signals in audio and communication systems.
2. Regulating the flow of current in digital circuits used for data processing and computation.
3. Use in Signal filtering, modulation, and oscillation in a variety of electronic devices.
4. Adjusting brightness in LED lighting systems.
5. Controlling the direction and speed of electric motors for use in vehicles and industry.