What is Transistor in Electronics and Its Types [2023]

Poly Notes Hub share a note on Transistor. Notes Topic What is Transistor in Electronics ? and Types of Transistor and How Transistor Works.

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.

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.

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 symbols of both types of transistors – NPN Transistor and PNP Transistor.

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 are three terminals in BJT transistor as shown in the above picture.

• Base
• Collector
• Emitter

Below, we discuss about these terminals.

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.

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.

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

• 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.
• The emitter is heavily doped. The base is lightly doped .The collector is moderately doped.
• 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.
• The emitter diode is always forward-biased whereas the collector diode is always reverse-biased. As shown in the below image.

• 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.

How Transistor Work?

Below we discuss How Transistor works. The Transistor NPN and PNP work depends on the charge carrier of the semiconductor material.

Working of NPN Transistor

The NPN transistor has a forward bias to the emitter-base junction and a reverse bias to the collector-base junction. The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current. As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%)combine with holes to constitute base current. The remainder (more than 95%)cross over into the collector region to constitute collector current. In this way, almost all the emitter current is in the collector circuit. It is clear that the emitter current is the sum of the collector and base current.

Working of PNP Transistor

Below shows the basic connection of the PNP transistor. The forward bias causes the holes in the p-type emitter to flow towards the base. This constitutes the emitter current. As these holes cross into the n-type base, they tend to combine with the electrons. As the base is lightly doped and thin, only a few holes (less than 5%) combine with the electrons. The remainder (more than 95%) cross into the collector region to constitute the collector’s current. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that the current conduction with the PNP transistor is by holes. However, in the external connecting wires, the current is still by electrons.

Why Transistor is called so?

The name of transistor is given to the device by combining the two terms, One is Transfer and another is Resistor. Combining of these two word the name came Transistor. Transfer ➕ Resistor ➡️ Transistor.

Uses of Transistor

• Used in Amplifier Circuit.
• Used in Integrated Circuit.
• In many digital circuits.

Conclusion | Diploma Engineering Notes | Poly Notes Hub

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