Junction transistors and their characteristics

Semiconductors are a type of device that has the properties of both a conductor and an insulator. Their electrical conductivity value falls between that of an insulator and a conductor. It is neither entirely conductive for electricity like a conductor (for example, copper wires),  but it can conduct electricity more than an insulator (e.g., glass).

Now, a junction transistor is a type of semiconductor with three terminals and two junctions. It can act as a switch or be used to amplify a signal. They are also known as Bipolar transistors.

In this article, we shall learn more about the different types of junction transistors and their characteristics.

What is a junction transistor?

A junction transistor is a three-terminal active device that is made up of semiconductors that have electrical conductivity between that of an insulator and a conductor.

The transistor was invented back in 1947 by a team of scientists consisting of W. Bratterin,

1. Barden  and W. Shockley.

A junction transistor or a bipolar junction transistor is formed by joining two single PN-junction diodes back to back. By doing this, it becomes a series that  shares a common positive (p) or negative (n) terminal.

The word transistor is made up of two words, which are: transfer and resistor. This defines the function of a transistor as it transfers an input current from a low resistance circuit to a high resistance circuit. The term was coined by John R. Pierce.

The three terminals of a junction transistor are namely:

1. Emitter ( E )
2. Base ( B )
3. Collector ( C )

A junction Transistor is a device that is controlled by current. It makes use of both electrons ( constitutes the negative charge) and holes (constitute the positive charge) as their charge carriers.

On applying an external DC power supply source, a signal of small amplitude is applied at the base. This signal gets amplified at the collector terminal of a transistor.

Types of Junction Transistor

Junction Transistors can be categorised into two types:

1. NPN Transistors
2. PNP Transistors

Let us have a look at both the above categories in detail.

NPN Bipolar Junction Transistor

The NPN transistor is formed by putting a thin layer of P-type semiconductor in between two layers of N-type semiconductors. NPN denotes the Negative-positive-negative type of bipolar junction transistor.

The majority of charge carriers in this type of transistor are electrons. At the same time, the minority are the holes.

Here, the electrons flow from the emitter terminal to the current terminal. Hence, electricity flows from the collector terminal to the emitter terminal.

NPN is the most commonly used type of bipolar junction transistor, as electrons are fast charge carriers. It can be used as a switch in high-powered devices such as motors and can also be used as amplifiers. As an amplifier, if you apply a small input voltage, it amplifies it into a large output voltage.

PNP Bipolar Junction Transistor

A PNP Bipolar Junction Transistor can be formed by sandwiching an N-type semiconductor between two P-type semiconductors. The PNP stands for Positive-Negative-Positive Transistor.

Here, the majority of charge carriers are the holes, and the minority are the electrons. The current flows from the emitter terminal to the collector terminal of the transistor.

PNP Transistors also have the same applications as NPN transistors. But the latter is preferred due to its higher mobility of electrons.

Junction Transistor Characteristics

The junction transistor characteristics can be defined based on the different modes of the configuration of a junction transistor. Junction Transistor characteristics can be represented by various graphical forms of the relationship between the current and voltage.

Since the bipolar junction transistor has three terminals, it can be connected in three ways.

Common Base Configuration

Here the base terminal remains common to both the input and output signals. The input signal is applied at the emitter-base terminal and the output signal is taken from the collector-emitter terminal.

The base remains connected to a fixed voltage. This is the reference voltage point.

This is used in a P-N-P transistor. Here, there is a voltage gain but no current gain.

Common Emitter Configuration

Here the emitter terminal remains common to both the input and the output signal. The input signal is given at the base-emitter terminal and the output signal is taken from the emitter-collector terminal.

This is the most common type of configuration used. It is used in NPN bipolar junction transistors.

It is mainly used for amplification purposes, as it produces the most power and current among the three configurations of transistors. This is due to its low impedance level as it is connected to a forward-biassed junction diode.

Common Collector Configuration

Here the collector terminal remains common to both the input and the output signal. The base is the chosen terminal for receiving the input. The emitter-load resistor terminal is responsible for the output signal.

It is also known as the voltage follower or the emitter follower circuit. 

It is often used in impedance matching devices as it has a very high input impedance.

Conclusion

The key take away from this article is that transistors can be of two types: bipolar junction transistors and field effect transistors.

Bipolar junction transistors are categorised into types: PNP and NPN transistors.

Transistors have three terminals, namely, collector, emitter and base. In different configurations of junction transistors, the input signal is applied on different terminals.

Bipolar junction transistors have their application in multitudes of ways. They can be used in amplifiers, switches, temperature sensors, etc.