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Physics > Semiconductor Electronics: Materials, Devices and Simple Circuits > Junction Transistor – Structure and Action
Semiconductor Electronics: Materials, Devices and Simple Circuits

Junction Transistor – Structure and Action

In 1951, William Schockley invented the first junction transistor with two back to back p-n junctions. Over the years, different types of transistors were invented and to differentiate the junction transistor from the new ones it is now called Bipolar Junction Transistor (BJT). In this chapter, since we are not talking about any other transistors we will use the words ‘transistor’, ‘junction transistor’ and ‘Bipolar Junction Transistor’ interchangeably.

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Introduction to a Junction Transistor

A junction transistor has three doped regions – emitter, base, and collector. These regions form two p-n junctions between them. Depending on the number of n and p-type semiconductors in the transistor, they are of two types:

n-p-n junction transistor

  • n-p-n transistor: A p-type semiconductor (base) separates two segments of the n-type semiconductor (emitter and collector).

p-n-p junction transistor

  • p-n-p transistor: An n-type semiconductor (base) separates two segments of the p-type semiconductor (emitter and collector.

As can be seen in both the figures above, all three segments have different thickness and doping levels. The schematic symbols of both these transistors are as follows:

junction transistor

The arrowhead shows the direction of the conventional current in the transistor. Let’s understand the three segments in detail:

Emitter This segment is on one side of the transistor. It has a moderate size and is heavily doped causing it to supply a large number of carriers for the flow of current.
Base This segment is at the centre of the transistor. It is thin and lightly doped.
Collector This segment is also on one side of the transistor. It is larger than the emitter and is moderately doped. Hence, it collects most of the majority carriers supplied by the emitter.

Also, in the case of a junction transistor, the depletion regions are formed at the emitter-base junction and the base-collector junction. To understand the action of the transistor, it is important to consider the nature of depletion regions formed at these junctions.

It is also important to note here that junction transistor was invented to produce an enlarged copy of a signal – an amplifier. Eventually, it became equally popular as a switch.

Browse more Topics under Semiconductor Electronics Materials Device And Simple Circuits

The Amplifying Capabilities of a Junction Transistor

A junction transistor works as an amplifier when,

  1. The emitter-base junction is forward biased and
  2. The base-collector junction is reverse biased.

The circuit diagram for the same is as shown below:

junction transistor

As can be seen above, the base is the common terminal for two power supplies whose other terminals are connected to the emitter and collector.

Some Terminologies

  • The voltage between emitter and base = VEB
  • The voltage between collector and base = VCB
  • VEE = Power supply connected between the emitter and base
  • VCC = Power supply connected between the collector and base

Let us now study the path of the current carriers in the junction transistor where –

  • The emitter-base junction is forward biased and
  • The Base-collector junction is reverse biased

The emitter is heavily doped and has a large concentration of the majority carriers. These majority carriers enter the base in large numbers. Since the base is very thin and lightly doped, it has very few majority carriers. Let’s look at the path of current in a p-n-p transistor:

p-n-p Transistor

The emitter has a large concentration of holes. The base, being an n-type semiconductor will have electrons as its majority charge carriers. When the majority carriers (holes) enter the base from the emitter, they swamp the majority charge carriers of the base (electrons). The base-collector junction is reverse biased.

Hence, these holes appear as minority carriers at the junction. Hence, they can easily enter the collector (which is a p-type semiconductor). The holes in the base can either:

  • Move towards the base terminal to combine with the electrons entering from outside or
  • Cross the junction and enter the collector.

Since the base is very thin, most of the holes find themselves near the base-collector junction (reverse biased). Hence, they cross over to the collector rather than move to the base terminal.

Observations

The forward bias leads to a large current entering the emitter-base junction. However, most of it diverts to the adjacent base-collector junction. Hence, the current coming out of the base is a small fraction of that entering the junction. The total current in a forward biased diode is Ih + Ie … where Ih is the hole current and Ie is the electron current.

The emitter current IE = Ih + Ie. However, the base current IB << Ih + Ie. This is because a big part of IE goes to the collector instead of the base terminal. Now, current enters the emitter from outside. Applying Kirchhoff’s law:

IE = IC + IB

where IC is the current emerging from the collector terminal. Also, IC is nearly equal to IE since IB is very small.

n-p-n Transistor

In an n-p-n transistor, current enters from the base to the emitter. The description of the paths followed by the majority and minority charge carriers is similar to that of the p-n-p transistor. However, the current paths are exactly the opposite.

In an n-p-n transistor, electrons are the majority charge carriers, supplied by the n-type emitter region. They cross the thin p-type base region and are able to reach the collector to give the collector current, IC.

Solved Examples for You

Question: Name and describe the three regions of a junction transistor

Solution: A junction transistor comprises three regions – emitter, base, and collector. The emitter is on one side of the transistor. It is moderate in size and heavily doped. Also, it supplies a large number of majority charge carriers. The base lies at the centre of the transistor. It is very thin and lightly doped. The collector is on the other side of the transistor. It is large in size and moderately doped. Also, it collects most of the majority carriers supplied by the emitter.

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