Transistor: NPN Transistor And PNP Transistor

A human brain is made of 100 billion cells known as neurons that help us think and remember. Similarly, computers contain billions of “brain cells” as well. They are known as transistors. Transistors have revolutionized electronics since the time they were invented by Walter Brattain, John Bardeen and William Shockley.

Table of Contents

Transistor DefinitionClassification of Transistor Based on ConstructionBipolar Junction TransistorTransistor Modes of OperationFAQs

What is a Transistor?

A transistor is a semiconductor device that transfers a weak signal from low resistance circuit to high resistance circuit. In simple words, what it means is that it regulates and amplifies electrical signals such as voltage or current. 
Transistors are special because they allow you to control how much current flows through a circuit. This can be achieved by controlling the voltage across two of the transistor leads. Each transistor has three leads.

Transistor History 

John Bardeen, William Shockley and Walter Brattain, the inventors of the transistor.

From left: John Bardeen, William Shockley and Walter Brattain, the inventors of the transistor, 1948.

 
A transistor is an influential little invention that changed the course of history for computers and all electronics. A transistor consists of two PN diodes that are connected back to back. It has three terminals that are named emitter, base and collector. John Bardeen, William Shockley, and Walter Brattain were scientists at the Bell Telephone Laboratories. The idea for transistors arose when they were working on an idea that used a field effect to control the current in a semiconductor.

Classification of Transistors According to Construction

Transistors are categorised into two types based on their construction as Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET).

What are Bipolar Junction Transistors?

Bipolar Junction Transistors are also known as junction transistors. These were the first type of transistors that were mass-produced in 1947 by the Bell Labs. These type of transistors are a combination of two junction diodes.
The three-layer structure of the junction transistors can contain either of the combinations:

  • an n-type semiconductor layer sandwiched between p-type layers forming a pnp configuration

or,

  • a p-type layer between n-type layers forming an npn configuration.

It has two junctions between p-type and n-type semiconductors. BJTs are current-controlled devices which means that a small amount of current flowing through the base of a Bipolar Junction Transistor results in a large current that flows from emitter to collector.

What are Field Effect Transistors (FET)?

A field-effect transistor uses an electric field to control the flow of current. They have three terminals which are named source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source. FETs are classified into three types as MOS (Metal-Oxide-Semiconductor) type FETs, and MES (Metal-Semiconductor) type FETs. 
Junction type FETs are used in analog circuits such as those in audio equipment. MOS type FETs are used in digital ICs such as those used in microcomputers. MES type FETs are used for amplification of microwaves.

Construction of Bipolar Junction Transistor

A transistor is a three-layer semiconductor device in which one type of semiconductor ( either P-type or N-type) is sandwiched between two other similar types of semiconductor.
A bipolar junction transistor is formed by three layers of semiconductor materials, if it is a PNP transistor, it will have two P-type regions and one N-type region, likewise, if it is an NPN transistor, it will have two N-type regions and one P-type region. 

transistor
 

Transistors have three terminals namely emitter, collector and base. We have explained the functionalities of each of these terminals below:

  • Emitter – In a transistor, emitter supplies a large section of majority charge carriers. The emitter is always forward biased with respect to the base so that it supplies the majority charge carrier to the base. The emitter of a transistor is heavily doped and moderate in size.
  • Collector – In a transistor, the section that collects the majority of the charge carrier supplied by the emitter is called a collector. The collector-base junction is always reverse biased. The collector section of the transistor is moderately doped, but larger in size so that it can collect most of the charge carrier supplied by the emitter. 
  • Base – The middle section of the transistor is known as the base. The base forms two circuits, the input circuit with the emitter and the output circuit with the collector. The emitter-base is forward biased and offers low resistance to the circuit. The collector-base junction is in reverse bias and offers higher resistance to the circuit. The base of a transistor is lightly doped and very thin due to which it offers the majority charge carrier to the base.

The action of n-p-n transistor

npn transistor

The n-p-n transistor consists of two n-type semiconductors that sandwich a p-type semiconductor. Here, electrons are the majority charge carriers while holes are the minority charge carriers.

 
In an n-p-n transistor, the majority of the charge carriers are electrons and holes are the minority charge carriers. A small amount of current at the base terminal causes a large amount of current to flow from emitter to collector. The figure below represents the circuit diagram of the n-p-n transistor:
npn transistor
From the circuit diagram of the n-p-n transistor, it is seen that the emitter-base circuit is forward biased while the collector-emitter circuit is reverse biased.
 

Due to the forward bias, the majority charge carriers in the emitter are repelled towards the base. The electron-hole recombination is very small in the base region because the base is lightly doped. Most of the electrons cross into the collector region.

The action of p-n-p transistor:

pnp transistor

The p-n-p transistor consists of two p-type semiconductors that sandwich an n-type semiconductor. Here, holes are the majority charge carriers while electrons are the minority charge carriers.
 
The figure below represents the circuit diagram of the p-n-p transistor:

pnp transistor

The emitter-base (\(V_{BE}\)) battery connects the p-type emitter which is forward biased whereas the collector-base (\(V_{BC}\)) battery connects the p-type collector which is reverse biased.
 
In this case, the majority charge carriers in emitter are holes which are repelled towards the base. As the base layer is thin, thus only little interaction occurs when electrons and holes combine. Most of the holes reach the collector. The current is carried by holes in p-n-p transistors.

Why are NPN transistors most preferred over PNP transistors?

There are several reasons for this:

  • Carrier Mobility – The majority carriers in NPN transistors are electrons unlike in PNP transistors where the majority charge carriers are holes. Electrons move far more easily than holes within the crystal lattice. As a result, they have higher mobility and operate faster providing a much better level of performance.
  • Production Costs – The manufacture of silicon-based transistors is most economically carried out using large N-type silicon wafers. The manufacture of PNP transistors requires three times more surface area of the wafer, and this significantly increases the costs.
  • Negative Grounding – Over the years, a negative ground has become standard and the polarity of NPN transistors means that the basic transistor configurations operate with a negative ground.

Transistor Modes of Operation

A transistor consists of two junctions which can be biased in different ways. The different working modes of the transistor based on different junction biasing is given in the table below:

Condition  Emitter Junction (EB) Collector Junction (CB) Region of Operation
FR Forward-biased Reversed-biased Active
FF Forward-biased Forward-biased Saturation
RR Reversed-biased Reversed-biased Cut-off
RF Reversed-biased Forward-biased Inverted

FR – In this case, the emitter-base junction is forward biased and the collector-base junction is reverse biased. The transistor is in the active region and the collector current depends on the emitter current. The transistor which operates in this region is used for amplification. 
FF – In this case, both the junctions are forward biased. \The transistor is in saturation and the collector current becomes independent of the base current. The transistors act like a closed switch.
RR –  In this case, both the junctions are reverse biased. The emitter does not supply the majority charge carriers to the base because of which the carrier current is not collected by the collector. Thus, transistors, in this case, acts like a closed switch.
RF – In this case, the emitter-base junction is reverse biased and the collector-base junction forward biased. As the collector is lightly doped compared to the emitter junction it does not supply the majority charge carrier to the base. Hence poor transistor action is achieved.

Frequently Asked Questions on Transistor

What does the arrow in the transistor symbol indicate?

The arrow in the transistor symbol indicates the direction of flow of holes i.e. the direction of conventional current.

List some applications of transistors.

Transistors are used for amplification and switching purpose

Phototransistors that operate based on the intensity of the incoming light are used in optoisolators and light-dependent controlling units

Transistors are used in microwave communication

Are all the regions in a transistor doped?

Yes, all the regions in a transistor are doped. 

Which region in the transistor is highly doped?

The emitter of the transistor is highly doped.

Which junction is forward biased when a transistor is used as an amplifier?

The emitter-base junction of the transistor should be forward biased to be used as an amplifier.

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