How the Transistor Works? – Explained

Introduction – How the Transistor Works?

The transistor is a main building block of all modern electronic systems.  Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor’s ability to change between these two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics). Output current, voltage and power are controlled by its input current. In communication systems it is the primary component in the amplifier.  Transistors are used in digital computers, satellites, mobile phones and other communication systems, control systems etc. Let’s see how the transistor works in detail with working of a npn transistor and also working of a pnp transistor.

Transistor Construction

A transistor has three regions known as emitter, base and collector.

Emitter:

• It is aregion situated in one side of a transistor, which supplies charge carriers (ie., electrons and holes) to the other two regions.
• Emitter is heavily doped region.

Base:

• It is the middle region that forms two P-N junction in the transistor
• The base of the transistor is thin as compared to the emitter and is alightly doped region

Collector:

• It is a region situated in the other side of a transistor (i.e.., side opposite to the emitter) which collects the charge carriers.
• The collector of the transistor is always larger than the emitter and base of a transistor.
• The doping level of the collector is intermediate between the heavy doping of emitter and the light doping of the base.

Symbol for NPN Transistor

• The transistor symbol carries an arrow head in the emitter pointing from the P- region towards the N- region
• The arrow head indicates the direction of a conventional current flow in a transistor
• The direction of arrow heads at the emitter in NPN and PNP transistor is opposite to each other
• The PNP transistor is a complement of the NPN transistor
• In NPN transistor the majority carriers are free electrons, while in PNP transistor these are the holes.

How the Transistor works in NPN Type?

In NPN transistors, majority carriers are electrons.

Working of a NPN Transistor

• The NPN transistor is biased in forward active mode ie., emitter – base of transistor is forward biased and collector base junction is reverse biased
• The emitter – base junction is forward biased only if V is greater than barrier potential which is 0.7v for silicon and 0.3v for germanium transistor
• The forward bias on the emitter- base junction causes the free electrons in the N –type emitter to flow towards the base region. This constitutes the emitter current. Direction of conventional current is opposite to the flow of electrons.
• Electrons after reaching the base region tend to combine with the holes.

• If these free electrons combine with holes in the base, they constitute base current.
• Most of the free electrons do not combine with the holes in the base.
• This is because of the fact that the base and the width is made extremely small, and electrons do not get sufficient holes for recombination.
• Thus, most of the electrons will diffuse to the collector region and constitutes collector current. This collector current is also called injected current, because of this current is produced due to electrons injected from the emitter region.
• There is another component of collector current due to the thermal generated carriers.
• This is called as reverse saturation current and also is quite small.

How the Transistor works in PNP Type?

A PNP transistor behaves the same way as an NPN transistor, with the difference that the majority carriers are holes.

Symbol for PNP Transistor

Working of a PNP Transistor

• Operation of a PNP transistor is similar to npn transistor
• The current within the PNP transistor is due to the movement of holes whereas, in an NPN transistor it is due to the movement of free electrons.
• In PNP transistor, its emitter – base junction is forward biased, and collector base junction is reverse biased.
• The forward bias on the emitter – base junction causes the holes in the emitter region to flow towards the base region.
• This constitutes the emitter current.
• The holes after reaching the base region, combine with the electrons in the base and also constitutes base current.

• Most of the holes do not combine with the electrons in the base region.
• This is due to the fact that base width is made extremely small, and holes does not get sufficient electrons for recombination.
• Thus, most of the holes diffuse to the collector region and constitutes collector region.
• This current is called injected current, because it is produced due to the holes injected from the emitter region.
• There is small component of collector current due to the thermally generated carriers.
• This is called reverse saturation current.

Transistor Configuration

• A transistor is a three terminal device, but we require four terminals ( two for input and two for output) for connecting it in a circuit.
• Hence one of the terminal is made common to the input and output circuits.
• The common terminal is grounded
• There are three types of configuration for the operation of a transistor

CB- Common base configuration

• This is also called grounded base configuration.
• In this configuration emitter is the input terminal, collector is the output terminal and base is the common terminal

Common emitter configuration -CE

• This is also called grounded emitter configuration.
• In this configuration base is the input terminal, collector is the output terminal and emitter is the common terminal

Common collector configuration -CC

• This is also called grounded collector configuration.
• In this configuration, base is the input terminal, emitter is the output terminal and collector is the common terminal.

LIKE WHAT YOU’RE READING?
CHECK OUT SOME OF OUR OTHER GREAT CONTENT HERE:

• APPLICATIONS OF A TRANSISTOR – BJT,UJT & FET
• LVDT- CONSTRUCTION, WORKING PRINCIPLE , APPLICATIONS, ADVANTAGES, AND DISADVANTAGES
• HOW AN IGBT WORKS?
• HOW THE MOSFET WORKS?
• SCR VI CHARACTERISTICS EXPLAINED IN DETAIL
• STRAIN GAUGE WORKING PRINCIPLE
• WHAT IS THE ZENER DIODE? – EXPLAINED
• WHAT ARE THE USES OF CAPACITOR
• POWER FACTOR IMPROVEMENT METHODS
• VARIOUS USES OF A RESISTOR

Share this post: on Twitter on Facebook on Google+