What is a Transistor? How does It Work?

Transistors are the building blocks of today’s electronics. The invention of the bipolar transistor that is often known as BJT, has resulted in numerous advancements in the globe. Bipolar transistors are now available in a variety of shapes and sizes. The basic transistor is offered as a leaded transistor or as a surface mount transistor. Transistors are commonly employed in integrated circuits (IC chips). Field effect technology is used in most digital ICs, although bipolar technology is used in many analogue ICs to give the requisite performance. Here you can learn about the transistor basics: what is a transistor, types of transistor and its working.

What is a transistor?

A transistor is a semiconductor device that can conduct as well as insulate electrical current or voltage. A transistor is a switch and an amplifier in one. A transistor is a small device that is used to control or regulate the flow of electronic impulses.

What is a Transistor? How do It Works?

A transistor converts a weak signal from a low to a high resistance circuit. The terms trans and istor refer to the connections’ transfer and resistance properties. To put it another way, it’s a switching device that regulates and amplifies electrical signals such as voltage and current. Two PN junction diodes are connected back to back in a transistor. Emitter, base, and collector are its three terminals. The central part, which is made up of thin layers, is the base. The emitter is on the right side of the transistor, while the collector is on the left. All these terms combine to make up the transistor basics.

Usually the transistor’s emitter-based junction is forward biased, while the collector-base junction is reverse biased, resulting in a large resistance. Transistors are one of the most important components in most modern electrical devices. Transistors are being most common as they have low cost and are smaller in size. They have low operating voltage and no power consumption. However transistors also have some limitations like they get damaged due to electrical and thermal rise events and they are affected by cosmic rays.

Symbol of transistor

NPN transistors and PNP transistors are the two types of transistors. NPN transistors are those transistors that have two blocks of n-type semiconductor material and one block of P-type semiconductor material. PNP transistors, on the other hand, are made up of one layer of N-type material and two layers of P-type material. The symbol of transistor NPN and PNP are depicted in the diagram below.

What is a Transistor? How do It Works?

When emitter-base junction is forward biased, the arrow in the symbol represents the direction of conventional current that flows in the emitter. The only difference between an NPN and a PNP transistors is the current direction.

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Parts of a Transistor

A typical transistor is made up of three layers of semiconductor materials, also called terminals, which help to link the transistor to an external circuit and carry current. The current through the other pair of terminals of a transistor is controlled by the voltage or current that is applied to any pair of terminals of the transistor. For a transistor, there are three terminals.

What is a Transistor? How do It Works?

  • Emitter: This is the left portion of transistor and it provides the majority charge carriers (electrons). As the name indicates that emitter will emit the charge carriers to the next region. Width of emitter is less than that of collector but greater than base.
  • Base: This is the central part of a transistor and it is very thin. When electrons are ejected from emitter, they move to base region. Since it is very thin so these carriers are attracted to the next region.
  • Collector: This portion has greater width than emitter and base, so charge carriers are attracted by this portion by applying reverse voltage to this junction. As the result, current flows through the circuit.

What is a Transistor? How do It Works?

Working of transistor

Silicon is commonly used to make transistors due to its high voltage rating, higher current and lower temperature sensitivity. The base current flows via the base area because the emitter-base section is forward biased. The base current has a relatively tiny magnitude. The electrons flow into the collector region as a result of the base current.

What is a Transistor? How do It Works?

In transistors, the emitter base junction is always forward biased and base collector junction is always reverse biased. The reason is that, by applying forward voltage electrons can be emitted from emitter and then high reverse voltage at collectors collects these electrons to make a current. Due to the thin area of base, only a few electrons get chance to get combined with holes so a very small value of base current flows. While collector current has much greater value than base or emitter current. One thing to note is that collector reverse voltage is always greater than that of emitter forward voltage, so that collector has ability to attract the emitted electrons. This is how do transistors work.

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Why NPN transistor is more preferred over PNP?

Usually, when we deal with electronic circuits then we consider NPN transistors, instead of PNP. The reason is that in NPN transistors, majority charge carriers are electrons. But in PNP transistors, majority carriers are holes and we know that holes actually don’t travel. So it is better to understand working with considering electrons as charge carriers, instead of holes.

Transistor operating conditions

What is a Transistor? How do It Works?

As mentioned that transistors work when its emitter base junction is made forward biased and base collector junction as reverse biased. But what will happen if other biasing is applied?

  • If emitter base junction is forward biased and base collector junction is reverse biased then transistors are said to be active.
  • If both junctions are forward biased then transistors will be in saturation.
  • If emitter junction is reverse biased and collector junction is forward biased then it is in inverted mode.
  • If both junctions are reverse biased then transistors will be in cut off region.

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