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What is a Transistor? - Definition, Function & Uses

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  • 0:02 Background on Transistors
  • 1:00 Types of Transistors
  • 1:34 Composition of Transitors
  • 3:48 Transistor Modes of Operation
  • 4:42 Uses of Transistors
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: Babita Kuruvilla

Babita has an electrical engineering degree and has taught engineering students and college students preparing for medical and dental college admissions tests.

In this lesson, we will learn about transistors by exploring their characteristics. We will then look at how they work and find out why they are the key to modern electronics.

Background on Transistors

While watering the garden, you notice that the water pressure is low, and so you turn the faucet valve to its maximum. Water gushes out in full strength, and you marvel at the power you hold literally at the tip of your fingers. You wonder ''If only everything else in life is that easy!''

That's probably what the electronics engineers hoped for when they set out to design a device that would allow them to control the flow of current in circuits. What they came up with is a semiconductor device called a bipolar junction transistor.

Transistors are special because they allow you to control how much current goes through - all of it, some of it, or none at all. You can do this by controlling the voltage across two of the transistor leads.

Bipolar junction transistor
Bipolar junction transistor

Each transistor has three leads: emitter, base, and collector. The collector 'collects' the current and the emitter 'emits' the current, while the base is the region between them.

Types of Transistors

Depending upon where the three terminals of the transistor are located on the semiconductor, we can have either an npn transistor or a pnp transistor. In the transistor symbol, an arrow between the emitter and base indicates the direction of current flow.

npn and pnp transistors symbols
npn and pnp transistor symbols

The sequence of doped regions in a pnp transistor is:

  • p-region collector
  • n-region base
  • p-region emitter

The sequence of doped regions in a npn transistor is:

  • n-region collector
  • p-region base
  • n-region emitter

npn and pnp transistors block diagrams
npn and pnp transistors block diagrams

Composition of Transistors

We can think of a transistor as made of two diodes pointing either towards or away from each other.

npn and pnp transistors using diodes
npn and pnp transistors using diodes

Before we dive into their significance, let's take a moment to familiarize ourselves with terminology that is used to describe diodes.

  • Electrons - Negatively-charged carriers of current
  • Holes - Positively-charged carriers of current
  • Current - Result of the flow of charged carriers - both holes and electrons
  • Doping - Addition of impurities into a semiconductor to form regions rich in electrons or holes - either electron donors (like Phosphorous) or electron acceptors (like Boron) are added into the semiconductor.
  • n-region - Negatively-charged area of the semiconductor into which electron donor atoms are added so that there is an extra electron that is free to float away per donor atom
  • p-region - Positively-charged area of the semiconductor into which electronic acceptor atoms are added so that there is an extra space or hole created per acceptor atom
  • pn junction - Region where the p- and n-regions meet allowing electrons and holes diffuse across

Top: pn junction of a diode; Bottom: symbol for a diode
Top: pn junction of a diode; bottom: symbol for a diode

  • Biasing - Process of adding an outside voltage source to make the diode behave a certain way
  • Forward-biased - Connecting a voltage source such that its positive terminal connects to the p-region and negative terminal to the n-region of the diode; voltage is greater at the tail end compared to the pointed end of the diode symbol
  • Reverse-biased - Connecting a voltage source such that its positive terminal connects to the n-region and negative terminal to the p-region of the diode; voltage is higher at the pointed end when compared to the tail end of the diode symbol

Because the number of electrons and holes is different across the pn junctions, they try to diffuse down their concentration gradient from one region into the other. This diffusion continues until there are equal numbers of each species on both sides, a state called equilibrium.

Transistor Modes of Operation

What makes transistors really useful is the fact that we can carefully calibrate them so that electrons and holes move the way we want them to. There are four different modes of operation that the transistor can be in:

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