NPN Transistor: Definition & Equations

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  • 0:04 How Do Robots Move?
  • 0:38 What's an NPN Transistor?
  • 1:26 How Does an NPN…
  • 4:31 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 learn about NPN transistors. How do they function and why are they so useful in modern electronics? We'll also explore their unique characteristics by deriving mathematical equations.

How Do Robots Move?

Ever wonder how robots are able to perform the tasks given them and move about so easily? Although there are many components that come together to make a robot work, a really interesting component called the bipolar junction transistor (BJT) is used to control how much current flows through circuits.

We can buy BJTs of two different types - NPN and PNP. The NPN transistor is more commonly used, and the way it works is really ingenious! So you may understandably ask, ''What's an NPN transistor, and what makes it so special?'' Well, let's get into it!

What's an NPN Transistor?

An NPN transistor is a semiconductor piece in which the positively charged P-region is sandwiched between two negatively charged N-regions, giving us a device with three distinct regions and two PN junctions:

  • Emitter-base junction (EBJ): In this type of PN junction, the very negatively doped emitter region meets with the positively doped base.
  • Collector-base junction (CBJ): In this type of PN junction, the negatively doped collector region meets with the more positively doped base.

We can think of an NPN transistor as the combination of two diodes pointing away from each other. In other words, the anodes of two diodes are joined at the base. A great mnemonic to remember in which direction the diodes are facing in an NPN transistor is 'Not Pointing iN'.

How Does an NPN Transistor Work?

An NPN transistor symbol shows an arrow from the base to the emitter. This means that the current flows from the collector through the base into the emitter. To understand exactly how an NPN transistor works, let's follow one in active-forward operation mode.

Note: As we go through the description, keep in mind that current is defined as the flow of positive charge. Although we now know that it is the movement of electrons that cause current flow, historically it was thought that the current was caused by the movement of positive charges.

Voltages

Let's start at the collector and work our way around. Using an external voltage source, the collector-base junction is reverse-biased, meaning that the collector is held at a higher potential than the base - (VCB). The voltage at C is greater when based against B. Because it is reverse-biased, there is no diffusion of electrons and no current flows between the two terminals.

Note: An easy way to see if a diode is forward-biased is to check whether the positive terminal of the voltage source is connected to the P-region of the diode. If it's connected to the N-region, then the junction is reverse-biased.

The base-emitter junction is forward-biased so that the voltage applied at the base is higher than at the emitter - VBE. Typically, VBE for silicon NPN transistor is 0.7 V and for germanium transistor is 0.3 V.

Currents

Electrons from the negative terminal of the voltage source VBE push electrons in the emitter (N-region) down the diffusion gradient into the base. Because the emitter is highly doped, it has a lot of electrons that diffuse into the base.

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