NPN vs. PNP Transistors

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 the physics behind transistors and why they are important as semiconductor devices. We will then compare the characteristics of transistors of the npn and pnp types.

Transistor Radios

Remember the last time you were relaxing at the beach listening to your favorite music station on your mini radio? Ever wondered how such a small device could pick up signals that you couldn't see or hear otherwise?

The secret's in the nifty electronic circuit part inside the radio called a transistor. It is part of the circuit. As it turns out, the invention of transistors using semiconducting material revolutionized the electronic industry.

First transistor radio - Regency TR1
transistor radio

Come along for an exciting ride as we take a look at how transistors work and then explore the two types: npn and pnp!

Semiconductors

Until the 1950s, computers and other electronic devices were built using bulky vacuum tubes. These tubes helped to direct current in one direction, but they weren't easy to maintain. So, we needed other ways to control the flow of electricity - electrical conductivity.

Scientists noticed that elements like Silicon have special conductivity properties because they let electricity pass through them only under certain conditions. These elements called semiconductors were a gift to the engineers because they could now build circuits using these materials.

Transistors

Transistors are one of these electronic components using semiconductors. Transistors were designed to make small currents, voltages, and signals become bigger. They were especially great for strengthening audio and video signals.

Transistors have three points, called terminals - emitter, base, and collector. All three make contact with the circuit.

Transistor
Transistor

Because transistors control flow of current through a circuit, these are called active devices. You could use a transistor to control current at one terminal based on how much voltage is applied across the other two terminals. This is why transistors are also called voltage-controlled current sources.

So, how does a transistor work? How do we tell it when to act as a conductor and when as an insulator? The answer lies in how a transistor is built.

Transistor Composition

A transistor is made of a single piece of Silicon (Si) which is separated into 'n' and 'p' regions by adding different types of impurities. This process of adding impurities is called doping and the impurities are either electron donors or electron acceptors.

n-region

Let's first look at what happens when donor atoms, like Phosphorus (P), are injected into Si.

Phosphorus has five electrons in its outermost shell, but can form only four strong bonds with Si. That leaves its extra electron to float freely, giving the region a negative charge overall and is appropriately called the n-region.

This n-region is a cathode because it acts as a negatively charged electrode through which electrons come to the device.

p-region

Next, let's look at what happens when acceptor atoms, like Boron, are injected into Si.

Boron has only three electrons in its outermost shell to form covalent bonds with neighboring Si atoms. This leaves a positively charged hole in the Si layer and so is called the p-region.

The p-region is an anode because it acts as a positively charged electrode through which electrons leave the device.

p-n Junction

So, what happens when we place the negatively charged n-region and the positively charged p-region right next to each other? We get areas called the p-n junctions.

There are more electrons in the n-region than the p-region. And, there are more holes in the p-region than in the n-region. Now, the charged electrons and holes try to balance out their numbers in each region by passing through the p-n junction into the opposite region. This movement of the electrons and holes is what we call current.

Transistor types

There are two types of bipolar junction transistors: npn and pnp. Both types have three distinct regions with different levels of doping. These areas are the emitter, base, and collector represented by E, B, and C, respectively.

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