Transistors make our electronics world go ‘round. They’re critical as a control source in just about every modern circuit. 

Transistors have been deemed the most important invention of modern times.

In simple terms, a transistor is an electronic switch.

What is a Transistor

A transistor basically acts as a switch and an amplifier. In simple words, we can say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals. Think of a transistor as the electronic equivalent of a valve on a pipe.

Three States of a Transistor

On-Short Circuit: A valve can be completely opened, allowing water to flow freely – passing through as if the valve wasn’t even present.

Off-Open Circuit: When it’s closed, a valve can completely stop the flow of water.

Linear Flow Control: With some precise tuning, a transistor can be adjusted to finely control the flow rate to some point between fully open and closed.

Types of Transistors

Use and Their Casings

The transistor is one of the most used components in modern electronics and logic circuits due to its two main functions: switching and amplification. They are used in both analog and digital circuits, low and high power, and frequency applications. There are various kinds of transistors having their own advantages and drawbacks. These different transistors come in as many different cases and packages as the functions they perform and the voltage, current, and power breakdowns that are required. 

Some common packages are:

These packages all perform a specific function to enable different sensitivities of voltage and current breakdowns or allow a higher power rating through the use of different materials or specialized packages to disperse excess power, usually in the form of heat.

The (BJT) NPN and PNP Transistor

In real life, there are hundreds of types of transistors for just as many applications. They change based on current, voltage, and total power requirements as well as shape and size for their desired location of use. That said, there are two main types of transistors: the NPN and the PNP. Since transistors act as a switch, we need two main types of switches: one whose starting condition will create an open circuit and one whose starting condition will create a closed circuit. For this, we have the NPN and PNP. The NPN transistor acts as if it was a NO (normally open) switch- this stays open until voltage is applied to the base leg of the NPN transistor. The PNP transistor acts as if it was a NC (normally closed) switch- this stays closed until voltage is applied to the base leg of the PNP transistor.

Collectors, Emitters, and the Base

Transistors are composed of three parts: a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is for the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. The Base leg is always in the middle while the Collector and Emitter are on the ends but knowing which is as tricky as it is important. If you swap them on accident you can break the transistor component or cause a malfunction in your circuit. Here is how you can tell which leg is which using two different methods:

Method One: Using the Datasheet

Datasheets are commonplace in engineering when considering parts and components in all different engineering disciplines. A datasheet will provide specific breakdown information on a part that is crucial when designing using those parts. In electronics, the Datasheet will cover things like typical voltage breakdowns, power ratings, resistance and current notes and graphs, and more. Below are two Datasheets for the BJT casing 3904 and 3906 Transistor. 


Method Two: Using a Multimeter

Once you know how a BJT is made, you will realize that the individual legs will have different characteristics that can be measured. Most multimeters have a "Diode Test Mode" that can be used for this since a BJT is made using similar methods and materials as many diodes. This symbol looks like the schematic symbol for a diode and on most Multimeters will be next to the 2K Resistance setting but some will have their own setting entirely. Follow the following steps for this method:

Making a BJT Transistor

Transistors rely on semiconductors to work their magic. A semiconductor is a material that is not quite a pure conductor (like copper wire) but also not an insulator (like air). The conductivity of a semiconductor – how easily it allows electrons to flow – depends on variables like temperature or the presence of more or fewer electrons. Semiconductors with different properties will conduct electricity in different ways. 

Engineers taking advantage of those differences can build transistors using two main properties: Doping and Depletion Zones.


Transistors are built by stacking three different layers of semiconductor material together. Some of those layers have extra electrons added to them (a process called “doping”), and others have electrons removed (doped with “holes” – the absence of electrons). A semiconductor material with extra electrons is called an n-type (n for negative because electrons have a negative charge) and material with electrons removed is called a p-type (for positive). Transistors are created by either stacking an n on top of a p on top of an n, or p over n over p.

Depletion Zones

As you can see from the picture to the left, a depletion zone forms when the negatively charged ions from an N-type attract to the positively charged ions of the P-type and vice-versa. This creates a zone in the middle that is a wall of charge that prevents any further charges from moving through.

Depletion Zones in an NPN Transistor

When we sandwich N-type and P-type semiconductors in an NPN orientation we get an example of an NPN transistor. Since the N-type semiconductors are negatively charged they have free electrons ready to move toward a positively charged material. Likewise, since the P-type semiconductor has positive charges that want to move toward a negatively charged material. Since an NPN transistor has doped semiconductors stacked negative-positive-negative you get a double band of depletion zones where negative charges from the N-types on the end move towards the middle and a split percentage of positive charges move to the outside. This creates an overwhelming negative charge in the depletion zones that prevent any further negative charges from moving toward them because like charges repel each other. An NPN transistor acts as a normally open switch meaning current does not flow. This is until electric potential (voltage) is applied to the base which will actually pull some of the charges away from the depletion layer allowing charges to flow through the transistor completing a circuit.

Examples of a Transistor in Use


Oscillators put out 100% power in a clean square wave. This allows for something to be either 100% on, or 100% off, otherwise known as 1's and 0's for binary. 

Motor Controller

Using transistors, a very low voltage and current input can allow a high voltage and current to flow. In this example, you can also see that it will change the motor's direction and speed from the polarity of the flow and using transistor flow controlling. 

The Past and Future

PAST: Vacuum Tube

PAST: Vacuum Tube Computer

PAST: First Transistor

FUTURE: The 1.4 Trillion Transistor Chip

For more information on Transistors and a few experiments to build to further understand them, check out our page on Transistors in a Circuit.