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What Is a Transistor and Why is it Important

Updated on November 22, 2012

What is a Transistor

First, the word transistor is a portmanteau of the words "transfer" and "resistor." They were conceived in the early part of the last century (1925 in Canada by Julius Edgar Lilienfeld), but the first real transistor was not built until about 1934 in Germany. As such it was experimental and German inventor Oskar Heil only created a very few transistors to study the effects. He never got around to formally publishing his findings or trying to come up with a means to mass produce them, but they did help in his research.

The first practical mass producible transistor was created by Gordon Teal at Texas Instruments in 1954. Teal was an expert in growing metallic crystals of high purity. Teal's work set the stage for mass produced transistors and many of his discoveries are still used today.

Motorola Transistor (in casing)
Motorola Transistor (in casing) | Source
Progression of Transistors (TO-3, T0-126, TO-92, SOT-23
Progression of Transistors (TO-3, T0-126, TO-92, SOT-23 | Source

What Does a Transistor Do?

Transistors have two functions. They act as switches or as amplifiers.

The transistor as switch concept is pretty simple; it is either on or off. A transistor, acting as switch, allows the flow of electricity or it does not. This flow is always a constant amount of current, never more or less than the amount on the source side of the circuit.

A transistor as amplifier allows a metered flow of electricity. As an amplifier a transistor can allow a variable amount of electricity to flow from one point to another. Acting as amplifier the transistor may power audio circuitry to produce music or sound through speakers.

For a transistor to operate as either switch or amplifier it must be able to control the amount of electricity flowing through it. If you look at the second photo at the right you'll see that all, but the largest transistor has three wires or leads trailing out of it. The two leads on either side are electrical input and electrical output. The center lead is the control wire.

In the very last package (from top to bottom) the control lead is on one side of the transistor and the input and output leads are on the other. This is a bit hard to see, but you can make them out.

The magic of the transistor is this. The amount of current sent to the center lead is considerably less than the amount of current the transistor blocks or allows to flow. So, for example up to fifty (50v) volts of electrical flow can be controlled by a fraction of a volt at the center lead. This example is for a transistor used as switch, but it also applies to a transistor as amplifier.

If the transistor is used as amplifier then the voltage on the center lead can be sent into the transistor at a variable rate with the corresponding voltage coming out of the transistor much greater, but by the same varying amount as that put into the center lead. Transistors as amplifiers can power audio circuits, radio transmission, and signal strength.

Vacuum Tubes in an Early Computer
Vacuum Tubes in an Early Computer | Source

What Came before the Transistor?

Before the transistor were vacuum tubes (thermionic valves in Britain). Vacuum tubes had the exact same function as the transistor, acting as switch or amplifier depending on how they were used. They were considerably larger and used a great deal more power to operate than the transistor.

This is due to the fact that the tubes regulated the amount of power flowing through them by means of a heated surface (the therm in thermionic) that sat between the incoming current and the outgoing current leads.

Because of their size a vacuum tube radio of the nineteen thirties might occupy a cabinet about the size of a toaster oven made today. A transistor radio with the exact same power requirements and audio output could fit in the palm a hand or just hang around the neck like a pendant.

Vacuum Tube Discovery
The first vacuum tube, arguably, was Thomas Edison's light bulb. Granted it only had two terminals and it's only purpose was to produce light, but it still needed a vacuum to operate. The second was Nicola Tesla's light bulb, which closely resembles the compact fluorescent bulb or light of today. The third was as an experiment by Joseph John Thomson. Thomson's vacuum tube had a third filament which was there to investigate "particles" of matter that seemed to be streaming from the glowing filament. These "particles" were named "corpuscles" by Thomson and their discovery eventually earned him the Nobel prize in Physics in 1906. Today we call these "particles" electrons.

Click thumbnail to view full-size
single gate transistor (field effect transistor)floating gate transistor
single gate transistor (field effect transistor)
single gate transistor (field effect transistor) | Source
floating gate transistor
floating gate transistor | Source

Transistors vs Vacuum Tubes

There are many advantages to using transistors that vacuum tubes could not attain. First and foremost the vacuum tube, even at the height of its popularity, was an expensive device to make when compared to a transistor. They also consumed more power, were larger, were not as reliable, and were prone to failure due to heat, breakage, and manufacturing techniques. Vacuum tubes were also problematic in the circuits they were used in. Due to the heat they produced they could cause other components nearby to fail or not operate as well.

Cost Differences
In the nineteen fifties a vacuum tube cost anywhere from $15 to $30 to make. A transistor designed to do the same job cost about $5 to make in the 1960s. Today, with hundreds of thousands of transistors making up integrated circuits, their cost to make is about one hundred thousandth of a cent each.

Power Differences
Transistors require considerably less power to operate than a vacuum tube. After all, a vacuum tube requires a filament, similar to a light bulb, to generate the electrons that need to be routed on or off. A transistor, on the other hand, simply needs a tiny amount of power (in comparison) to function as a switch.

Size Differences
A transistor, when compared to a comparable vacuum tube, is perhaps one one thousandth the size of the tube. This is a bit hard to actually quantify since transistors continue to shrink in size every year to eighteen months.

Transistor Types

There are many types of transistors. The first, called a point contact transistor, is rarely used today. There are also bipolar junction transistors, field effect transistors, diffusion transistors, uni-junction transistors and so on.

The reason there are so many types has to do with size and application. As transistors have gotten smaller their function has been affected. Point contact transistors no longer work effectively as switches or amplifiers once they get down to a certain size. This resulted in redesigns using different techniques to get current to flow when a small amount of power is applied to the gate. In fact almost every transistor type is the direct result of making them smaller and having to overcome problems in their function due to smaller size.

Click thumbnail to view full-size
4004 layout4004 on Dual Inline Package
4004 layout
4004 layout | Source
4004 on Dual Inline Package
4004 on Dual Inline Package | Source

Transistors in Integrated Circuits

Single (called discrete) transistors are still around, but transistors are most useful as a collection on a single chip of silicon. These are called integrated circuits and the very first one was designed by Bob Noyce and Gorden Moore (this is the same Moore of Moore's law). Moore and Noyce were tasked with designing and creating twelve specialty chips for a Japanese firm called Busicom. Integrated Electronics (Intel) did not have the facilities to create twelve separate chips, but they did have the engineering smarts to create one chip that could fill all twelve roles with careful programming.

This was the birth of the Intel 4004. This chip has 2,300 transistors on it that served as computer, memory, mathematical unit and so on. Though other scientists had predicted the birth of the "computer on a chip" no one had thought of a way to make good use of it. Intel did.

The 4004, the first microprocessor made, had over two thousand transistors on it and took up 1/8" of an inch width and 1/3" of an inch in height. (see images at right) The 4004 had the same computing power as the ENIAC computer. ENIAC, built in 1947, took up a basketball court and needed eighteen thousand vacuum tubes to operate. One of the functions of ENIACs staff was to replaced burned out or non-functioning vacuum tubes daily.

Intel wisely bought back the rights to the 4004 chip from Busicom for $60,000. Despite the infusion of cash Busicom went under and Intel prospered.

Wrapping Up

I hope this article helps the reader understand a bit more about transistors, how they operate, and how they have become so important.


The author was not compensated in any way, monetarily, with discounts, or freebies by any of the companies mentioned.

Though the author does make a small profit for the word count of this article none of that comes directly from the manufacturers mentioned. The author also stands to make a small profit from advertising attached to this article.

The author has no control over either the advertising or the contents of those ads.


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