From Andrew Zimmerman Jones,
Your Guide to Physics.
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A. At its core, a transistor is an electronic component used in a circuit to control a large amount of current or voltage with a small amount of voltage or current.

It does so by sandwiching one semiconductor between two other semiconductors. Because the current is transferred across a material that normally has high resistance (i.e. a resistor), it was a "transfer-resistor" or transistor.

The first practical point-contact transistor was built in 1948 by William Bradford Shockley, John Bardeen, and Walter House Brattain. Patents for the concept of a transistor date as far back as 1928 in Germany, though they seem to have never been built, or at least no one ever claimed to have built them.

Basic Point-Contact Transistor Structure

There are essentially two basic types of point-contact transistors, the npn transistor and the pnp transistor, where the n and p stand for negative and positive, respectively. The only difference between the two is the arrangement of bias voltages.

To understand how a transistor works, you have to understand how semiconductors react to an electric potential. Some semiconductors will be n-type, or negative, which means that free electrons in the material drift from a negative electrode (of, say, a battery it's connected to) toward the positive. Other semiconductors will be p-type, in which case the electrons fill "holes" in the atomic electron shells, meaning that it behaves as if a positive particle is moving from the positive electrode to the negative electrode. The type is determined by the atomic structure of the specific semiconductor material.

Now, consider a npn transistor. Each end of the transistor is an n-type semiconductor material and between them is an p-type semiconductor material. If you picture such a device plugged into a battery, you'll see how the transistor works:

• the n-type region attached to the negative end of the battery helps propel electrons into the middle p-type region.
• the n-type region attached to the positive end of the battery helps slow electrons coming out of the p-type region.
• the p-type region in the center does both.

By varying the potential in each region, then, you can drastically affect the rate of electron flow across the transistor.

Benefits of Transistors

Compared to the vacuum tubes that were used previously, the transistor was an amazing advance. Smaller in size, the transistor could easily be manufactured cheaply in large quantitites. They had various operational advantages, as well, which are too numerous to mention here.

Some consider the transistor to be the greatest single invention of the 20th century, since it opened so much in the way of other electronic advancements. Virtually every modern electronic device has a transistor as one of its primary active components. The computer, in all of its modern forms, could not exist without transistors.

Other Types of Transistors

There are a wide variety of transistor types that have been developed since 1948, and a description of all of them would require more space and technical expertise than is I have. Here's a list (not necessarily exhaustive) of various types of transistors:

• Bipolar junction transistor (BJT)
• Field-effect transistor (FET)
• Heterojunction bipolar transistor
• Unijunction transistor
• Dual-gate FET
• Avalanche transistor
• Thin-film transistor
• Darlington transistor
• Ballistic transistor
• FinFET
• Floating gate transistor
• Inverted-T effect transistor
• Spin transistor
• Photo transistor
• Insulated gate bipolar transistor
• Single-electron transistor
• Nanofluidic transistor
• Trigate transistor (Intel prototype)
• Ion-sensitive FET
• Fast-reverse epitaxal diode FET (FREDFET)
• Electrolyte-Oxide-Semiconductor FET (EOSFET)