A transistor is often described as a switch, but this is only partially true. It’s more like a valve or a faucet. If your faucet has only two choices, “on” and “off,” then it behaves as a switch, but most faucets have a range in the middle, and so do most transistors. If the range in the middle is used, we say the transistor circuit is analog. If all that’s used is “on” and “off” then it’s digital. In an analog circuit, the difference between 1/2 on and 2/3 on or 1/3 on is important. In fact, the difference between 1/2 on and 9/16 on might be important. This means that analog transistors need to behave very precisely. It’s also important for the transistor’s response to be “linear.” If a faucet is linear, the flow of water will increase smoothly as you turn the handle one way, and decrease smoothly as you turn it the other way. If turning the handle one way caused the flow to increase and decrease in fits and starts between “all on” and “all off” then it’s non-linear. The same would be true for a transistor. To change from analog to digital, we need to represent the whole range between “all on” and “all off” using a lot of “on” and “off” transistors. That means a digital system needs many more transistors than an analog system, but each transistor can be less precise, non-linear, and much cheaper to make.

There are two kinds of transistors that work in two completely different ways. A bipolar transistor consists of three doped regions of alternating N and P type, with the middle region quite small. The middle region, called the base, is like the handle on the faucet; changing the voltage on the base controls the amount of current that flows from the collector to the emmiter. Bipolar transistors are usually used in analog applications. The other kind of transistor is a Field Effect Transistor, or FET. It consists of alternating doped regions with the middle one being small, like the bipolar transistor, but there is no direct connection to the middle region, and it is lightly doped. A thin insulating layer sits above this middle region, covered by a conductor. This connection is known as the gate. A voltage on the gate effectively reverses the polarity of the lightly doped region between the other two regions, called the source and drain, and this reversal forms an electrical connection between the source and drain, turning the transistor on. FETs are usually used in digital applications.

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