The Sandwich Transistor 

The sandwich transistor was William Shockley's brainchild. It's also called the junction transistor. While the rest of the lab was busy researching Bardeen and Brattain's point-contact transistor, Shockley secretly worked ahead on his own project. 

The sandwich transistor does in fact look like a little sandwich: two "bread" layers surrounding a piece of "meat." If you attach a wire with current to one side of the bread and measure what comes out the other side, not much happens. But trickle a little current into the meat and suddenly the contraption springs to life. Electricity can race through. 

The transistor base (the "meat" of the sandwich) acts like the handle on a faucet. Turn it one way and current will gush through like water through a hose. Turn it the other and the current will stop altogether. 

The two slabs of "bread" (one is called the emitter and one is called the collector) have excess electrons. These electrons can scoot around jumping from atom to atom, even into the center if circumstances are right. The meat, on the other hand, is missing electrons. (The sandwich could just as easily have bread with few electrons and meat with an excess, but Shockley's first version is described here.) 

The sandwich transistor works because of the curious things that happen at the border between the bread and the meat. A boundary between semiconductors with different amounts of electrons like this is called a P-N junction. And P-N junctions can do interesting things -- they create a one-way road in a crystal. The crystal can conduct in one direction, but not in the other.

A positive voltage in the base pulls the electrons across in the correct direction. It's like squeezing a tube of toothpaste -- the electrons rush out. Changing the voltage to one that makes the electrons travel the wrong way is the equivalent of putting the cap back on. No matter how much you squeeze, the toothpaste won't come out. The current has been stopped completely. 

These days, nobody makes "sandwich transistors" as Shockley designed them. But the concepts he developed with his design created a new class of transistors called junction transistors, some of which are still used today in specialized applications.

Other transistor types:
-- Field-Effect Transistor
-- Point-Contact Transistor

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