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The transistors we studied earlier have no more than two junctions, to form NPN or PNP bipolar transistors. What happens if we build devices with more junctions? What might a PNPN device be able to do? How about a 5-layer device, either PNPNP or NPNPN? Could we make any practical use of such a device?

The Four-Layer Diode

Let's start by adding one layer to a bipolar junction transistor. This gives us a PNPN structure.

The Silicon Controlled Rectifier

Now let's add a connection to the internal P-type layer.

The Silicon Controlled Switch

Next, we add a connection to the internal N-type layer, to gain access to all regions of the device.

The Diac and Triac

Now it's time to get fancy. Can we arrange it so the device can conduct current in either direction, and still be controlled?

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