The Silicon Controlled Switch

Home	www.play-hookey.com	Sat, 02-04-2012
Digital \| Logic Families \| Digital Experiments \| Analog \| Analog Experiments \| DC Theory \| AC Theory \| Optics \| Computers \| Semiconductors \| Test HTML

Direct Links to Other Semiconductors Pages:
Basic Semiconductor Structures:	[Basic Semiconductor Crystal Structure] [The PN Junction] [The Transistor] [The Early Effect]
Field Effect Transistors (FETs):	[Junction FET] [Depletion Mode MOSFET] [Enhancement Mode MOSFET]
Adding More Junctions:	[The Four-Layer Diode] [The Silicon Controlled Rectifier] [The Silicon Controlled Switch] [The Diac and Triac]
Specialized Devices:	[A Touch of Physics] [Specialized Diodes] [The Unijunction Transistor]

The Silicon Controlled Switch

The silicon controlled switch (SCS) is the next step beyond the silicon controlled rectifier (SCR). It is still a four-layer diode, but this time all four regions are accessible to the external circuit.

The basic construction of the SCS is the same as for the SCR, with the addition of a second gate lead. We thus have an anode, a cathode, an anode gate, and a cathode gate. The equivalent circuit is the same as shown to the right, and even the schematic symbol of the SCS is similar to the SCR symbol.

The SCS has two advantages over the SCR and the four-layer diode. First, because both gate regions are accessible, they can be biased so as to completely cancel the rate effect we described with the four-layer diode. This is now possible because we have access to both gate junctions, so we can design the external circuit to prevent either outside junction from becoming forward biased until we want to trigger the SCS. No matter how fast the applied voltage rises, it now cannot cause the top junction to become forward biased, so it cannot trigger the device on prematurely.

The second advantage is that since we can now control both end junctions, we can actively turn the SCS off without having to reduce the applied voltage or current. All we have to do is force both end junctions into cutoff. This will deny current carriers through the center junction, and turn the SCS off. Thus, the SCS really is a switch, and can be used as one.