What Is the Difference Between a Thyristor & a Transistor?

Transistor Construction

Semiconductor devices contain two or more active regions. Each semiconductor region is chemically altered to allow the movement of either free electrons or spaces in which electrons can fit, called holes. Regions that contain excess electrons are called "n-type" regions, whereas regions with an abundance of available electron holes are called "p-type" regions. The area where the two regions meet is called a "p-n junction."

Bipolar junction transistors are made up of three semiconductor regions -- called the collector, base, and emitter -- and contain two p-n junctions. The collector and emitter regions are doped -- chemically altered -- as the same type of region, whereas the base region is doped differently from the other two regions. Bipolar junction transistors come in two varieties; the NPN transistor contains a p-type base region and n-type collector and emitter regions, while a PNP transistor contains an n-type base and p-type collector and emitter regions.

Thyristors contain four active regions -- two p-type regions and two n-type regions. Thyristors contain three p-n junctions. A thyristor has three leads just as a bipolar junction transistor has; however, the three electrical leads of a thyristor are called the anode, cathode, and gate.

Electrical current will flow through an NPN bipolar transistor if current is applied to the transistor's base lead, and if the voltage at the base lead is higher than the voltage at the emitter lead. If the electrical current is removed from the base lead, the transistor will turn off and cease to conduct electrical current.

A thyristor works like a transistor in that it allows electrical current to flow from the anode to the cathode when a small electrical current is applied to the gate lead. However, unlike a transistor, current will continue to flow through the thyristor after the current is removed from the gate lead. Once activated, the thyristor will continue to conduct electrical current until the thyristor is completely deprived of electrical power.