Silicon-controlled rectifier in the context of N-type semiconductor

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

The process is known as rectification, since it "straightens" the direction of current. Physically, rectifiers take a number of forms, including vacuum tube diodes, wet chemical cells, mercury-arc valves, stacks of copper and selenium oxide plates, semiconductor diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches. Historically, even synchronous electromechanical switches and motor-generator sets have been used. Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector".

A thyristor (/θaɪˈrɪstər/, from a combination of Greek language θύρα, meaning "door" or "valve", and transistor ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage of current in one direction but not the other, often under control of a gate electrode, that is used in high power applications like inverters and radar generators. It usually consists of four layers of alternating P- and N-type materials. It acts as a bistable switch (or a latch). There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its gate lead controls the larger current of the anode-to-cathode path. In a two-lead thyristor, conduction begins when the potential difference between the anode and cathode themselves is sufficiently large (breakdown voltage). The thyristor continues conducting until the voltage across the device is reverse-biased or the voltage is removed (by some other means), or through the control gate signal on newer types.

Some sources define "silicon-controlled rectifier" (SCR) and "thyristor" as synonymous. Other sources define thyristors as more complex devices that incorporate at least four layers of alternating N-type and P-type substrate.

The TO-220 is a style of electronic package used for high-powered, through-hole components with 0.1 inches (2.54 mm) pin spacing. The "TO" designation stands for "transistor outline". TO-220 packages have three leads. Similar packages with two, four, five or seven leads are also manufactured. A notable characteristic is a metal tab with a hole, used to mount the case to a heatsink, allowing the component to dissipate more heat than one constructed in a TO-92 case. Common TO-220-packaged components include discrete semiconductors such as transistors and silicon-controlled rectifiers, as well as integrated circuits.