SILICON-CONTROLED RECTIFIER

The SCR (silicon-controlled rectifier or semiconductor-controlled rectifier) is the parent and still the main member of the thyristor family. It is the workhorse of high-power electronics and is sometimes simply referred to as a thyristor. (A thyristor is loosely defined as a device having a four-layer p-n-p-n structure, leading to bistable behavior.) The first p-n-p-n structure was described as a bipolar transistor with a p-n hook-collector by Shockley in 1950. The current-gain mechanism of this hook-collector transistor was further analyzed by Ebers in 1952. The switching characteristics of a two-terminal p-n-p-n structure were first explored by Moll et al. in 1956. Subsequently, the control of switching using a third terminal was examined by Mackintosh and by Aldrich and Holonyak in 1958.

Since the 1960s, the SCR has significant commercial value in the power electronic industry. The SCR is the basis for many specialized but closely related devices.

An SCR has a four-layer p-n-p-n structure. The outermost n- and p-terminals are called cathode and anode, respectively, and the contact to the p-base is the gate (or cathode gate). The top view of the device usually has a circular shape, with the gate located at the center. Notice that the n-base layer is much thicker, from a few microns to a few hundred microns, has a much lower doping density ( ), and is designed to support a large blocking voltage. The impurities usually are incorporated by diffusion into lightly doped starting material, and the two p-regions (( ) can be diffused at the same time. SCRs with high-power capability are discrete devices. They are mounted on pedestals with a good heat sink to dissipate generated heat. The anode is usually bonded onto the package since the gate terminal is near the cathode and needs to be connected separately. As the name implies, SCRs are made of silicon because of its good thermal conductivity, high-voltage and high-current capability, and more-mature technology.