Your part is a silicon controlled rectifier. (or semiconductor-controlled rectifier)
This device is generally used in switching applications.
In the normal "off" state, the device restricts current to the leakage current. When the gate-to-cathode voltage exceeds a certain threshold, the device turns "on" and conducts current.
The device will remain in the "on" state even after gate current is removed as long as current through the device remains above the holding current. Ref:
http://en.wikipedia.org/wiki/Silicon...lled_rectifier.
Click the link and read on down the page.
SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium to high-voltage AC power control applications, such as lamp dimming, regulators and motor control.
SCRs and similar devices are used for rectification of high power AC in high-voltage direct current power transmission. They are also used in the control of welding machines, mainly MTAW and GTAW processes.