This part is a kind of semiconductor called AN1003.

Function of this product has Thyristors.

Manufacturers : Littelfuse

Image and pinout :

Some of the text within the PDF file :

Teccor® brand Thyristors AN1003 Introduction Due to high-volume production techniques, Thyristors are now priced so that almost any electrical product can benefit from electronic control. A look at the fundamentals of SCR and Triac phase controls shows how this is possible. Output Power Characteristics Phase control is the most common form of Thyristor power control. The Thyristor is held in the off condition -- that is, all current flow in the circuit is blocked by the Thyristor except a minute leakage current. Then the Thyristor is triggered into an “on” condition by the control circuitry. For full-wave AC control, a single Triac or two SCRs connected in inverse parallel may be used. One of two methods may be used for full-wave DC control -- a bridge rectifier formed by two SCRs [ ... ]

AN1003 PDF Datasheet Download