This part is a kind of semiconductor called BAS21W.

Function of this product has SURFACE MOUNT FAST SWITCHING DIODE.

Manufacturers : Diodes Incorporated

Image and pinout :

Some of the text within the PDF file :

Features • Fast Switching Speed • Surface Mount Package Ideally Suited for Automated Insertion • For General Purpose Switching Applications • High Conductance • Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2) • Halogen and Antimony Free. “Green” Device (Note 3) • Qualified to AEC-Q101 Standards for High Reliability BAS19W - BAS21W SURFACE MOUNT FAST SWITCHING DIODE Mechanical Data • Case: SOT323 • Case Material: Molded Plastic, "Green" Molding Compound, UL Flammability Classification Rating 94V-0 • Moisture Sensitivity: Level 1 per J-STD-020D • Terminal Connections: See Diagram • Terminals: Solderable per MIL-STD-202, Method 208 e3 • Lead Free Plating (Matte Tin Finish annealed over Alloy 42 leadframe) • Weight: 0.006 grams (approximate) SOT323 T [ ... ]

BAS21W PDF Datasheet Download