This part is a kind of semiconductor called BAS21W.

Function of this product has Surface Mount Switching Diode.

Manufacturers : Comchip Technology

Image and pinout :

Some of the text within the PDF file :

Surface Mount Switching Diode BAS16W, BAS19W, BAS20W, BAS21W Voltage: 75 - 200 Volts Power: 200 mWatts COMCHIP www.comchiptech.com Features Fast SwitchingSpeed Surface Mount Package Ideally Suited for Automatic Insertion Electrically Identical to Standard JEDEC High Conductance SOT-323 .087 (2.2) .070 (1.8) .016 (.40) Top View .054 (1.35) .045 (1.15) Mechanical data Case: SOT -23, Plastic Terminals : Solderable per NIL-STD -202, method 208 Approx. Weight: 0.008 gram SINGLE 3 1 2 .004 (0.1) max. .006 (0.15) .002 (0.05) .056(1.4) .047(1.2) .016 (.40) .016 (.40) max. .087 (2.2) .078(2. 0) Dimensions in inches (millimeters) BAS16W, BAS19W, BAS20W, BAS21W Maximum Ratings and Electrical Characterics Ratings at 25°C ambient temperature unless otherwise specif ied. Single phase, ha [ ... ]

BAS21W PDF Datasheet Download