This part is a kind of semiconductor called 55N03LT.
Function of this product has PHB55N03LT.
Manufacturers : NXP
Image and pinout :

Some of the text within the PDF file :
Philips Semiconductors Product specification N-channel TrenchMOS™ transistor Logic level FET FEATURES • ’Trench’ technology • Very low on-state resistance • Fast switching • Low thermal resistance • Logic level compatible PHP55N03LT, PHB55N03LT PHD55N03LT QUICK REFERENCE DATA d SYMBOL VDSS = 25 V ID = 55 A RDS(ON) ≤ 14 mΩ (VGS = 10 V) RDS(ON) ≤ 18 mΩ (VGS = 5 V) g s GENERAL DESCRIPTION N-channel enhancement mode logic level field-effect power transistor in a plastic envelope using ’trench’ technology. Applications:• High frequency computer motherboard d.c. to d.c. converters • High current switching The PHP55N03LT is supplied in the SOT78 (TO220AB) conventional leaded package. The PHB55N03LT is supplied in the SOT404 (D2PAK) surf [ ... ]