This part is a kind of semiconductor called 3N40K-MK.

Function of this product has N-CHANNEL POWER MOSFET.

Manufacturers : Unisonic Technologies

Image and pinout :

Some of the text within the PDF file :

UNISONIC TECHNOLOGIES CO., LTD 3N40K-MK Preliminary 3A, 400V N-CHANNEL POWER MOSFET  DESCRIPTION The UTC 3N40K-MK is an N-channel mode power MOSFET using UTC’s advanced technology to provide customers with planar stripe and DMOS technology. This technology specializes in allowing a minimum on-state resistance and superior switching performance. It also can withstand high energy pulse in the avalanche and commutation mode. The UTC 3N40K-MK is universally applied in electronic lamp ballast based on half bridge topology and high efficient switched mode power supply.  FEATURES * RDS(ON) < 2.0Ω @ VGS=10V * High switching speed * 100% avalanche tested  SYMBOL Power MOSFET  ORDERING INFORMATION Ordering Number Lead Free Halogen Free 3N40KL-TF3-T 3N40KG-TF3-T 3N40KL-TF1 [ ... ]

3N40K-MK PDF Datasheet Download