This post explains for the transistor IRFZ44N.
Part Number : IRFZ44N
Description : HEXFET Power MOSFET ( Vdss = 55V, Rds(on) = 17.5mOhm, Id = 49A )
Package : TO-220AB
Manufactures : International Rectifier
1. Advanced Process Technology
2. Ultra Low On-Resistance
3. Dynamic dv/dt Rating
4. 175°C Operating Temperature
5. Fast Switching
6. Fully Avalanche Rated
Advanced HEXFET Power MOSFETs from International Rectifier utilize advanced processing techniques to achieve extremely low on-resistance per silicon area. This benefit, combined with the fast switching speed and ruggedized device design that HEXFET power MOSFETs are well known for, provides the designer with an extremely efficient and reliable device for use in a wide variety of applications. The TO-220 package is universally preferred for all commercial-industrial applications at power dissipation levels to approximately 50 watts. The low thermal resistance and low package cost of the TO-220 contribute to its wide acceptance throughout the industry.