Question:
Two conditions are described for the total dissipation parameter in the power ratings of a power transistor or a power MOS FET: Ta = 25°C and Tc = 25°C. What is the difference between these conditions?

Answer:
The specification at Ta = 25°C in the power ratings refers to the total power dissipation of a discrete semiconductor element in an environment with an ambient temperature of 25°C. In this case, the thermal resistance from the heat source to the ambient air is expressed as Rth(j-a).

The specification at Tc = 25°C in the power ratings refers to the total power dissipation when the semiconductor element (case) itself has been forcibly cooled, i.e., when temperature of the package surface is kept at 25°C. Note that the ratings may include the note «with infinite heat sink». However, in actual use, it is very difficult to make the package surface temperature exactly 25°C, and if you also take derating into account, the allowable power will in fact be somewhere in between Ta = 25°C and Tc = 25°C.

Ta = Ambient temperature. This is a still air temperature reading for the environment that the semiconductor is in.

Tc = Case temperature. This is the temperature reading of the case of the semiconductor device.

Ta would normally mean the environment is cooled and kept at 25°C whereas Tc would mean the device is forcibly cooled through a directly attached heatsink and cooled to the 25°C.

Tj = Operating Junction temperature. This is the temperature of the device circuit itself under given conditions. Tj must be calculated from the Tc and/or Ta.

TJmax = Maximum Junction temperature. This is the maximum temperature that the device tolerate.

BSS138, FDV301N, WM02N20G (большая ёмкость), WM02N08G (получше, но большая ёмкость)

FDV301N лучше BSS138

FDV303N хороший, ёмкость больше, чем FDV301N

https://ru.wikipedia.org/wiki/составной транзистор