BS250, P-channel MOSFET transistor
A metal-oxide-semiconductor field-effect transistor (MOSFET) is an active component with at least three terminals (electrodes): G (gate), D (drain) and S (source.) In some designs, an additional B (bulk) terminal is fed to the outside. Usually though, B is connected internally with S.
Like other field-effect transistors, a MOSFET acts as a voltage-controlled resistor, meaning that the voltage between gate and source can be used to change the resistance (and thus the electrical current) between drain and source by several orders of magnitude.
Depending on doping type, MOSFETs are classified in two types: n-channel and p-channel MOSFETs.