MOSFET

In an enhancement-mode N-channel MOSFET, a positive gate–source voltage above the threshold $V_{th}$ attracts electrons and forms a channel.

Saturation region (square-law model):

$I_D = \frac{1}{2}\mu_n C_{ox}\frac{W}{L}\left(V_{GS} - V_{th}\right)^2$

As a switch the key spec is $R_{DS(on)}$, with conduction loss $P = I_D^2 R_{DS(on)}$.

• N-channel — usually on the low side (source to ground); lower $R_{DS(on)}$.
• P-channel — convenient on the high side.
• Power MOSFETs contain a body diode from source to drain.