MOS Device

Q1: What is a MOSFET?
A1: A MOSFET is a metal oxide semiconductor field-effect transistor. They're often abbreviated as MOS devices. A MOSFET has 4 terminals:

1. Gate
2. Body
3. Drain
4. Source

Q2: What are some types of MOS devices?

A2: There's n-channel metal oxide semiconductor (NMOS) and p-channel metal oxide semiconductor (PMOS).

Current direction is reversed in the PMOS

Note the 4 terminals of the NMOS

In practice, the body terminal is often connected
to the source terminal of the NMOS

Same goes for the PMOS

Combining the body and source terminals
simplifies the NMOS to a 3 terminal device

Same for the PMOS

Q3: What circuit variables do I need to look for in a MOSFET?
A3: For a MOSFET in which the body and source terminals are connected, there are only 3 currents and 3 voltages to deal with. By KVL, you know that 1 voltage may be written in terms of the other 2. Same goes for current. So it looks like we need to account for 4 circuit variables, as was the case for the BJTs. However, MOSFETs differ from BJTs in that one current is already specified: $i_G$ is always 0. Hence, you only need to keep track of 3 circuit variables.

For an NMOS you might choose to keep track of:

1. $i_D$
2. $v_{GS}$
3. $v_{DS}$

For a PMOS, the current direction is reversed and the voltage subscripts are flipped.