Enhancement MOSFET Cheatsheet
Mostly a cheatsheet for my friends and myself
The functional bits
I chose to use a few abbreviations.
- N-channel enhancement MOSFET, let's call it NMOS
- P-channel enhancement MOSFET, let's call it PMOS
- Each MOSFET has three terminals, Drain, source & gate. I'll refer to them as D, S & G from now on.
Each MOSFET can be either turned off or turned on.
- When a PMOS is turned on, current can flow from S to D.
- When a NMOS is turned on, current can flow from D to S.
Regardless of a MOS being turned on or off, there is always one direction current can flow in, like a diode. Look for the diode on the schematics, it'll tell you which direction current will always flow. In most cases, you won't need to make the transitor operate in reverse like that. This also has potential for short circuits and burnt fingers.
- When a PMOS is turned off, current can still flow from D to S if positive voltage is applied to D in respect to S.[1]
- When a NMOS is turned off, current can still from from S to D if positive voltage is applied to S in respect to D.[1:1]
Turning a them on (or off) in terms of binary logic is essentially pulling the G high or low respective to the S voltage.
- To turn a PMOS on, you need to apply a low voltage at G when S is on the high voltage side. Applying high voltage to G will turn it off.
- To turn a NMOS on, you need to apply a high voltage at G when D is on the high voltage side. Applying low voltage to G will turn if off.
Excuse the shoddy KiCAD drawings. I tend to draw schematics in a way that positive rails are usually on top, and ground rails are at the bottom, so almost all the current "flow"[2] from top to bottom. This helps me visualize a lot quite clearly. In this case, the I flipped the PMOS so the direction of current from S to D is downwards.
Background
Recently, a friend of mine wanted to build a half H-bridge, and I quickly realised talking about it, I forgot how to use MOSFETs. Thanks to four way symmetry between N-channel, P-channel, enhancement mode, depletion mode, it can get quite confusing when trying to validate your design or mental model. Most text I found online starts talking about all the things all the time, including the "Metal-Oxide-Semiconductor" part. Valuable knowledge, but it does not help very well when designing simple circuits. I get easily confused by all the "Vds" and "Vgs".
So I decided to write this simple cheatsheet for enhancement type MOSFETs used as a simple switch. This is by no means complete. MOSFET-based design is nuanced, like most things in life. But at least if you just need to use it as a switch, this guide is for you. This cheatsheet is mostly for me and my friends. I skipped a lot of important information that I take for granted, but I would appreciate feedback if you have any.
Maybe I will write some words about reading MOSFET datasheets & what are the important values to look for next. I'll also include a selection of nice and convenient MOSFETs that I will compile from someone else's hard-work.
15-01-2026