0
\$\begingroup\$

I was reading my physics book and found a description of mosfets that was different than the one I'm used to. Here is the picture from the physics book: enter image description here

In summary the physics book describes it's operation as this:

  • Gate = OFF = No potential difference applied to gate. This allows electrons to drift through the n channel (which is already there at the beginning). Current is maintained because of applied potential difference between source and drain.
  • Gate = ON = Potential difference is applied to the gate, in such away that repels the electrons in the n channel into the p substrate. This reverse bias causes the depletion zone between the p-n junction to increase, causing the n-channel to decrease, and the current to stop due to a low amount of charge carriers.

mosfet that I'm used to (taken from a digital design book)

enter image description here

  • Gate = OFF = No current due to no conduction path.
  • Gate = ON = Electrons attracted to the gate, forming a n channel path for electrons to drift through, so current flows.
\$\endgroup\$

1 Answer 1

2
\$\begingroup\$

Your physics book example is that of a depletion-mode MOSFET, as evidenced by the phrase "This reverse bias causes the depletion zone...".

Your digital logic example uses the more familiar enhancement-mode MOSFET.

For a very quick discussion of the differences, see https://en.wikipedia.org/wiki/Depletion_and_enhancement_modes

\$\endgroup\$
2
  • \$\begingroup\$ Nice, I was looking for the vocabulary because neither book mentions "depletion" or "enhancement" modes. Thanks. \$\endgroup\$
    – Michael
    Sep 10, 2015 at 2:32
  • \$\begingroup\$ @Michael - but also note that the first example also has a built-in n-type channel between the drain and source, while the second has only the substrate. \$\endgroup\$ Sep 10, 2015 at 3:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.