Depletion-Mode MOSFETs
So far we've examined Enhancement Mode MOSFETs, that is, MOSFETs that will NOT conduct electricity until the VGS reaches a particular threshold.
However, something interesting when you rotate the leads around the internal structure of a MOSFET so that the Source and Drain are connected through the main channel, and the Gate is put on both sides of that channel:
[illlustration of N-Chan and P-Chan depletion mode MOSFETs]
Depletion Mode
In this design, by default, when no voltage potential is applied to the Gate, the MOSFET will already be conductive:
[illustration of current flowing]
However, if you apply voltage to the Gate, now the field created between the two internal Gate structures when the minority charge particles congreate between the Gates will cut off the flow of current!
[illustration of current stopped because minority charge particles are in the way]
These types of MOSFETs are known as Depletion Mode MOSFETs and using them is the exact same as Enhancement Mode MOSFETs, except that whether or not they conduct is opposite.
Depletion Mode MOSFETs are particularly useful when the default state of a circuit should be to conduct (be ON), and only when voltage is applied to the Gate, does the circuit turn OFF.