Why the threshold voltage of PMOS is different from NMOS?
The p-channel MOSFET, or the PMOS, is the complementary device to the NMOS. This requires applying a negative potential to the gate with respect to the source to turn on the PMOS. In other words, the PMOS threshold voltage is negative.
What is PMOS and NMOS?
NMOS is built with n-type source and drain and a p-type substrate, while PMOS is built with p-type source and drain and a n-type substrate. In a NMOS, carriers are electrons, while in a PMOS, carriers are holes. When a high voltage is applied to the gate, NMOS will conduct, while PMOS will not.
Can a NMOS be used as a voltage switch?
With no signal at the NMOS gate, both the NMOS and the PMOS are OFF. When you apply 3.3V at the gate of the NMOS, that will turn it ON, and will pull the gate of the PMOS down to 0V—this will turn ON the PMOS since now VG-VS (e.g -5V, -12V) is enough. Just need to pick the two the MOSFETs.
How is a NMOS connected to a PMOS?
If you use a PMOS the source pins are connected directly to the positive supply rail which makes it relatively easy to provide a gate-source voltage difference to switch the MOSFET. But if you need an NMOS for efficiency, speed, or power handling, the source pin is not connected to a fixed rail.
When do you apply 3.3V at the NMOS gate?
With no signal at the NMOS gate, both the NMOS and the PMOS are OFF. When you apply 3.3V at the gate of the NMOS, that will turn it ON, and will pull the gate of the PMOS down to 0V—this will turn ON the PMOS since now VG-VS (e.g -5V, -12V) is enough.
Can a high side MOSFET be switched with a PMOS?
In some circuits like an H-bridge, the high side MOSFETs provide some challenges. If you use a PMOS the source pins are connected directly to the positive supply rail which makes it relatively easy to provide a gate-source voltage difference to switch the MOSFET.