Electrical – Why PMOS act like “close switch” when zero voltage

cmosmosfetpmostransistors

enter image description here

in PMOS that Vtp less than 0 , from what I understand if We give Vg less than Vtp it has current flow from source to drain ,on the other hand if we give Vg more than Vtp it cut-off.

but in digital Circuit it said "when the control voltage, VC on the gate is zero and is thus more negative with respect to either input terminal (source) or the output terminal (drain), the transistor is “ON” and in its saturation region acting as a closed switch. If the input voltage, VIN is positive and greater than VC current will flow from the source terminal to the drain terminal, that is ID flows out of the drain thus connecting VIN to VOUT."

so that mean we give Vg is zero right? . I confuse PMOS act like close switch when it has Vg is zero or lower than Vtp??

Best Answer

The gate voltage is relative to the source. So when the Vgs is less than* the threshold voltage, significant current can flow from source to drain (often threshold is specified as something like 250uA).

In your example where Vg is zero, Vgs is -Vin. So if, say, Vin is +5V then Vgs is -5V and the Rds (assuming a logic-level MOSFET) can be very low.

* greater in magnitude, but negative in sign

Example datasheet:

enter image description here

So if Vin is +5V then Rgs(on) will be less than 60m\$\Omega\$ when Vg = 0.

When off (Vg = +5V) the leakage is guaranteed to be less than -1uA at 25°C.

Related Topic