When the gate-source voltage of a MOSFET exceeds a certain voltage (threshold voltage, |Vth|), the drain-source resistance decreases, causing the MOSFET to turn on. This drain-source resistance is called on-resistance.
The direction of the voltage applied across gate and source differs between n-channel and p-channel MOSFETs. The following figure shows the condition under which a MOSFET turns on.
N-channel MOSFET: When the gate voltage is higher than the source voltage by |Vth|, the n-channel MOSFET turns on.
P-channel MOSFET: When the gate voltage is lower than the source voltage by |Vth|, the p-channel MOSFET turns on.
The following figure shows an inverter, a basic building block of CMOS logic ICs.
When VIN is at the VCC or GND level, either the p-channel or n-channel MOSFET is off. Therefore, very little current (ICC) flows between VCC and GND. When the input is in the steady state (at the VCC or GND level), ICC is very low.
The following figure shows the VIN-ICC curve of CMOS.
When VIN is between ０and |Vth| or between VCC－|Vth| and VCC, very little current (ICC) flows between VCC and GND. However, when VIN is between |Vth| and VCC－|Vth|, shoot-through current flows from the p-channel MOSFET to the n-channel MOSFET, increasing ICC. Therefore, care should be exercised to ensure that excessively slowly changing input is not applied to VIN.