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Basic Operations of CMOS Logic ICs

When the gate-source voltage of a MOSFET exceeds a certain voltage (threshold voltage, |V_th|), 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 |V_th|, the n-channel MOSFET turns on.

P-channel MOSFET: When the gate voltage is lower than the source voltage by |V_th|, the p-channel MOSFET turns on.

Building block of the CMOS logic IC (inverter)

The following figure shows an inverter, a basic building block of CMOS logic ICs.
When V_IN is at the V_CC or GND level, either the p-channel or n-channel MOSFET is off. Therefore, very little current (I_CC) flows between V_CC and GND. When the input is in the steady state (at the V_CC or GND level), I_CC is very low.

V<sub>IN</sub>-I<sub>CC</sub> curve of the CMOS logic IC — V_IN-I_CC curve of the CMOS logic IC

The following figure shows the V_IN-I_CCcurve of CMOS.
When V_IN is between ０and |V_th| or between V_CC－|V_th| and V_CC, very little current (I_CC) flows between V_CCand GND. However, when V_IN is between |V_th| and V_CC－|V_th|, shoot-through current flows from the p-channel MOSFET to the n-channel MOSFET, increasing I_CC. Therefore, care should be exercised to ensure that excessively slowly changing input is not applied to V_IN.