Let Vtn and Vtp denote the threshold voltages of the n and p-devices respectively. The following voltages at the gate and the drain of the two devices (relative to their respective sources) are all referred with respect to the ground (or VSS), which is the substrate voltage of the n -device, namely
Vgsn =Vin , Vdsn =Vout, Vgsp =Vin -VDD , and Vdsp =Vout -VDD .
The voltage transfer characteristic of the CMOS inverter is now derived with reference to the following five regions of operation :
Region 1 : the input voltage is in the range 
. In this condition, the n -transistor is off, while the p -transistor is in linear region (as 
).
Figure 2.11: Variation of current in CMOS inverter with Vin