Basic CMOS inverter is the ratioless inverter. It consists of pmos and nmos FET with input from VDD and VSS/ground, respectively. Terminals 'I' and 'O' are gate voltage and output respectively. High voltage (~VDD) at the input, turns-on nmos and switches-off pmos, pull-down output to VSS. Similarly, low gate-voltage I (~0) pull-up output to VDD.