Re: transmission gate
Put an n-MOS and a p-MOS connected together : S of n-MOS to D of p-MOS and D of n-MOS to S of p-MOS.
Their gates are controled complementary in such a way that the gate is either closed or open: i.e. if you apply a PD signal to n-MOS and a PD_c to p_MOS the gate will be open and vice-versa.
The question is why use a Transmission gate that is a switch and not just n-MOS or p_MOS switch.
If you do the math you will see that in order to bring p or n - MOS to saturation you need some time. In transmission gate this time is elapsed since when the one FET is in linear or in weak inversion the other one will be in saturation and vice versa. Hence, the signal will pass quickly and with minimum losses from the gate, since the Ohmick resistance of the switch is minimized ( the FETs are connected in parallel).
These are the advantages of the transmission gate. For more detailed description, either use the FET equations or check Sendra - Smith.
D.