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I think you are referring to 4016 or 4066 type switches (metal gate CMOS era).
These are FET switches and the switch (between Drain and Source) is
controlled by gate voltage.
Faster versions available like 74AHC4066.
Even faster versions are made from GaAs FET for microwave application.
These are analog switches and you can think as a smll resistor when it
is turned on. But you need to pay attention to power supply voltage and
signal swing range.
S. H.
Transmission gates acts as relays in a circuit. It allows a signal to pass through when swith on.
A simple example of its use is in flip-flop circuits. Clock signal will trigger the transmission gate to switch 'on' or 'off' therefore allowing or disallowing a signal at its input to pass through.
The link bellow has some explanation on cmos transmission gates: **broken link removed**
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.
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