Here is a diagram of a typical analog switch. The two field-effect transistors (FETs) are connected in parallel to reduce an effect called “RDS-on modulation”. The effective resistance, drain-to-source, is a function of the signal voltage that the transistor is trying to pass. The graph at the bottom of the page has two curves. One plot is for an N-channel FET, while the other is for a P-channel FET. When the user puts the two FETs in parallel, the fact that their on-resistances are in parallel gives them a much flatter transfer function as the signal voltage changes. That is, the value for RDS(ON) will almost be constant with respect to the input voltage to the switch. Incidentally, the measure of the variation (the RDS(ON) flatness specification) is seldom supplied for a microcontroller.