Hi,

MOSFET's as well as BjT's can be used as switches as well as linear devices, but both has got parameters that you need to take into account, to get them to work as well as possible, whether used as linear or "digital" devices.

For a MOSFET, you need to consider things like the Miller plateau at some V_gs (and hence Q_g and Q_gd), V_gs(th). C_iss, C_oss, (in some apps even C_rss), for starters and if you ignore them, you can just as well ignore the equally important BjT parameters.

All kinds of transistors, whatever the type, needs math for well designed circuits and those that won't do the math are forever destined to just assemble puzzles allready solved by others - and in this case... There is probably several times as many allready designed BjT switches, since the BjT has existed for longer - MOSFET's has some advantages, BjT's some other and it's up to the designer to choose - the puzzlers just need to select from those designs various specs to get the problem solved.