There are three issues. Whatever the voltage drop it is when on, what the   
   leakage current when off,, and the power dissipated during the turnoff. (the   
   last is usually the most significant in high speed circuits because there are   
   more turnoffs per unit of    
   time)   
      
   MOSFETS and BJTs alog with other devices (capable of somewhat linear operation   
   usually require base/gate drive optimization for efficiency. Thrysistors do   
   not suffer that need. (to any great degree)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)