Im using a photovoltaic cell to charge up a supercap. When the supercap voltage reaches 2.2 volts, a voltage detector turns on a Darlington TIP120 and dumps the supercaps charge into a hydrogen fuel cell to produce small amounts of hydrogen/oxygen. It works. However, in a quest for efficiency, I tried using a Mosfet (L1404Z) in place of the TIP120, but running into complexities, for example, different size loads seem to alter the turn on point that the Mosfet begins to conduct (as well as other problems). The TIP120 doesnt have these problem. In terms of providing as much available v and current from the super cap to drive the fuel cell, my question for this forum are there advantages of using a Mosfet instead of using the TIP120 to transfer the supercap charge to the fuel cell? Is the TIP120 a good choice? Is there a better choice?