Why specifically use a MOSFET and Schottky diode on a buck or boost converter and not other transistors or diodes?
MOSFET's are used because the voltage drop across the MOSFET, when on, is lower than the voltage drop across a BJT. This makes the MOSFET more efficient as a switching device.
Schottky diodes are used because the voltage drop across the Schottky diode under forward bias is lower than the voltage drop across an ordinary silicon diode, once again, improving efficiency.
In some cases, instead of a Schottky diode, a second MOSFET may be used. If that is the case, then the second MOSFET would normally be switched on and off in complimentary fashion to the first MOSFET. This is referred to as &quot;synchronous&quot; operation. Synchronous operation is generally more efficient than using a Schottky diode.