MOSFET or Transistor in Zetex circuit?

People suggested the use of low RDS(ON)MOSFET instead of the transistor in those DC-DC circuit. I pulled the specs (see bottom) from the Zetex page and got these numbers. I have few questions here:

1) Can I just replace the FMMT617 with a ZXMN2A01F in the famous Zetex circuit?
2) If question one is yes, how to handle those connectors (base, collector, emitter etc.)?
3) Zetex suggested the ZXMN2A01F in their new circuit ZXSC300 Step Down Converter for 3W LED, FMMT617 was in the older circuit. But when comparing the specs, I thought that the transistor had better performance and had lower resistance. Any advice?


Specifications (portion):

FMMT617

RCE(SAT) 0.05 Ohm at 3A
IC CONT 3A
IPeak Pulse Current 12A
Base Current 500mA
Power Dissipation at Temp 25 C 625 mA

ZXMN2A01F

V(BR)DSS = 20V
RDS(ON) = 0.12 Ohm
Continuous drain current 2.2A
Idm Pulse drain current 8A
Power Dissipation at 25 C 625/806 mW

It is likely with a better MOSFET you will need a mosfet driver, as to take full advantage of it, you need , which can be a single transistor, or much more complex.

The other nice thing would be a way to get rid of that lossy schottky diode.

Last thing comes up first. Looking at the ZXSC300 Step down converter for 3W LED, there is also a schottky diode and capacitor as in the ZXSC310 Solution for 1W High Power White LED. The difference of two circuit is that the former is a buck circuit while the latter is a boost circuit. If only want to get rid of the lossy diode, we can choose the efficiency maxium circuit with the output current not filtered (Would it be the same in the MOSFET circuit?).


Second,
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It is likely with a better MOSFET you will need a mosfet driver, as to take full advantage of it, you need , which can be a single transistor, or much more complex.

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What is the difference of MOSFET and transistor drivers and how much advantage we can have? Can I use the ZXSC300 as the driver?

I have these questions because I have a lot of the ZXSC300 and FMMT617. Must use them to build something, build it better if I can.

Edit: Link fixed.

I get the MOSFET converter circuit on this thread I can see that there is a R 0.12Ohm connected between the MOSFET and ground. Would this R cause some power loss and reduce the efficiency?

EDIT: R 0.12Ohm connected between the MOSFET, LED and ground.