What is so special in the power MOSFETS IRFD120 and IRFD110?

The 100V could be a reason: high voltage MOSFET tend to be quite expensive. But I think in this case, the reason is the strange package the IRFD110 uses. This is non standard and certainly uses specific production lines.

And usually, the reason is just that: they use older technology that is more expensive to produce than newer technology. The older products then become more expensive, although they are not better speced (usually, they are worse, as you have noticed for the current handling ).

Now, if you look for a replacement MOSFET, look at the datasheet

• The voltage handling (both in VGS and VDS) should be the same or above (in your case, it's below, so double check that you can do this in your circuit)
• The current handling should be the same or above.
• The VGs gate threshold should be similar (in your case, it slightly below, but that is less of a problem than if it's above)
• The RDSon should be below
• The gate charge should be in the same range (if it's a lot more, you may have to use bigger drivers).
• Make sure the dissipation is appropriate.

If you check all this, I think you're clear.

That package is not widely available from different makers (though I see plenty in stock- eg. 20.5 cents Qty 50 from Future).

If you want the lowest price, now and in the future, newer parts in SMT will likely be the best value, with the bulky TO-220 package being the main through-hole alternative.

The original part has quite a high Rds(on), a not-so-low gate charge and an oddball package. Not great performance.

Generally, all other things being equal, the die size (and thus manufacturing cost) of a MOSFET is related to maximum Rds(on) and Vds rating. But the acreage of silicon does not necessarily track selling price, especially for parts on extremes of the life cycle and in tiny quantities. You may find the price dips as a part is going obsolete, as the stock is dumped onto the market, followed inevitably by unavailability at any kind of reasonable price (brokers and middlemen as well as users doing lifetime buys soak up the stock, with the former looking to make a tidy profit by waiting until the users run out and are forced to choose between huge engineering and testing costs vs. paying a fortune for obsolete parts).

MOSFET technology in particular has been evolving steadily, and newer better and cheaper parts are regularly introduced.

Try doing a parametric search at a distributor- of course you have to understand all about the circuit to know which parameters are important- Vds, Rds(on) at what gate voltage, gate charge, power dissipation capability in a realistic situation (not the stupidly high number on the datasheet), and a number of other factors.

Obviously if you can relax the Vds requirement to 50V or 60V you can get a better performing MOSFET and/or a better price. If the power dissipation turns out to be very low, you might even be able to use an SOT-23 MOSFET which could be very small and cheap.

But it`s always possible, in any given circuit, there is some parameter that does not allow safe substitution and serious problems will arise, as they tend to do if they have not been fully examined quantitatively.

Would it be a good idea to replace the stock of IRFD110 and IRFD120 with FQP30N06L and FQP27P06 for hobby project related applications?

Nobody but a fool is going to say yes to that question. It's all down to the application, the load, the power rails, the switching frequency and the driving circuit, none of which is specified in your question.