I have an electromagnetic door that I enable/ disable with my relay. I'd like to know what do you think about replacing it with a MOSFET npn. This would save me space and current draw. I'm a bit worried about the tip120 overheating as it'd be on all day. At max value,255
What do you think about it
Voltage is 12vdc x 800mah ish
N-channel MOSFET, not MOSFET npn.
Yes, the TIP120 is an old darlington transistor, don't bother with it as its best used by date was about thirty years ago. This should handle the job just fine without a heatsink: N-Channel MOSFET 60V 30A - COM-10213 - SparkFun Electronics
You always want a LOGIC level MOSFET for use with an Arduino, otherwise the device will likely overheat (best case) or let the smoke out (worst case) due to the lower applied gate voltage.
why is the relay on all day? And the coil current is 12v @ 800mA? That seems quite high! Also a MOSFET you're talking about is N channel NPN is for BJT's... Also TIP120 is not a MOSFET.
We need specifics. If you have a drawing with all the details it would be helpful.
The Tip120 is not a mosfet, but a Darlington transistor.
Mosfets are either n-chanel or p-chanel.
NPN or PNP usually refers to bipolar transistors (BJT)
mah is not a unit of current, mA is.
There's no reason an appropriate mosfet would overheat at 800mA, but if you really want to use a Darlington transistor, you are dealing with ~1-2W of heat, which calls for a little heatsink.
wolframore:
why is the relay on all day?
Because it is as described, an electromagnetic door latch.
Used in medical and aged care facilities. Fail safe.
Do not forget the back-EMF diode across the latch if you have not verified that there is one in the latch (in which case you must be very careful of the polarity. And the indicator will not work if the polarity is wrong either. 
)
I have four of them in my rooms.
TIP120 is not a good choice.
As many people have stated the TIP120 is a darlington Transistor and is not actually meant for switching applications like yours. Also since you are planing to use Arduino here you would need a Logic Level MOSFET so as to control it directly from your 5V Arduino Pin.
Since your Voltage is 12V and current is 800mA which I assume to be continuous current and peak current to be somewhere near 2A. For this purpose you might want look at the IRF540N MOSFET which most people use it with Arduino.
Although it has a high Gate to Source Voltage for 800mA current it should work fine with a Vgs of just 5V. Also it will not have a problem when left turned on for a long time. You can refer the Vgd vs Drain current chart form the IRF540N datasheet.
The link will also help you in using the FET in your application.
I asked for a MOSFET at my local shop, this was the only one they had. https://www.tancredi.it/modulo-driver-mosfet-irf520. I gave it a shot for a minute and it's working well.. yes I have one electromagnetic exactly like paulB described
IRF520 is not a logic level part, it'll have significant losses.
Aren't you switching the coil of the relay that switches the current to the door magnet, rather than the current of the door magnet itself? That relay coil should be in the 50-100 mA range, for which a TIP120 will do just fine. Probably about as well as a partially opened MOSFET such as an IRF520 with 5V at the gate.
Aswinth:
For this purpose you might want look at the IRF540N MOSFET which most people use it with Arduino.
Euhm... Don't think so. That'd be the IRL540 (the L is for logic level), and that in turn is an end of life part. Not recommended for new designs.
I still have a few in my parts box, don't use them much as they don't work at 3.3V.
wvmarle:
IRF520 is not a logic level part, it'll have significant losses.
Actually, it turns out it is nowhere near as bad as that! If you look at the datasheet, figure 2 which is worst case, at 1 A switched by 4.5 V on the gate, the voltage drop is less than 200 mV.
These are the ones available on nicely built and convenient modules cheap from eBay. They are in fact very useful for currents under 1 A as here. I have not yet, but will check them out for automating my latches.
wvmarle:
Aren't you switching the coil of the relay that switches the current to the door magnet, rather than the current of the door magnet itself? That relay coil should be in the 50-100 mA range, for which a TIP120 will do just fine. Probably about as well as a partially opened MOSFET such as an IRF520 with 5V at the gate.
Waste of time, controlling a relay to control the electromagnet which is easily controlled directly. KISS!
Agree that either of these FETs is useless at 3.3 V.