High current - via ULN2803, or not

I have a ATMEGA8L-8PU operating at 5V controlling multiple IR LEDs which are used for transmitting instructions. The signal frequency is roughly 100 kHz. The LEDs are currently hooked up via an ULN2803, but it does not meet my current requirements. I need about 10A at 12V.

What would be the easiest way to increase power output?
One approach would be to replace the ULN with something like IRFZ44E which I have lying around. But my partner who will be doing the actual soldering has reservations about messing about near the microcontroller and would prefer to use the ULN as additional protection. That comes with its own set of issues, though.
How would I go about inverting the drain of the ULN to source so I can drive something bigger?

Do you guys have a suggestion?

Using the ULN2803 introduces unnecessary complexity. So the design will get worse actually.

The mosfet you mentioned is not a logic level mosfet. Like other normal mosfets, it is 'on' at 9V gate voltage.

Use a number of logic level mosfets; a protective resistor from Arduino to gate of mosfet; and a pull-down resistor to keep the gate low during power-on.

I hope you don't want to use 100kHz with the logic level power mosfets. If you need such a high frequency, you have to deal with a lot extra problems. You might need a mosfet driver (a mosfet driver can be used with normal mosfets).

I do need to blink the LEDs at 100kHz. That part is not something I can change.

Either use a logic level MOSFET or get a MOSFET driver chip such as a MIC4422 to power the gate of the IRFZ44E - you'll need 12V supply for the MIC4422 and lots of decoupling (0.1uF + 10uF ceramic minimum) on the MIC's supply rails right next to the chip.

logic level is a simpler way to go, but for 10A you'll need a low Rds(on) value (0.01 ohm or less would be great - don't go for a device with more than twice the voltage you need, Rds(on) increases with device voltage rating.

I'm still thinking about the 100kHz. Why is that needed ? Do you want to transmit data via the leds ? Could it be 100Hz instead of 100kHz ?

For such a high frequency, a resistor to a logic level gate might not be fast enough. You have to look for mosfet driver chips. There are a number of those, some common and some rarely used. I can look into it if you like (I have used them myself before). The MIC4422 as mentioned by MarkT has no problem with 100kHz, but you need one of those per mosfet.

If you really want to pulse the LEDs at 100kHz then you definitely need to use a mosfet driver, such as MCP1407 or TC4420. But with the driver, you don't need to use a logic level mosfet, because you can run the driver from the 12V supply so as to give the mosfet 12V gate drive.

It’s 100 kHz, as in 100 000 Hz. It has to be that because the rest of the system operates on that. XD
It is for controlling 1:87 model road vehicles on a model railroad layout. The system continuously sends instructions to all the cars on the layout, so I guess the designer decided on such high freq to increase the bandwidth.

I’ll look for drivers then.

you'll need 12V supply for the MIC4422 and lots of decoupling (0.1uF + 10uF ceramic minimum)

Yes and look at capacitors in the 1,000uF region at the very least. Ones designed for switch mode power supplys and that can take the ripple current. If you just have ceramic caps they will burn out with the ripple current.