arduino - Is it normal for a transistor running a motor to be very hot?

I've researched for a few hours but I'm still concerned.

I'm building a very low budget quadcopter (think toothpicks) and as I've seen on this site, a D10N05 mosfet would do the job. I've visited 3 of the largest electronics shops in my area but unfortunately, they don't have this specific part, or any mosfet as a matter of fact.

Then, I saw this post here on se and lucky enough I have some 2222's collecting dust in a container. I tried it, and it works like a charm. But, one thing I noticed was the transistors would run so hot to the point that I couldn't touch them. Is this to be expected? I have some small heatsinks and some thermal adhesive that I can attach once I know it's okay.

Thanks in advance!

EDIT: The transistor does not reach high temps immediately. It heats up over time.

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Here are the the parts I'm using:

1. 
   

   Small motors, rated at 3.7v 100ma

   
   


2. 
   

   2n2222 transistors

   
   


3. 
   
   

   1n4001 as a flyback diode

   
   


4. 
   

   5v power supply (but I'll use a 3.7v battery on the final build)

   
   


5. 
   

   Arduino nano (PWM control)

   
   

Answer

To answer your question: Yes, it is normal for (power) transistors under load to become very hot while operating. Most are rated for temperatures well above 100 deg C.

Even 60 deg C is too hot to touch, at least for exposed metal tabs and such.

Note that it is only normal for transistors to become hot when operated with substantial current. The transistors used in the question are rated at 1A, and 100mA is substantial enough to cause heating for them.

When operating very small current, for instance for low-speed logic, a hot transistor is indicative of a fault.

Note that the transistors in a modern CPU DO get very warm, but that is not because of high current per transistor but because there are so very many of them in a small constrained package.

When designing a circuit, of course it is desirable that it not run too hot. Keeping the temperature down increases the longevity of the device. However, keeping temperature down can mean choosing beefier transistors, and this costs more. For a quad copter it can also be noted that bigger transistors lead to smaller losses, but also higher weight and cost.

For optimal performance, the transistors shouldn't be too small (short life, danger to operator, high power loss) and not too big (heavy weight, high cost).