What is an epitaxial transistor? I have a simple circuit that calls for a 2SD669A and it's referred to as that. Is it anything special or can I substitute one of my run of the mill 2N3904 that I have a million of ? (I'm guessing I can).

The circuit simply applies .5V output (from a ham xcvr, on transmit) to the collector and the emitter is grounded, then a 10K on the base to ground and another 10K on the base to an output which gives a high/low for transmit on/off. -- thanks

 

Google is your friend " epitaxial transistor / (ˌɛpɪˈtæksɪəl) / noun a transistor made by depositing a thin pure layer of semiconductor material (epitaxial layer) onto a crystalline support by epitaxy. The layer acts as one of the electrode regions, usually the collector". Basically, it is how modern transistors are made... So, yes, you can.

 

http://www.suzushoweb.com/pdf_file/5433ad6be89ce.pdf
Look in a cross-reference database for a possible replacement.
This looks to be a possible replacement device.
https://weisd.com/uploads/product/sheet_pdf/4037/NTE373.pdf

 

Ok. That's what I thought. Sort of redundant to call one that.

 

It was Hot New Stuff back then with a fancy new name! LOL

 

yes -- this was old. "2SD" anything is antique.

 

As to modern equivalent replacements, find the PDF datasheet for the old component to see what its parameters are (Spooky gave it to you in #3). Then here are a couple of replacement tools/guides. One can do a parametric search for similars.
El-Component
All Transistors. Datasheet. Cross Reference Search. Transistor Database.

 

What is an epitaxial transistor?

It refers to how the transistor was constructed. Epitaxial is the same as planar.

From TI Power Semiconductor Databook, first edition:

Is it anything special or can I substitute one of my run of the mill 2N3904 that I have a million of ? (I'm guessing I can).

The 2SD transistor is a power transistor. 2N3904 won't be a good substitution.