As long as you have nothing visible in the background (like a backdrop) it should be transparent in the render. I usually use a coloured GROUND and a Skydome, but when I want a transparent background, I just make sure these are not visible and export the render to a PNG file. It should work.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)